The structure of our new online Groundwater Atlas is as follows: here on the main CGS website we include a Table of Contents that links all the various sections and platforms. A master GIS map frames the various basins and aquifers along with their geologic settings. Also available is ON-010D Colorado Groundwater Atlas (Data) – v20210304, a compilation of all GIS data that we had available as of 04 March 2021. All content in this website will be subject to ongoing updates as new information and data become available. Groundwater in Colorado is an ever-changing discipline as many agencies, public entities, and academic institutions are constantly gathering new information to better understand and manage this critical resource. As a web-based information source, this Atlas provides a flexible platform where data can be added as it becomes available. The CGS plans to identify, process, and present pertinent additional data so that the users have access to the most useful information on a timely basis. We consider you, the users, to be our best source for feedback and potential information that may not be readily available through normal literature search platforms. If you have access to, or know about, new data on any of the aquifers, basins, or regions that you feel would enhance the content of this atlas, please contact us.
01 – GROUNDWATER RESOURCES OF COLORADO – AN INTRODUCTION 01.01 Groundwater 01.02 Climate and Water 01.03 Uses and Withdrawals 02 – GROUNDWATER BASICS 02.02 Aquifer Characteristics and Groundwater Flow 02.03 The Hydrologic Cycle 02.04 Surface Water / Groundwater Interaction 02.05 Wells 03 – ADMINISTRATION AND REGULATION OF WATER 03.02 History of Water Development/Water Rights 03.03 Statutory Groundwater Categories 03.04 Well Permitting and Construction 04 – MANAGED AQUIFER RECHARGE 04.02 Technologies 04.03 Managed Aquifer Recharge 04.04 Underground Water Storage Potential 05 – COLORADO WATER PLAN 06 – GROUNDWATER QUALITY 06.02 Water Standards 06.03 Groundwater Protection/Aquifer Vulnerability 06.04 Impacted Groundwater 07 – GEOTHERMAL RESOURCES 08 – UNDERGROUND INJECTION 08.02 Wastewater Disposal 08.03 Aquifer Storage & Recovery 08.04 Carbon Sequestration 09 – AQUIFERS OF COLORADO 09.01 Aquifer Types 09.02 Hydrogeologic Units 09.03 Aquifer Location 10 – MAJOR ALLUVIAL AQUIFERS 10.01 South Platte River Basin 10.02 Republican/Arikaree River Basin 10.03 Arkansas River Basin 10.04 Rio Grande River Basin 10.05 Gunnison River Basin 10.06 Colorado River Basin 10.07 Yampa River Basin 10.08 White River Basin 10.09 North Platte River Basin 10.10 San Juan River Basin 10.11 Dolores River Basin 11 – SEDIMENTARY BEDROCK AQUIFERS 11a – Regional Sedimentary Bedrock Aquifer Systems 11a.01 Colorado Piedmont Region 11a.02 High Plains Aquifer 11a.03 Colorado Plateaus Region 11b – Structural Basin Aquifers 11b-01 – Ancestral Rocky Mountains Basins 11b.01-01 Eagle Basin-Central Colorado Trough 11b.01-02 Paradox Basin 11b.01-03 Ancestral Denver Basin 11b-02 – Laramide Basins 11b.02-01 Denver Basin 11b.02-02 Cheyenne Basin 11b.02-03 Raton Basin 11b.02-04 San Juan Basin 11b.02-05 Piceance Basin 11b.02-06 Sand Wash Basin 11b.02-07 South Park Basin 11b.02-08 Colorado Headwaters Basin 12 – MOUNTAINOUS REGION AQUIFERS 12a – Mountainous Valleys 12a.01 South Park 12a.02 San Luis Valley 12a.03 Upper Arkansas Valley 12a.04 Wet Mountain/Huerfano Park 12a.05 Blue River Valley 12a.06 North-Middle Parks 12b – Mountainous Region Volcanic Center Rocks 12c – Crystalline Bedrock Aquifers 13 – Geologic Evolution of Colorado as the Foundation for its Groundwater Resources 13.01 Precambrian Basement 13.02 Paleozoic Carbonates 13.03 Ancestral Rocky Mountains Event 13.04 Mesozoic Sandstones 13.05 Interior Seaway 13.06 Laramide Mountain Building 13.07 Cenozoic Extension 13.08 Quaternary Landscape BIBLIOGRAPHY and GROUNDWATER GLOSSARY GROUNDWATER ATLAS GIS MAP CONTACT US
A primary purpose for the Atlas is to collate and present a wide range of GIS data relating to groundwater resources across the state wherever available. ON-010M — Colorado Groundwater Atlas (Map) – v20210304 — Includes the full extent of groundwater data currently available along with detailed information on the geological environment that affects groundwater: its availability, quality, and quantity. ON-010D — Colorado Groundwater Atlas (Data) – v20210304 — For those who would like to work with a static downloadable dataset of the full map contents as of 04 March 2021, we provide a ZIP file that also contains a full set of all figures and tables included on the Atlas web site. We recently added direct links to particular GIS web mapping apps that also appear in the main Atlas map. These include: Selected web mapping apps ON-010-04 Managed Aquifer Recharge — See Section 04 Managed Aquifer Recharge for background. ON-010-10 Major Alluvial Aquifers — See Section 10 Major Alluvial Aquifers for background. ON-010-11a Regional Sedimentary Aquifers — See Section 11a Regional Sedimentary Bedrock Aquifer Systems for background. ON-010-11b-01 Ancestral Rocky Mountain Basin Aquifers — See Section 11b-01 Ancestral Rocky Mountains Basins for background. ON-010-11b-02 Laramide Basin Aquifers — See Section 11b-02 Laramide Basins for background. ON-010-12 Mountainous Region Aquifers — See Section 12 Mountainous Region Aquifers for background. ON-010-13 Geologic Evolution of Colorado — See Section 13 Geologic Evolution of Colorado as the Foundation for its Groundwater Resources for background. From one of our collaborators: Colorado Groundwater Vulnerability Atlas — Developed by the Colorado Environmental Public Health Tracking Program, this map — a valuable resource for private well owners, well drillers, consultants, among other constituencies — integrates location information for sites with vulnerable groundwater under various agencies’ regulatory authority. It also provides analytical tools for determining the proximity of a selected location to those sites with vulnerable groundwater conditions.
See ON-010 Colorado Groundwater Atlas (Data) for a full set of GIS data and all the following Figures and Tables. 01 – GROUNDWATER RESOURCES OF COLORADO – AN INTRODUCTION Figure ON-010-01-01 :: Annual precipitation patterns in Colorado Figure ON-010-01-02 :: Annual runoff patterns in Colorado Figure ON-010-01-03 :: “Snake Diagram” schematic showing relative annual average stream flow of Colorado Rivers Figure ON-010-01-04 :: Annual lake evaporation patterns in Colorado Figure ON-010-01-05 :: Mean annual water balance in Colorado Figure ON-010-01-06 :: Distribution of water source by surface water or groundwater Figure ON-010-01-07 :: Principal use of groundwater withdrawals by county 02 – GROUNDWATER BASICS Figure ON-010-02-01 :: Aquifer material types Figure ON-010-02-02 :: Recharge pathway in unconfined aquifer Figure ON-010-02-03 :: Main types of aquifers Figure ON-010-02-04 :: The hydrologic cycle Figure ON-010-02-05 :: Gaining stream schematic diagram Figure ON-010-02-06 :: Losing stream schematic diagram Figure ON-010-02-07 :: Schematic diagram A) groundwater discharge to stream, B) pumping well intercepting discharge to stream Figure ON-010-02-08 :: Water supply well diagram 03 – ADMINISTRATION AND REGULATION OF WATER Figure ON-010-03-01 :: Colorado water divisions. Figure ON-010-03-02 :: Designated groundwater basins Figure ON-010-03-03 :: Well types Figure ON-010-03-04 :: Water supply well diagram 04 – MANAGED AQUIFER RECHARGE Figure ON-010-04-01 :: Objectives of managed aquifer recharge Figure ON-010-04-02 :: Surface infiltration Figure ON-010-04-03 :: Subsurface infiltration Figure ON-010-04-04 :: Direct injection into an unconfined aquifer Figure ON-010-04-05 :: Direct injection into an confined aquifer Figure ON-010-04-06 :: Statewide managed aquifer recharge sites Figure ON-010-04-07 :: Statewide sites for water management objectives Figure ON-010-04-08 :: Statewide potential for underground storage in alluvial aquifers. Figure ON-010-04-09 :: Statewide potential for underground storage in bedrock aquifers 05 – COLORADO WATER PLAN Figure ON-010-05-01 :: Colorado Roundtable Basins 06 – GROUNDWATER QUALITY 07 – GEOTHERMAL RESOURCES 08 – UNDERGROUND INJECTION 09 – AQUIFERS OF COLORADO Figure ON-010-09-01 :: Aquifer material types Figure ON-010-09-02 :: Main types of aquifers Figure ON-010-09-03 :: Aquifer distribution by primary type Figure ON-010-09-04 :: Physiographic regions of Colorado Figure ON-010-09-05 :: Aquifer regions of Colorado 10 – MAJOR ALLUVIAL AQUIFERS Figure ON-010-10-01 :: Statewide alluvial aquifers Figure ON-010-10-02 :: Elevations of older river terraces above modern rivers Figure ON-010-10-01-01 :: South Platte River Basin aquifers Figure ON-010-10-01-02 :: Saturated thickness of the lower South Platte River alluvium Figure ON-010-10-02-01 :: Republican/Arikaree River Basin alluvial aquifers Figure ON-010-10-03-01 :: Arkansas River Basin alluvial aquifers Figure ON-010-10-04-01 :: Rio Grande River Basin alluvial aquifers Figure ON-010-10-05-01 :: Gunnison River Basin alluvial aquifers Figure ON-010-10-06-01 :: Colorado River Basin alluvial aquifers Figure ON-010-10-07-01 :: Yampa River Basin alluvial aquifers Figure ON-010-10-08-01 :: White River Basin alluvial aquifers Figure ON-010-10-09-01 :: North Platte River Basin alluvial aquifers Figure ON-010-10-10-01 :: San Juan River Basin alluvial aquifers Figure ON-010-10-11-01 :: Dolores River Basin alluvial aquifers 11 – SEDIMENTARY BEDROCK AQUIFERS Figure ON-010-11-01 :: Statewide sedimentary bedrock aquifers 11a – Regional Sedimentary Bedrock Aquifer Systems 11a.01 – Colorado Piedmont Region Figure ON-010-11a-01-01 :: Colorado Piedmont region sedimentary bedrock aquifers Table ON-010-11a-01-01 :: Colorado Piedmont region stratigraphic chart 11a.02 – High Plains Aquifer Figure ON-010-11a-02-01 :: High Plains region aquifer Figure ON-010-11a-02-02 :: High Plains region and well distribution Table ON-010-11a-02-01 :: High Plains region stratigraphic chart 11a.03 – Colorado Plateaus Region Figure ON-010-11a-03-01 :: Colorado Plateaus region sedimentary bedrock aquifers Table ON-010-11a-03-01 :: Colorado Plateaus region stratigraphic chart 11b – Structural Basin Aquifers 11b-01 – Ancestral Rocky Mountains Basins Figure ON-010-11-02 :: Primary Ancestral Rocky Mountains features 11b.01-01 – Eagle Basin-Central Colorado Trough Figure ON-010-11b-01-01-01 :: Eagle Basin-Central Colorado Trough sedimentary bedrock aquifers Figure ON-010-11b-01-01-02 :: Generalized cross section through the Eagle Basin Table ON-010-11b-01-01-01 :: Eagle Basin-Central Colorado Trough sedimentary stratigraphic chart 11b.01-02 – Paradox Basin Figure ON-010-11b-01-02-01 :: Paradox Basin-sedimentary bedrock aquifers Figure ON-010-11b-01-02-02 :: Generalized cross section through the Paradox Basin Table ON-010-11b-01-02-01 :: Paradox Basin-sedimentary stratigraphic chart 11b.01-03 – Ancestral Denver Basin Figure ON-010-11b-01-03-01 :: Ancestral Denver Basin sedimentary bedrock aquifers Table ON-010-11b-01-03-01 :: Ancestral Denver Basin stratigraphic chart 11b-02 – Laramide Basins 11b.02-01 – Denver Basin Figure ON-010-11b-02-01-01 :: Denver Basin extent and water well distribution Figure ON-010-11b-02-01-02 :: Denver Basin core hole and cross section geophysical log locations Figure ON-010-11b-02-01-03 :: Denver Basin cross section showing different geologic units and aquifers Table ON-010-11b-02-01-01 :: Denver Basin stratigraphic chart 11b.02-02 – Cheyenne Basin Figure ON-010-11b-02-02-01 :: Cheyenne Basin extent Table ON-010-11b-02-02-01 :: Cheyenne Basin stratigraphic chart 11b.02-03 – Raton Basin Figure ON-010-11b-02-03-01 :: Raton Basin extent and well distribution Figure ON-010-11b-02-03-02 :: Generalized cross section of the Raton Basin Table ON-010-11b-02-03-01 :: Raton Basin stratigraphic chart 11b.02-04 – San Juan Basin Figure ON-010-11b-02-04-01 :: San Juan Basin extent and well distribution Figure ON-010-11b-02-04-02 :: Generalized cross section of the San Juan Basin Table ON-010-11b-02-04-01 :: San Juan Basin stratigraphic chart 11b.02-05 – Piceance Basin Figure ON-010-11b-02-05-01 :: Piceance Basin extent and well distribution Figure ON-010-11b-02-05-02 :: Generalized cross section of the Piceance Basin Table ON-010-11b-02-05-01 :: Piceance Basin stratigraphic chart 11b.02-06 – Sand Wash Basin Figure ON-010-11b-02-06-01 :: Sand Wash Basin extent and well distribution Figure ON-010-11b-02-06-02 :: Generalized cross section of the Sand Wash Basin Table ON-010-11b-02-06-01 :: Sand Wash Basin stratigraphic chart 11b.02-07 – South Park Basin Figure ON-010-11b-02-07-01 :: South Park Laramide basin extent Table ON-010-11b-02-07-01 :: South Park Laramide Basin stratigraphic chart 11b.02-08 – Colorado Headwaters Basin Figure ON-010-11b-02-08-01 :: Colorado Headwaters Laramide basin extent Table ON-010-11b-02-08-01 :: Colorado Headwaters Laramide Basin stratigraphic chart 12 – MOUNTAINOUS REGION AQUIFERS Figure ON-010-12-01 :: Mountainous Region 12a – Mountainous Valleys 12a.01 – South Park Figure ON-010-12a-01-01 :: South Park mountainous valley Figure ON-010-12a-01-02 :: Generalized cross section of the South Park Basin Table ON-010-12a-01-01 :: South Park stratigraphic chart 12a.02 – San Luis Valley Figure ON-010-12a-02-01 :: San Luis Valley Figure ON-010-12a-02-02 :: Generalized cross section of the San Luis Valley Table ON-010-12a-02-01 :: San Luis Valley stratigraphic chart 12a.03 – Upper Arkansas Valley Figure ON-010-12a-03-01 :: Upper Arkansas Valley Figure ON-010-12a-03-02 :: Schematic cross section of the Upper Arkansas Valley Table ON-010-12a-03-01 :: Upper Arkansas Valley stratigraphic chart 12a.04 – Wet Mountain / Huerfano Park Figure ON-010-12a-04-01 :: Wet Mountain Valley and Huerfano Park Figure ON-010-12a-04-02 :: Huerfano Park generalized cross section Table ON-010-12a-04-01 :: Wet Mountain Valley / Huerfano Park stratigraphic chart 12a.05 – Blue River Valley Figure ON-010-12a-05-01 :: Blue River Valley Table ON-010-12a-05-01 :: Blue River Valley stratigraphic chart 12a.06 – North-Middle Parks Figure ON-010-12a-06-01 :: North and Middle Parks **Figure ON-010-12a-06-02 :: Table ON-010-12a-06-01 :: North Park stratigraphic chart Table ON-010-12a-06-02 :: Middle Park stratigraphic chart 12b – Mountainous Region Volcanic Center Rocks Figure ON-010-12b-01 :: Mountainous volcanic region Table ON-010-12b-01 :: Mountainous volcanic region stratigraphic chart 12c – Crystalline Bedrock Aquifers Figure ON-010-12c-01 :: Mountainous region crystalline bedrock Figure ON-010-12c-02 :: Hydrologic model in fractured crystalline bedrock 13 – Geologic Evolution of Colorado as the Foundation for its Groundwater Resources Figure ON-010-13-01 :: Generalized geologic map of Colorado, by event Figure ON-010-13-02 :: Schematic cross section of strata from each major event in the geologic evolution of Colorado Table ON-010-13-01 :: Major geologic events in Colorado and their influence on groundwater
(01.01) Sustainable use of water is a priority for Colorado today. The state’s rapid and continued growth and its popularity as a tourist destination has focused attention on the natural resources required to sustain its wildlife, communities, businesses, and recreational sites. Demands on Colorado’s water supply are dominated by agricultural, domestic, and industrial use but also increasingly include requirements for recreation and ecosystems. With its ready access and storage capability, both historic and current, surface water provides the bulk of the water supply across the state. Over-appropriation of this resource, combined with increased demand and a lack of suitable or acceptable future storage reservoir sites, has directed attention to groundwater. Water is essential for life on this planet. It is all around us, in vapor form in the atmosphere, in solid form as ice and snow, and in its liquid form as precipitation, streams and rivers, ponds lakes and oceans. Some water inevitably seeps into the ground where it becomes groundwater. Underground, it fills and moves through open spaces, or pores, between the grains of soil or rock. Grains of soil and rock may be loose, or unconsolidated in geologic materials near the ground surface. Alternatively, the grains may be tightly compacted or cemented together by other minerals, forming solid bedrock. Many bedrock formations have little or no pore space between grains and must have fractures, or open cavities to hold and transmit water. An aquifer is a geologic material that holds groundwater that can be released to springs and wells. Aquifers in Colorado can be classified by whether they are made up of loose unconsolidated materials, consolidated bedrock, or fractured crystalline bedrock. They are found throughout the state in its many river basins and geologic settings with the type depending on its setting and geologic history. The use of groundwater in Colorado for public supply and domestic and industrial purposes began before 1900. Groundwater resources currently supply approximately 18 percent of the state’s needs and groundwater development is continuing at a fast pace. Historic patterns of groundwater development reflect the state’s climate and population distribution patterns. Future patterns of groundwater use will undoubtedly have to respond to evolving climate and population changes. Back to Table of Contents
The climatic conditions of a region ultimately control its overall water resources. Colorado’s location, far inland from any ocean, and its variable topography greatly influence weather patterns, producing a semiarid climate with hot summers and cold winters. These conditions yield a mean annual statewide precipitation of 17 in (43 cm). Precipitation patterns are not evenly distributed (Figure 01-01), with most precipitation falling in the mountainous areas. Much of that falls west of the Continental Divide. A large portion of precipitation that falls on the land surface during storm events runs off directly into surface water drainages. Yet some infiltrates the soil to recharge the underlying aquifer with much of that eventually making its way to the surface water as baseflow. Direct runoff and baseflow contribute to surface water. Average annual runoff patterns mimic precipitation patterns, with the greatest runoff originating high in the mountains, and on the west side of the Continental Divide (Figure 01-02). Because of this uneven distribution, precipitation runoff through the major river systems headwatered in the state is very uneven, with a far greater portion heading west via the Colorado River and its tributaries (Figure 01-03). Climatic conditions also control groundwater resources because the primary source of groundwater is the infiltration of precipitation. A great portion of precipitation returns to the atmosphere by direct evaporation (Figure 01-04) and through transpiration of vegetation, collectively called evapotranspiration. On average, 81 percent of the precipitation that falls on the land surface is lost through evapotranspiration, which is a combination of direct evaporation and transpiration through vegetation. The difference between precipitation and potential evapotranspiration can be either positive or negative, depending on location. Much of the state, with the exception of the higher mountainous regions, is in deficit with evaporation exceeding precipitation (Figure 01-05). In areas of deficit the potential for direct recharge to the aquifers comes mainly during infrequent periods of prolonged precipitation, and particularly winter and spring snowfall events. While the amount of water recharged to a local aquifer is dependent upon climatic conditions, land surface characteristics, and aquifer characteristics, the delicate water balance in Colorado limits the water available for long-term storage from fractions to a few inches per year. Given these low recharge rates, it is readily understood why groundwater resources should be considered a finite and very limited resource.
Because of Colorado’s climatic conditions and topography, most of the easily accessible surface water is found in the western half of the state. Unfortunately, the demand for water in the eastern portion of the state where 85 percent of the population resides exceeds the available supply in that region. This disparity between water supply and population centers has sparked tremendous controversy because Western Slope water is being apportioned and diverted across the Continental Divide to the thirsty urban corridor of the Front Range. Availability and relative ease of diversion makes surface water the dominant water source in Colorado for both public and industrial supplies. Groundwater, however, is being developed at an increasing rate to keep up with demand. As illustrated in Figure 01-06 — the distribution and amount of both groundwater and surface water used by counties in 1995 — groundwater withdrawals are typically less than 40 percent of total water use, with the exception of some counties where surface streams are sparse, such as in the eastern plains. As of early 2019, over 285,000 water supply wells of record were drilled in Colorado, serving a variety of uses (Figure 01-07). Groundwater withdrawals are estimated to exceed 2.5 million acre-feet per annum with the dominant use in Colorado for agricultural application, which includes irrigation of commercial crops and watering of livestock. Groundwater withdrawals by wells for domestic/residential uses are estimated to range from 81.6 million gallons per day to 142.4 million gallons per day, based on average use numbers from the Water Foundation. Colorado needs more water storage and aquifers are gaining recognition as an alternative to surface storage. Aquifer storage has many advantages over surface storage: no evaporative losses, ability to build incrementally, and fewer environmental impacts. Increased recognition of the vital role that groundwater plays in Colorado’s water portfolio also increases the recognition of how important it is to protect groundwater from contamination. Threats to groundwater quality are many, and many historic activities have left a lasting impact on the usability of local groundwater resources. Colorado’s planning for future growth needs to recognize and account for protection of groundwater. Not only are aquifers considered sources of groundwater, but in some instances they may serve other functions. In some areas deep aquifers are used for wastewater disposal or are being considered for carbon sequestration. Many have potential as a geothermal resource.
(02.01) Groundwater is water present beneath the surface of the earth where it fills open pore spaces between soil particles, rock grains in sedimentary rocks or open fractures in crystalline rocks (Figure 02-01). Because groundwater is hidden from view, many people think of its occurrence in the form of underground lakes, streams, and veins. Only in very special geologic settings, such as cavernous limestone and lava tubes in certain types of volcanic rocks, does groundwater occur as viewed in this common perception. The ultimate source of groundwater is precipitation in the form of rain, snow, sleet, or hail. Precipitation that does not immediately evaporate or flow away as surface water infiltrates into the ground. Rate and efficiency of water infiltration through soil and rock, commonly referred to as percolation, depends on many factors, including land use, soil physical properties and moisture content. It also depends on the intensity and duration of precipitation. Precipitation that falls as snow or hail may melt slowly, allowing more to infiltrate than if it fell as intense rain. When rainfall is intense, exceeding the rate of infiltration, water accumulates on the surface and runs off downhill as overland flow. As gravity pulls the water through the pore spaces or fractures in the soils, sediments, or rocks some may be taken up by plant roots. What is left continues to infiltrate until it reaches a depth where all of the available pore space is completely filled with water. At this depth (Figure 02-02) the material is considered saturated, and if it is permeable enough, it may be considered an aquifer. Water that infiltrates to the aquifer is considered recharge. An aquifer is a groundwater reservoir composed of geologic materials that are saturated with water and sufficiently permeable to yield water in a usable quantity to wells and springs. Sand and gravel deposits, sandstone, limestone, and fractured, crystalline rocks are examples of geologic formations that form aquifers. Back to Table of Contents
Aquifers provide two important functions: (1) they transmit groundwater from areas of recharge to areas of discharge, and (2) they provide a storage medium for useable quantities of groundwater. The amount of water a material can hold depends upon its porosity, the relative amount of open space between grains, or in open fractures. The size and degree of interconnection of those openings determine the material’s permeability, or its ability to transmit fluid. Geologic formations are often classified as either aquifers or confining units, depending on their ability to transmit water and yield it for beneficial use. Confining units are those geologic materials, such as clay and shale, with insufficient porosity and/or permeability to transmit and yield water for beneficial use. The term hydrogeologic unit is used for a geologic formation, or group of formations, with characteristics that make it either an aquifer or a confining unit. The arrangement of pore space in a geologic material that determines levels of porosity and permeability depends on many factors. If the material is granular the most important factors are sorting and roundness of grains; the more rounded and sorted the better the porosity and permeability. Grain size is also important with very fine grained granular material having limited permeability even though the grains may be well sorted because of capillary forces. Crystalline rocks are rocks where mineral crystals interlock with little or no open space in between. Pore space is gained by fracturing that type of rock. Porosity and permeability in crystalline rock depends on how many fractures cross-cut the rock, how open they are, and how well interconnected they are. In many sedimentary deposits minerals crystallize in the pore spaces between grains over time, filling, or partly filling the pore spaces. The minerals often bond with the grains to lock them together as one solid mass. In most sedimentary formations this is part of the process that changes an unconsolidated deposit into a consolidated bedrock formation. What was once a well-sorted, well-rounded, sedimentary deposit became, over geologic time, a solid rock with little open and connected pore space. Fractures in a crystalline rock can fill with mineral deposits depending on the chemistry of water passing through, much like buildup of calcium in pipes in areas with hard water. Aquifers that are open to the atmosphere are termed unconfined aquifers (Figure 02-03). The upper portion of the geologic material that forms the aquifer, where the pore spaces are only partially filled, is referred to as the unsaturated zone. It is also called the vadose zone. A perched aquifer represents a limited unconfined aquifer with an underlying confining layer that lies above and is separated from the regional water table by an unsaturated zone. Water in an aquifer has pressure caused by its own weight in the pore spaces and any flow conditions affecting it. The water table is a depth in an unconfined aquifer where the pressure in the pore spaces is high enough to allow the water to flow into wells. That pressure is approximately equal to atmospheric pressure. There is an interval above the water table where the pore spaces may be fully saturated but the water is held in place by capillary force, or the force that keeps a sponge moist after wringing out. Thickness of that interval, or capillary fringe, varies with shape and size of the pore spaces. Smaller pore sizes typically result in a thicker capillary fringe. Confined aquifers are completely saturated, permeable geologic units overlain by low permeability confining layers that prevent the free movement of air and water between the layers. Water in this type of aquifer is thus confined under pressure and if tapped by a well rises to a level above the top of the aquifer. The level to which the water rises in a well is called the potentiometric surface. Where the potentiometric surface is above the ground surface the aquifer is considered artesian and wells tapping the aquifer will naturally flow. The most important hydraulic properties of aquifers are their ability to store and transmit water. Hydraulic conductivity of an aquifer is a measure of its ability to transmit water. It is dependent on the porosity and permeability of the material (Table 02-01) as well as on the dynamic characteristics of the fluid (water). A measure of the volume of water that can be transmitted horizontally by the full saturated thickness of an aquifer is referred to as its transmissivity. The transmissivity is the product of the hydraulic conductivity and the saturated thickness of the aquifer. The hydraulic conductivity, hydraulic gradient, and transmissivity are all characteristics of aquifers that are used to describe the flow of water. The ability of an aquifer to store or release water is quantified by the value of its storage coefficient. The magnitude of the storage coefficient depends upon whether an aquifer is confined or unconfined. The storage coefficient in confined aquifers is small compared to that of unconfined systems. The term storage coefficient is usually associated with the storativity of a confined aquifer, where water release from storage depends upon the elasticity of the aquifer and the compressibility of the fluid. The term specific yield is normally applied to the storativity of an unconfined aquifer. Unconfined aquifers provide water by physical dewatering (via gravity drainage) of the material through which the decline in the water table occurs. Some water stays behind in the pore spaces because of capillary retention, making specific yield less that porosity. Transmission of water through an aquifer requires a driving force. Fluid flow in an aquifer is driven by changes in pressure of the water within the pore spaces over a distance, or length. Change in total pressure over a specific distance or length along an aquifer is termed the hydraulic gradient. Hydraulic gradient can be a function of many factors but it is most commonly related to elevation differences across topography where precipitation infiltrates to the aquifer. Hydraulic gradient can also be a response to greater infiltration of water in a particular region of an aquifer because of differences in exposure.
The concept of the hydrologic cycle is central to understanding the occurrence of groundwater. As the name implies, the hydrologic cycle is an endless dynamic process of the circulation of water between the atmosphere, the oceans, and the land (Figure 02-04). Evaporation from the surface of open water bodies and exposed soil, along with transpiration from plants, sources water to the atmosphere where it forms clouds. Precipitation from the clouds either evaporates, becomes surface runoff, or infiltrates to become groundwater. Both surface runoff and shallow groundwater eventually flow under gravity back to the open water to begin the cycle again. Seventy-one percent of all U.S. precipitation originates from land surface evaporation; whereas, the remaining 29 percent is produced by evaporation from the oceans. In Colorado approximately 81 percent of precipitation that falls returns to the atmosphere through evaporation and transpiration. Combined, evaporation and transpiration is called evapotranspiration, a critical component of any water budget. Water is always in motion through the hydrologic cycle. This cycle integrates surface water, groundwater, and the atmosphere. Groundwater may discharge to the surface as a spring, surface water can infiltrate into an aquifer through a streambed, and surface water and groundwater can evaporate into the atmosphere. This exchange process may occur repeatedly as water moves through the entire system.
The development of land, and the water resources associated with it, requires a clear understanding of the interaction of groundwater and surface water. The dynamic interaction of these components of the hydrologic system will influence water supply, water quality, and aquatic environments. Withdrawal of water from streams or pumping of groundwater can influence or even deplete the water resources contained in the other medium. Due to the interaction of these two resources, pollution in one can cause degradation of water quality in the other. The interaction between groundwater and surface water occurs at the streambed interface with the underlying aquifer. The stream reach is considered to be gaining if groundwater flows from the underlying aquifer through the streambed and into a surface stream (Figure 02-05). In contrast, a losing stream results when water flows through the streambed of a surface water body into the underlying aquifer (Figure 02-06). Along the course of a stream, it is possible for both conditions to exist, with a gaining stream in some reaches and a losing stream in other reaches. Seasonal and major precipitation events also influence surface-water/groundwater interactions. The elevation of the water table typically exhibits seasonal fluctuations, being high during spring runoff and low during winter. Significant water-table declines can occur during prolonged dry spells or periods of drought. This condition can have profound effects on the groundwater flow to a stream resulting in the drying up of a stream reach. The withdrawal of groundwater through wells can produce a similar result by capturing groundwater that would normally reach the stream. Depending upon its location and rate of discharge, a pumping alluvial well can capture flow directly from the stream into the well (Figure 02-07). 02.05 Wells A well is a man-made feature constructed into an aquifer for the purpose of yielding water. Wells are typically drilled with specialized tools, or rigs, to depths where water is produced at sufficient rates to meet anticipated needs. In the past many wells were hand-dug to the depth of the first water intercepted. Hand-dug wells were often lined with bricks or rock. Modern wells may be drilled to great depths where they might intercept multiple water-bearing formations depending on the geology at the location. The aquifer tapped by a deep well depends on many factors including water rights, water quality, or potential yield of the geologic formation. A typical well construction (Figure 02-08) includes a length of solid casing set to the depth of the aquifer being tapped. At the depth of production, the casing has openings, or perforations that allow the water to enter the well. The space between the borehole wall and casing is filled with either a sealing material where the casing is solid, or permeable filter pack where the casing is perforated. A pump is necessary for wells where the water level does not naturally rise to the surface. Pumps are typically installed to depths near the bottom of the well. Operation of the pump draws water out of the well which causes the water level in the casing to drop below its natural level outside of the well. This creates a gradient between the aquifer and well that drives water from the aquifer into the well. Continuous operation of the pump sustains flow into the well. The rate at which the well can produce water depends on the transmissivity of the aquifer at the location of the well.
Water is a limited, but essential, resource. Accordingly, Colorado has developed over time a comprehensive system for allocating and administering its use. Surface water allocations are driven by the doctrine of prior appropriation, or “first in time, first in right”. Generally, tributary groundwater is considered directly tied to the surface water system and rules and regulations follow the same doctrine. The State Engineer for the State of Colorado (SEO) is authorized by statute to administer the waters of the state. This authority includes responsibility for the administration and distribution of the state’s waters, the promulgation of rules and regulations, the collection of data on water supply, the compliance of interstate compact commitments, and the enforcement of the laws imposed by statute and by order of the courts. To accomplish these responsibilities, the state is divided into seven water divisions with boundaries that correspond to the major river watershed boundaries (Figure 03-01). Each water division is staffed with its own division engineers and water commissioners. The State Engineer’s Office in Denver oversees the seven water divisions. Groundwater use in Colorado is increasing, both in absolute withdrawals and in relation to surface-water uses. In spite of its significance as a resource, the legal rules governing the allocation, use, and protection of groundwater are of relatively recent origin and continue to evolve as our understanding and knowledge of the dynamics of this resource improve. The primary objectives of groundwater law are to: Regulate the rate of groundwater depletion in aquifers for which recharge is insufficient to meet demands; Regulate the rate of depletion in aquifers that contribute to the supply and flow of adjacent surface water; Protect aquifers from pollution; and Regulate the extraction or injection of groundwater in areas prone to geologic instability. Back to Table of Contents
The history of Colorado’s water development began in the late 1780s, before it was designated as a state. Colorado has the distinction of being the first state to publicly administer water development. The oldest — and most senior — water right in Colorado dates from 1852 and is for diversion from Culebra Creek in Costilla County. The use and development of water in Colorado is governed by a system or doctrine of prior appropriation. This doctrine of “first in time, first in right” is common to the majority of western states. An appropriation is made when water is diverted from a stream or groundwater and put to beneficial use. The first person to appropriate the water becomes the senior water-right holder after receiving a court decree verifying their priority status. Although groundwater use in Colorado for public supply, domestic use, and industrial use dates back to before the turn of the century, its management and administration were not addressed until 1957. The Colorado Ground Water Law of 1957 required obtaining a permit from the State Engineer prior to construction of a new large capacity (>50 gallons per minute, gpm) well, and had provisions for the registration of existing wells. Small capacity wells remained exempt and essentially unregulated until 1971. In 1965 and 1969, the Ground Water Management Act (Colorado Revised Statutes (CRS) 37-90-101 to 104) and the Water Rights Determination and Administration Act of 1969 (CRS 37-92-101 to 602) were enacted. The Ground Water Management Act was enacted to address groundwater-based irrigation issues, involving declining water levels, arising from rapid development on the high plains of eastern Colorado. The 1965 Act created a permit system for groundwater development within designated groundwater basins, and established the Ground Water Commission. The Water Rights Determination and Administration Act of 1969 required that surface and tributary groundwater rights be administered together. This required two actions: (1) the adjudication of groundwater rights to protect their priority and (2) the filing of augmentation plans for tributary wells to mitigate material injury to senior water rights.
Under Colorado law, groundwater is classified under five primary categories: geothermal tributary non-tributary not non-tributary designated groundwater All subsurface geothermal fluids are part of the state’s groundwater resources and are subject to the Colorado Geothermal Resources Act. Tributary groundwater is water in the unconsolidated alluvial aquifer and water hydrologically connected thereto. Non-tributary groundwater is groundwater located outside the boundaries of any designated groundwater basin in existence on January 1, 1985, the withdrawal of which will not deplete the flow of a natural stream at a specified rate over a specified time period. Not non-tributary is a special class of groundwater specific to the Denver Basin that is similar to tributary groundwater. Designated groundwater is non-tributary groundwater within the geographic boundaries of a designated groundwater basin. The establishment and management of designated groundwater basins are delegated to the Colorado Ground Water Commission (http://water.state.co.us/cgwc/). Currently, there are eight designated basins (Figure 03-02) that contain 13 Ground Water Management Districts.
Well Permitting By law, every new well in the state that diverts groundwater must have a valid well permit. The Division of Water Resources (DWR) has several types of well permit application forms. To assist individuals in obtaining a permit to construct a well, the DWR has made these forms available on their website. As special rules and regulations apply, a distinction is made if the well is located within the Denver Basin and/or a Designated Groundwater Basin. For residential uses there are generally three types of uses, depending on your particular needs and physical land situation. Form GWS-44 may be used for all these types of uses: Household Use Only Livestock Watering Only Residential Use (may include lawn/garden irrigation and/or domestic animal watering) All residential real estate transactions that include a well transfer require a Change of Owner Name/Address form to be completed and existing wells must be registered. For all other situations and types of uses, the forms to use are: Commercial Wells Dewatering Well Dewatering System Gravel Pits Irrigation, Municipal, Industrial, and Other Large Capacity Wells Monitoring and Observation Hole (temporary) Monitoring and Observation Well (permanent) Recovery/Remediation Well Registration of an Existing Well (construction prior to 1972) Test Hole Well Construction Requirements All water wells must be constructed in accordance with the rules and regulations promulgated by the State Board of Examiners of Water Well Construction and Pump Installation Contractors. The Board of Examiners, within the Division of Water Resources, promulgated the “Water Well Construction Rules” pursuant to the authority granted the State Board of Examiners of Water Well Construction and Pump Installation Contractors (CRS 37-91-104, -106, -109, and -110). The current version of the rules and regulations became effective on September 1, 2016. The purpose of those rules is as follows: To enable the Board to carry out the provisions of Article 91 of Title 37, C.R.S. To safeguard the public health of the people of the State of Colorado and to protect the groundwater resources of the State of Colorado To set minimum standards for the construction, repair, plugging, sealing, and abandonment of all wells, test holes, monitoring and observation holes and wells, and dewatering wells To allow certain types of monitoring and observation holes, monitoring and observation wells, temporary dewatering wells, and test holes to be constructed, utilized, plugged, sealed, and abandoned by persons other than a licensed well construction contractor To set minimum standards for the installation and repair of pumping equipment and cisterns To set minimum standards for the reporting, testing, sampling, measuring, and disinfection of all wells and associated water well supply systems, to the extent such standards are required for the proper construction and repair of water wells The minimum well construction standards specified by the rules are intended to assist in the development of Colorado’s groundwater resources, and to protect public health and ambient groundwater quality by requiring that all wells be constructed, maintained and repaired in such a manner as to: maintain existing natural protection against pollution of aquifers; prevent the entry of contaminants through the borehole; limit groundwater production to a single aquifer unless otherwise permitted; and prevent the intermingling of groundwater from different sources through the borehole. To achieve these objectives, the rules address selection of a well location, well casing materials, construction procedures, grout types and placement methods, well development and cleaning requirements, and minimum disinfection standards. Well construction requirements are dependent upon the type of aquifer penetrated. The Board has identified four aquifer types: Type I is a confined aquifer, Type II is an unconfined bedrock aquifer, Type III is an aquifer composed of unconsolidated material such as alluvial and/or colluvial deposits and severely weathered crystalline rocks. A fourth, and special type, are wells in the Laramie-Fox Hills aquifer found below the shales of the Laramie Formation and above the Pierre Shale, including the basal sandstone units of the Laramie Formation and the siltstones/sandstones of the Fox Hills sandstones. (Figure 03-03). Other nationally recognized organizations, such as the American Water Works Association (AWWA), have also published standards (A100-90) for the construction of a well. While these standards do not serve as well design specifications, they typically provide much more detail than information published by the Board of Examiners. The National Groundwater Association (NGWA) maintains wellowner.org, to inform consumers about groundwater and water wells. The objectives of a proper well design are to produce an optimum combination of performance and longevity within reasonable economic constraints. The specific performance considerations in proper well design involve selection of appropriate construction materials, proper well sizing and depth for the end objective, and proper choice of the completion method(s). The components that must be specified in a properly designed well and their basic functions include: Well casing—prevents hole collapse and provides a housing for the pumping equipment; Well screen—the intake section where the water enters the well bore; Sand filter or gravel pack—siliceous sand or gravel that is placed in the annular space to prevent formation material from entering the screen; Grout—sealant material to isolate the open section of the well from the overlying formation and ground surface. Appropriate material selection is dependent upon considerations of both the ambient water quality and the physical properties of the geologic medium and water-bearing zone. Water-well performance will be influenced by the drilling and completion techniques used and the degree of well development. Wells are developed after completion to mitigate formation damage caused by the drilling activity and to optimize flow from the aquifer into the well. A typical domestic well water supply diagram in a semi-consolidated, sedimentary formation is presented as Figure 03-04.
(04.01) Managed Aquifer Recharge (MAR) is the process of enhancing recharge of groundwater to an aquifer. It may be implemented for a variety of objectives and through many methods depending on objectives and aquifer characteristics. Artificial recharge is a term commonly used for the practice in the past, but MAR is gaining preference because it implies a broader spectrum of applications. Enhanced recharge can potentially be applied to any type of aquifer with the main limitations being (1) obtaining suitable source water, (2) cost of implementation, and (3) regulatory, legal, or other institutional considerations. Development of MAR has been expanding globally over the past 20 years and MAR is gaining recognition in Colorado as an option for water management. Objectives The objectives (Figure 04-01) of most MAR applications fall into one, or a combination, of the following categories: Manage water supply, including short-term water supply regulation, seasonal storage, long-term storage (drought mitigation), emergency supply, and conjunctive use; Meet legal obligations, such as providing augmentation water, supplementing downstream water rights, or facilitating compliance with interstate agreements; Manage water quality through the improvement of surface water or groundwater quality or treated wastewater disposal; Restore aquifers by restoring groundwater levels, limiting aquifer compaction and surface subsidence resulting from excessive groundwater withdrawals, or mitigating saltwater intrusion; Protect the environment by maintaining wetland hydrology, enhancing endangered species habitat, or controlling the migration of groundwater contamination. Back to Table of Contents
Managed aquifer recharge technologies are broadly grouped according to whether water is recharged at the surface or underground, and then by whether water is recharged into the unsaturated zone or directly into the saturated zone of the aquifer. The selection of a particular technology requires detailed site investigation and depends on the hydrogeologic setting of the target aquifer, land availability and uses, and the project objectives. Surface infiltration is the impoundment of water at the ground surface (Figure 04-02) for the purpose of infiltration to the underlying near-surface, unconfined aquifer. Subsurface infiltration is the application of water below the ground surface for infiltration to the underlying unconfined aquifer (Figure 04-03). Direct injection differs from infiltration systems by recharging water directly into the saturated zone of the aquifer (Figure 04-04 and Figure 04-05). Aquifer storage recovery (ASR) wells are wells through which water is injected into aquifer storage during times of low demand and high surface-water supply and subsequently recovered by pumping at a later date when demand exceeds surface supply. Modification of natural recharge involves man-made changes to the land surface or hydrogeologic conditions to increase the amount of recharge from natural and local sources. Underground (non-aquifer) water storage technologies apply to storage and retrieval of water in natural or manmade voids in the subsurface, such as abandoned mines or natural caverns. Until recently the acronym ASR has been restricted to a particular type of well used for both injection and recovery of water. Common use of ASR in Colorado has recently expanded to be a more general reference to the active storing water in an aquifer for eventual recovery without specifying the particular technology.
MAR has been implemented in Colorado in one way or another since as early as 1939. At that time it was recognized that seepage from Olds Reservoir in Adams County was recharging the alluvium beneath Lost Creek Valley. Since then intentional recharge operations have been applied at a number of locations as part of water management plans with emphasis on offsetting depletions to surface water from agricultural pumping. An inventory of MAR sites within Colorado in 2004 identified 39 projects that were either active operations, demonstration projects, conceptual investigations, or short term operations terminated for a number of reasons (Figure 04-06). Each of the five primary objectives has been addressed by one or many of the projects identified. Most MAR activity in the state at that time has been implemented to meet legal obligations as part of augmentation plans or manage water supply. Regional MAR Operations in Colorado for Agriculture Two regional operations consisting of many individual sites exist in heavy agricultural areas: Lower South Platte River Basin where water diverted from the South Platte River is directed to infiltration ponds, dry streambeds, leaky reservoirs, and leaky ditches for recharge for augmentation of out-of-priority pumping from irrigation wells. Augmentation plans held by several canal companies and the Central Colorado Water Conservancy District account for timing of flow of recharged water through the alluvial aquifer to the river for replacement of depletions from pumping. San Luis Valley where water diverted from the Rio Grande is directed to a number of excavated pits at the corners of center pivots and through leaky canals for seasonal storage and maintenance of water levels. Many canal segments are maintained to enhance leakage. The operations have evolved into a large-scale conjunctive system managing use of groundwater and surface. Underground Water Storage in Colorado With limited options for additional surface water storage, underground water storage using MAR has many advantages. There are virtually no evaporation losses and a MAR project can be phased in incrementally, spreading costs out over a long period of time. MAR sites can have a much smaller environmental footprint than surface storage facilities and can even be integrated with wildlife habitat enhancement. Pilot studies for long term water storage in the Denver Basin began in the mid-1980s at Parker Water and Sanitation District and Willow Water District. Full scale long term water storage was first implemented in 1992 by Centennial Water District in Highlands Ranch. Tributary surface water is drawn directly from the South Platte River and treated for mixed uses in the district. During times of surplus surface water, the treated water is injected into Denver Basin bedrock aquifer wells through a series of ASR wells for future use. Since then other districts have either started, tested, or initiated studies for ASR operations in the Denver Basin. Colorado Division of Water Resources has implemented rules and regulations governing how ASR can be managed in the Denver Basin. These important regulations allow banking of water in place as well as banking of recharged water imported from other sources. Rules and regulations have also been implemented for underground water storage in Groundwater Management Districts. Recognition of the potential for underground water storage as an important tool for overall water supply management has been increasing since the early studies and pilot operations. As of the end of 2018 there are 24 MAR active projects for water storage and aquifer restoration in the state (Figure 04-07). Many of these are still in the study or pilot stage, but thirteen have been fully implemented. There are an additional twenty-one projects for water storage and aquifer restoration that are considered non-active. Most were short-term pilot studies but several were fully implemented projects that were terminated for various reasons.
Given the advantages for storing water underground there is increasing interest in finding other aquifers with potential for storage elsewhere in Colorado. In 2004 the CGS completed a statewide assessment of available aquifer storage across the State in a number of geologic settings: unconsolidated sedimentary deposits, bedrock sedimentary aquifers, natural cave systems, and abandoned mines. This assessment was done at a reconnaissance scale to provide an initial indication of where further investigations would be appropriate to better define storage potential. The amount of storage available in an aquifer is dependent upon the aquifer’s (1) storage coefficient (storage ability), (2) areal extent, and (3) freeboard (amount the water level could rise above present water level). In general, unconfined aquifers have smaller areal extent, tens of feet of freeboard, and a high storage coefficient. Confined aquifers, on the other hand, often have a large areal extent and hundreds of feet of available freeboard, but a very low storage coefficient. A weighted ranking system was established to evaluate the key physical properties of the state’s 16 highest-potential unconsolidated aquifers (Figure 04-08) and 29 highest-potential consolidated aquifers (Figure 04-09). Hydrogeologic parameters taken into account in the “aquifer ranking value” include areal extent, depth, saturated thickness, head freeboard, storage coefficient, and hydraulic conductivity. In addition to calculating a final ranking for the aquifer, the quality of the input data was also assessed. The alluvial deposits of the South Platte River, its tributary Bijou Creek, and the Arkansas River are the top three ranked unconsolidated aquifers. The High Plains Aquifer, Dakota-Cheyenne Group of southeast Colorado, and the Denver Basin aquifers are the top three ranked consolidated bedrock aquifers. The evaluation of the available storage capacity in Colorado’s highest-potential aquifers was guided by the desire to find opportunities to develop large-scale artificial recharge projects, defined as having storage capacity in excess of 100,000 acre-feet. Thirteen of the 16 primary unconsolidated aquifers have sufficient storage capacity to accommodate a large-scale project. In aggregate, the lower South Platte River alluvium and the San Luis Valley alluvium have the capacity to store in excess of one million acre-feet. All but two of the 26 primary consolidated rock aquifers have sufficient storage capacity available to meet the 100,000 acre-feet criterion. Because of their large areal extent and head freeboard, the majority of these aquifers can store millions of acre-feet of water. Three types of non-aquifer underground water storage possibilities were assessed statewide: abandoned coal mines, abandoned metal mines, and caves. Storage of water in abandoned underground coal mines is not a new concept, but has only recently been tried in Colorado, most notably by the City of Arvada at the former Leyden coal mine. Overall, the estimated storage capacities of non-aquifer alternatives are much smaller than those of aquifers. An estimated 55,000 acre-feet of underground water storage is available statewide for artificial recharge in inactive coal mines. Major technical challenges to water storage projects in coal mines include maintaining hydraulic control of stored water, poor water quality (high salinity), and mine subsidence. The potential water storage volumes for abandoned metal mines and natural cave systems are much smaller than for coal mines. Metal mines and natural caves are not considered a viable option for water storage because of their limited storage capacity, water quality issues, leakage of stored water, and land ownership issues. Interest in MAR grows with each year as attention turns to the need for additional water storage. In 2006 the Colorado State Legislature passed a bill, SB06-193, authorizing and funding a detailed followup to the 2004 statewide assessment by CGS. The SB06-193 Underground Water Storage Study by the CWCB looked at multiple sites in the South Platte and Arkansas river basins. Potential sites were ranked using 10 criteria covering hydrogeologic, environmental, and implementation considerations. The study identified ten priority alluvial aquifer storage sites and six sedimentary bedrock sites with a total storage capacity of almost 130 million acre feet. Two other studies have also taken place on the Upper Black Squirrel and Lost Creek designated groundwater basins. Underground water storage is also an option considered in the South Platte Storage Study authorized by House Bill 16-1256. Managed aquifer recharge projects can increase the total amount of stored groundwater in a very specific and calculated fashion. In addition, indirect or passive methods of groundwater recharge such as vegetation control, stormwater retention basins, and leaky ditches are non-specific in application, but can significantly increase overall groundwater storage. Similar to water conservation measures, some changes in legislation and water facility design and engineering, combined with passive recharge structures, would benefit both groundwater and surface-water resources. Considerations for Underground Water Storage in Colorado Underground water storage requires several essential components to be implemented including (1) a suitable aquifer, (2) suitable source water, (3) infrastructure, (4) an operating entity, and (5) regulatory compliance. A suitable aquifer is one with hydrogeologic characteristics favorable for permitting recharge, storage, and recovery within the objectives of a given project. Characteristics to be considered include permeability, depth, available storage in unsaturated thickness, and ambient chemical conditions. It is also important to consider location relative to source water and areas where the water is ultimately needed. Suitable source water is water that is of suitable quality to meet applicable regulatory conditions, that does not degrade ambient water quality of the receiving aquifer, and that does not degrade the aquifer hydrology by clogging or chemical reactions. It should also be delivered at an adequate rate and total volume to meet the objectives. Diversion of source water must comply with conventional water rights administration. Infrastructure will be necessary to deliver source water to the point of recharge and to extract it from the aquifer at the time of need. Infrastructure for delivery may include pipelines, pump stations, canals, settling basins, or advanced treatment facilities, depending on the technology used for recharge. Infrastructure will also be necessary for extracting the water from storage, although in may be possible to use existing water supply wells or wellfields. Costs of infrastructure may limit the distance to the point of origin of source water. An operating entity is one that will design, test, obtain necessary regulatory permits, and, most likely, operate a MAR facility. This entity should be responsible for identifying and securing adequate water for storage. Entities for existing MAR operations in Colorado are either water providers, both public and private, ditch companies, or conservancy districts. Regulatory compliance applies to nearly every aspect of a MAR operation. This can include land use considerations, environmental considerations, drinking water regulation considerations and so forth. Perhaps the most relevant aspects deal with introducing water to an aquifer from a source different from natural recharge. In Colorado regulation of discharging water into an aquifer depends on the methodology. For surface infiltration, discharge into an aquifer is regulated by the Colorado Department of Public Health and Environment; subsurface injection falls into the US Environmental Protection Underground Injection Control Program. Regardless of the regulatory environment, regulations guide sampling and concentration limits for many parameters. Extraction of recharged water is covered by water rights administration by the Colorado Division of Water Resources. Rules and regulations for extraction in the Denver Basin Bedrock aquifers have been in place since 1995. Colorado has just recently expanded extraction rules to non-tributary aquifers outside of the Denver Basin (2 CCR 402-11). New rules have also been promulgated for aquifer storage and recovery in designated groundwater basins.
(05.01) Colorado’s Water Plan 2023 is focused on achieving the right balance of water resource management strategies. It recognizes that water is important for all sectors and regions in Colorado, and greatly affects Coloradans’ livelihoods. It stresses collaboration between stakeholders with a variety of interests: agriculture, municipal, industrial, recreational, and environmental. It also strives for collaboration between different regions and river basins. Colorado’s Water Plan takes into account the state’s history, legal system, policy structure (which includes local, state, and federal laws, institutions, and stakeholders), and institutional arrangements that influence decisions about available water resources. Colorado’s Water Plan affirms the private ownership of water rights under the state’s prior appropriation system. Furthermore, this plan supports the authorities and responsibilities of local governments and water providers established by state law. It recognizes the limited statutory role of state agencies in decisions regarding the allocation and reallocation of water to various beneficial uses, and the overlay of federal regulatory and permitting processes that pervade water resources management decisions in Colorado. Thus, the plan advocates for cooperation among parties so that no one governmental agency, water provider, or private party is compelled to go it alone and make unilateral decisions. This plan is a framework to guide future decision making and to address water challenges with a collaborative, balanced, and solutions-oriented approach. The State recognizes that Coloradans have accomplished innovative and creative work, and acknowledges that there is still much work to do. Colorado strives for vibrant and sustainable cities, viable and productive agriculture, a robust recreation and tourism industry, and a thriving natural environment. The goals of the Colorado Water Plan are to meet the water supply gap, defend Colorado’s compact entitlements, improve regulatory processes, and explore financial incentives—all while honoring Colorado’s water values and ensuring that the state’s most valuable resource is protected and available for generations to come. Legislation in 2005, known as the Colorado Water for the 21st Century Act, created a grassroots process for multiple stakeholders to determine water needs and propose solutions to meet those needs. This process included establishment of nine regional roundtables based on river basins (Figure 05-01) and a statewide Interbasin Compact Committee to facilitate cooperation between individual basins. Implementation of the Colorado Water Plan will originate mainly through the basin roundtables through their individual basin implementation plans (BIPs). Technical resources essential to implementing the original 2014 Colorado Water Plan are found in the Analysis and Technical Update. This process provides data and analytical tools for determining water needs, water supplies, and gaps to be used by basin roundtables in their planning activities. The process incorporates water supply and demand conditions considering potential effects of population growth, climate change, and other factors. Aquifer data available through this atlas can be a powerful tool for analyzing groundwater supply and underground water storage options. Back to Table of Contents
(06.01) A comprehensive approach to protecting surface and groundwater quality was enacted in the 1970s with the passage of the Federal Clean Water Act (CWA) and Safe Drinking Water Act (SDWA). Many states, including Colorado, manage and enforce these laws at the state level. The Colorado Department of Public Health and Environment is the lead agency responsible for the administration, management, and enforcement of water quality regulations. The focus of the CWA was the regulation of “end of the pipe” discharges to address pollution in streams, rivers, and lakes. The SDWA is administered by the EPA and provides national standards to protect the public from harmful effects of some contaminants in our drinking water. Groundwater quality in Colorado is addressed in the Colorado Water Quality Control Act. Water-quality data from public water systems supplied by groundwater indicate the most common contaminants in Colorado are nitrate, fluoride, selenium, iron, manganese, alpha radiation (radon), and uranium. Sources may be natural or may result from either planned or unplanned discharges and can be categorized as waste- and non-waste derived.
Water quality standards apply to all waters within Colorado, with the exception of those waters that are within Indian Country, as defined in 18 U.S.C. Section 1151. In some limited instances, Colorado’s definition for “waters of the State” as part of its water quality standards may be more comprehensive than required under federal regulations. Similarly, state water standards have to match federal standards, but may be more stringent. In Colorado, water standards are administered by the Colorado Department of Health and Environment (CDPHE). The Water Quality Control Commission is responsible for adopting new water quality standards and provides a list of regulations for surface water and groundwater in Colorado. Regulation 11 contains the Colorado primary drinking water regulations which apply to public water systems. Surface water quality classifications and numeric water quality criteria are defined for specific rivers and basins within Regulations 31 through 39. Groundwater quality classifications and standards are within both Regulations 41 and 42. Regulation 41 establishes statewide standards and a system for classifying groundwater and adopting water quality standards for such classifications to protect existing and potential beneficial uses of groundwater. The classification system is a framework of uses of groundwater which the Water Quality Control Commission assigns on a site-specific basis, so that standards for chemical pollutants can be assigned at levels necessary to protect the use. There are five groundwater uses in Colorado: Domestic Use – Quality Agricultural Use – Quality Surface Water Quality Protection Potentially Usable Quality Limited Use and Quality All but the last category have groundwater standards listed in tables in Regulation 41. This classification system is based on existing and potential future uses and actual water quality data. Groundwater may be assigned more than one class because it may have more than one existing or potential use. Excluded from Regulation 41 are locations defined in Regulation 42, having site-specific groundwater quality classifications and standards. These site-specific standards are limited to defined geographic areas and in some cases specific geologic formations, such as in those where extensive oil and natural gas exploration and development have occurred. As of June 2018, 28 counties in the state are listed in Regulation 42 as having one or more locations with site-specific standards. Back to Table of Contents
There are many federal laws designed to protect groundwater, including the Federal Safe Drinking Water Act (SDWA), the Resource Conservation and Recovery Act (RCRA), the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), the Toxic Substances Control Act (TSCA), and the Clean Water Act (CWA). These have been implemented within Colorado. Aquifer vulnerability is a measure of how easy or how hard it is for pollution or contamination at the land surface to reach a production aquifer. Factors used to assess vulnerability include characteristics of the intrinsic geological and hydrogeological features. These characteristics include the soil and unsaturated zone material, the aquifer material itself and its permeability, the depth to the water table or aquifer, whether the aquifer is confined or unconfined, the amount of recharge to the aquifer, and any preferential pathways such as fractures which contaminants may follow. Characterization of vulnerability may vary from qualitative indexing methods to process-based, quantitative hydrogeologic assessments with numerical modeling. A common qualitative indexing method used worldwide is the DRASTIC method, which shows the relative differences in vulnerability across regional scales (mapped areas of 100 acres or larger). The DRASTIC method components are Depth to water, Recharge, Aquifer media, Soil media, Topography, Impact of the vadose zone, and hydraulic Conductivity of the aquifer. DRASTIC indexing was applied by the USGS for all of Colorado and New Mexico. Their findings for Colorado demonstrated that greater relative aquifer vulnerability was centered in south-central Colorado and along riparian corridors—all areas where the water table is relatively close to the land surface and the aquifer is more susceptible to surface influences.
Groundwater can contain contaminants from both natural and human sources that make it unsuitable for consumption without treatment. These can be local to regional problems. Natural contaminants are sourced from rock formations, which can include metals and radionuclides, methane, and high salinity and sulfate that cause elevated total dissolved solids. There are several common anthropogenic contaminant categories, such as petroleum, solvents, and agriculture, which can result in groundwater impact. These are regulated and monitored by various state agencies, as follows: Petroleum: The Colorado Department of Labor and Employment, Division of Oil and Public Safety (OPS) has a Petroleum Program which regulates petroleum storage facilities with USTs (underground storage tanks) that hold 110 gallons or more and ASTs (aboveground storage tanks) that hold between 660 and 40,000 gallons. There have been petroleum releases resulting in over 600 contaminated sites within Colorado, recorded by OPS as “events”. The OPS Remediation Section’ goal is to identify, assess, and clean up contaminated sites while protecting human health and the environment. When remediation or clean up is considered complete, an event is closed. OPS provides both GIS-based datasets and maps of open petroleum release sites. OPS has a Petroleum Brownfields Program to promote environmental protection, provide economic benefits, create jobs, and support community revitalization through the assessment, cleanup and reuse of abandoned and underutilized petroleum storage and dispensing sites throughout the State of Colorado. These brownfields are often abandoned gas stations with known or perceived contamination that complicates the future sale, reuse, or redevelopment of the property. Another petroleum (oil and gas) related agency is the Colorado Oil and Gas Conservation Commission (COGCC) whose mission is to regulate the development and production of the natural resources of oil and gas in the state of Colorado in a manner that protects public health, safety, welfare, the environment and wildlife resources. The COGCC maintains a GIS map where some of the layers show the different kinds of environmental sites and environmental sampling sites. The COGCC Environmental Unit is responsible for groundwater activities as follow: Management of the COGCC Environmental Database for analytical results derived from groundwater, surface water, produced water, soil, and gas samples Special environmental projects to remediate or reclaim orphan oil and gas locations and baseline water quality studies. Solvents: Solvents are predominantly used as degreaser agents in commercial or industrial settings. Since 2007 the CDPHE requires registration of businesses that generate 3 or more gallons per year of solvent wastes. Unfortunately, there have been spills and releases of solvents to the subsurface in Colorado, which have the potential to impact groundwater. Solvents in groundwater do not degrade easily or quickly, and depending on the geology can have long contaminant plumes. Several solvent plume sites in Colorado have Regulation 42 specific groundwater standards. Agricultural Nitrogen and Pesticides: The Colorado Department of Agriculture (CDA) monitors groundwater for agricultural contaminants, which are predominately nitrogen species, like ammonia and nitrate (sourced from fertilizers and manure), and pesticides. Their Agricultural Chemicals and Groundwater Protection program goal is to protect groundwater and the environment from impairment or degradation due to the improper use of agricultural chemicals while allowing for their proper and correct use. They collect groundwater samples from various regions around the state, and provide annual summary reports for that data. The water quality data is hosted through the Colorado State University Extension eRAMs GIS map. Mining Metals: Metal mining activities have the potential to mobilize metals into surface water predominantly, but also groundwater. Water draining out of mines (effluent) tends to have elevated metal concentrations plus other elements. Water percolation through waste rock and tailing piles can mobilize metals into the soil and groundwater. However, this tends to be a localized (small aerial footprint) impact in remote mountainous areas. The Colorado Division of Reclamation, Mining and Safety (DRMS) regulates these mining activities and also their geologists and hydrologists collect sampling data. High Salinity: In areas situated on Cretaceous marine sediments, irrigation accelerates the mobilization of elements like selenium and uranium, and can result in elevated salinity into surface water and potentially groundwater. Two regional areas in Colorado known to exhibit this problem are along the Lower Arkansas River and the Gunnison and Colorado rivers. 06.05 Vulnerable Groundwater Mapping As a resource for private well owners, well drillers, consultants, among other constituencies, the CDPHE maintains the Groundwater Vulnerability Atlas GIS map that integrates location information for sites with vulnerable groundwater under regulatory authority by multiple agencies. The map provides analytical tools for determining proximity of a selected location to those sites with vulnerable groundwater conditions.
(07.01) Colorado has many thermal springs and wells that have water temperatures measurably above the land surface temperatures. There are many different definitions for the temperature of a hot or a warm spring, but, in practice, application of the definitions depends on the surface temperature and use of the water. The generic term thermal water is therefore used here. Temperature increases with increasing depth in the earth. Therefore, in general, the deeper the aquifer, the hotter the groundwater in the aquifer. In some areas more heat is escaping from the earth: areas with active volcanoes are an example. In those areas temperature increase with depth more rapidly than in areas without volcanoes. Temperatures increase more slowly in areas where the rocks allow the heat to pass easily (high thermal conductivity) and more rapidly where the rocks resist the flow of heat (low thermal conductivity). If there is no water flow, the temperature increases at depth in proportion to the depth and the magnitude of the heat flowing through the rocks and in inverse proportion to the thermal conductivity of the rocks. In Colorado the heat flow is generally low in the eastern plains and moderately high west of the mountain front. There is one particular rock layer that has a low thermal conductivity, the Pierre Shale, also known as the Mancos Shale. It typically has a thermal conductivity less than half of the rocks that lie beneath it. In the higher heat flow areas in western Colorado, temperatures increase rapidly in the Pierre Shale, 40°C/km (2.2°F/100 feet) or higher, and then much less rapidly in underlying layers, 20°C/km (1.1°F/100 feet). Thermal wells may be pumping water from depth at the ambient temperature of the water under these regional temperature conditions. Thermal springs and many thermal wells produce water from systems where the thermal water is unusually shallow. None of Colorado’s thermal springs and wells have water chemistry that would be indicative of having interacted with a volcanic source. The alternative source of heat is deep circulation to depths where temperatures are higher and return to the surface where they discharge in springs or through wells. For many of the thermal springs, the recharge areas are difficult to constrain, but from the chemistry of the waters their depths of circulation are estimated to be a minimum of 2 km (6,500 feet). The waters appear to rise to the surface through relatively well-defined fractures at a speed of a few mm/s (around 1/8 inch/sec). At much slower speeds the water cools on its way to the surface. At much faster speeds water flow cools the rocks at depth. Many thermal spring/well systems have unique characteristics. At Pagosa Springs a portion of the thermal water spreads laterally in the Dakota aquifer with the remainder rising to the surface spring. This has resulted in a circular thermal anomaly centered on the surface spring. Water from thermal springs at Glenwood Springs is saline because the groundwater circulates through the Eagle evaporites at depth. Discharge from these springs is very high and surface temperatures are thought to be the same as the maximum temperature at depth because flow through the system is rapid. In the Mt. Princeton/Hortense thermal spring system water chemistry indicates a much hotter geothermal resources at depth and water is thought to rise at least in part by thermal buoyancy. At the northern end of the San Luis Valley, a number of artesian wells are thermal wells, but a plot of the temperatures of the wells against their depths indicates that their temperatures are normal for the depths of the wells and that there is no heat source associated with the wells. Thermal waters in Colorado have been used since prehistoric times for spiritual and healing purposes and they continue to be used for spas, bathing, geothermal greenhouses, aquaculture, domestic and commercial heating and hot water, and other direct-heat uses. Pagosa Springs has a geothermal district heating system. There is plenty of room for expansion of these green, sustainable, carbon-free direct-uses of geothermal energy. Studies indicate that geothermal electricity may be generated from some of the higher temperature resources. However, the development of these resources awaits economic investment and incentives equivalent to those given to solar and wind. Back to Table of Contents
(08.01) There are six classes of underground injection wells (I through VI), as described below. In Colorado all classes are administered by the US EPA, except for Class II – administered by the COGCC. Class I is for industrial and municipal waste disposal wells, which are used to inject hazardous and non-hazardous wastes into deep, confined rock formations. Class II is for oil and gas related injection wells, which are used only to inject fluids associated with oil and natural gas production. Class III is for injection wells for solution mining, which are used to inject fluids to dissolve and extract minerals. Class IV is for shallow hazardous and radioactive injection wells, which are shallow wells used to dispose hazardous or radioactive wastes into or above a geologic formation that contains an underground source of drinking water (USDW). Class V is for injection of non-hazardous fluids into or above underground sources of drinking water. Most Class V wells are shallow disposal systems that depend on gravity to drain fluids directly in the ground. More sophisticated Class V wells may rely on gravity or use pressure systems for fluid injection, including systems used to inject and store water for later reuse. Class VI is for wells used to inject carbon dioxide (CO2) into deep rock formations. This long-term underground storage is called geologic sequestration (GS). Geologic sequestration refers to technologies to reduce CO2 emissions to the atmosphere and mitigate climate change. Class IV and V injection wells are the only permit types for groundwater aquifers containing an underground source of drinking water. It is recognized that this disposal can pose a threat to groundwater quality if not managed properly. Back to Table of Contents
There are two types of Class II wells in Colorado, disposal wells and enhanced recovery wells. Class II disposal wells are used for waste fluids generated during oil and natural gas production. Injected wastes can include produced water, drilling fluids, spent well treatment or stimulation fluids, pigging wastes, and gas plant wastes (including amine and cooling tower blowdown). Produced waters are typically brines (salty water). Brines generated during oil and gas extraction are separated from hydrocarbons at the surface and then disposed of by re-injecting into the same or similar underground formations. For enhanced recovery wells, fluids consisting of brine, freshwater, steam, polymers, or carbon dioxide are injected into oil-bearing formations to recover residual oil and in limited applications, natural gas. Oil and gas related injection wells are managed in Colorado solely by the COGCC when injecting into saline formations having total dissolved solid (TDS) concentration of greater than 10,000 milligrams per liter (mg/L). Groundwater that has TDS greater than 10,000 mg/L fall outside of the EPA definition of Underground Source of Drinking Water (USDW) (40 CFR § 144.3). COGCC Rule 324(B) governs injection wells that are completed in brackish formations that have TDS concentrations between 3,000 mg/L and 10,000 mg/L. To obtain injection permit approval, these wells go through an aquifer exemption process which involves consultation between the CDPHE WQCD and COGCC. Aquifer Exemptions are only granted if the injection zone: 1) is not currently a source of drinking water, and 2) is unlikely to become one, because it is or may be a hydrocarbon producing interval, is too deep to be economically or technically practical.
Class IV shallow hazardous and radioactive waste injection wells were banned by the EPA in 1984. The only allowable Class IV wells are used to clean up groundwater contaminated by hazardous chemicals. These wells may only operate as part of an EPA- or state-authorized groundwater clean-up action. The US EPA indicates that there are less than 32 waste clean-up sites with Class IV wells existing in the entire United States. Groundwater clean-up actions by pump and treat technology can involve pumping back some or all of the treated water back into the aquifer. This is often done to encourage flushing or increase the pumping gradient. The injection well is a Class IV well when hazardous contaminants are not completely removed from the injected water. Class V aquifer remediation wells support groundwater cleanups deemed to be non-hazardous. 08.04 Aquifer Storage and Recovery Class V wells are defined by EPA’s regulations as injection wells not included in other well classes. This includes wells used for aquifer storage and recovery (ASR), where water is injected and stored for later reuse. In Colorado, the use of aquifers to capture and store water has the benefit of not being subject to the large evaporative losses that occur in surface water reservoirs. In 2018, the South Metro Water Supply Authority (SMWSA) completed an ASR feasibility assessment at a local and regional scale of implementation in the Denver Basin as a water management strategy to meet future demand. A 2007 study conducted for the CWCB under Colorado Senate Bill 06-193 evaluated potential locations for underground water storage in the South Platte and Arkansas River Basins. The study identified several areas within the Denver Basin bedrock aquifers, which included most of the SMWSA area, as good candidate underground water storage locations. Within the SMWSA region, the Centennial Water and Sanitation District (CWSD) has a long and successful ASR program.
Class VI permits are required for injecting carbon dioxide (CO2) into deep rock formations for the purpose of carbon sequestration. In 2007, the CGS evaluated CO2 sequestration potential in Colorado. Although CO2 sink potential is widely distributed across the state, characterization efforts focused on seven “pilot study regions” defined on the basis of maximum diversity in potential sequestration options relatively close to large CO2 sources. The highest CO2 sequestration capacity potential for Colorado lies within the oil, gas, coalbed, and saline aquifer reservoirs of the Denver, Cañon City Embayment, Piceance, and Sand Wash basins
(09.01) An aquifer is a geologic material with pore spaces saturated with water and that is permeable enough to yield water to springs and wells. At the highest level aquifers in Colorado fall into three primary groups, based on geologic material type: (1) unconsolidated granular deposits, (2) sedimentary bedrock formations, and 3) fractured crystalline bedrock. Differences in structural characteristics of these primary types can lead to very different ways that water moves through and is stored within them (Figure 09-01). (Refer to Groundwater Basics for more details on the dynamics of subsurface water movement.) Unconsolidated granular deposits include sand, gravel, silt, and clay where the particles are not bound together and pore spaces between the open pore spaces between them can hold water. When the pore spaces are large enough, water can move through them with ease. These materials have been deposited by water, wind, and gravity. Sedimentary bedrock formations include sandstone, conglomerate, siltstone, and shale (or mudstone), where the particles are bound together to varying degrees. This group also includes many types of carbonate formations, typically limestone and dolomite. Sedimentary bedrock formations can be semi- to well consolidated due to compaction or the presence of other minerals that bind the particles together. Pore space can be diminished or blocked because of the consolidation so that there is less capacity to hold and transmit water. Fractured crystalline bedrock includes many types of rock with the most common being igneous and metamorphic rocks, such as granite, gneiss, and schist. It can also include many volcanic rocks, such as basalt and welded tuff. Mineral grains in these rocks interlock with little to no pore space that could hold or transmit water. The rocks must be fractured, and the fractures must be wide enough to allow water to move through to make an aquifer. Not all geologic formations fit neatly into these classifications. Volcanic rocks can include flows that are clearly crystalline in nature that are interbedded with unconsolidated volcanic ash or unconsolidated to consolidated water- and debris flow- deposited sediments. Some sedimentary formations are so well consolidated that they behave very similarly to igneous and metamorphic rocks that must be fractured to form aquifers. Many carbonate sedimentary rocks have a crystalline structure that lacks primary porosity and permeability. Similarly, in some sandstone formations the pore spaces are completely filled with mineral “cement” and the rock, often called quartzite, cannot hold or transmit water. Like igneous and metamorphic rock, these sedimentary rocks require fractures or cavities formed by dissolution to allow the transmission of water. Unconfined aquifers are aquifers that are not completely saturated with water and there is connection to the atmosphere (Figure 09-02). The upper portion of the aquifer, where the pore spaces are only partially filled, is referred to as the unsaturated zone. Confined or artesian aquifers are completely saturated, permeable geologic units overlain by low permeability confining layers that prevent the free movement of air and water between the layers. The water is thus confined under pressure and if tapped by a well rises to a level above the top of the aquifer, but not necessarily above the land surface. A perched aquifer represents a limited unconfined aquifer with an underlying confining layer that lies above and is separated from the regional water table by an unsaturated zone. Confining units are geologic units that have permeability low enough to inhibit the flow of water. The most common confining units are shale and mudstone formations that are made up of very small particles of clay. These materials may have porosity, but the pore spaces are so small that water can not move freely through them. The pore spaces may not be well connected and capillary forces hold the water in place. Back to Table of Contents
Geologic formations are geologic materials, or strata, with specific characteristics that set them apart as mappable features that can be correlated from one location to another. They are the fundamental unit of lithostratigraphy. Hydrogeologic units are geologic formations, or groupings of formations, with similar characteristics to hold and transmit water. Hydrogeologic units can be either aquifers or confining units within the context of geologic formations.
All of the primary aquifer types occur in Colorado and where each is located depends on the geologic setting. Colorado’s geology records a long and diverse history of evolving landscapes where many sedimentary formations deposited over 500 million years over crystalline bedrock that is as old as old as 2.4 billion years. Episodes of volcanic activity added layers of volcanic rock or injected magmatic bodies into this stratigraphy. Over such a long period of time these rocks can be complexly deformed and divided into many large structural basins, complicated uplifts, and broad plateaus. This has created a colorful geologic mosaic featuring a variety of sedimentary and fractured crystalline bedrock formations. A network of relatively young rivers and streams, with their associated unconsolidated alluvial deposits, overprints this mosaic (Figure 09-03). The Geologic Evolution of Colorado as the Foundation for its Groundwater Resources outlines the relationship between geology, long-term tectonics, and groundwater aquifers. Geologic formations, or their hydrogeologic unit equivalents, can be grouped according to geologic setting. Doing so simplifies the complex geologic mosaic Colorado presents. At first pass, unconsolidated alluvial deposits in Colorado can be seen as one state-wide aquifer system (Figure 10-01). Even though alluvium along individual rivers may cover relatively narrow bands, in aggregate, the network is quite extensive. As such, alluvial aquifers can be grouped as one major aquifer type. The other hydrogeologic unit types, sedimentary bedrock and crystalline bedrock, are best organized by major geologic structures and geographic location within the state. Whether hydrogeologic units are grouped by geologic setting or geographic region depends on the overall extent of the units. Structural basins are a geologic setting useful for grouping hydrogeologic units. Structural basins are large-scale downwarps formed by tectonic forces that are filled with strata. Some strata, particularly that which was deposited as a basin formed, may be unique to that basin. Colorado has many structural basins (Figure 11-01) formed during two phases of its geologic evolution: three formed during the Pennsylvannian-Permian Ancestral Rockies event, and eight during the Upper Cretaceous-Paleogene Laramide event. Many geologic formations are not limited to specific structural basins, but instead, extend across large regions. Colorado spans three main physiographic provinces: (1) Great Plains, (2) Southern Rocky Mountains, and (3) Colorado Plateau. It also includes small parts of the Wyoming Basin and Middle Rocky Mountains Provinces (Figure 09-04). These provinces, characterized by different climate and topography, reflect an underlying complex geologic framework. Hydrogeologic units in Colorado can be grouped by regions that generally correspond to these provinces (Figure 09-05). The High Plains and Colorado Piedmont regions are parts of the Great Plains Province. In this context the Colorado Piedmont includes the Raton Basin subprovince. The Mountainous region represents the Colorado portion of the Southern Rocky Mountains Provinces and the Colorado Plateaus region covers the Colorado Plateau-Wyoming Basin-Middle Rocky Mountains Provinces. Colorado Piedmont This region makes up most of the east side of the State and is dominated by sedimentary bedrock formations. Some of these formations are widespread and form regional hydrogeologic units. Overall, structural deformation in this region is subdued with the exception of the Denver, Cheyenne, and Raton Laramide structural basins. These are prominent structural downwarps filled with sedimentary formations that form major basin hydrogeologic unit systems, including many recognized aquifers. Deposits of unconsolidated alluvium follow each of the main rivers, the South Platte and Arkansas, and their tributaries forming important surface aquifer systems. Large areas are also blanketed by wind deposited unconsolidated sand and silt, that in places, are part of the surficial aquifer systems. The High Plains region laps over the Colorado Piedmont region in a manner that the regional is underlain by the regional hydrogeologic units of the greater region. Colorado Plateaus This region makes up the westernmost one-fourth of the State, and is also dominated by sedimentary bedrock formations. It is characterized by high plateaus and mesas and deep valleys and canyons in contrast to the Colorado Piedmont. Many widespread sedimentary bedrock formations form regional hydrogeologic units, several of which are the same as found in the Colorado Piedmont. Structural deformation in this region includes broad uplifts and folds and it includes the Sand Wash, Piceance, and San Juan Laramide structural basins as well as the Ancestral Rocky Mountains Paradox structural basin. Each of these structural basins is filled with sedimentary formations which form basin hydrogeologic unit systems that may include several important aquifers. Deposits of unconsolidated alluvium follow rivers that include the Colorado River and its many tributaries forming localized surface aquifer systems. Mountainous This region bisects the central part of the state and has the greatest topographic relief with elevations exceeding 14,000 feet at the summits of its 53 venerable 14ers. It is also the most geologically complex region in the state. Within this region are deep valleys formed by geologically young faulting that are filled with sedimentary bedrock formations; complexly deformed sedimentary bedrock formations of many ages; and a href=”#gwa12b”>volcanic areas covered with lava flows, ash deposits, and sediments shed off of eroding volcanoes. Ancient crystalline bedrock formations and more recent igneous intrusive rocks form the cores of many of the mountain ranges and can be found in deep river valleys and canyons. The river systems that extend across the Colorado Piedmont and Colorado Plateaus regions originate within the Mountainous region and have associated alluvial deposits that form local unconsolidated aquifers. In each of these regions the geologic mosaic of geologic materials is three-dimensional and can be quite complex. Specific geologic formations may be found throughout the state, or they may be limited to only parts of regions or structural basins. Episodes of tectonic deformation within a region may have formed one, or many structural features. Structural basins formed at different times can even overlap, with younger features cross-cutting older (see Figure 13-02). Many wide-spread geologic bedrock formations can span multiple structural basins, geographic regions, and river basins. Geographic regions often include multiple river basins and geologic basins. River basins can cross multiple geologic basins and geographic regions.
Most people are familiar with Colorado’s major rivers, all of which originate within the State. The Continental Divide splits the state with river basins heading east to the Atlantic, south to the Gulf of Mexico, and west to the Pacific. Sand and gravel deposited by the many rivers and their tributaries in these river basins form a network of unconsolidated aquifers that have historically been a critical source of groundwater supplying the State’s vital agricultural economy. Because the major alluvial aquifers in Colorado are all associated with active rivers and streams they are grouped by the major river basins. Generally, these river basins determine the boundaries of the Colorado Division of Water Resources administrative divisions (Figure 10-01). Nearly every river, stream, arroyo or water course of any size has some alluvium associated with it. Flowing water transports the sediments down from upstream areas either as suspended sediment or bed load in the channels. As the water slows down, or the river changes course, the material settles out as alluvium. Sand and gravel may predominate, but layers of silt and clay are common. Organic material such as wood debris may also be present. Groundwater in alluvial aquifers fills pore spaces between loose grains of sediment and is typically connected directly with the surface water in the stream (see Groundwater Basics). Many perennial streams only carry water during Spring runoff or storm events, yet the alluvium beneath the dry bed may carry groundwater year round. Alluvial aquifers rarely extend deeper than 100 feet and are limited to areas along the rivers that may extend from hundreds of feet to many miles wide. Although often limited in extent, alluvial aquifers may be the most productive aquifer type in the state with yields commonly over 1000 gpm. Alluvial deposits associated with modern river systems were deposited during the Quaternary Period, or the last 2.4 million years, making them young geologic formations (see Geologic Evolution of Colorado). During this period of time the rivers have generally carved downward through the older bedrock as the landscape evolved. In some areas, where bedrock is easily eroded, the rivers moved from side as they cut downward to side forming broad valleys. In others, where bedrock is harder and more resistant to erosion, the valleys are narrow, or even deep canyons. The pace of downcutting has not always been the same, nor was the sediment load carried by the streams constant as time passed. Repeating periods of glacial advance followed by periods of retreat made for a cycle of downcutting followed by alluvial deposition over and over again. This cycle of downcutting followed by deposition, overprinted on a gradual overall trend of downcutting, left behind series of terraces climbing higher in the landscape away from the modern river. River terraces that become progressively older as you climb away from the river can be seen along almost any river in Colorado. Typically, each is mantled by alluvial sand, gravel, clay, and silt with a layer of soil developed on the surface. Older terraces, or those highest in the landscape, can have well developed soil profiles and calcium carbonate “caliche” hard-pan deposits can be common. The number and ages of individual terraces found above rivers throughout the state can vary depending on bedrock conditions and the histories of incision. In some areas at least seven terraces, dating back through the middle Pleistocene, have been identified. Naming conventions for the terrace systems vary from region to region. Older terraces, high above the modern rivers and streams, are often carved on bedrock (Figure 10-02). On these older terraces, alluvial deposits may not be directly connected, hydraulically, to the modern rivers below. Depending on the type and permeability of the underlying bedrock, groundwater fed by local precipitation, irrigation, or other human activities can form localized perched groundwater systems. Occasionally, that perched groundwater is sufficient to supply domestic, stock, and irrigation wells. Quaternary deposits from other processes may be associated with the alluvium deposited directly from rivers and streams. These may include wind-blown sand and silt, landslide deposits, glacial drift, local debris fan deposits, alluvial fan deposits from small tributaries. They can overlap with the alluvial deposits of the trunk stream, potentially expanding the extent of the main alluvial aquifer. For example, windblown sand deposits extend for miles beyond both the South Platte and Arkansas Rivers. These vast dune deposits can mask the extent of alluvium beneath, but can also be very effective at capturing precipitation for recharge with very high permeability and having limited soil development. Back to Table of Contents
The South Platte River basin drains an 18,924 square mile area in the northeastern quarter of Colorado (Figure 10-01-01). The basin along with the Laramie River and Republican/Arikaree Rivers defines Water Division 1, with the divisional office in Greeley. As of early 2001, there were nearly 12,000 alluvial wells of record in the South Platte River basin. Originating high in the Rocky Mountains, the mainstem of the South Platte River and its many tributaries descend through high, glaciated mountain valleys before incising deep canyons through the foothills. Well known tributaries include the Big Thompson, Cache la Poudre, and St. Vrain Rivers, and Boulder, Clear, and Cherry Creeks. From Colorado, the South Platte River continues east to join the North Platte River at North Platte, Nebraska and then on to join the Missouri River south of Omaha. Nine trans-mountain diversions import over 400,000 acre-feet of water annually into the South Platte River watershed from west-slope basins, exceeding the imports into all other basins in the state. Over 1.1 million acre-feet of water are stored in 22 reservoirs within the basin. According to the 2000 Census, approximately three million people, or 70 percent of the state’s population, live within the basin along the Front Range urban corridor. Aside from the urban and growing suburban land use, the region is an important agricultural area with over 1 million acres of irrigated cropland.
Deposits of sand, gravel, and silt associated with the South Platte River and its tributaries, combined with deposits of wind-blown sand, form an extensive aquifer system covering an area of over 4,000 square miles. In the mountainous upper South Platte River basin, alluvial deposits tend to be thin and discontinuous and serve as a water resource on a very local basis. Well depths in the upper South Platte River basin, reported in the Colorado Division of Water Resources well permit records, average about 36 feet below ground surface. In the lower South Platte River basin, east of the hogback and extending across the eastern plains, the alluvial deposits thicken and form a continuous aquifer network that is a major groundwater resource for agricultural and municipal uses. Grain size tends to decrease whereas sorting and grain rounding increase away from the mountains. Beds of gravel and cobbles, typically found near the base of the alluvium, are less common downstream. The saturated thickness of the alluvium is close to 20 feet near Denver and increases to over 200 feet downstream near Julesburg, the state line (Figure 10-01-02). Eolian sand and silt cover much of the land surface outside of the stream valleys and overlap the alluvial deposits. Well depths in the lower South Platte River basin alluvium average about 75 feet below ground surface. Water Levels/Aquifer Characteristics Groundwater within the alluvial aquifers of the South Platte River basin is in hydraulic connection with the surface water, and therefore tributary to the surface water system. The alluvial aquifer system is generally unconfined and under water-table conditions. Infiltration from precipitation, irrigation, canal seepage, and pond seepage recharge the alluvial aquifers whereas groundwater tends to discharge to the main channel of the river. Discharge to the river channel creates base flow for the river. The overall water balance in the alluvial aquifer system is complex and changes as the volume of water in storage in the aquifer varies with changes in water levels. Water levels in the upper South Platte River basin can be anywhere between 0 and 80 feet, and in the lower basin, water levels be over 200 feet. Where the Denver Basin bedrock aquifers subcrop beneath the alluvium, they are in hydraulic connection with, and discharge into, the alluvial aquifers of the lower South Platte River basin. In the upland areas of the Denver Basin, and near the foothills where the streams enter the Denver Basin, the alluvial aquifers recharge the bedrock aquifers. Users of Denver Basin groundwater in these overlapping areas are required to replenish depletions to the surface aquifer systems following formulas specified in individual water court decrees. Reported values of hydraulic parameters for the alluvium of the South Platte River and its major tributaries are listed in a downloadable file. Water Use/Withdrawals The lower South Platte River aquifer is estimated to hold as much as 8.3 million acre-feet of water in storage. In 1930 there were 200 wells in the alluvial aquifer of the lower South Platte River basin with the capacity to produce over 100 gpm. By 1970 that number had swelled to more than 3,200. Well discharge accounts for nearly 0.85 million acre-feet per year from the alluvial aquifer system with this water placed to beneficial uses. In the lower South Platte River basin there are approximately 10,880 permitted wells of record listed as producing between 1 and 3,000 gallons per minute (gpm). The lower capacity wells are primarily used for domestic and livestock watering purposes. The higher capacity wells, with reported rates above 100 gpm, include irrigation, industrial, and municipal wells. Of the approximately 930 wells of record in the upper South Platte River basin, there are very few with reported yields over 100 gpm. Most of the upper South Platte River basin wells are permitted for domestic and livestock uses. Water Quality Water quality in the alluvial aquifer is quite variable. While being relatively good in the upper South Platte River basin and the lower South Platte River basin above the Denver metropolitan area, water quality downstream of Denver is degraded due to a number of factors. As groundwater in the lower South Platte River basin aquifer passes through areas of different land use, the quality changes to reflect different contaminant sources. Volatile organic compounds, such as benzene and methyl-tri-butyl-ether (MTBE) from fuels, enter the groundwater in the urban area while nutrients, such as nitrate and phosphate, along with pesticides enter in the agricultural and mixed-use areas. In the historically agricultural areas downstream of the Denver metropolitan area, nitrate concentrations exceeded the drinking water standard of 10 parts per million in about 50 percent of the wells in the alluvial aquifer. The relatively shallow water table makes this aquifer vulnerable to both surface and subsurface contamination. Although the alluvial aquifer has historically been used as a source for drinking water, the water in the downstream segments of the lower South Platte River basin is not well suited for this purpose because of its taste, high total dissolved solids (TDS) concentrations, hardness or localized concentrations of iron, nitrate, or sulfate. In most areas, however, the water quality is suitable for irrigation use.
In the Upper South Platte River Basin the main stem passes through the South Park mountainous valley before carving through the Front Range, an area dominated by crystalline bedrock of the Mountainous region. The prominent hogback bordering the west side of the Front Range urban corridor marks the abrupt transition to the Lower South Platte River Basin where the river leaves the more geologically complex mountains and enters the Colorado Piedmont region. At the border with Nebraska the South Platte River enters the High Plains region. Along its the way the river, and many tributaries, pass over the Cheyenne and Denver Laramide basins.
The Republican/Arikaree River basin in eastern Colorado encompasses an area of 8,775 square miles and comprises four major streams: the South Fork Republican River, the Arikaree, the North Fork Republican River, and Frenchman Creek. Management of the waters in the Republican and Arikaree River basins are under the jurisdiction of Colorado Water Division 1, with offices located in Greeley. As of February 2001, only 67 wells were recorded within the confines of the mapped alluvium of the Republican River and its tributaries. The majority of the alluvial wells are located on the South Fork Republican River and the Arikaree River. The occurrence and distribution of alluvium along the Republican and Arikaree Rivers are shown in yellow on Figure 10-02-01. As is characteristic of the Eastern Plains, there is little variation in elevation in the Republican River basin. Elevation ranges from about 5,000 feet at the headwaters to about 3,500 feet where the streams exit the state. Northeastern Yuma County from Wray northward is composed of sand hills, with no surface drainage except on the periphery. The sand hills extend into southeastern Phillips County and westward into eastern Washington County. The landscape is characterized by flat to gently rolling grasslands. Agriculture is the dominant land use in the basin. Population in the area is sparse, with 3 or fewer persons per square mile in Lincoln and Washington Counties, and 4 to 9 persons per square mile in Kit Carson, Phillips, and Yuma Counties. Back to Table of Contents
Alluvial sediments in the basin are constrained to the immediate vicinity of the drainage valley. The hydrogeologic units in the Republican/Arikaree River basin are Pleistocene alluvial and eolian deposits. The alluvial deposits consist of poorly sorted gravel, sand, and clay with caliche. Eolian sand and silt cover much of the land surface outside of the stream valleys and overlap the alluvial deposits. The alluvial deposits in these river basins are generally less than 100 feet in thickness, and are often in hydraulic connection with the underlying bedrock formation. Reported well depths generally range from 20 feet to more than 65 feet. Ninety percent of the alluvial wells of record are completed at depths of less than 60 feet, with a mean depth of 46 feet. Water Levels/Aquifer Characteristics Reported depths to water, below ground surface, for the alluvial wells of record range from 5 to 64 feet, with an average of 18 feet. For the 57 wells of record with reported water levels, the water table generally lies between 10 and 20 feet below ground surface. The reported alluvial well yields in the Republican/Arikaree River basin display a distinct bimodal distribution with slightly over 60 percent of the wells producing less than 55 gallons per minute (gpm). These wells are suitable for domestic use, livestock watering, and small acreage irrigation. Reported well yields from 250 to 950 gpm are also common. The higher yield wells are most likely utilized for large-scale irrigation. Hydraulic conductivity values range from 30 to 270 feet per day, and annual recharge to the aquifer is estimated to range from 0.25 to 0.50 inches. Reported values of hydraulic parameters for the alluvium of the Republican/Arikaree Rivers and their major tributaries are listed in a downloadable file. Water Use/Withdrawals Kit Carson, Lincoln, Phillips, Washington and Yuma are some of the few counties in the state in which groundwater use exceeds surface-water use. In 1995, groundwater withdrawals from wells by county were 19,520 acre-feet for Lincoln, 50,290 acre-feet for Washington, 95,680 acre-feet for Phillips, 300,500 acre-feet for Yuma, and 186,015 acre-feet for Kit Carson County. These figures reflect pumping from the High Plains aquifer, the major supply for groundwater, as well as from the alluvium. The overwhelming use of groundwater in this basin is for agriculture. To a lesser extent, groundwater is also used for municipal supplies, individual domestic, and livestock wells. No public information is available regarding the water quality of the alluvial aquifers. Back to Table of Contents
The Republican/Arikaree River Basin lies entirely within the High Plains region. In Colorado, the South Fork Republican and the Arikaree Rivers are within the Ogallala Formation, and the Arikaree River has downcut into the Pierre Shale, which is the uppermost of the Colorado Piedmont regional hydrogeologic units, west of US Highway 385. Frenchman Creek also passes through the Arikaree Formation.
The Arkansas River basin drains a 28,273 square mile area in the southeastern quarter of Colorado (Figure 10-03-01). The basin defines Water Division 2 with the divisional office in Pueblo. As of early 2001, there were over 5,450 alluvial wells of record in the Arkansas River basin. The Arkansas River originates high in the Rocky Mountains near Leadville, Colorado, flowing south-southeast through the mountains before it turns east and enters the plains near Pueblo. Well known tributaries include the Purgatoire, Huerfano, Cucharas, and Apishapa Rivers, along with Fountain and Big Sandy Creeks. Total annual flow in the Arkansas River within Colorado is approximately one million acre-feet. There are ten major trans-basin diversions of surface water into the upper Arkansas River that provide an additional source of recharge to the underlying alluvial aquifer. The diversions consist of tunnels and ditches that routed 144,288 acre-feet of water into the basin in 1998. Over 1.8 million acre-feet of water are stored in 19 reservoirs within the basin. Current land use in the upper Arkansas River valley is primarily recreation and tourism with limited agriculture and industry. Land use in the lower Arkansas River valley is heavily agricultural, with both surface and groundwater being utilized to grow a significant amount of farm crops, including the famous Rocky Ford cantaloupes. The alluvial aquifer is an important source of groundwater in the plains. Back to Table of Contents
The primary alluvial aquifer along the Arkansas River consists of unconsolidated river-deposited sediments. The sediments are more varied in size in the upper basin, upstream of Pueblo, ranging from glacial silts to large boulders. In many areas along the upper Arkansas the alluvium is missing where the river is actively eroding in deep bedrock canyons. Alluvium in the lower Arkansas valley is composed of a heterogeneous mix of interbedded sands, gravels, silts, and clays. In the lower valley the aquifer belt can reach 10 miles in width and can be as thick as 200 feet. Alluvium is not a significant aquifer along many of the Arkansas tributaries. Well depths along the upper Arkansas River range from less than 10 feet to greater than 100 feet below ground surface, with a mean depth of 53 feet. The Division of Water Resources well permit database contains over 3,400 wells that have been completed in the lower Arkansas River valley alluvium. Over 90 percent of these wells are completed at depths less than 120 feet below ground surface with a mean depth of only 58 feet. Recharge to the Arkansas River alluvium is primarily through infiltration of surface water through the streambed of the river. Infiltration from irrigation canals and surface application of irrigation water also provides a significant amount of recharge to the alluvium downstream from the Pueblo/Crowley County line. Water Levels/Aquifer Characteristics Arkansas River alluvium is an unconfined, water-table aquifer in direct hydraulic connection with the Arkansas River surface water system. The water table typically slopes toward the valley center and downstream along the river, except where localized pumping causes a gradient reversal. Depth to water ranges from about 5 to 30 feet below ground surface along much of the lower Arkansas River and its tributaries. In the upper Arkansas River Basin, depths to water ranged from 5 to 58 feet below ground surface during the 1990s. Many of the upper basin wells record strong seasonal fluctuations with the highest water levels corresponding to snowmelt and spring runoff in the mountains. Reported values of hydraulic parameters for the alluvium of the South Platte River and its major tributaries are listed in a downloadable file. Water Use/Withdrawals Current land use in the upper Arkansas River valley is primarily recreation and tourism with limited agriculture and industry. Individual household and domestic along with public supply are the primary uses of groundwater in Chaffee County and probably in Lake County, with just over 5,000 acre-feet of fresh groundwater withdrawn from the two counties in 1995. Mining was historically the major industry in the valley, with silver and gold mines dominating the area in the late 1800s into the 1900s. Land use in the lower Arkansas River valley is heavily agricultural, with both surface and groundwater being utilized to grow a significant amount of farm crops. About 2.0 million acre-feet of river water were diverted for irrigation in 1998 and 174,383 acre-feet of groundwater were withdrawn in 1995 in Prowers County along the lower Arkansas at the Colorado-Kansas state line. Water Quality Groundwater in the lower Arkansas River basin alluvial aquifer is classified as sodium-calcium, sulfate-bicarbonate in character and is typically of fair to good quality, although it becomes increasingly saline and marginally unusable downstream due to heavy irrigation use. Water quality within the upper Arkansas River basin alluvium is generally potable with a few exceptions of elevated metals produced by natural acid rock drainage and anthropogenic septic effluent contamination.
The upper reaches of the Arkansas River and its tributaries pass through the Upper Arkansas mountainous valley and highlands formed by crystalline bedrock of the Mountainous region. Grape Creek and Huerfano River originate in the Wet Mountain-Heurfano Park Mountainous valley. After leaving the mountains the river extends across the Colorado Piedmont region and lower valley tributaries reach up into the High Plains region.The Purgatoire, Huerfano, Cucharas, and Apishapa Rivers pass through the Raton Laramide basin.
The Colorado portion of the Rio Grande Basin, located in south-central Colorado, encompasses approximately 7,500 square miles and constitutes Colorado Water Division 3 (Figure 10-04-01). Administration of water in the Rio Grande Basin is governed not only by Colorado state law, but also as stipulated by the Rio Grande Compact with New Mexico and Texas. San Luis Valley spans much of the Rio Grande River Basin and is an open, almost treeless, intermontane basin bounded by the foothills of the San Juan Mountains on the west and the Sangre de Cristo Range on the east. The Rio Grande and Conejos River originate in the eastern San Juan Mountains and are the dominant watersheds in the river basin. Many other tributaries to the Rio Grande River flow out of the San Juan Mountains as well as the Sangre de Cristo Range to join the Rio Grande before it flows south into New Mexico at the head of the Rio Grande Gorge. The 3,200 square mile San Luis Valley encompasses the counties of Saguache, Rio Grande, Alamosa, Conejos, and Costilla. The San Luis Valley represents a significant groundwater resource that is recognized nationally. The overall Rio Grande watershed also includes Mineral and Hinsdale counties, as well as a small area in the southeastern corner of Archuleta County. Total population in the San Luis Valley is about 45,000 and the population in the entire Rio Grande Basin is only slightly higher, as few people reside in the mountainous areas. The San Luis Valley is an open, relatively treeless, flat valley floor with some hills in the southern end, whereas the remaining part of the Rio Grande watershed, the mountainous areas outside the central valley, are generally heavily vegetated, rugged, and steep. Irrigation is widely practiced in the valley, with the major crops being potatoes, barley, vegetables, and alfalfa. Back to Table of Contents
Alluvium along the Rio Grande River consists of unconsolidated sand and gravel deposited by rivers. In the upper valleys the alluvium can include glacial outwash deposits up to at least 100 feet in thickness. In the mountains, alluvium can be an important local aquifer supplying mountain communities that are experiencing rapid growth where new vacation homes rely on groundwater from individual wells. Water Levels/Aquifer Characteristics [No data available presently] Reported values of hydraulic parameters for the alluvium of the Rio Grande River and its major tributaries are listed in a downloadable file. Water Use/Withdrawals [No data available presently] Water Quality [No data available presently]
The Rio Grande River and its tributaries originate in the Mountainous region and flow into the San Luis Valley, where the alluvial aquifer is considered part of the upper, unconfined valley aquifer. Many tributaries originating in the Sangre de Cristo Range flow through parts of the Eagle Basin-Central Colorado Trough Ancestral Rockies basin.
The Gunnison River basin of southwestern Colorado encompasses approximately 8,000 square miles (Figure 10-05-01), extending from the Continental Divide in the east to Grand Junction in the west where the Gunnison joins the Colorado River. It passes out of the Mountainous region into the Colorado Plateaus region before joining the Colorado River. The watershed comprises portions of seven counties: Mesa, Delta, Montrose, Ouray, Hinsdale, Gunnison, and Saguache. Management of the waters in the Gunnison River basin is under the jurisdiction of Colorado Water Division 4, with its divisional office located in Montrose. As of February 2001, there were approximately 1,844 alluvial wells of record in the Gunnison River basin. Elevations in the basin are greater than 13,000 feet in the headwater areas along the Continental Divide, and less than 4,600 feet at the confluence of the Gunnison with the Colorado River. Principal tributaries of the Gunnison River include the Uncompahgre, North Fork Gunnison, Lake Fork Gunnison, East, and Taylor Rivers; and Ohio, Tomichi, and Cochetopa Creeks. Surface water is the main water resource in the basin. The historic average annual discharge of the Gunnison River into the Colorado River at Grand Junction is approximately 1.9 million acre-feet. Diversions for irrigation use were also of this magnitude and represented 99 percent of the total surface-water diversions. Alluvial groundwater, although relatively insignificant in terms of total volume withdrawn, is important for irrigation, public and domestic water supply, and livestock uses. The principal agricultural areas in the Gunnison River basin include the Gunnison area, East River valley, North Fork Gunnison River valley, Delta area, Orchard City-Cedaredge area, and the Uncompahgre River valley. The estimated population of the Gunnison River basin, based on the 2000 Census for the five principal counties, is approximately 80,000 persons. Back to Table of Contents
The alluvium of the Gunnison River basin consists of clay, silt, sand, gravel, and cobbles. Alluvial deposits are very thin or nonexistent in the canyon areas of the main stem of the Gunnison River and tributaries, for example down river of the town of Gunnison to the confluence with North Fork Gunnison River. The Division of Water Resources well permit database contains over 1,800 wells that have been completed in the Gunnison River alluvium. Over 90 percent of these wells are completed at depths less than 100 feet below ground surface with a mean depth of only 49 feet. Water Levels/Aquifer Characteristics The depth to water (water level) in the alluvial aquifer also depends on the time of year as well as location within the alluvial valley. Water levels for the North Fork Gunnison alluvium ranged from 2 to 111 feet and averaged 28 feet. For the lower Gunnison River basin, Brooks and Ackerman (1985) reported alluvial water levels ranging from 1 to 130 feet, averaging 26 feet. Reported well yields from the alluvial aquifer can be variable, depending to some degree on intended use, sediment type, and saturated thickness. Reported well yields for the lower Gunnison River basin range from 1 to 750 gpm and average 39 gpm. In the Ohio Creek-Gunnison area, reported yields exceed 100 gpm. For the Gunnison-Crested Butte area reported yields ranged from 8 to 35 gpm and averaged 16 gpm. In the North Fork Gunnison valley, well yields ranged from 1 to 150 gpm and averaged 20 gpm. Reported values of hydraulic parameters for the alluvium of the Gunnison River and its major tributaries are listed in a downloadable file. Water Use/Withdrawals Yield data for Gunnison River basin wells suggest the majority of these wells are used for domestic purposes. Water use studies by the U.S. Geological Survey indicate that in 1995 the five principal counties of the basin used 9,454 acre-feet of groundwater for irrigation purposes. Total groundwater withdrawal in 1995, including bedrock wells, for the five principal counties in the basin was 18,516 acre-feet. There are at least 25 alluvial wells utilized for public water supply, including municipal and commercial wells. The communities of Gunnison, Lake City, Orchard City, Ouray, Paonia, and Ridgeway use groundwater for public supply; however, this may include bedrock wells. Water Quality The alluvial water quality is generally suitable for agriculture, domestic, and industrial purposes. Groundwater in the North Fork Gunnison valley is a calcium bicarbonate type, with total dissolved solids (TDS) concentrations ranging from 110 to 2,300 milligrams per liter (mg/L). In the Gunnison-Crested Butte area reported values of TDS range from 47 to 533 mg/L, averaging 199 mg/L.
The watershed originates within the Mountainous region before crosses into the Colorado Plateaus region.
The Colorado River basin watershed encompasses an area of approximately 9,830 square miles (Figure 10-06-01). The basin defines Water Division 5 with the divisional office in Glenwood Springs. As of early 2001, there were approximately 1,370 alluvial wells of record in the Colorado River basin. The headwaters of the main stem are within Rocky Mountain National Park in eastern Grand County. The Colorado River flows southwest some 230 miles through Grand, Eagle, Garfield, and Mesa counties and exits the state at the Utah border. Principal tributaries are the Fraser, Blue, Eagle, and Roaring Fork Rivers. Surface water is the principal water resource in the basin. The Colorado River represents the largest surface-water outflow in the state with over 4.6 million acre-feet of water leaving annually. Agriculture dominates the use of the basin’s water resources, with diversions of approximately 2.2 million acre-feet for the irrigation of 300,000 acres. Between 450,000 and 600,000 acre-feet of water are also diverted annually from the basin to eastern Colorado. Alluvial groundwater resources are used for public water supply and agricultural irrigation, and represent an important resource in rural areas for domestic supplies. The principal agricultural area is the Grand Valley from Palisade to Fruita; other agricultural areas include Plateau Creek (Collbran area), Colorado River Valley from New Castle to Parachute, and the Roaring Fork Valley. The estimated population of the Colorado River basin, based on the 2000 census of the six major counties within the basin, is slightly over 253,000 people. Back to Table of Contents
The valley fill deposits or alluvium in the Colorado River basin consist generally of unconsolidated boulders, cobbles, gravel, sand, silt, and clay. The thickness of the alluvium can be extremely variable depending on location. Alluvium in the upper reaches of the basin tends to be thin due to increased slopes and higher flow velocities. Thicker deposits tend to accumulate in the lower reaches. Alluvium is very limited or nonexistent in the canyon sections of the Colorado River, such as the Gore, Glenwood, DeBeque, Ruby, and Horsethief Canyons where bedrock is exposed. Well depths along the Colorado River range from less than 10 feet to greater than 175 feet below ground surface. The Division of Water Resources well permit database contains records of approximately 1,370 wells that have been completed in the Colorado River alluvium. Over 90 percent of these wells are completed at depths less than 130 feet below ground surface with a mean depth of 72 feet. Water Levels/Aquifer Characteristics Static water levels in alluvial deposits are related to the adjacent river or creek stage. Generally, the alluvial water levels will be high in the spring and early summer due to snowmelt and increased runoff, dropping through the summer and fall, and will remain low throughout the winter. Within the Colorado River Basin water levels ranged between 1 and 70 feet. Reported yields from wells completed in the alluvial aquifer are dependent upon the intended use of the well, well construction design, type of sediment, and saturated thickness. Reported yields for the water wells of record range from 1 to 1,600 gpm with an average of 34 gpm and the 90th percentile reporting yields of less than 50 gpm. These lower well yields suggest that the majority of wells are used for domestic purposes. Reported values of hydraulic parameters for the alluvium of the Colorado River and its major tributaries are listed in a downloadable file. Water Use/Withdrawals Most of the water used within the Colorado River basin comes from surface-water sources. Annual groundwater withdrawal data from 1995 indicate that groundwater use by the counties encompassing the river basin varies from less than one percent in Grand and Mesa counties to a maximum of nine percent in Summit County. Because of the shallow well depths and water levels, alluvial groundwater is readily developed in rural areas for agricultural and domestic purposes. Water Quality Published water quality data for the Colorado River alluvial aquifers include concentrations of total dissolved solids (TDS), hardness, and measurements of radioactivity. In the Eagle River valley where alluvium overlies the Eagle Valley evaporite sequence, the water can be high in sulfates, producing high concentrations of TDS. Flows associated with hot springs also have typically high dissolved solids concentrations. As a result of these discharges, the alluvium downstream from Glenwood Springs has elevated TDS, sulfate, sodium, magnesium, manganese, calcium, and chloride levels. The hot springs at Glenwood Springs annually add 475,000 to 534,000 tons of dissolved solids to the Colorado River. In addition, irrigation-return flows are another source for increasing the concentrations of dissolved solids, especially in the Grand Valley.
Watersheds of the main stem and tributaries originate within the Mountainous region and include the Blue River and Middle Park Mountainous valleys. The main stem of the Colorado, Eagle River, and Roaring Fork River cross the Eagle Basin-Central Colorado Trough Ancestral Rocky Mountains basin. The main stem then crosses the Piceance Laramide Basin before entering Colorado Plateaus region as it heads to the border with Utah.
The Yampa River basin is located in north-central and northwestern Colorado (Figure 10-07-01). With its tributaries, the Yampa River drains all of Routt, most of Moffat, northeastern Rio Blanco, and a small portion of northeastern Garfield counties. Based on the 2000 census, the Yampa River basin contains a population of approximately 33,000. The Colorado portion of the drainage basin encompasses an area of approximately 6,765 square miles, and is a part of Water Division 6 with the divisional office in Steamboat Springs. The headwaters of the main stem are within the Park Range in western Routt County. The Yampa River drains the Colorado portion of the Wyoming Basin. The Yampa is a tributary of the Green River and their confluence is near the western border of Moffat County in Dinosaur National Monument. The historical average annual outflow for the Yampa River leaving Colorado is approximately 1.66 million acre-feet, or about 15 percent of the total stream flow that leaves Colorado. Principal tributaries of the Yampa are the Little Snake River, Williams Fork River, Fortification Creek, and Elk River. Back to Table of Contents
The alluvium in the Yampa River basin typically consists of unconsolidated deposits of clay, silt, sand, and gravel. The saturated thickness of the Yampa River alluvium ranges from 10 to 100 feet. In the tributary valleys, the saturated portion of the alluvium is generally less than 20 feet thick. Alluvium can be thin or absent where the streams cross hard, resistant bedrock such as sandstone, and thick and wide where the streams cross less resistant bedrock such as shale. Recharge of the alluvial aquifer occurs mainly from bank storage during spring runoff, leakage of irrigation ditches and laterals, and underflow from sedimentary rock aquifers. As of early 2001, there were approximately 340 alluvial wells of record in the Yampa River basin. Published depths for alluvial wells in this basin range from 5 to 190 feet and average about 30 feet deep. Ninety percent of these wells have been completed at depths less than 140 feet, with a mean depth of 63 feet. Water Levels/Aquifer Characteristics Published water levels in alluvial wells range from 0 (at land surface) to 41 feet below ground surface, averaging about 10 feet. The alluvium is generally a water table aquifer and water levels will fluctuate seasonally with stages in the adjacent surface water courses. Yields from alluvial wells in this basin have been reported from five to several hundred gallons per minute with the highest yields from the Yampa River alluvium near Steamboat Springs, Hayden, and Craig. A close inspection of alluvial wells in the Yampa River basin indicates that domestic water supply wells with yields of 15 gpm or less predominate. Although information about hydrologic aquifer characteristics for the alluvium in the Yampa River basin are scarce, published values for hydraulic conductivity of the alluvium ranges from 1.9 to 28.8 feet per day. These values are typical for fine- to coarse-grained sand. Reported values of hydraulic parameters for the alluvium of the Yampa River and its major tributaries are listed in a downloadable file. Water Use/Withdrawals Alluvial groundwater resources in this basin are used for domestic, livestock, and low-demand commercial purposes. The low reported yields (<21 gpm) of the alluvial wells in the Yampa River basin indicate that these wells are used chiefly for domestic, livestock, and commercial purposes. The towns of Steamboat Springs, Phippsburg, Hayden, and Yampa use alluvial groundwater for a part of their public water supply. Approximately 3,000 acre-feet of groundwater are withdrawn annually from the Yampa River basin. One-third of the water withdrawn is used for irrigation purposes. Most of the alluvial wells are concentrated around Craig, between Hayden and Steamboat, along Elk River, and in the Yampa area. Water Quality Alluvial groundwater in the Yampa River basin is generally a calcium and sodium bicarbonate type when the alluvial material is derived from the erosion of sandstone or granitic rocks. The water is a calcium sulfate type when the alluvium is composed of reworked Fort Union Formation or where the Fort Union discharges into the alluvium. The concentration of total dissolved solids (TDS) in the alluvial water ranges from 82 to 2,970 milligrams per liter (mg/L) and averages 724 mg/L. Hardness ranges from 10 to 1,000 mg/L, averaging 300 mg/L.
Watersheds of the main stem and its tributaries originate within the Mountainous region. The main stem continues across the Sand Wash Laramide basin before entering Colorado Plateaus region as it heads to the border with Utah.
The White River flows from east to west parallel to and south of the Yampa River in northwestern Colorado (Figure 10-08-01). The White River basin drains approximately 3,770 square miles encompassing nearly all of Rio Blanco County, a small portion of southern Moffat County, and small areas of northern Garfield County. The White River basin is sparsely populated and contains a population of approximately 6,000 people. The drainage basin is part of Colorado Water Division 6 with the office of the Division Engineer in Steamboat Springs. However, the Water Judge and Clerk assigned to the White River are a part of the Water Division 5 administration. The White River drains part of the Colorado Plateau, Wyoming Basin, and Southern Rocky Mountains. Like the Yampa, the White River is a tributary of the Green River and the confluence with the Green is south of Ouray, Utah. The historical average annual outflow for the White River leaving Colorado is approximately 590,100 acre-feet, or about 5% of the total streamflow that leaves Colorado. Principal tributaries of the White River are Douglas Creek, Yellow Creek, Piceance Creek, and the North and South Forks of the White River. Back to Table of Contents
The White River alluvium generally consists of silty sand and rounded gravel and cobbles derived from sandstone, quartzite, basalt, and granite from the eastern mountains, whereas alluvium in the tributaries is finer-grained material of a local origin. Width of the alluvial aquifer ranges from 0.1 to 1.5 miles and averages 0.5 miles. The saturated thickness of the White River alluvium ranges from 0 feet at the valley edge to more than 140 feet, with an overall average of 22 feet. In the Meeker area the average saturated thickness is 54 feet. West of Meeker the saturated thickness ranges from 14 to 90 feet and averages 17 feet. The Colorado Division of Water Resources well permit database identifies 75 alluvial wells of record that are completed at a depth of less than 150 feet in the White River basin. The majority of the alluvial wells are concentrated east and west of Meeker. Reported alluvial well depths ranged from a minimum of 7 feet to a maximum of 147 feet, with the majority of these wells completed at depths between 10 and 70 feet. Water Levels/Aquifer Characteristics For the alluvial wells of record with reported water levels, those levels ranged from a minimum of 3 feet to a maximum of 90 feet below ground surface. Though the alluvium is generally a water table aquifer, near-surface accumulations of alluvial silts and clays can produce local semi-confined conditions for the underlying coarse-grained alluvial aquifer. Van Liew and Gesink (1985) report yields from alluvial wells generally less than 25 gallons per minute (gpm). East of Meeker, the average yield is 17 gpm, and between Meeker and Rangely the average yield is 10 gpm with a few wells able to yield 60 gpm for short periods of time. For the alluvial wells of record with the Division of Water Resources, the data indicate that nearly 90 percent of the wells produce less than 33 gpm. Reported values of hydraulic parameters for the alluvium of the White River and its major tributaries are listed in a downloadable file. Water Use/Withdrawals Alluvial groundwater resources in this basin are used for domestic, municipal, and irrigation purposes. Hatton (2000) reports that 1,000 acre-feet of groundwater are pumped annually from the alluvium. Yield data indicate that the majority of alluvial wells are used for domestic or livestock purposes. Water Quality Water quality data for the White River alluvium are sparse. Van Liew and Gesink (1985) report that water in the alluvium in the eastern part of the basin is a calcium-bicarbonate becoming sodium-bicarbonate to the west. Total dissolved solids range from 200 to 2,500 mg/L.
The White River originates within the Eagle Basin-Central Colorado Trough Ancestral Rockies basin and then continues across the Piceance Laramide basin as it heads to the border with Utah.
The North Park River Basin encompasses headwaters of both the North Platte and Laramie Rivers. The North Platte River originates in North Park and flows north into Wyoming. North Park is a broad, intermontane valley of flat to rolling topography nested between the Park Range to the west, the Medicine Bow Range to the east and the Rabbit Ears Range to the south. It encompasses about 1,190 square miles of Jackson County (Figure 10-09-01). According to the 2000 Census, Jackson County recorded a population of approximately 1600, with agriculture being the dominant business. The Laramie River lies on the east side of the Medicine Bow Range in a sparsely populated valley in Larimer County. This river also flows north into Wyoming to join the North Platte River nearly 140 miles to the northeast close to the border between Wyoming and Nebraska. From there the North Platte flows eastward and is part of the Mississippi River drainage system. North Park, and the North Platte River, fall under the administration of Water Division 6, with offices located in Steamboat Springs. The Laramie River, on the other hand, falls under the administration of Water Division, with offices located in Greeley. Back to Table of Contents
Water Levels/Aquifer Characteristics [No data available presently] Reported values of hydraulic parameters for the alluvium of the North Platte River and its major tributaries are listed in a downloadable file. Water Use/Aquifer Characteristics [No data available presently] Water Quality [No data available presently]
The North Platte River flows through the North Park mountainous valley before flowing across the border with Wyoming.
The San Juan River originates along the Continental Divide in southern Colorado, just north of the town of Pagosa Springs. The watershed (Figure 10-10-01) encompasses about 26,000 square miles of Colorado, New Mexico, and Arizona before merging with Lake Powell and the Colorado River system in Utah. The entire San Juan River system in Colorado is located within Water Management Division 7, with the division office in Durango. In Colorado, the San Juan River system is fed by a series of sub-parallel rivers that drain the San Juan and La Plata Mountains. These rivers, from east to west, include the Piedra, Los Pinos, Florida, Animas, La Plata, and Mancos Rivers. The Mancos and Mc Elmo Rivers head west-southwest to join the San Juan in New Mexico and Utah respectively. Altitudes in the San Juan River system range from greater than 13,000 feet near the headwaters of the San Juan and Piedra Rivers to nearly 4,500 feet where the Mancos River exits the state just east of Four Corners. The San Juan River contributes about 10 percent of the total flow in the Colorado River Basin. The average annual flow exiting the state through the San Juan River represents approximately 7 percent of the state’s total annual stream outflows. Back to Table of Contents
Alluvial aquifers in the San Juan River system are typically limited arealy in the higher mountain areas and become more expansive as they exit the mountain fronts. In general, the San Juan and Piedra Rivers form one combined alluvial system, as do the Florida and Animas Rivers. Each of the rivers in the San Juan River system contains alluvial valley fill and extensive terrace deposits. As with most alluvial deposits, they typically consist of varying amounts of gravel, sand, silt, and clay, depending upon distance from the mountain front and location within the depositional system. In general, the San Juan River system alluvium does not exceed 150 feet in thickness, with most areas containing less than 100 feet. As of February 2001, over 1,800 wells were on record as being completed in the alluvial aquifers of the San Juan River basin. The majority of the alluvial wells are concentrated in the Los Pinos, Animas, and La Plata River valleys. Of the alluvial wells of record, over 90 percent are completed at depths less than 170 feet, with an average depth of 103 feet. Reported alluvial well depths ranged from a minimum of 2 feet to greater than 200 feet. Water Levels/Aquifer Characteristics Water levels within the alluvial aquifer vary tremendously depending upon the stream reach or tributary, saturated thickness of the aquifer, vertical position along the stream reach, and horizontal position within the alluvial valley. Reported depths to water range from a minimum of 1 foot to a maximum of 181 feet below ground surface. Reported well yields are typically low, ranging from 0 to 50+ gallons per minute (gpm) with a mean yield of 17 gpm. Eighty-five percent of the alluvial wells of record have well yields of less than 18 gpm, suggesting that these wells are used predominantly for domestic purposes and livestock watering. Little data on the hydraulic properties of the San Juan River system alluvium are available. Reported hydraulic conductivities from the river system upstream of Farmington, New Mexico range from 0.006 to 200 feet per day. Reported values of hydraulic parameters for the alluvium of the San Juan River and its major tributaries are listed in a downloadable file. Water Use/Withdrawals Alluvial groundwater from the San Juan River system has typically been utilized for agricultural irrigation, stock-watering, and domestic purposes. Irrigated lands are common along the river valleys and irrigation return water accounts for much of the alluvial recharge along the La Plata, Animas, Florida, and Los Pinos Rivers. Valley fill deposits provide an important source of potable water for many rural residences, although total withdrawals from the alluvial aquifer are small when compared to the bedrock aquifers and surface water. Water Quality Water quality in the San Juan River system alluvium is highly variable and dependent upon local factors in each of the individual river watersheds. Total dissolved solids (TDS) in the alluvial aquifers is typically less than 1,000 milligrams per liter (mg/L) and can be less than 500 mg/L in reaches of the San Juan, Animas, and La Plata valleys where irrigation water recharges the alluvium. Published data for the Animas River alluvium indicated that TDS ranged from 326-572 mg/L and sulfate ranged from 112-121 mg/L in domestic water wells.
Watersheds of the San Juan River and its tributaries originate within the Mountainous region. They cross the Paradox Basin before reaching the San Juan Laramide basin where they flow south into New Mexico. The San Juan River basin includes parts of Colorado Plateaus region.
The Dolores and San Miguel Rivers originate in the Rico, La Plata, and San Juan Mountains of southwest Colorado (Figure 10-11-01). The Dolores River basin is about 95 miles long from northwest to southeast and encompasses an area of just over 5,300 square miles, including parts of Montezuma, Dolores, San Miguel, Montrose, and Mesa counties. The San Miguel River basin is tributary to the Dolores River, is 68 miles long, and encompasses about 1,600 square miles, including portions of San Miguel and Montrose counties. The Dolores River basin falls within both Water Management Divisions 4 and 7. Altitudes in the Dolores and San Miguel River basins range from about 14,200 feet near the Dolores River headwaters to 4,100 feet at their combined confluence with the Colorado River in Utah. Although restricted in extent, the alluvium is an important aquifer to those people who utilize it for domestic, stock, and minor irrigation use. Population is sparse in the Dolores and San Miguel River basins, with the 2000 Census indicating that approximately 65,700 people live in the combined Dolores, San Miguel, Montezuma, and Montrose Counties. Back to Table of Contents
Alluvial aquifers in the Dolores River basin are very limited in extent and are restricted to areas immediately adjacent to the main river channels. Within Division 7, most of the mapped alluvial deposits are located at, or upstream of, the town of Dolores. In Division 4, mapped alluvial deposits are localized around Gateway and in West Creek. Alluvium within the Dolores and San Miguel River basins is comprised of typical Quaternary alluvial valley fill. These deposits consist of gravel, sand, silts, clay, and various mixtures. The alluvial extent is limited to areas near the rivers and their tributaries and disappears entirely in areas where active canyon downcutting occurs. The greatest extent of alluvium along the southern portion of the Dolores River is located upstream from the town of Dolores and McPhee Reservoir. Less than 100 alluvial wells have been recorded with the Division of Water Resources. The depths of the alluvial wells of record range from 9 to greater than 140 feet below ground surface. Ninety percent of the alluvial wells have been completed at depths less than 120 feet, with the majority of wells completed at depths less than 70 feet. The mean completion depth is 66 feet below ground surface. Water Levels/Aquifer Characteristics Little data are available for the Dolores and San Miguel River basin alluvial aquifers. A search of The Colorado Division of Water Resources well permit database yielded just under 100 alluvial wells of record completed to depths of less than 140 feet. Reported water levels for various well depths range from 2 to 90 feet in the two river basins with springs present in some locations. Groundwater in the alluvium is under water table, or unconfined, conditions in both river basins. The alluvial aquifers are only capable of yielding low to moderate quantities of groundwater. For the wells of record within the DWR database, reported yields range from 1 to 200 gallons per minute (gpm). Over 90 percent of the wells completed in the associated river alluvium yield less than 50 gpm with the average well yielding only 22 gpm. Reported values of hydraulic parameters for the alluvium of the Dolores River and its major tributaries are listed in a downloadable file. Water Use/Withdrawals Public water supply is the primary use of groundwater in San Miguel and Dolores Counties, whereas agriculture is the primary use of groundwater in Montrose and Mesa Counties. Alluvial groundwater is most commonly used for domestic and stock watering purposes, although irrigation could be accomplished in the near-river areas. Water Quality Groundwater from the Dolores River alluvium is characterized as calcium sulfate or calcium bicarbonate-type water and often exceeds the EPA secondary drinking water standards of 500 mg/L for sulfate and 250 mg/L for total dissolved solids (TDS). Discharge from the underlying Paradox Basin salt formations is thought to be the source of the lower quality waters, as the Dolores River is a gaining river within the Paradox Basin. In general, the San Miguel River alluvium exhibits better quality water than the Dolores River alluvium.
Watersheds of the two rivers lie within the Mountainous region before crossing into the Paradox Basin.
Sedimentary bedrock aquifers are composed primarily of consolidated or semi-consolidated granular materials deposited in river, wind, shoreline, or deep-sea environments. These originated as unconsolidated sediments, but over time were consolidated by compaction or cementation as minerals formed in the pore spaces. The most common cement minerals are silica, calcite or other carbonates, and iron oxides. Because of consolidation, well productivity in sedimentary bedrock aquifers tends to be lower than in unconsolidated alluvium. Mean well yield from sedimentary bedrock aquifers is generally less than 30 gpm, and rates over 500 gpm only occur under rare conditions. However, these aquifers can extend over much larger areas than alluvial aquifers. Sedimentary bedrock formations may be quite variable in composition and the way different types of sediment are layered. The layering affects how well water is stored and can move through the formations and determines what type of hydrogeologic unit a formation may be. Some bedrock formations may be made up entirely of a well-sorted and permeable material, such as wind blown sandstone, that can form a fairly uniform aquifer through which water can move easily. Other geologic formations may consist of strata of varying size and composition in multiple layers with confining units separating aquifers where water may move easily in a horizontal direction but not so vertically. Some may have individual permeable bodies limited lateral extent imbedded in impermeable material. An example of the later are lens-shaped deposits of sand deposited by meandering streams surrounded by mudstone deposited in marshes and floodplains. Sedimentary bedrock aquifer distribution, depth, and thickness is quite variable depending on geologic setting. Sedimentary formations in the state can be grouped into aquifer systems with common characteristics based on geologic setting (Figure 11-01). Several aquifer systems are regional, where multiple geologic formations can be wide-spread with only minor variation in characteristics. Structural deformation of the formations across a region may be present, but structural style tends to consist of a limited number of large scale, well defined features. What formations are present at, or near the surface depends on structural and topographic setting across the region. Other aquifer systems are defined by distinct, and often very deep, structural basins. There may be formations unique to individual basins with characteristics that might change dramatically across the basin. Specific formations may be present at, or near the surface in one location, but at a very great depth in a relatively short distance. Back to Table of Contents
Colorado’s aquifers can be grouped into four main regions: Colorado Piedmont, High Plains, Colorado Plateaus, and Mountainous regions. Regional aquifer systems are those that are found spanned across large parts, or all, of any of these regions. Some aquifers, for example the Dakota Aquifer, are found in each of the regions. Some geologic strata with similar characteristics and geologic origins can be found in different regions, yet they have different names. Much of the Pierre Shale of the Colorado Piedmont region is stratigraphically equivalent to the Mancos Shale of the Colorado Plateaus region; the two formations and they share sedimentary characteristics. Hydrologically, both the Pierre Shale and Mancos Shale form regional confining units. Back to Table of Contents
The Colorado Piedmont region covers over 30,000 square miles of eastern Colorado and is characterized by broad open valleys separated by low hills. It extends from the abrupt face of the Rocky Mountains east to the escarpment of the High Plains (Figure 11a-01-01). Topographic expression within the Colorado Piedmont tends to be gentle, interrupted by local features with higher relief where geologic formations resistant to erosion form mesas and buttes. Elevation and topographic relief increase closer to the west edge of the region closer to the Rocky Mountain foothills. The South Platte and Arkansas River Basins carve their way from west to east across the Colorado Piedmont region. Over 80% of Colorado’s population lives within the Colorado Piedmont region and most of that is in the Front Range urban corridor from Fort Collins to Pueblo. The region supports a robust economy driven by agriculture, transportation, industry, service, and construction. It also serves a vital transportation hub. Back to Table of Contents
Hydrogeologic units within the Colorado Piedmont region include marine and non-marine sedimentary bedrock formations of Permian through Upper Cretaceous age (Table 11a-01-01). Marine formations deposited while the Upper Cretaceous Interior Seaway occupied much of Colorado dominate region. These are predominantly thick accumulations of shale that form a regional confining unit with the Pierre Shale having the greatest lateral extent at, or near the surface. Organic-rich shales, such as the Niobrara Formation near the base of the sequence, are important sources of oil and gas throughout the Rocky Mountain region. Beds of sandstone and limestone within the shales can form local aquifers that may be the only source of groundwater over large areas. The Fort Hays Member of the Niobrara Formation and the Codell Sandstone Member of the underlying Carlile Shale are recognized as an aquifer in the Lower Arkansas River Basin. Recently, an interval near the top of the marine Pierre Shale that consists of fine-grained sandstone interbedded with silt and shale has gained recognition as a potential groundwater source. It is recognized as the Upper Pierre Aquifer and supplies a number of wells in Weld County. The Dakota-Cheyenne Aquifer is a regional name for the Dakota Group, which is an extensive regional formation deposited during advance of the Upper Cretaceous seaway flooding event. It is perhaps the most reliable, extensive hydrogeologic unit within this region where it consists of the Dakota Sandstone and underlying Cheyenne Sandstone of the Purgatoire Formation. The group was deposited in a variety of coastal and marginal marine environments with compositions ranging from well-sorted sandstone to fine-grained shale. Beds of conglomerate and coal may also be present. The Dakota Group can be over 500 feet thick in northeastern Colorado. Depth to the top of the aquifer ranges from zero where it crops out along the Front Range on the western side of the region to greater than 9,000 feet below ground surface near the center of the Denver Basin. The depth is less than 2,000 feet in the southern part of Colorado between Pueblo and Trinidad and along the Purgatoire River. Older, and deeper, Mesozoic formations may also be present, but often at very great depths. The Entrada-Dockum Aquifer is at, or near the surface mainly along the western edge of the region where older and deeper formations rise to the surface at the edge of the Rocky Mountains. It is also present in the upper reaches of the Purgatoire River and Muddy Creek south of the Arkansas River. Water Levels/Aquifer Characteristics Aquifer characteristics of the many hydrogeologic units are quite variable because of the wide variability in geologic material type and potential changes across such a large region. Probably the best recognized hydrogeologic unit of the region is the Dakota-Cheyenne Aquifer. In eastern Colorado, groundwater in the Dakota-Cheyenne Aquifer generally moves in a northeast direction. Yields from wells completed in the Dakota-Cheyenne Aquifer sandstones are highly variable and are generally greatest in southeastern Colorado where the transmissivity and storage coefficients are high. Domestic well yields commonly range from 5 to 50 gallons per minute (gpm), and some irrigation wells in Baca County are reported to yield more than 1,000 gpm. The hydraulic conductivity of various units within the Dakota-Cheyenne Aquifer ranges from 20 feet per day (ft/d) in the well-sorted sandstones to less than 0.001 ft/d in poorly sorted, clay-rich portions of the aquifer. Average hydraulic conductivity values range from 0.001 ft/d along the western margins of the Denver Basin to 2.0 ft/d in the shallow aquifer zone near the Arkansas River and Apishapa Arch. Reported values of hydraulic parameters for the hydrogeologic units of the Colorado Piedmont region are listed in a downloadable file. Water Use/Withdrawals The Dakota-Cheyenne aquifer provides a reliable source of water in most areas of Colorado. The primary water use is for irrigation purposes and domestic water supply in southeastern Colorado and domestic use elsewhere. In some cases, municipal or industrial supplies may be developed in the Dakota-Cheyenne aquifer; however, restricted yields make it an unreliable high-volume producer in most areas. Its potential use is very dependent upon the chemical and physical characteristics of the water. Water temperatures in the aquifer range from 60° F in the shallow portions to more than 200° F beneath the Denver metropolitan area. Water Quality Water quality within the Dakota-Cheyenne aquifer is generally good, but is dependent upon the geologic composition of the unit. The total dissolved solids (TDS) concentrations of the groundwater in this aquifer typically range from 200 to 25,000 milligrams per liter (mg/L) with the higher concentrations associated with oil and gas fields. The complex stratigraphy in the northern portion of the aquifer produces highly variable water chemistry. In areas of southern Colorado, TDS concentrations decrease as the water moves eastward due to recharge mixing, ranging from 250 to 500 mg/L in much of Baca and southeastern Prowers counties. Back to Table of Contents
Alluvial aquifers of the South Platte and Arkansas River Basins overprint the Colorado Piedmont region. Where sedimentary bedrock aquifers are at the surface and intersect alluvium, there can be direct hydraulic connection between alluvial aquifer and bedrock sedimentary aquifer. The Denver and Raton Laramide Basins overlie the Colorado Piedmont regional aquifer system, and can be considered parts of the region. Regional aquifers continue deep below the Laramide Basins. Similarly, the regional aquifers extend beneath the High Plains Aquifer, which effectively extends the Colorado Piedmont region to over 45,000 square miles. The Ancestral Denver Basin underlies the entire region at very great depths in most places. Hydrogeologic units, including the Lyons and Fountain Aquifers rise to the surface at the very west edge of the region at the base of the Rocky Mountains.
The High Plains aquifer is an extensive regional aquifer that underlies approximately 174,000 square miles of the Great Plains states extending from South Dakota on the north to Texas and New Mexico on the south (Figure 11a-02-01). The aquifer is of significant economic importance as it provides groundwater to approximately 20 percent of the irrigated cropland in the United States. In Colorado, the High Plains aquifer is present beneath all or parts of Weld, Logan, Sedgwick, Phillips, Washington, Yuma, Lincoln, Kit Carson, Cheyenne, Kiowa, Prowers, Las Animas, and Baca counties (Figure 11a-02-02). Land use in eastern Colorado is almost exclusively agricultural with most groundwater withdrawal used for irrigation and crop-related purposes. The topography is characterized by flat to gently rolling terrain that is bisected by mostly eastward-flowing rivers and streams. The population in eastern Colorado remains low, with population densities rarely exceeding four to nine residents per square mile. Colorado counties containing the High Plains aquifer are included in the Office of the State Engineer Water Divisions 1 and 2. Most of the aquifer is a part of Colorado’s Northern or Southern High Plains Designated Ground-Water Basins and their associated groundwater management districts. Virtually thousands of wells (15,600 wells of record as of February 2001) are completed in the High Plains aquifer. Back to Table of Contents
The High Plains aquifer system covers a large portion of this region and is made up of permeable sand and gravel shed off of the Rocky Mountains during regional uplift in the Neogene period. It is a vital regional water resource.The High Plains aquifer is composed principally of unconsolidated to semi-consolidated sands, gravels, clays, and silts of the Miocene-aged Ogallala Formation (Table 11a-02-01). Quaternary-age alluvial, valley-fill, dune sand, and loess deposits are also considered a part of the High Plains aquifer where they are hydraulically connected to the underlying Ogallala Formation. The High Plains aquifer in Colorado ranges from less than 50 feet thick along the western, eroded outcrop edge to more than 500 feet thick in the paleo-river valleys of Washington County. Thicknesses average about 250 to 350 feet in the northern counties and 50 to 150 feet in the counties south of the Arkansas River. Erosion and downcutting have removed the High Plains aquifer along the main channels of the Arkansas and South Platte rivers, effectively separating the northern and southern portions of the aquifer in Colorado. Currently, the saturated thickness ranges from zero to greater than 250 feet in the northern portion of the aquifer. The average saturated thickness ranges from 25 to 200 feet in the north and 25 to 75 feet in the south. Water Levels/Aquifer Characteristics The High Plains aquifer is typically under unconfined conditions throughout Colorado. Localized confined conditions may occur where clay beds or caliche horizons provide a confining layer. The primary source of recharge to the High Plains aquifer in Colorado is from infiltration of precipitation in the form of rain and snow. Recharge is limited by the low precipitation and high evaporation rates that are common to the eastern plains. Discharge typically exceeds recharge in the aquifer, with the primary source of discharge being groundwater extraction for agricultural purposes. Groundwater withdrawal from wells was approximately 1 million acre-feet in 1979. Discharge to rivers is estimated at 40,000 acre-feet per year and subsurface outflow to neighboring states is estimated at 390,000 acre-feet per year. In general, the well depths tend to increase eastward, as the aquifer dips deeper below the ground surface. Some of the deepest wells extend to depths greater than 500 feet below ground surface, with most wells completed between 200 and 350 feet. Groundwater flow is generally towards the east at a hydraulic gradient ranging from 0.004 to 0.05. Water levels in the High Plains aquifer have been steadily dropping with increasing groundwater withdrawals. The greatest declines are evident in the Yuma and Burlington areas with declines of over 40 feet. Well yields in the Colorado High Plains aquifer range from less than 25 gallons per minute (gpm) to more than 1,000 gpm. Wells reporting yields of less than 25 gpm typically represent domestic and stock use, while yields greater than 500 gpm represent irrigation use. In Colorado, hydraulic conductivities range from less than 25 to more than 200 feet per day, with an average of about 60 feet per day. Approximately 85 percent of the aquifer exhibits hydraulic conductivity values between 25 to 100 feet per day. Reported values of hydraulic parameters for the alluvium of the region are listed in a downloadable file. Water Use/Withdrawals Groundwater from the High Plains aquifer is primarily used for agricultural irrigation throughout the region. Over 600,000 acres of land in the High Plains area was under irrigation in 1980 with water produced from 4,800 wells. This acreage accounts for approximately 17 percent of the state’s total irrigated acreage. Areas with well yields greater than 1,000 gpm are limited to the far northeastern portions of the state, centered in Yuma and Washington counties – an area of significant center-pivot irrigation. In comparison, relatively minor quantities of water are used for domestic, livestock, and industrial purposes. Groundwater extraction in the eastern plains of Colorado in 1995 ranged from a low of 19,500 acre-feet per year in Lincoln County to almost 300,000 acre-feet per year in Yuma County. With limited surface-water resources, groundwater represents the dominant (almost exclusive) water supply in the eastern plains. The High Plains aquifer is the primary source of that supply. Water Quality The water from the Colorado High Plains aquifer is generally of good quality, and the total dissolved solids (TDS) range from 100 to 600 milligrams per liter (mg/L). The waters tend to be moderate to very hard, containing 100 to 350 mg/L of calcium carbonate. Since the early 1900’s, TDS concentrations in many portions of the aquifer, including Colorado, have risen significantly. The most significant increases appear to be associated with valley-fill areas, and may be the result of agricultural irrigation recharge and evaporative concentration. Naturally occurring concentrations of sulfate, chloride, fluoride, and iron may sometimes exceed the Federal and State drinking water standards. These constituents may be derived from underlying rock formations or from ash lenses within the High Plains aquifer. Arsenic concentrations are elevated in some areas of the northern part of the High Plains aquifer in Colorado and may exceed drinking water standards in places. The source of arsenic may be naturally derived from associated rocks or may have been introduced by older pesticides containing arsenic compounds.
Alluvial aquifers of the Republican/Arikaree river basin cross over the High Plains region, and are considered part of the High Plains Aquifer. The Cheyenne Laramide basin underlies part of the northern High Plains Aquifer in northern Weld County. Regional aquifers of the Colorado Piedmont region underlie all portions of the High Plains Aquifer, and indeed, many modern streams have carved through the High Plains Aquifer into underlying bedrock formations.
Western Colorado covers portions of the Colorado Plateau and Wyoming Basin Plateau region (Figure 11a-03-01), an area characterized by deep canyons and high plateaus. Geologic formations at the surface in this area are dominated by Pennsylvanian through Quaternary sedimentary deposits with scattered Neogene volcanic flows and intrusions. Upper Cretaceous marine Mancos Shale, the western equivalent of the Pierre Shale, dominates much of this region. The deeper Dakota Group underlies large areas and can be an important regional aquifer. Deformation of the sedimentary formations in this area include broad uplifts and large-scale folds formed during the Ancestral Rocky Mountains and Laramide events. Back to Table of Contents
Hydrogeologic units within the Colorado Plateaus region include marine and non-marine sedimentary bedrock formations of Permian through Upper Cretaceous age (Table 11a-03-01). Marine formations deposited while the Upper Cretaceous Interior Seaway occupied much of Colorado are common in the lower elevations. These areas are where most of the population has settled and where agriculture has prospered. The marine formations are predominantly thick accumulations of shale that form a regional confining unit with the Mancos Shale having the greatest lateral extent at, or near the surface. Organic-rich shales in this sedimentary sequence are important sources of oil and gas throughout the Rocky Mountain region. Beds of sandstone and limestone within the shales can form local aquifers that may be the only source of groundwater over large areas. Beneath the shales is the Dakota Aquifer which is an extensive regional aquifer deposited during advance of the Upper Cretaceous seaway flooding event. It is perhaps the most reliable, extensive hydrogeologic unit within this region. The unit was deposited in a variety of coastal and marginal marine environments with compositions ranging from well-sorted sandstone to fine-grained shale. Beds of conglomerate and coal may also be present. Older, and deeper, Mesozoic formations may also be present, particularly in the rugged mesa and canyon areas. The Entrada and Navajo Aquifers are widespread in the mesas and have the potential to hold considerable amounts of water. The sandstones tend to be intermittently saturated, with perched water tables common. Springs frequently occur at the base of the more permeable units, such as the Navajo Aquifer. The Mesozoic sandstone aquifers are often under the influence of surface water and they discharge to the surface water where it is deeply incised. In the northern part of the region Neogene sandstones and conglomerates of the Browns Park Formation and Bishop Conglomerate blanket the area and form important local aquifers. Many of these are tuffaceous and overlap onto the Laramide Sand Wash Basin. Because of their widespread extent in this area they are included with the Colorado Plateaus regional aquifers. Locally Neogene volcanic flows cap the regional aquifers. Water Levels/Aquifer Characteristics [No data available presently] Reported values of hydraulic parameters for the alluvium of the region are listed in a downloadable file. Water Use/Withdrawals [No data available presently] Water Quality [No data available presently]
Alluvial aquifers of the Colorado, Yampa, Dolores, and San Juan River basins overprint the Colorado Plateaus region. Where sedimentary bedrock aquifers are at the surface and intersect alluvium, there can be direct hydraulic connection between alluvial aquifer and bedrock sedimentary aquifer. The Sand Wash, Piceance, and San Juan Laramide basins overlie the regional aquifer system, and can be considered parts of the region. Regional aquifers continue deep below the Laramide Basins. The Paradox and Eagle-Central Colorado Trough Ancestral Rockies basins underly parts of the region typically at very great depths. Hydrogeologic units within the deeper basin are exposed in deeper canyons and at the edges of uplifts.
Large quantities of groundwater occur in deep basins formed during several of the major geologic events that define the evolution of the geologic landscape of Colorado. These basins typically contain multiple geologic formations made up of sand, gravel, and mudstone shed off of nearby uplifts. Stratigraphic setting, or the way different layers are stacked, can be complex with some formations changing characteristics rather quickly with distance away from an active rangefront. Deposition close to the active range front may be dominated by high-energy alluvial fans where the deposits can be mostly coarse grained sandstone and conglomerate. Sorting may be very poor near the range front. Away from the active uplifts, stream transport tended to slow down and grain size decreases, but sorting increases. Fine-grained shale and mudstones can be dominant with greater distance from the rangefront. In Colorado three episodes of basin development created important hydrogeologic settings. The earliest was the Pennsylvanian to Permian Ancestral Rocky Mountain event. Next came the Upper Cretaceous to Paleogene Laramide mountain building event which created the foundation for many of the mountain ranges present today. Basins formed during these two events form large-scale aquifer systems recognized today. The third, and most recent, episode of basin development is the development of deep basins during extension of the Rio Grande Rift. These basins form many of the deep valleys included in the Mountainous region. Back to Table of Contents
The oldest major basin-forming episode in the progression of events for Colorado that is significant for groundwater resources is the Pennsylvanian-Permian ancestral Rocky Mountain event. This major tectonic event occurred between approximately 250 and 320 million years ago when uplift occurred along two northwest-trending belts, the Ancestral Front Range-Apashipa range on the east and the Uncompaghre-San Juan uplift on the west (Figure 11b-01-01). A deep basin, the Central Colorado Trough separated the two uplifts. Boundaries of these features are often difficult to define because later tectonic events modified defining structural elements and overprinted new ones. Sediments eroded from the uplifts filled basins flanking the uplifts, the Paradox Basin to the west, Ancestral Denver Basin to the east, and the Central Colorado Trough in between. Total thickness of the sediments that accumulated can be over 10,000 feet in places. The stratigraphy represents transition from active mountain fronts, dominated by alluvial fan systems, basinward to quieter depositional settings that, at times, were open to the ocean and flooded with seawater. Sediments can include sandstone, siltstone, conglomerate, shale, and limestone with proportions and bedding relationships that change with distance from the adjacent rangefront. At times circulation with the open ocean was restricted and evaporation of ponded water in the basins led to the buildup of thick evaporite deposits, chiefly salt and gypsum. Older Paleozoic sedimentary formations blanketed the region prior to the Ancestral Rocky Mountain event. These formations, dominated by limestone and dolomite with lesser amounts of quartz sandstone and shale, were eroded off of the uplifts but remain within the basins. Because they are mainly restricted to the Ancestral Rocky Mountains basins, they can be considered part of those basins. Later tectonic events, primarily the Laramide mountain building event, brought parts of the old basins up to be exposed at the surface. Sedimentary formations from the Ancestral Rocky Mountains event include the colorful Maroon and Minturn Formations of the Eagle Basin, the Fountain Formation of the Ancestral Denver Basin, and the Hermosa Formation of the Paradox Basin. Formations also include thick accumulations of salt and gypsum like the Eagle Valley Evaporite Member exposed in the Eagle and Roaring Fork Valleys in the Colorado River Basin. Not only do salt and gypsum impair water quality locally, but their dissolution by groundwater can cause the ground to collapse in areas where they occur. Back to Table of Contents
The Central Colorado Trough (Figure 11b-01-01-01) covers a wide, northwest-trending swath across central Colorado. Remnants of it are found throughout the Mountainous region with its greatest exposure is recognized as the Eagle Basin. The Eagle Basin represents that part of the Central Colorado Trough that was uplifted during the Laramide event along the White River and Sawatch uplifts. The Central Colorado Trough continues to the northwest with part deep within the Sand Wash Basin and part exposed in the Axial Basin-Uinta uplifts. Fragments of the Central Colorado Trough can also be found southeast in the Mountainous region as structural blocks within valleys and high in the mountain ranges. Eagle Basin underlies approximately 1,500 square miles in north-central Colorado along the western flank of the Continental Divide. It lies primarily in Eagle County, but also extends into Routt, Rio Blanco, Garfield, Pitkin, and Grande counties. The Eagle, Roaring Fork, and Colorado Rivers form the primary surface water drainages in the basin, with the headwaters of those rivers located along the Continental Divide. The Eagle River generally follows the axis of the basin to the town of Wolcott, where it turns west and meets with the Colorado River at Dotsero. The bulk of the Eagle Basin is included in Water Division 5. Land use in Eagle County is primarily recreation and tourism with limited ranching outside the Interstate 70 corridor. Recreational opportunities and the proximity of major ski resorts have resulted in tremendous development along the I-70 corridor between Vail and Edwards. According to the 2000 Census, 41,659 people permanently reside in Eagle County, with at least one-quarter of these located in the resort towns of Vail and Avon. Over 2,600 non-alluvial wells are permitted in the Eagle Basin. Most of these water supply wells are clustered near the population centers along the Interstate 70 and Roaring Fork River corridors.
Hydrogeologic units within the Eagle Basin-Central Colorado Trough fall into two main groupings (Table 11b-01-01-01). The first, and uppermost group, are sedimentary formations directly associated with the development of the basin. It includes the Weber Aquifer, Maroon-Minturn Aquifer, and the Belden-Molas confining unit. Water-yielding rock types in this group are mainly beds of sandstone. It also includes the Eagle Valley Evaporite unit, which is generally a confining unit but also can yield brackish to briney water that contributes to salt loading in the Colorado River Basin. The second group are the Mississippian through Cambrian formations that are dominated by carbonate aquifers. This includes the Mississippian-Devonian carbonate aquifer, which includes the Leadville Limestone and Chaffee Group, and the Ordovician-Cambrian carbonate aquifer. Many of these units crop out along the edge of the Eagle Basin (Figure 11b-01-01-02), which represents the regional recharge area. Although limestone and dolomite dominate both aquifers, quartz sandstone is also present in thick beds. Intergranular porosity and permeability is limited in both the carbonates and sandstones but fracturing of both enhances water movement. Solution channels and caverns in the carbonates can hold and transmit considerable amounts of water. A widespread paleo-karst system in the Leadville Limestone is a major aquifer and is the source of thermal waters at Glenwood Springs. Water Levels/Aquifer Characteristics Water levels indicate that groundwater flow is controlled by the basin’s structural features. In the southern portions of the basin, groundwater flows follow the Eagle and then the Colorado Rivers. Groundwater appears to flow to both the Sand Wash and Piceance Basins in the northern portions of the basin. Springs are common along the uplifts and the aquifers discharge to streams and rivers where they are incised. According to DWR permit files, over 2,600 permitted, non-alluvial wells tap the hydrogeologic units of the Eagle Basin. Of these wells, over 90% are completed at depths less than 375 feet. The average well depth is 175 feet, and the deepest well recorded in the area is 1,402 feet below ground surface. Well yields vary significantly throughout the basin, with flows of up to 3,000 gallons per minute (gpm) recorded from the Mississippian and Devonian carbonates. Natural discharges of up to 50 gpm are common from the regional aquifers. Local aquifers rarely produce more than 50 gpm unless intensely fractured, with an average yield of 22 gpm reported from the DWR well permit database. Transmissivities for the Mississippian and Devonian carbonate aquifers are typically much greater along the uplifted areas than near the basin centers, exceeding 75,000 gpd/ft along the White River uplift near Glenwood Springs. Hydraulic conductivities for the carbonates are also highly variable, ranging from 0.01 ft/day near McCoy to greater than 170 ft/day near Glenwood Springs. Reported values of hydraulic parameters for the alluvium of the basin are listed in a downloadable file. Water Use/Withdrawals Groundwater in the Eagle Basin has historically been utilized for a variety of uses, including domestic, livestock, irrigation, and industry. Surface water is the predominant source of water utilized by municipalities due to the ready availability of large quantities of water in the Eagle, Roaring Fork, and Colorado Rivers. In 1995, groundwater accounted for 1 to 11 percent of total water use for those counties within the Eagle Basin, ranging from a low of 995 acre-feet per year in Pitkin County to 14,960 acre-feet per year in Rio Blanco County. Public water supply and domestic use accounts for most of the groundwater use in Eagle and Grand counties. Water Quality Groundwater quality of the Eagle Basin aquifers is extremely variable and highly dependent upon connectivity to the evaporitic rock sequences. In general, the highest quality groundwater is located in the alluvial aquifers adjacent to the major rivers, select springs, and wells completed in the Leadville Limestone. Geothermal discharges from the Leadville Limestone near Glenwood Springs are an exception. Here total dissolved solids (TDS), sulfate, and chloride all exceed applicable drinking water standards. Groundwater in the Eagle Valley Evaporite is highly saline and of a sodium chloride composition. TDS routinely exceeds 10,000 mg/L and the water is not suited for domestic, agricultural, or livestock use.
Alluvial aquifers of the Colorado River Basin, Yampa River Basin, upper Arkansas River Basin, upper South Platte River Basin, Rio Grande Basin overlie parts of the Eagle Basin-Central Colorado Trough. In places, regional aquifers of the Colorado Plateaus region overlie units of this basin. The younger Sand Wash and Piceance Laramide basins overprint the Central Colorado Trough. Structural remnants of the basin can also be found within the mountainous region Blue River Valley, Upper Arkansas Valley, Wet Mountain Valley/Huerfano Park, and San Luis Valley.
The Paradox Basin, as recognized today, is an elliptical-shaped structural depression located within the Colorado Plateau physiographic province in southwestern Colorado and southeastern Utah. At the time of the Ancestral Rocky Mountains event the Paradox Basin extended to the southeast where the Laramide San Juan structural basin later developed. In Colorado, the Paradox Basin is bounded on the northeast side by the Uncompahgre uplift (Figure 11b-01-02-01), and it extends beyond the borders into Utah to the west and New Mexico to the south. The Laramide San Juan Dome separates today’s surface expression of the basin from the San Juan Basin, where the Paradox Basin units continue at great depths. The basin covers approximately 14,000 square miles of which, 40 percent or, 5,600 square miles are located in Colorado. The terrain is mostly composed of high plateaus with deeply incised canyons and dry arroyos. The San Miguel and Dolores Rivers, which are part of the Colorado River system, form the main surface-water drainage system in the Colorado portion of the Paradox Basin. Administratively, the basin lies in both Water Divisions 7 and 4. In Colorado, the Paradox Basin is primarily located in Dolores, San Miguel, and western Montrose counties. Few people live in the Paradox Basin, with the 2000 Census recording only 1,844 people in Dolores County and 6,594 in San Miguel County. Agriculture has historically been an important industry in this region. According to the Colorado Division of Water Resources (DWR) well permit database, less than 2,000 bedrock wells are located in the basin, with most clustered around the population centers and river courses. Back to Table of Contents
Hydrogeologic units within the Paradox Basin fall into two main groupings (Table 11b-01-02-01). The first, and uppermost group, are sedimentary formations directly associated with the development of the basin. It includes the Cutler Aquifer and the Molas unit, with later being mostly a confining unit. Water-yielding rock types in this group are mainly beds of sandstone. It also includes the Paradox unit, which is dominated by evaporitic beds and is generally a confining unit. The paradox unit yields brackish to briney water that contributes to salt loading in the Dolores River Basin. The second group are the Mississippian through Devonian formations that are dominated by carbonate formations. This group includes the Mississippian-Devonian carbonate aquifer, in which the Leadville Limestone is the primary water-yielding unit, the Elbert confining unit, and the Ignacio Aquifer. The Ignacio Aquifer is primarily a quartz-rich sandstone. Many of these units are exposed in the San Juan Mountains (Figure 11b-01-02-02), which represents the regional recharge area. Water Levels/Aquifer Characteristics Quantified hydrologic data are scarce for the Paradox Basin aquifers primarily due to the basin’s remote location and low level of groundwater development. The Mississippian-Devonian carbonate aquifer is composed primarily of relatively porous and permeable limestone and dolomite. Fracturing often enhances water movement through these rock types. Solution channels and caverns in the carbonates can hold and transmit considerable amounts of water. It typically occurs under confined conditions. Some groundwater leakage from the overlying salt units occurs along faults and fractures; therefore, most groundwater in the lower aquifer is saline and unsuitable for human consumption. Porosity and permeability of the Ignacio Aquifer depend on the degree of cementation and fracturing. Well completion depths range from less than 10 feet to greater than 1,000 feet below ground surface. The average well depth is 180 feet below ground surface and at least 90 percent of the wells of record are completed to depths of less than 350 feet. Well yields are typically low, with 90 percent of the wells yielding less than the average of 20 gallons per minute (gpm). Highly productive wells yielding up to 225 gpm are also found in the region, most of which are completed in the Navajo Sandstone. No known groundwater supplies are developed in the lower Paleozoic aquifer due to its depth and high salinity. Reported values of hydraulic parameters for the alluvium of the basin are listed in a downloadable file. Water Use/Withdrawals Public water supply is the primary use of groundwater in San Miguel and Dolores counties while agriculture is the primary use of groundwater in Montrose and Mesa counties. Winter wheat, pinto beans, oats, and alfalfa or other hays are common commercial farm products in the area. The total population served by groundwater resources in the Colorado portion of the Paradox Basin ranges from less than 4 percent and 8 percent in Mesa and San Miguel counties, respectively, to greater than 46 percent in Dolores County. Water Quality Groundwater quality in the Paradox Basin is variable, with the best quality water typically found in the shallower and/or more highly productive units. In general, concentrations of total dissolved solids (TDS), chloride, and sulfate all increase with depth. Water in the Dakota Sandstone, Burro Canyon, and Morrison Formations exceeds the federal secondary drinking water standard of 500 milligrams per liter (mg/L) for TDS, and water in the Morrison Formation exceeds the secondary drinking water standard of 250 mg/L for sulfate. Groundwater from shallower portions of the Navajo Sandstone (500 feet bgs) is highly saline, and chemical concentrations significantly exceed federal drinking water standards. Groundwater from the confining salt beds and Leadville Limestone is highly saline and unfit for human or stock consumption without substantial treatment.
Alluvial aquifers of the Dolores River Basin and upper San Juan River Basin overlie parts of the Paradox Basin. Regional aquifers of the Colorado Plateaus region overlie much of the basin. The older Paradox Basin extends beneath the Laramide San Juan Basin to the southeast and part extends into the Mountainous region to be overlain by voluminous volcanic rocks.
The Ancestral Denver Basin is a large structural basin formed during the Ancestral Rocky Mountains that is in the South Platte and Arkansas River Basins (Figure 11b-01-03-01). It flanked the east side of the Ancestral Front Range uplift and in places now underlies the Denver and Cheyenne Laramide Basins that formed later. It also lies within the Colorado Piedmont region, and High Plains regions. With this placement it is not commonly recognized, yet the strata preserved within it share characteristics with other Ancestral Rocky Mountains basins to justify including it as a specific hydrogeologic basin. The basin is bound on the west by the Front Range, the southwest by the Apishapa-Sierra Grande uplift, and it extends to the east beyond the stateline without a defining boundary. Formations deposited while it was active are exposed along the hogback and include the prominent Lyons Sandstone and Fountain Formation. Back to Table of Contents
As with the other Ancestral Rocky Mountains basins, hydrogeologic units within the Paradox Basin fall into two main groupings (Table 11b-01-03-01). The first, and uppermost group, are sedimentary formations directly associated with the development of the basin. It includes the Lyons and Fountain Aquifers. The Madera Aquifer is mainly considered part of the Central Colorado Trough where it passes into the Laramide Raton Basin, south of the Apishapa Uplift. That hydrogeologic unit appears to lap over the uplift that separates the Central Colorado Trough from the Ancestral Denver Basin and is continuous with the Fountain Aquifer. The second group are the Mississippian through Cambrian formations that are dominated by carbonate formations. This group includes the Mississippian-Ordovician carbonate aquifer that includes the Leadville Limestone and Manitou Formations that source historic Manitou Springs. It slo includes the localized Sawatch Aquifer that is primarily a quartz-rich sandstone. These units have very limited exposure along the Front Range. Water Levels/Aquifer Characteristics [No data available presently] Water Use/Withdrawals [No data available presently] Reported values of hydraulic parameters for the alluvium of the basin are listed in a downloadable file. Water Quality [No data available presently]
Alluvial aquifers of the South Platte and Arkansas River Basins, cross over the outcrop belt of the Ancestral Denver Basin hydrogeologic units in the Front Range hogback belt. Regional aquifers of the Colorado Piedmont region overlie much of the basin. The older Paradox Basin extends beneath the Denver and Cheyenne Laramide Basins.
The second episode of basin development with relevance to groundwater resources is the Late Cretaceous-Paleogene Laramide mountain-building event that occurred between approximately 45 and 70 million years ago. Seven major basins formed during this event, including the Denver Basin, which often contain coal, oil, and gas resources as well as large quantities of groundwater. These basins formed in between a series of uplifting fault-bound blocks cored with older sedimentary formations and Precambrian crystalline bedrock. The region had been part of the Upper Cretaceous Interior Seaway which retreated as the uplifts began, gradually fragmenting the large downwarp that accommodated the seaway. Sedimentary formations in each basin record an upward succession from marine shoreline environments to coastal plain and delta environments and then on to very active fluvial basin environments (Figure 11b-02). Complexity in the stratigraphy increases upward in the active basin environments which represent lateral transition from high energy fluvial systems near the active range fronts to lower energy river and marshland areas in the basin centers. Coal formed in many of the lower energy basin interiors and many of the porous sandstones are important petroleum reservoirs in the deeper parts of several of the basins. At times, the subsiding basins filled with intermountain lakes that resulted in accumulations of shale, carbonates, and evaporite deposits. The largest of these lakes were Lake Uinta in the Piceance Basin and Lake Gosiute in the Sand Wash Basin. Oil shale, which is a lake deposit rich in kerogen makes up part of these deposits and is recognized as a major potential source of energy. Not only do the Laramide basins share a general succession of sedimentary formations related to the basin evolution, but they also share a general spoon-shaped geometry. Each formed near an active uplift with its deepest part close to the uplift, giving the basin an asymmetrical profile with depth. At the edge nearest the active uplift, which is often defined by a major fault or fault zone, the sedimentary formations within the basin rise to the surface with steep dips. Sometimes the beds can be overturned near the basin-bounding faults. On the opposite side, away from the uplift, the same formations come back to the surface with much more gentle dips. The spoon-shape places the youngest aquifers at the surface in the basin center ringed by successively older aquifers away from the center. Laramide differ in other ways. Some are more structurally deformed than others and can have faults and folds in their interiors. The types of rocks exposed in the bounding uplifts can also differ so that the compositions of the sedimentary formations can be markedly different. In this atlas Laramide basin outlines, as seen in the web-maps, are based on the extents of the lowermost shoreline bedrock formations above the main Interior Seaway shale body. On the east side of Colorado this includes shoreline retreat deposits of the Fox Hills and Trinidad Sandstones. On the west side of the state, this is the Mesaverde Group. Back to Table of Contents
The Denver Basin is a Laramide structural basin that encompasses much of the Front Range urban corridor metropolitan area from Denver to Colorado Springs. It is entirely within the Colorado Piedmont region of the Great Plains physiographic province. The center of the basin lies just west of Parker, where the lowermost of four aquifers, the Laramie-Fox Hills Aquifer, is approximately 3,000 feet deep. The administrative groundwater portion of the basin underlies a 6,700 square mile area extending into Weld County on the north, El Paso County on the south, Jefferson County on the west, and the eastern portions of Adams, Arapahoe, and Elbert Counties on the east (Figure 11b-02-01-01). The basin straddles the boundary between Water Divisions 1 and 2. The northern part of the Denver Basin extends into an area of known oil and gas resources in the deep part of the basin with the Wattenberg Field being the largest recognized producing field. Producing formations are mainly in the Upper to Lower Cretaceous sandstones beneath the Pierre Shale. As much as 7,000 feet of nearly impermeable Pierre Shale separates the main producing zones from the Denver Basin bedrock aquifers. The Denver Basin is an important non-renewable source of groundwater for municipal, industrial, agricultural and domestic uses. The eight-county Denver metropolitan area contains 56% of Colorado’s population, or slightly over 2.4 million people according to the 2000 census. The lack of available surface-water rights and accelerated urban growth has resulted in extensive development of the Denver Basin aquifers as both primary and supplemental sources of water supply. As of February 2001, approximately 33,700 wells of record have been completed in the sedimentary rock aquifers of the Denver Basin. Back to Table of Contents
The water-yielding strata of the Denver Basin aquifer system consists predominantly of Paleogene and Upper Cretaceous sandstone, conglomerate, and shale (Table 11b-02-01-01). The units are statutorily defined as four distinct bedrock aquifers. At the base are the genetically linked Laramie Formation and Fox Hills Sandstone. Together with the underlying Pierre Shale these units represent the transition from the marine environment of the Interior Seaway to shoreline and on to coastal plain as the seaway retreated. Shale dominates the sequence with the Pierre Shale being marine and the Upper Laramie being non-marine; both are confining units. The Fox Hills Sandstone is the most extensive formation across the basin, and it, combined with the Lower Laramie Formation make the Laramie-Fox Hills Aquifer. It is at, or near the surface, around the outer perimeter of the basin and can be up to 3,000 feet deep at the structural center in Douglas County. Resting above the Laramie Formation are formations in the Denver Basin Group D1 sequence with the more familiar names of the Arapahoe and Denver Formations north of Palmer Divide. Aquifer names throughout the basin follow the formation names north of the Palmer Divide as the Arapahoe and Denver Aquifers. The Dawson Arkose, also known as the Denver Basin Group D2 sequence, is the uppermost Denver Basin bedrock aquifer. As the uppermost aquifer in the basin, the Dawson Aquifer is at the surface over the central part of the basin. Both the Arapahoe and Dawson Aquifers are split into upper and lower sub-units in the northern part of the basin. Characteristics of sediments making up the hydrogeologic units of the Denver Basin are quite variable both with vertical position and map location across the basin. Continuous core samples have been obtained from two boreholes drilled specifically to provide data about hydrogeologic characteristics of the Denver Basin units. These two important sets of information are from the Castle Pines core hole in Douglas County at the west side of the basin and the Kiowa core hole in Elbert County in the center of the basin. Data is also available by interpreting from hundreds of oil and gas boreholes, water well boreholes, and uranium exploration boreholes (Figure 11b-02-01-02). Data from the coreholes and geophysical logs indicate that the three-dimensional relationship of the many types of geologic material is complex (Figure 11b-02-01-03). Simply speaking, the Fox Hills Sandstone and Laramie Formation, or the Laramie-Fox Hills Aquifer, forms the most extensively continuous unit across the basin, yet it consists of a series of overlapping sand bodies, or shingles, that climb up stratigraphically from west to east. The D1 sequence of the Denver Basin Group, that forms the Arapahoe and Denver Aquifers, consists of a series of fluvial fan deposits that are more numerous, tend to be made up of coarser grained material, and are thicker on the west side of the basin. Heading east, sandstone bodies, that are more favorable for producing water, tend to become thinner, finer grained and more frequently separated by mudstone. Water Levels/Aquifer Characteristics Because of the more regional setting along the retreating seaway shoreline, the Laramie-Fox Hills Aquifer has perhaps the most consistent characteristics across the basin of the Denver Basin bedrock aquifers. Typically it consists of one, or multiple planar bodies of fine-grained sandstone interbedded with shale and sometimes coal. The characteristics of the D1 and D2 sequences, and hence, Arapahoe, Denver, and Dawson Aquifers, are quite variable across the basin. These formations were being deposited in a basin adjacent to an actively rising range front. Because of the change in environment heading away from the range front, the amount of permeable sand relative to finer-grained, and less permeable, shale and mudstone changes with distance. Aquifer characteristics on the west side of the basin tend to be much more favorable than on the east. Over the majority of their respective areas, the Denver Basin aquifers are under confined conditions. The quantity of recoverable water stored within the Denver Basin is estimated at 200 million acre-feet. All four Denver Basin aquifers have experienced net declining water levels during the past 20 years due to increasing withdrawals. During the period 1995-2000, water levels recorded by Division of Water Resources for the Dawson and Denver aquifers have shown both rises and declines. Water levels in the Arapahoe and Laramie-Fox Hills aquifers, being used more extensively for municipal water supply, have declined throughout that period by as much as 30 ft/yr. During that five-year period, water level fluctuations in the Denver Basin aquifers ranged from a maximum rise of 270 feet in the Denver aquifer to a maximum decline of up to 300 feet in the Arapahoe aquifer. The Denver Basin aquifers exhibit large differences in transmissivity. Transmissivity ranges from zero at the edge of each of the aquifers to over 7,500 gpd/ft in the Laramie-Fox Hills aquifer, 15,700 gpd/ft in the Arapahoe aquifer, 3,000 gpd/ft in the Denver aquifer, and 9,000 gpd/ft in the Dawson aquifer. Denver Basin wells may produce in excess of 500 gpm, though 300 gpm well yields are more common. Reported values of hydraulic parameters for the alluvium of the basin are listed in a downloadable file. Water Use/Withdrawals With the artesian pressure supplying the driving force to operate the elevators in the Brown Palace Hotel and run the bellows for the Trinity Church organ, the underground waters of the Denver Basin have been put to beneficial use since the late 1800’s. Due to the availability of surface water resources and the cost of deep well completions, historic withdrawals have been largely limited to domestic, public, and commercial uses. The USGS reported that total withdrawals from the Denver Basin aquifers in 1985 was 36,000 acre-feet, of which 53 percent of the water was used for public supply and 34 percent was used for agriculture. While current total withdrawal volumes for the Denver Basin are not available, nearly 445,000 acre-feet of groundwater was withdrawn in Adams, Arapahoe, Denver, Douglas, and Elbert counties, which lie almost entirely in the basin, in 1995. Water Quality Water in the Denver Basin aquifer system is generally of good quality, and with few exceptions, meets state and federal drinking water standards. The exceptions are usually for secondary drinking water constituents that affect the color, odor, or taste of the water. Dissolved solids concentrations range from less than 100 milligrams per liter (mg/L) in the Dawson aquifer to nearly 2,000 mg/L in the Laramie-Fox Hills aquifer.
Alluvial aquifers of the South Platte and Arkansas River Basins, overlie the Denver Basin. Alluvium can be in contact with each of the aquifers in locations depending on position above the outcrop belt for each aquifer. Regional aquifers of the Colorado Piedmont region extend beneath the basin.
The Cheyenne Basin is part of a greater Laramide basin known as the Denver-Cheyenne Basin. This larger basin is also called the Denver Julesburg Basin, or DJ Basin in the oil and gas community. The greater structural basin is divided into two sub-basins, the Denver Basin on the south and the Cheyenne basin on the north. A subtle, northeast-trending structural divide known as the Greeley Arch separates the two (Figure 11b-02-02-01). Cheyenne Basin continues north into Wyoming. Cheyenne Basin in underlies over 2,000 square miles in the northern part of Weld County and parts of Larimer and Morgan Counties. The area is primarily agricultural and industrial with Greeley as its main urban center. It is at the northern end of the Front Range Urban corridor and underlies an area experiencing rapid growth. Until recently most groundwater use has been from the alluvial aquifers for agriculture; however, recent growth patterns are increasing need for bedrock groundwater to meet domestic and other demands. Back to Table of Contents
The water-yielding strata in the Cheyenne Basin consist of Upper Cretaceous sandstone and shale of the genetically linked Laramie Formation and Fox Hills Sandstone (Table 11b.02-02-01). The Fox Hills Sandstone is continuous with the Fox Hills Sandstone of the Denver Basin and is at the surface on the Greeley structural arch that separates the two basins. Together with the underlying Pierre Shale the Fox Hills Sandstone and Laramie Formation represent the transition from the marine environment of the Interior Seaway to shoreline and on to coastal plain as the seaway retreated. Shale dominates the sequence with the Pierre Shale being marine and the Upper Laramie being non-marine; both are confining units. As much as 7,000 feet of nearly impermeable Pierre Shale underlies the Fox Hills Sandstone. The Laramie Formation and Fox Hills Sandstone are each considered separate aquifers instead of as one as in the Denver Basin. The Fox Hills Aquifer is also subdivided into an upper and a lower member. The Laramie Aquifer is much thicker and contains more bodies of sand that the similar strata in the Denver Basin. Recently, an interval near the top of the Pierre Shale consisting of fine-grained sandstone interbedded with marine silt and shale is being recognized as a potential groundwater source. It is recognized as the Upper Pierre Aquifer and supplies a number of wells in Weld County. Water Levels/Aquifer Characteristics [No data available presently] Reported values of hydraulic parameters for the alluvium of the basin are listed in a downloadable file. Water Use/Withdrawals [No data available presently] Water Quality [No data available presently]
Alluvial aquifers of the South Platte River Basin, overlie the Cheyenne Basin. Alluvium can be in contact with each of the aquifers in locations depending on position above the outcrop belt for each aquifer. Regional aquifers of the Colorado Piedmont region extend beneath the basin and the High Plains Aquifer overlies the northern part of the basin in northern Weld County.
The Raton Basin is a north-northwest trending structural basin located in south-central Colorado within Las Animas and Huerfano counties. The basin extends from the vicinity of the Huerfano River valley in its northwest corner southward into New Mexico (Figure 11b-02-03-01). The Colorado portion of the basin is bounded on the west by the Culebra Range, on the east by the Park Plateau, on the north by the extent of the Poison Canyon Formation, and on the south by the state line. The basin is located within the jurisdiction of Water Division 2. Topography ranges from fairly flat along the Cucharas River west of Walsenburg to very steep and rugged in the vicinity of the igneous stocks of the Spanish Peaks and Mount Mestas. The Park Plateau, which covers most of the area south of the Spanish Peaks, is highly dissected with elevations over 8,400 feet at the upper plateau surface. Las Animas and Huerfano Counties are sparsely populated. Land use is primarily forest with some agricultural use. The major industry in the basin is coal mining. Currently, coal-bed methane production is very active in the basin. As of February 2001, almost 1,370 permitted water wells of record are present in the Colorado portion of the Raton Basin. Back to Table of Contents
The major bedrock aquifers in the Raton Basin are Paleogene and Upper Cretaceous sandstones and siltstones associated with the retreat of the Interior Seaway followed by formation of the structural basin (Table 11b.02-03-01) and (Figure 11b-02-03-02). The Trinidad Sandstone and the Vermejo Formation of the Raton Basin are stratigraphic equivalents of the Fox Hills Sandstone and Laramie Formation of the Denver Basin and represent the retreat of the Interior Seaway. The Raton Formation was deposited as the basin started to form. The Raton, Vermejo, and Trinidad Formations are classified as the Raton-Vermejo-Trinidad Aquifer. This aquifer is the deepest aquifer of the basin and is at or near the surface around the perimeter as well as much of the Purgatoire valley in the middle of the basin. It can be up to 3,000 feet deep at the basin’s structural center. Sandstones and siltstones of the Cuchara and Poison Canyon Formations are classified as the Cuchara-Poison Canyon Aquifer. The Poison Canyon and Cuchara Formations were deposited as the Laramide basin formed. This aquifer is at the surface in two large areas in the center of the basin, one north of the Purgatoire River and the other south. Water Levels/Aquifer Characteristics Studies by Geldon (1989) indicate that groundwater flow is generally from west to east in the southern part of the basin. Locally, groundwater flows from topographic divides to valleys. Water moves laterally through pore spaces and along bedding planes and vertically through fractures in impermeable or semi-permeable rocks connecting permeable layers. In Huerfano County, water can generally be found in the Cuchara-Poison Canyon aquifer at depths of less than 200 feet in the outcrop areas. In the vicinity of the Spanish Peaks, however, this aquifer may be dry to a depth of several hundred feet as a result of dissection by igneous intrusive rocks. The depth to water is deepest in the southeastern part of the basin, where it is more than 200 feet below ground surface at higher elevations. The majority of the permitted wells in the Raton Basin are less than 150 feet deep, with 90 percent of all wells completed at depths less than 450 feet. The mean depth for the wells in this data set is 188 feet. Eighty-five percent of the water supply wells of record in the Raton Basin have reported yields of less than 16 gallons per minute (gpm), while nearly 10 percent have reported yields greater than 50 gpm. Reported values of hydraulic parameters for the alluvium of the basin are listed in a downloadable file. Water Use/Withdrawals Most of the nearly 1,370 water supply wells of record presented in Figure 11b-02-03-01 are for domestic and stock use. Community water systems using groundwater for public water supply include: the town of Aguilar; Huajatolla Valley L & C WC; Echo Canyon Guest Ranch; and The Shop’n Bag in Weston. Alluvial and bedrock wells also provide water for several subdivisions in the Raton Basin. Water Quality The quality and classification of the groundwater in the Raton Basin varies with the aquifer. Water in the Cuchara-Poison Canyon aquifer generally meets the secondary drinking water standard of 500 mg/L for total dissolved solids (TDS) around the basin edges, and decreases in potability towards the center of the basin. Water within the Raton-Vermejo-Trinidad aquifer is also generally potable, however areas of high TDS and poor quality water do exist in the vicinity of Trinidad Lake and south of the Purgatoire River.
Alluvial aquifers of the Arkansas River Basin overlie the Raton Basin. Alluvium can be in contact with each of the aquifers in locations depending on position above the outcrop belt for each aquifer. Regional aquifers of the Colorado Piedmont region extend beneath the basin. The geologic structure that defines the Raton Basin extends north into the Mountainous region Wet Mountain/Huerfano Park valley.
The San Juan Basin is an asymmetric Laramide structural basin that encompasses a surface area of approximately 21,600 square miles in Colorado, New Mexico, Utah, and Arizona. The basin occupies the eastern third of the Colorado Plateau physiographic province and is characterized by rugged terrain. Landscape features include mesas, terraces, escarpments, canyons, dry washes (arroyos), and mountains. In Colorado, the San Juan Basin extends into Archuleta, La Plata, and Montezuma counties (Figure 11b-02-04-01). Much of the land located in the Colorado portion of the basin belongs to the Southern Ute Indian Reservation. Land use in southwestern Colorado is highly variable including irrigated-agriculture, ranches and residential lots clustered around the main community centers. The San Juan Basin is a significant producer of coal, oil, and natural gas. As of February 2001, almost 5,000 permitted water wells of record are present in the Colorado portion of the San Juan Basin. Several major rivers, including the San Juan, Los Pinos, Florida, Animas, and La Plata, flow through Colorado’s portion of the San Juan Basin. All of the rivers flow southward into New Mexico and are part of the Colorado River system via the San Juan River. The entirety of the Colorado portion of the basin is included in Water Division 7. Back to Table of Contents
The San Juan Basin contains numerous aquifers throughout its stratigraphic sequence (Table 11b.02-04-01). Sandstones typically provide the aquifer resources, whereas shales of varying thickness act as confining layers. The Mesaverde Group is made up of several member formations within which are bodies of sandstone, shale, and coal. It represents a shoreline and delta environment at the west edge of the Interior Seaway. Even though it contains several sandstone bodies as well as shale confining layers, it is collectively called the Mesaverde Aquifer. Characteristics can vary depending on location within the basin. It is the deepest of the aquifers in the basin and is only exposed at the surface along the hogback edge of the basin. It is over 6,000 feet deep in the structural center of the basin where it is a source for petroleum production (Figure 11b-02-04-02). The Lewis Shale, Pictured Cliffs Sandstone, and Fruitland Formation were deposited during a westward advance followed by the final eastward retreat of the Interior Seaway. The Lewis Shale is a confining unit while the Pictured Cliffs Sandstone and Fruitland Formation are collectively called the Fruitland-Pictured Cliffs Aquifer. These units are only exposed at the surface along the hogback edge of the basin and the Fruitland-Pictured Cliffs Aquifer is as deep as 3,000 feet in the structural center of the basin. In the deeper part of the basin it is the target for coal bed methane (CBM) production. Development of the Laramide basin led to deposition of the Kirtland Shale, Animas, Nacimiento, San Jose, and Blanco Basin Formations. Distribution of these named geologic units depends on age and locations in the basins, some formations such the San Jose and Blanco Basin Formations are stratigraphically equivalent where there are lateral changes for sediments of the same age, or the names are different depending on where they have been mapped. Of these formations hydrogeologic units include the Kirtland confining unit, Animas Aquifer, and San Jose Aquifer. The Animas and San Jose Aquifers are at, or near, the surface across much of the interior of the basin. Water Levels/Aquifer Characteristics Most of the basin’s sandstone aquifers exhibit confined conditions within the central part of the basin. Unconfined conditions occur at the basin outcrop margins, which includes most areas in Colorado. Extensive recharge areas are associated with these outcrops which, when combined with the greater amounts of precipitation enjoyed along the La Plata and San Juan mountains, provides a significant source of recharge to the San Juan Basin. Well densities are greatest near the river valleys and population centers, with fewer wells located in the more rural areas. Most water wells in the basin are completed at depths ranging from 100 to 225 feet below ground surface, with an average well depth of 178 feet. The water-yielding capabilities of the bedrock aquifers are highly variable, and are largely dependent on fracture porosity and permeability. Typical yields of San Juan Basin aquifer wells range from 10 to 25 gallons per minute (gpm), although some yields of up to 50 gpm have been reported. The permit record data support this, and indicate that approximately 90 percent of the wells yield less than 20 gpm. The average reported well yield is 11 gpm. Water Use/Withdrawals Alluvial and shallow bedrock groundwater in the Colorado portion of the San Juan Basin is typically used for agricultural, stock-watering, and domestic purposes. Groundwater withdrawals in 1995 ranged from 1,174 acre-feet per year in Archuleta County to over 4,360 acre-feet per year in La Plata County. This represents low groundwater usage compared to other regions of the state, and is the result of readily available surface-water sources. The fraction of the total population served by groundwater resources in the Colorado San Juan Basin ranges from approximately 3.4 percent in Archuleta County to 16 and 17.3 percent in La Plata and Montezuma counties, respectively. Population increases since 1995 have prompted state and local leaders to try to protect groundwater resources and existing water rights in this semi-arid region. Water Division 7 recently developed a “water critical” map for La Plata County that delineates a system for evaluating if domestic well permits should be issued. Water Quality Water quality in the San Juan Basin is highly variable by aquifer, with total dissolved solids concentrations typically increasing with depth towards the center of the basin. Naturally occurring concentrations of iron, chloride, fluoride, and total dissolved solids may locally exceed the drinking water standards. These constituents are often associated with the presence of shale and coal units.
Alluvial aquifers of the San Juan River Basin overlie the San Juan Basin. Alluvium can be in contact with each of the aquifers in locations depending on position above the outcrop belt for each aquifer. Regional aquifers of the Colorado Plateaus region extend beneath the basin as do hydrogeologic units of the Paradox Basin.
The Piceance Basin is an elongated structural depression trending northwest-southeast in western Colorado (Figure 11b-02-05-01). It is an extension of the greater Uinta Basin that continues to the northwest into Utah. The Douglas Creek Arch is a structural high that separates the Piceance Basin from the Uinta Basin even though many of the bedrock formations carry over from one basin to the other. The basin is more than 100 miles long and has an average width of over 60 miles, encompassing an area of approximately 7,110 square miles. The Piceance structural basin encompasses varying portions of Moffat, Rio Blanco, Garfield, Mesa, Pitkin, Delta, Gunnison, and Montrose counties. Being part of the Colorado Plateau physiographic province, the Piceance Basin is characterized by a series of high plateaus and deep valleys. Downcutting of the Colorado River has divided the Piceance Basin into a northern and southern province. The southern province is marked by two significant erosional remnants, Grand Mesa and Battlement Mesa. The northern province, that portion of the Piceance Basin between the Colorado and White Rivers, still retains basin-like features with rocks dipping inward from the margins toward the deepest part of the basin at the northern end. The Piceance Basin is part of the upper Colorado River basin that includes the area drained by the Colorado River and its tributaries upstream from Lee’s Ferry, Arizona. The principal rivers that drain the Piceance Basin are the Colorado, Gunnison, North Fork Gunnison, and White. Three Colorado Water Divisions administer the water in the Piceance Basin. Water Division 6 manages the north portion of the basin drained by the White River and its tributaries. Water Division 5 manages the central part of the basin drained by the Colorado River and its tributaries. Water Division 4 manages the southern portion of the basin from the crest of Grand Mesa south to about the Gunnison River. There are no significant population centers within the Piceance Basin. Well permit records with the Division of Water Resources indicate that approximately 2,200 water supply wells have been drilled as of early 2001. Back to Table of Contents
Hydrogeologic units in the Piceance Basin fall into two main groups, the deeper units associated with the Interior Seaway, and shallower units associated with the development of the basin (Table 11b-02-05-01). The Mesaverde Group is made up of several member formations within which are bodies of sandstone, shale, and coal. It represents a shoreline and delta environment at the west edge of the Interior Seaway. Even though it contains several sandstone bodies as well as shale confining layers, it is collectively called the Mesaverde Aquifer. Characteristics can vary depending on location within the basin. It is at, or near the surface only around the perimeter of the basin where it is only used locally for groundwater. In the center of the basin it is over 12,000 feet deep and is a major source of natural gas production (Figure 11b-02-05-02). Thickness of Paleogene-age formations that accumulated as the basin developed varies from 2,000 to approximately 12,000 feet. The Fort Union and Wasatch Formations consist of sandstone, conglomerate, and shale in the lower part of this sequence. These formations form the Wasatch-Fort Union aquifer that is at, or near the surface, south of the Colorado River where younger formations have been largely removed by erosion. North of the Colorado River this aquifer is exposed only along the basin perimeter. Principal aquifers in the northern portion of the Piceance Basin consist of younger, porous members of the Paleogene Uinta Formation and Parachute Creek Member of the Green River Formation. These formations were deposited as the basin evolved, at times accommodating a deep, restricted lake. Thick intervals of shale-dominated sediments form confining units that separate the aquifers into an Upper Piceance Basin Aquifer and a Lower Piceance Basin Aquifer. The Mahogany confining unit that separates the two is also the main oilshale-bearing stratigraphic interval in the Green River Formation. Evaporitic minerals nahcolite and halite in the Parachute Creek Member also impact water quality and are an economic resource. Water Levels/Aquifer Characteristics Water levels for the upper and lower Piceance Basin aquifers in the northern portion indicate that the general direction of groundwater flow is to the north towards the White River. Aquifers in the Piceance Basin are typically under confined conditions, except along outcrops at the basin edge. The potentiometric surface indicates that the pressure head is at or very near the surface within the drainage valleys. This suggests that groundwater is moving from the aquifers to the creek alluvium. The potentiometric surface for the Mesa Verde aquifer in the southern portion of the basin indicates groundwater flow to the northwest towards the Colorado River. Paleogene sedimentary rocks in the Piceance Basin are generally fine-grained and well-cemented, resulting in small hydraulic conductivity values. Fracturing during structural deformation and dissolution of minerals has enhanced hydraulic conductivities of the Piceance Basin aquifer system. Hydraulic conductivities of the upper aquifer unit range from 0.8 to 1.2 feet/day, while the lower aquifer unit is slightly tighter with ranges from 0.1 to 1.1 feet/day. Ninety percent of the wells of record have reported well yields of less than 22 gallons per minute (gpm), which is also the statistical mean yield value. The higher mean yield suggests that some wells have reported yields in excess of 50 gpm. Water Use/Withdrawals Groundwater withdrawal data for specific basins or aquifers are not generally available. County-wide data is available, however, and provides an insight into the demands on groundwater resources in the basin. Well permit records indicate that approximately 2,200 wells tap aquifers in the Piceance Basin. Ninety percent of these wells are completed at depths of 300 feet or less. The minimum well depth reported is 2 feet and the maximum well depth is 2,395 feet, with a mean depth of 162 feet. In 1995, groundwater withdrawals ranged from a high of 15,000 acre-feet in Rio Blanco County to a low of 5,500 acre-feet in Gunnison County. The total groundwater withdrawal in 1995 for the five counties, represented in the Piceance Basin, was about 45,993 acre-feet. The principal uses of groundwater vary by county, but irrigation, public supply, and mining are the dominant uses. From a regional perspective, the withdrawal of groundwater from bedrock aquifers within the Piceance Basin appears to be minimal. Water Quality Groundwater in the Paleogene aquifer system for the northern portion of the Piceance Basin gains dissolved solids and shows changes in major-ion chemistry as it moves along the basin flow paths from the upland recharge areas to discharge areas. In the upper aquifer unit, the dissolved-solids concentration increases from 500 to 1,000 milligrams per liter (mg/L). In the lower aquifer unit, the dissolved-solids concentration increases from about 1,000 to 10,000 mg/L along the basin flow paths; well above the secondary drinking water standard of 500 mg/L.
Alluvial aquifers of the Colorado and White River Basins overlie the Piceance Basin. Alluvium can be in contact with each of the aquifers in locations depending on position above the outcrop belt for each aquifer. Regional aquifers of the Colorado Plateaus region extend beneath the basin as do hydrogeologic units of the Eagle Basin-Central Colorado Trough.
The Sand Wash Basin of northwestern Colorado straddles the Wyoming state line between the Park Range on the east and the Uinta uplift on the west. It is located within the northeastern half of Moffat County and western two-thirds of Routt County, as well as a small section of northeastern Rio Blanco County and the northern tip of Garfield County. The Sand Wash Basin is part of the Wyoming Basin physiographic province and the Colorado portion encompasses approximately 4,760 square miles (Figure 11b-02-06-01). Most of the basin is a rolling plain with elevations above 6,000 feet, although the eastern part of the basin grades into foothills and mountains with elevations above 10,000 feet. The Yampa River and its tributary, the Little Snake River, flow into the Green River, and represent the principal surface drainage systems for the basin. Administration of water within the Sand Wash Basin is under the jurisdiction of Water Division 6, located in Steamboat Springs. This portion of Colorado is sparsely populated. Back to Table of Contents
Hydrogeologic units of the Sand Wash Basin fall into three main groups (Table 11b-02-06-01). Deeper units associated with the Upper Cretaceous Interior Seaway and shallower units associated with the Paleogene development of the basin are the most wide-spread group of aquifers. Near-surface sedimentary deposits associated with the Neogene-age extensional tectonics make up a third which have a more limited extent but are an important local groundwater source. The Mesaverde Group is made up of several member formations within which are bodies of sandstone, shale, and coal. It represents a shoreline and delta environment at the west edge of the Interior Seaway. Even though it contains several sandstone bodies as well as shale confining layers, it is collectively called the Mesaverde Aquifer. Characteristics can vary depending on location within the basin. The Mesaverde Aquifer is likely to be the principal aquifer along the southern, southeastern, and eastern margins of the basin. In these areas, the Cretaceous-age target aquifers exist at depths less than 2,000 feet and their outcrop areas are exposed to recharge from precipitation, resulting in good water quality. Coal seams in the Mesaverde Group are an important energy resource and there has been limited coalbed methane production where water is pumped from the coal-bearing intervals to release entrapped methane. In the center of the basin the Mesaverde Group is over 10,000 feet deep and is a major source of natural gas production (Figure 11b-02-06-02). Advance of the Interior Seaway led to deposition of the Lewis Shale that forms a confining unit above the Mesaverde Aquifer. The Fox Hills Sandstone and Lance Formation above the Lewis Shale represent the final retreat of the seaway just as the Sand Wash Basin began to develop. Paleogene-age formations that accumulated while the basin developed include the Fort Union, Wasatch, and Green River Formations. These units consist of sandstone, conglomerate, and shale with shale dominant in the Green River Formation. These geologic formations lie at or near the surface throughout most of the basin, and as such the Wasatch-Fort Union aquifer is the uppermost regional aquifer in the Sand Wash Basin. Thickness of Paleogene rocks in the basin increases from a feather edge at the margins to about 12,000 feet in the center of the basin. Neogene extensional tectonics created several deep fault-bound basins in Colorado that are recognized as intermountain valleys. The San Luis Valley is the best recognized of these deep, sediment-filled basins. Neogene sediments that include the Bishop Conglomerate and Browns Park Formation were deposited in the Sand Wash Basin region at the same time as the other deep basins formed. These formations consist of semi-consolidated sandstone, conglomerate, siltstone and volcanic ash. The sediments were not deposited in distinct deep, fault-bound basins similar to those elsewhere, but instead, tend to be blanket formations over large areas. The units form near-surface aquifers that straddle the boundary of Sand Wash Basin with the Colorado Plateaus region in parts of Moffat and Routt Counties. Water Levels/Aquifer Characteristics The principal regions of groundwater recharge in the Sand Wash Basin are the outcrop areas of each aquifer unit. Groundwater discharge from the basin is thought to be through the alluvium of the Little Snake River. Wells in the valley bottoms, west of the Little Snake River, indicate that water levels in the Wasatch-Fort Union aquifer are at or near land surface. East of the Little Snake, water levels in the Wasatch zone are generally below the land surface by several to 100 feet. Water Use/Withdrawals The Colorado Division of Water Resources well permit database indicates that there are approximately 2,157 bedrock aquifer wells of record in the Sand Wash Basin. The majority of which are concentrated around the major population centers of Craig and Steamboat Springs. Records indicate that 90 percent of the water supply wells in the basin are 500 feet or less in depth. The average well depth is 245 feet, and the deepest well of record is 3,000 feet below ground surface. Reported well-yield values range from a few tenths of a gallon per minute (gpm) to 2,700 gpm. Ninety percent of the water-supply wells of record have a reported yield of 18 gpm or less, suggesting these wells are intended for domestic or livestock purposes. Hydraulic conductivities for the Wasatch-Fort Union aquifer range from 0.02 to 938 ft/day, based on aquifer pump tests. Basin-specific groundwater withdrawal data are not available for the Sand Wash Basin; however, the USGS publishes groundwater withdrawals by county as part of their national water use studies. In 1995, Moffat County’s total groundwater withdrawal was approximately 2,655 acre-feet and Routt County’s withdrawal was about 5,870 feet. Water Quality Published water quality data for the Sand Wash Basin are minimal. Glover and others (1998) indicate that the total dissolved solids (TDS) in the recharge areas for the Wasatch-Fort Union aquifer are less than 500 mg/L, but concentrations increase down the flow paths. Based on this interpretation, good water quality should exist along the western and eastern margins of the basin, with increasing TDS toward the Little Snake River. In general, the TDS concentration of groundwater in the Mesozoic rocks is less than 1,000 mg/L, along the southeastern and eastern part of the basin where there is good potential for recharge from precipitation. As groundwater in these older rocks moves toward the center of the basin it becomes briny, with TDS greater than 35,000 mg/L.
Alluvial aquifers of the Yampa River Basin overlie the Sand Wash Basin. Alluvium can be in contact with each of the aquifers in locations depending on position above the outcrop belt for each aquifer. Regional aquifers of the Colorado Plateaus region extend beneath the basin as do hydrogeologic units of the Eagle Basin-Central Colorado Trough. Neogene units of the Colorado Plateaus region overlap from the region onto the Sand Wash Basin and are considered units of both areas.
The South Park Basin is a structural basin bounded by the Front Range uplift on the east and the Sawatch uplift on the west (Figure 11b-02-07-01). The structural basin lies entirely within Park County. Back to Table of Contents
Within the basin are complexly deformed Cambrian through Neogene sedimentary formations, volcanic rocks, and igneous rocks (Table 11b-02-07-01). Sedimentary formations associated with the retreat of the Interior Seaway and the development of the basin are limited to a small area in the central part of the greater South Park structural basin. To maintain the context of the many hydrogeologic units within the Laramide structural basin, the entire area is considered as the South Park Basin Mountainous region valley.
Alluvial aquifers of the South Platte River Basin overlie the South Park Basin. Regional aquifers of the Colorado Piedmont Region are found within structural components of the complex basin. It also includes units of the Eagle Basin-Central Colorado Trough.
The Colorado Headwaters Basin is a structural basin bounded by the Front Range\Medicine Bow uplift on the east and the Park Range uplift on the west (Figure 11b-02-08-01). It is better recognized as the Middle and North Parks; however, as a Laramide basin it can be considered as a single structural basin. The north part of the structural basin lies within Jackson County and the south part lies within Grand County. The Rabbit Ears Mountains separates the two into the topographic North and Middle Parks. Back to Table of Contents
Within the Laramide basin are complexly deformed Permian through Neogene sedimentary formations, volcanic rocks, and igneous rocks (Table 11b-02-08-01). Because of the topographic division by the Rabbit Ears Range the basin is treated as North Park and Middle Park Mountainous region valleys.
Alluvial aquifers of the North Platte River Basin overlie the basin in North Park while alluvial aquifers of the Colorado River Basin overlie the basin in Middle Park. Regional aquifers of the Colorado Piedmont region are found within structural components of the complex basin.
The Southern Rocky Mountains bisect the state, covering nearly one third of its area. This mountainous region (Figure 12.01) has the greatest topographic relief with 54 summits exceeding 14,000 feet, the venerable 14ers of the state. It is also the most geologically complex region in the state. Within this region are deep valleys formed by Neogene faulting that are filled with sedimentary formations; complexly deformed sedimentary formations of many ages; and areas of intense volcanic activity covered with lava flows, ash deposits, and sediments shed off of eroding volcanoes. Fractured crystalline bedrock, the third common aquifer type in Colorado, is at or near the surface over large portions of this region where it forms the only available aquifer. Rocks forming this bedrock consist of interlocking mineral crystals with no interconnected pore space to create porosity or permeability. Groundwater only occurs and moves through where fractures and faults crosscut the rock. Because of this, groundwater production is generally low as compared to the sedimentary aquifers. Mean well yields are generally less than 10 gpm and rates over 50 gpm are rare. Production and depth to groundwater can also be variable from well to well. Crystalline bedrock includes Precambrian igneous bodies that intruded in molten form deep within the earth at high temperatures. The most commonly recognized rock is granite, but there are many other varieties. Precambrian metamorphic rocks also make up a large portion of these rocks and include gneiss and schist of varying composition and texture. These are rocks that were originally sedimentary or volcanic in origin but were changed in form during deep burial under pressure and high temperature. Volcanic and igneous intrusive rocks make up the remainder of the crystalline bedrock within the Mountainous region. These rocks include lava flows and ash deposits that welded under their own heat into very dense rock after coming to rest. Other rocks formed from explosive eruptions and erosion of the volcanoes in this region are often interlayered with the crystalline flows. They can be very similar to sedimentary bedrocks in that they consist of consolidated, semi-consolidated, and even unconsolidated material derived directly from the volcanic centers. However, these rocks are granular and often have better connected pore spaces between grains. Back to Table of Contents
South Park is a north-south trending, elongated, intermontane basin located in Park County in central Colorado. The basin is approximately 45 miles long and 20 miles wide, encompassing some 893 square miles. South Park is a high-altitude, relatively flat, generally treeless plain located between the Mosquito Range and the Front Range (Figure 12a-01-01). It is traversed by several ridges that rise hundreds of feet above the floor of the Park and extend for a number of miles down the center of the basin (e.g. Red Hill and Reinecker Ridge). South Park sources the South and Middle Forks of the South Platte River and other tributaries include Tarryall and Jefferson Creeks. Tarryall Creek and its tributaries drain the northern portion of the Park, while the South Platte River and its tributaries drain the remainder of the Park. Important water storage reservoirs include Antero, Elevenmile Canyon, Spinney Mountain and Taryall. South Park falls into the management of Water Division 1 with offices located in Greeley. South Park encompasses about 40 percent of Park County’s total area, whose population density in 2000 was about seven persons per square mile. Back to Table of Contents
South Park is a highly complex structural and depositional basin. A generalized geologic cross-section (Figure 12a-01-02) illustrates the complex nature of this synclinal basin and shows the structural relationships of the sedimentary rock units. The structural basin bounded on the east side by the Elkhorn Thrust, which brings pre-Cambrian rocks east of the fault against Paleogene and older rocks on the west. It is bounded on the west by the Mosquito Range where east-dipping Cambrian through Mississippian formations flank the uplift. These older formations continue at great depth at least half-way across South Park. With the exception of the pre-Cambrian, all geologic formations exposed in the lower elevations of South Park are post-Mississippian in age and generally dip to the east. The geology of South Park is further complicated by igneous activity, generally of Paleogene age. Most of the sedimentary rock formations within South Park are potential aquifers (Table 12a-01-01). Fractured crystalline bedrock aquifers cover large areas where Precambrian bedrock or Paleogene volcanic and intrusive rocks are present at the surface. The youngest sedimentary rock aquifers in South Park are within the Wagontongue/Trump Formations and the oldest are Paleozoic carbonate and sandstone aquifers along the flank of the Mosquito Range. Pennsylvanian sedimentary bedrock formations underlying the west side of South Park contain beds of gypsum and salt that can impact water quality locally. The South Park Formation is equivalent to the Denver Formation of the Denver Basin. Water Levels/Aquifer Characteristics Water well records from the Division of Water Resources (February 2001) indicate there are approximately 2,100 permitted wells in the South Park Basin. Data indicate that well depths range from 2 to 1,400 feet with 90% of the wells completed at depths equal to or less than 350 feet. The mean depth from this data is about 185 feet. The majority of the water wells completed in the South Park Formation are used for domestic purposes, so aquifer specific data are limited. Ninety percent of the wells have a yield equal to or less than 15 gallons per minute (gpm). Jehn Water Consultants, Inc. (1997) report yields from the lower South Park aquifer unit ranging from 5 to 10 gpm and hydraulic conductivity ranging from 1 to 3 feet per day. Estimated saturated thickness ranges from 50 to 3,000 feet for the lower South Park aquifer and 50 to 2,000 feet for the upper South Park aquifer. Monitoring well data indicates the average transmissivity is 475 ft2/d in the upper aquifer unit and is 260 ft2/d for the lower aquifer unit. Water Use/Withdrawals South Park encompasses about 40 percent of Park County’s total area, whose population density in 2000 was seven persons per square mile. Specific water use data for South Park is not available, but total water use in all of Park County (2000), including surface water, was approximately 31,500 acre-feet, of which only 6 percent represents groundwater. The USGS estimates that domestic and public water supply represent 90 and 10% of the total groundwater withdrawals, respectively. Based on the approximate 2,100 permitted South Park bedrock wells of record and using a domestic use consumption of approximately 280 gallons per day, groundwater withdrawals are calculated to be about 214,620,000 gallons or approximately 659 acre-feet per year – not a great demand on the resource. Rocky Mountain Consultants, Inc. (1998) estimate the South Park aquifers, upper and lower, have approximately 16-million acre-feet of groundwater in storage. Water Quality The limited water quality data available indicate the aquifers in South Park have good water quality, suitable for potable purposes. Water quality data from the literature indicate that water from sedimentary rock wells between Jefferson and Hartsel have a median total dissolved solids (TDS) concentration of 257 milligrams per liter (mg/L), with 10 percent of the wells exceeding the drinking water standard of 500 mg/L. An item of concern is the high concentration of radon-222 in the South Park Formation, in some locales greater than 6,000 picocuries per liter (pCi/L). The proposed U.S. Environmental Protection Agency drinking water standard for radon in groundwater is 300 pCi/L.
Alluvial aquifers of the South Platte River Basin overlie the South Park Basin. Regional aquifers of the Colorado Piedmont Region are found within structural components of the complex basin. It also includes units of the Eagle Basin-Central Colorado Trough.
The Colorado portion of the Rio Grande Basin, located in south-central Colorado, encompasses approximately 7,500 square miles and constitutes Colorado Water Division 3. The Rio Grande and Conejos River originate in the eastern San Juan Mountains and are the dominant watersheds in the basin. These rivers flow through the San Luis Valley, which is an open, almost treeless, intermontane basin bounded by the foothills of the San Juan Mountains on the west and the Sangre de Cristo Range on the east. The 3,200 square mile San Luis Valley encompasses the counties of Saguache, Rio Grande, Alamosa, Conejos, and Costilla (Figure 12a-02-01). The San Luis Valley represents a significant groundwater resource that is recognized nationally. Back to Table of Contents
The San Luis Valley is a structural basin that is part of the Rio Grande Rift, a north-trending series of interconnected, down-faulted, and rotated blocks, known as grabens. The graben forming the San Luis Valley tilts to the east with the main bounding faults at the base of the Sangre de Cristo Range. Formation of the San Luis Valley graben has accommodated as much as 30,000 feet of basin-fill deposits at its deepest part (Figure 12a-02-02). Basin-fill deposits within the graben form a vital aquifer system throughout the San Luis Valley (Table 12a-02-01). Basin-fill deposits are hydraulically interconnected with the alluvium of the Rio Grande and its tributaries within the valley. Due to this hydraulic interconnectivity, the Rio Grande alluvial aquifer within the valley is considered part of the near-surface basin fill. The two major hydrogeologic units in the San Luis Valley are the upper unconfined aquifer and the lower confined aquifer, predominantly within the Alamosa Formation. A series of clay layers in the upper Alamosa Formation forms the confining layer between the two aquifers. Depth to the confining clay layers varies from about 100 feet in the northern part of the basin to about 40 feet in the southern part of the basin. The unconfined aquifer consists of Quaternary alluvium and alluvial fan deposits with some well-sorted aeolian sands and the uppermost sandy layers of the Alamosa Formation. The alluvial deposits are underlain, in most areas, by the Alamosa Formation which is composed of fine dark sands interbedded with discontinuous blue, gray, and green clays and silts. The geologic materials comprising the upper portions of the confined aquifer vary with location within the valley. In the northern and central part of the San Luis Valley, this unit consists of unconsolidated sand and gravels of the lower Alamosa Formation. At the western edge of the valley, in the Monte Vista Graben and on the Alamosa Horst, the confined aquifer consists of unconsolidated sands and gravels of the Los Pinos Formation, and in the Baca Graben, it is composed of unconsolidated to semi-consolidated sands and sandstone of the Santa Fe Formation. The mountains surrounding the San Luis Valley expose a wide variety of geologic formations that are hydrogeologic units included as other sedimentary basins or regional aquifer systems. Many of these units may be found deep within the San Luis Valley basin. These other units include Paleozoic sedimentary formations along with sedimentary formations associated with the Ancestral Rocky Mountains event and the Laramide mountain building event. Other regional units include crystalline bedrock Paleogene volcanic units and Precambrian igneous and metamorphic rocks. Water Levels/Aquifer Characteristics The principal components of groundwater recharge to the valley are mountain-front recharge, precipitation, irrigation return flow, streambed infiltration, and groundwater inflow. Groundwater moves out of the Rio Grande Basin in Colorado through evapotranspiration, withdrawals from wells and drains, discharge to streamflow, and underflow to New Mexico. As of February 2001, water well permit records indicate that nearly 10,000 wells have been completed in the San Luis Valley, 90 percent of which are used for irrigation of commercial crops. Historically, depth to water in the unconfined aquifer has been generally less than 12 feet below ground surface. Extensive irrigation in the valley using groundwater wells has resulted in depletion of the aquifer. In the period 1969 to 1980 water level declines of up to 40 ft. were documented in the unconfined aquifer. Since 1976, the Water Division engineer estimates that the unconfined aquifer has lost 1 million ac-ft of storage. Based on well permit records, 90 percent of the wells have reported completion depths of less than 400 feet. The mean well depth is 172 feet, and the median well depth is 100 feet. These statistics include wells in both the unconfined and confined aquifers. Many of the wells completed in the confined aquifer in the central part of the basin are flowing artesian wells. In general, the shape and configuration of the water level surfaces of the unconfined and the confined aquifers are similar, indicating some degree of hydraulic connectivity. Water level elevations for the unconfined aquifer in the northern part of the valley range from approximately 7,700 feet on the edges of the valley to approximately 7,500 feet in the valley center near the San Luis Hills. Yields of the nearly 10,000 wells of record completed in the San Luis Valley range from less than 5 to a maximum of 8,000 gallons per minute (gpm). Over 50 percent of the wells have reported yields less than 100 gpm, and 90 percent of the wells have reported yields less than 1,600 gpm. Transmissivity in the confined aquifer is generally much greater than in the unconfined aquifer, ranging from less than 100,000 to greater than 1,200,000 gal/day/ft. Water Use/Withdrawals The San Luis Valley is estimated to contain over 2 billion acre-feet of groundwater in storage, with over 140 million acre-feet estimated to be recoverable. The principal use of groundwater is agricultural. Estimated average withdrawals for irrigation are 2 million acre-feet annually, of which an estimated 800,000 acre-feet is from groundwater sources. An estimated 85 to 90 percent of the irrigation water in the central portion of the valley is from managed recharge and pumping of the unconfined aquifer. Depletion of groundwater resources in the valley spurred the Colorado Legislature to adopt legislation mandating the State Engineer promulgate new rules on future appropriations from the deeper, artesian confined aquifer. These appropriations now require an augmentation plan. Groundwater is used for public water supply in most of the municipalities within the San Luis Valley. As of 2000, there were 76 permitted municipal wells in the valley, with a total permitted pumping rate of 32,552 gpm. Water Quality The quality of the water in the San Luis Valley varies by location and the aquifer. Water quality in the unconfined aquifer of the San Luis Valley ranges from very good along the periphery of the valley to very poor in the sump area in the vicinity of San Luis Lakes, northeast of Alamosa. The concentration of total dissolved solids is generally low to moderate except in the sump area around the San Luis Lakes, where the concentration ranges from 2,000 to over 10,000 milligrams per liter (mg/L). The salinity hazard in the unconfined aquifer is medium to very high. The waters of the confined aquifer are generally lower in dissolved solids and nitrogen, and thus of higher quality.
Alluvial aquifers of the Rio Grande River Basin are considered part of the unconfined aquifer within the San Luis Valley but continue up into the mountainous area watersheds. Regional aquifers of the Colorado Piedmont Region are found as structural components deep within the San Luis Valley graben. Hydrogeologic units of the Eagle Basin-Central Colorado Trough are present in the Sangre de Cristo Mountains bordering the east side of the valley. Cenozoic Volcanic Rock aquifers are present in the valley both near the surface in uplifted structural blocks in the valley and in the western uplands. The fractured crystalline Precambrian basement rock aquifer is present in the uplifted Sangre de Cristo Range and its foothills on the east side of the valley.
Upper Arkansas Valley is a narrow, north-south trending, intermontane basin encompassing Lake and Chaffee counties in central Colorado (Figure 12a-03-01). It is bounded on the west by the lofty Sawatch Range, home to 14 of Colorado’s peaks over 14,000 feet high, and on the east by the somewhat lower Mosquito Range. The basin is approximately 76 miles long and up to 30 miles wide at its widest, encompassing some 1,500 square miles. Watersheds in the surrounding mountain ranges bring the area to over 20 miles wide in some places. Low hilly areas in the valley are studded with piñon and juniper forests and the rich farmland characterizes the valley bottoms. The area has a rich mining and agricultural history and is becoming a year-round recreation focal point for the state. With headwaters at the north end of Lake County, the Arkansas River flows south through the Upper Arkansas Valley to exit through Bighorn Sheep Canyon on its way to the spectacular Royal Gorge. The valley falls into the management of Water Division 2 with offices located in Pueblo. Back to Table of Contents
The Upper Arkansas River Valley is part of the Rio Grande Rift, a north-trending series of interconnected, down-faulted, and rotated blocks, known as grabens. The graben forming the valley generally tilts to the west with the main bounding fault zone at the base of the Sawatch Range. Late Paleogene through Neogene sediments (Table 12a-03-01) that accumulated in the basin as it developed along with Quaternary alluvium and glacial deposits make up the valley-fill aquifer that may be over 5,000 feet thick on the west side of the valley next to the bounding fault zone (Figure 12a-03-02). Other units in the valley proper include Paleogene volcanic and intrusive igneous rocks that generally sit high in the landscape. Sedimentary formations associated with the Ancestral Rocky Mountains event and Paleozoic units dominated by limestone, dolomite with some quartzitic sandstones can be found in the Mosquito Range. Discharge from the Lower Paleozoic carbonate aquifer into Trout Creek generates consistent flow supporting a rich trout habitat. Precambrian igneous and metamorphic rocks form much of the bounding ranges but also crop out in many areas of the east side of the valley forming an important local fractured crystalline bedrock aquifer. Water Levels/Aquifer Characteristics [No data available presently] Water Use/Withdrawals [No data available presently] Water Quality [No data available presently]
Alluvial aquifers of the Arkansas River Basin form a network throughout the valley and are in places directly connected with valley-fill sediments. Hydrogeologic units of the Eagle Basin-Central Colorado Trough are present in the Mosquito Range on the east side of the valley. Cenozoic Volcanic Rock aquifers are present in the valley both near the surface in uplifted structural blocks in the valley and in the eastern uplands. The fractured crystalline Precambrian basement rock aquifer is present in the ranges and foothills on the both sides of the valley.
The Wet Mountain Valley and Huerfano Park are located in south-central Colorado in southwestern Fremont, western Custer, and northwestern Huerfano counties between the Sangre de Cristo Mountains on the west and the Wet Mountains on the east (Figure 12a-04-01). Both these intermontane basins contain thick Tertiary-age, basin-fill sediments. These two areas fall within and are under the administration of Water Division 2 of the Colorado Division of Water Resources. The Promontory Divide forms the boundary between the Wet Mountain Valley and Huerfano Park, as well as Custer and Huerfano counties. The two areas are discussed together because their basins are adjoining and their geology and hydrology are similar. The basin-fill deposits of the Wet Mountain Valley cover an area of approximately 230 square miles. Huerfano Park lies directly southeast of the Wet Mountain Valley and is underlain by many of the same aquifers. Land use in this area is primarily agriculture, rangeland, or forest. The area is sparsely populated with an average population density of approximately five residents per square mile. Surface water streams and creeks in the Wet Mountain Valley flow north to the Arkansas River, while those in Huerfano Park generally flow south toward the Huerfano River which is itself tributary to the Arkansas. Back to Table of Contents
Near-surface rocks in the Wet Mountain Valley and Huerfano Park range in age from Precambrian to recent (Table 12a-04-01), but the primary aquifers are basin-fill deposits of Paleogene through Quaternary in age. The Wet Mountain Valley is an extensional valley that formed along with the Rio Grande rift, but is off axis of the main rift which passes through the San Luis Valley just to the west across the Sangre de Cristo Range. In both areas, these deposits exceed 6,000 feet in thickness. Huerfano Park is a very complex structural basin on the east side of the Sangre de Cristo Mountains. Low-angle Laramide faults characterize the west side of the valley and the area is further overprinted by Neogene faults. The majority of groundwater withdrawals in the area are from the basin-fill deposits. These include, in descending order, rock units within the Dry union formation, Farisita, Huerfano, Cuchara, and Poison Canyon Formations (Figure 12a-04-02). Essentially, this basin is a northern extension of the Raton Laramide Basin beneath younger Paleogene and Neogene sediments. Neogene deposits, described as a salmon-pink basin-fill alluvium consisting of irregularly stratified stream-deposited fine- to coarse-grained sand with pebbles, cobbles, and boulders, are the primary source of water in the Wet Mountain Valley and Huerfano Park. Quaternary deposits in the Wet Mountain Valley consist of terrace gravels, valley fill, and glacial till and outwash. The size of this material ranges from clay to boulders, with considerable variability in aquifer characteristics. Paleogene volcanics and intrusive rocks and widespread Precambrian intrusive and metamorphic rocks make up the fractured crystalline bedrock aquifer beneath much of the east side of the basin. Water Levels/Aquifer Characteristics The basin-fill sediments are recharged by runoff from both the Sangre de Cristo and Wet Mountains. Precipitation in the valley also is an important source of recharge. Groundwater discharge is to the surface water system. Discharge also occurs by groundwater withdrawal from wells and flow to springs. In the Wet Mountain Valley, depth to water is less than 10 feet below ground surface in the central part of the valley and less than 100 feet in most of the remainder of the valley. Water levels in Huerfano Park range from near surface to a maximum of about 70 feet below ground surface. Groundwater flow in the basin-fill deposits of the Wet Mountain Valley is toward the center of the basin along Grape Creek and Texas Creek. The potentiometric surface in Huerfano Park has not been documented. As of February 2001, there were nearly 640 and 136 water wells of record completed in the Wet Mountain Valley and Huerfano Park, respectively. In the Wet Mountain Valley, 50 percent of the wells are less than 100 feet deep and 90 percent are less than 350 feet deep. The depth of most wells in Huerfano Park is even less; about 70 percent of the 136 wells of record are less than 100 feet deep, and 90 percent are less than 325 feet deep. Cumulative data from the Division of Water Resources indicate that 90 percent of the wells of record in the Wet Mountain Valley have reported yields of less than 30 gpm. About one-third of the wells in Huerfano Park have yields of less than 5 gpm, and 90 percent of the wells have reported yields of less than 20 gpm. Water Use/Withdrawals Surface water provides the predominant water supply for the area; however, groundwater is used for irrigation in the Wet Mountain Valley, and for public supply, domestic use, and livestock watering in both the Wet Mountain Valley and Huerfano Park. Groundwater represents less than two percent of total water use in these areas. Both the towns of Westcliffe and Silver Cliff utilize groundwater for municipal supply. Their wells are generally 120 feet deep and produce from hundreds to thousands of gallons per minute. Water Quality Water quality in the basin-fill deposits generally meets the drinking water standards, but is hard.
Alluvial aquifers of the Arkansas River Basin form a network throughout the basin and are in places directly connected with valley-fill sediments. Hydrogeologic units of the Raton Laramide Basin may extend up from the south beneath much of the basin and Eagle Basin-Central Colorado Trough are present in the Sangre de Cristo Range on the east side of the valley. Paleogene and Neogene Volcanic Rock aquifers are present in the valley both near the surface in uplifted structural blocks in the valley and in the eastern uplands. The fractured crystalline Precambrian basement rock aquifer is present in the ranges and foothills on the both sides of the valley.
Blue River Valley is a narrow, north-south trending, intermontane basin encompassing Summit County and extending north into Grand County as far as Kremmling in central Colorado (Figure 12a-05-01). It is bounded on the west by the Gore and Ten-mile Ranges and on the east by the Front Range and Williams Fork Range. The basin is approximately 45 miles long and 8 to 25 miles wide, encompassing some 700 square miles. Watersheds in the surrounding mountain ranges bring the area to over 25 miles wide in its southern end, narrowing to about 8 miles in its northern end. It is a high, generally forested valley rich mining history. It has become a center for winter sports recreation with four major ski resorts, while also being a year-round destination for many mountain activities. Its namesake river Blue River Valley is a major headwater tributary to the Colorado River and water use is administered through Division 5 with offices on Glenwood Springs. The valley is home to two major water storage reservoirs, Green Mountain Reservoir and Lake Dillon. Transmountain diversions export water from Dillon Reservoir to the South Platte watershed through the 23-mile long Roberts Tunnel that bores up to 4,000 feet beneath the Colorado Front Range. Back to Table of Contents
The Blue River Valley is the northernmost topographic expression of the Rio Grande rift and is a half graben with the boundary fault on the west side. It differs from the other extensional grabens in Colorado by having very little sedimentary fill associated with its development as a graben preserved within it. Sediments that would have accumulated have since been removed by erosion. The geometry of the basin is further complicated by the eastern bounding Williams Range Thrust Fault that places the Precambrian basement rocks of the Front Range over the sedimentary units in the graben. With this geometry, the valley center has many of the Paleozoic through Cretaceous sedimentary formations that form hydrogeologic units of the Ancestral Rocky Mountains Eagle Basin-Central Colorado Trough and Colorado Piedmont units of the Cretaceous Interior Seaway (Table 12a-05-01). The Pierre confining unit dominates much of the northern part of the valley. Paleogene intrusive rocks and widespread Precambrian intrusive and metamorphic rocks make up the fractured crystalline bedrock aquifer beneath the bounding ranges. As with other intermountain valleys Quaternary deposits cover large areas and consist of terrace gravels, valley fill, and glacial till and outwash. These deposits form the most common and accessible aquifer in areas where the population is concentrated. The size of this material ranges from clay top boulders, with considerable variability in aquifer characteristics. Water Levels/Aquifer Characteristics [No data available presently] Water Use/Withdrawals [No data available presently] Water Quality [No data available presently]
Alluvial aquifers of the Colorado River Basin form a network throughout the basin and are in places directly connected with valley-fill sediments. Hydrogeologic units of the Eagle Basin-Central Colorado Trough are present in the Tenmile Range. Much of the interior of the valley is underlain with units of the Colorado Piedmont region. The fractured crystalline Precambrian basement rock aquifer is present in the ranges and foothills on the both sides of the valley.
North Park and Middle Park are intermontane basins located in north-central Colorado lying between the Front Range on the east and the Park Range to the west (Figure 12a-06-01). These basins are in the Southern Rocky Mountains physiographic province and represent a single synclinal structural basin some 70 miles long and about 35 miles wide in Jackson and Grand counties. Although the Parks are considered a single structural basin, they are separate topographic, surface drainage, and groundwater basins, since the Continental Divide, in the form of the Rabbit Ears Range, bisects the syncline. North Park encompasses about 1,190 square miles of Jackson County, and is a broad, intermontane valley of flat to rolling topography that is drained by the North Platte River and its principal tributaries. The North Platte flows northward into Wyoming and is part of the Mississippi River drainage system. According to the 2000 Census, Jackson County recorded a population of approximately 1600, with agriculture being the dominant business. North Park falls under the administration of Water Division 6, with offices located in Steamboat Springs. Middle Park Basin is located in Grand County and encompasses about 1,030 square miles of complex valleys and sub-basins that are traversed by belts of overthrust faulting. The 2000 Census population for Grand County was approximately 12,500, with tourism accounting for nearly 66 percent of the employment. Middle Park is drained by the Colorado River and its tributaries. Groundwater and surface water in Middle Park are administered in Water Division 5, with offices located in Glenwood Springs. Back to Table of Contents
As much as 19,000 feet of sedimentary rocks of Permian to Neogene age lie within North Park, representing a number of hydrogeologic units (Table 12a-06-01), many which are known aquifers elsewhere in Colorado. The principal bedrock aquifer in North Park is the Coalmont Formation that is as much as 6,500 feet thick. The Coalmont Formation is a basin-fill unit derived from the surrounding uplifted mountains, and consists of a complex, interfingering of coarse- and fine-grained sediments. The hydraulic characteristics and water quality of this aquifer are dependent upon the compositional variability of the Coalmont Formation within the basin. An estimated 39 million acre-feet of recoverable, good quality water are available for withdrawal. There are up to 13,000 feet of sedimentary rocks in Middle Park of Jurassic to Neogene age(Table 12a-06-02). Groundwater in the hydrogeologic units within this section is highly influenced by the depositional complexity, thrust faulting, and variable thickness of the rock units. The Troublesome Formation (lake bed deposits) represents the most important bedrock aquifer for many parts of the Middle Park Basin. The Troublesome attains a thickness of at least 800 feet in the Fraser sub-basin and possibly 1,000 feet in the Granby area. This aquifer contains an estimated two million acre-feet of groundwater in storage. The underlying Middle Park Formation can be from 2,500 to 5,000 feet thick (Voegeli, 1965) and is an aquifer in the upper part where it consists of conglomerate and sandstone. Water Levels/Aquifer Characteristics As of February 2001, there were 422 water wells of record in North Park. The depths to water reported for those wells range from a minimum of 1 foot to a maximum of 298 feet. Groundwater in the upper part of the Coalmont Formation is generally unconfined. Well completion depths within North Park range from less than 20 to greater than 400 feet, with 90 percent of the wells reporting completion depths less than 200 feet. Reported well yield information from the Division of Water Resources indicates that 90 percent of the 420 wells with reported yield data have a yield equal to or less than 24 gallons per minute (gpm). Transmissivity values, for the upper 1,000 feet of the Coalmont aquifer, range from less than 20 ft2/d towards the middle of the basin to greater than 3,000 ft2/d along the basin edges. The water well density in Middle Park is significantly greater with more than 4,000 completed wells in Grand County, including alluvial, sedimentary bedrock, crystalline rock, and volcanic rock wells. Data indicate that 90% of the wells are completed at depths less than 325 feet below ground surface. The mean depth is about 183 feet, while the median depth is 141 feet. Water levels for the Middle Park wells range from above ground level (flowing artesian) to 525 feet below ground. In general, water levels in the Troublesome Formation aquifer range from 10 to over 100 feet. In the Fraser area, water levels in this aquifer are at or above ground level. Well yields from the sedimentary rock aquifers in Middle Park are variable and, as a general rule, low. Well yield values in the State’s well permit database indicate that 90 percent of the wells drilled in Middle Park have reported yields of less than 15 gpm, with yield values ranging from less than 1 to more than 1,100 gpm. Municipal wells completed in the Troublesome Formation in the Fraser area have reported yields ranging from 20 to 200 gpm. Water Use/Withdrawals In Jackson County, groundwater withdrawals of 80,000 gallons per day represented only a few hundredths of one percent of the total fresh water use. Of the groundwater withdrawals in North Park, over 87 percent is used for domestic supply with the remainder providing public water supply. In Grand County, groundwater withdrawals of 1.06 million gallons per day represented less than one percent of the total fresh water use. Of the over 4,000 completed water wells in Middle Park, 55 percent of the groundwater withdrawals are for domestic use and 45 percent for public supply. Water Quality Limited water quality information has been reported by the USGS from 30 wells, 100 to 800 feet deep, in the North Park Coalmont Formation. The median total dissolved solids (TDS) concentration of the Coalmont aquifer is about 400 mg/L, which is below the secondary drinking water standard of 500 mg/L. The concentration of TDS ranges from 200 to 3,000 mg/L, with the water generally soft to moderately hard. Publicly available water quality data for the bedrock aquifers in Middle Park is sparse. Analysis of groundwater from alluvial wells indicates the water quality is suitable for domestic and stock use. Limited water quality data from wells completed in the Troublesome Formation, in the Fraser area, indicate total dissolved solids are less than 200 mg/L. Radioactivity is a concern, however, with reported radon concentrations ranging from 305 to 5,943 picocuries per liter (pCi/L). These concentrations are typical for Colorado, but exceed the proposed EPA drinking water standard of 300 pCi/L.
In North Park, alluvial aquifers of the North Platte River Basin form a network throughout the basin. In Middle Park alluvial aquifers of the Colorado River Basin overlie the bedrock aquifers. Hydrogeologic units of the Colorado Piedmont region are present at the surface along the edges of the basins and may be found deep within the basin interiors. Paleogene and Neogene Volcanic Rock aquifers are present in Middle Park and along the Rabbit Ears Range that divides Middle Park from North Park. The fractured crystalline Precambrian basement rock aquifer is present in the ranges and foothills on the both sides of the valley.
Through geologic time Colorado has been the site of sometimes intense volcanism. As a result, volcanic rocks of varying ages can be found in many parts of the state (Figure 12b-01). Perhaps the most significant volcanic event in the State, from the perspective of current hydrogeologic setting, occurred in the Paleogene. This event began about 37 million years ago and lasted until about 24 million years ago and from it are several large volcanic fields. This episode was characterized by an early phase of intermediate, or andesitic, volcanism with eruptions from large stratovolcanoes. Little evidence remains of the actual volcanic centers, but the rocks from them are very common. This phase was followed by more silicic, or rhyolitic, volcanism with the eruptions from huge “super volcanoes” and the development of caldera complexes. Volcanism continued after this but changed character to widespread mafic, or basaltic, eruptions from cinder cones and fissures. The most recent eruption of this later type was from the Dotsero Crater in Eagle County only about 4,150 years ago. Volcanic fields all lie within the Mountainous region and include the San Juan Volcanic field, one of the largest in the world, that covers parts of ten counties in the southern part of the state. Other smaller fields include the Central Colorado volcanic field, the Flat Tops, and many other smaller outliers of volcanic rocks. Many form highlands mostly in public lands, while the larger fields extend across lowland areas now experiencing rapid growth dependent on groundwater. Back to Table of Contents
Rocks found within volcanic fields (Table 12b-01) tend to be very heterogenous to include eruptive lava flows, brecciated lava flows, unconsolidated ash deposits, ash deposits reworked by flowing water and wind, ash flows welded by their own heat, debris flow deposits, stream deposits, and lake deposits. Volcanic fields may also include intrusive igneous rocks near the eruptive centers. Because of the pattern of evolution of Paleogene through Neogene volcanism in Colorado, hydrogeologic units can be divided into two main groups. The earliest is the Paleogene unit that includes the intermediate phase followed by silicic caldera development. They can be treated as one unit because volcanic rocks from both are typically found together in overlapping stratigraphy. These tend to be concentrated in the larger volcanic fields. The next hydrogeologic unit are the Neogene, generally mafic, volcanic rocks. These are more widespread, found at the top of the Paleogene volcanic rocks, and scattered as smaller centers over much of the Mountainous region. Water Levels/Aquifer Characteristics The physical characteristics of volcanic rocks vary greatly. Chemical composition, mineralogy, volatile content, temperature, and mode of extrusion greatly affect their hydraulic characteristics. At the highest level the rocks are either mafic, intermediate, or silicic, based on silica content. Next are the types of flows and physical characteristics of flows. These can vary widely and result in many different types of textures. Some basaltic flows are vesicular, full of open holes from gas in the lava. And some basaltic flow have open lava tubes formed when the lava continued to flow beneath a solidifying crust. Other deposits interbedded with flows such as debris flows, water and wind transported deposits, and lake deposits can have varying hydrogeologic characteristics. This overall setting within volcanic fields can be quite complex and variable over short distances. While there is great potential for high hydraulic conductivity in some of these rocks, it typically takes fracturing to provide adequate permeability in most volcanic rocks. Fracturing can be from either cooling soon after eruption or subsequent tectonic deformation. Volcanic rocks tend to be unconfined with recharge directly from precipitation, with snowmelt predominant in the higher mountains. It can also be said that characterizing groundwater availability in a given location can be very site-specific. Water Use/Withdrawals As of February 2001, there were nearly 3,700 wells of record completed in the outcrop areas of Paleogene and Neogene volcanic rocks. While a few wells have been drilled in excess of 1,000 feet, 90 percent of the wells completed in these rocks are less than 400 feet deep. The reported well completion depths, for the majority of these wells, is less than 200 feet with a mean depth of 191 feet. Most wells completed in the volcanic rocks have low yields with 90% of the wells of record reporting yields of less than 18 gpm. The mean yield is 14 gpm and the median yield is 10 gpm. The State’s well permit database suggests that wells completed in Paleogene and Neogene volcanic rocks are shallower and have slightly higher yields than wells in Precambrian crystalline rocks. Water Quality Many of the volcanic areas in Colorado are highly mineralized. Hydrothermal systems accompanied and followed volcanic activity resulting in alteration of the host rocks, some leading to concentration of base and precious metals. Often mineralization included precipitation of sulfide minerals and other constituents potentially harmful to the environment. Mining activities further impacted the mineralized areas by disrupting natural groundwater flow systems enhancing discharge or changing ambient water conditions. The most common scenario has been suppression of water tables in mineralized areas that changes redox conditions from reducing to oxidizing which leads to mobilizing of metals bound in sulfide minerals. As a consequence many volcanic areas, particularly those near old eruptive centers, have impacted water quality.
With the widespread extent of volcanic rocks across central Colorado, they come into contact with nearly all other aquifer types. Alluvial aquifers of each river basin cross over volcanic areas, particularly in watershed areas. The Rio Grande River and San Juan River Basins are sourced in the vast San Juan Volcanic Field. Volcanic rocks may overlie any of the structural basins and both the Colorado Piedmont and Colorado Plateaus regions. Fractured crystalline Precambrian basement rock may also be present beneath parts of the volcanic areas.
Colorado’s crystalline rocks represent a unique and expansive aquifer system. The crystalline rocks are Precambrian aged (950 to 1800 million years old) igneous and metamorphic rocks; composed mostly of granites, gneisses, and schists. They also include Late Cretaceous and Paleogene igneous intrusive rocks. These rock types occupy approximately 19 percent of the state’s total surface area, and represent the fractured, crystalline-rock aquifers that supply much of the domestic water supply needs in the mountainous portion of Colorado (Figure 12c-01). Unlike sedimentary rock aquifers, igneous and metamorphic crystalline rocks have no primary porosity; water is stored in fractures within the rocks. Crystalline rocks are exposed at the surface throughout the Mountainous region. The general physiographic features of this area include high peaks, great relief, rugged terrain, steep slopes, shallow soils, and extensive areas of exposed bedrock. These rocks are exposed at the surface to some extent in all seven Water Divisions. Back to Table of Contents
Bedrock in the fractured, crystalline-rock aquifers consists of intrusive igneous and metamorphic rocks of Precambrian age, and intrusive igneous rocks of Upper Cretaceous and Paleogene ages. A thin veneer of soil, with moderate to high permeability, generally less than 5 feet thick, overlies the crystalline bedrock. The surficial deposits are not extensive enough to yield suitable quantities of water, but are an important unit for recharge and shallow, seasonal groundwater discharge. A conceptual model of the fractured, crystalline-rock aquifer system is shown in Figure 12c-02. Many of these rocks are extensively contorted through folding and faulting, producing joints and fractures that provide openings for storage of water. The fracture porosity of crystalline rocks is very low; as a general rule, less than 1 percent. Fractures provide the only significant porosity and flow conduits within the unweathered crystalline rocks of Colorado. Groundwater discharge and storage in crystalline rocks predominantly occurs in fracture networks. Vertical or steeply dipping fractures provide recharge, while near-horizontal fractures provide storage capacity and some degree of hydraulic continuity. Water Levels/Aquifer Characteristics In general, groundwater within the fractured, crystalline-rock aquifers is unconfined with water levels fluctuating seasonally and correlating with precipitation events. The predominant recharge is from snowmelt occurring between the middle of May and the first part of July. Recent studies indicate that some 84 percent of precipitation is returned to the atmosphere by evapotranspiration. Only a fraction of the remaining 16 percent recharges the groundwater storage system, the majority producing runoff. This limited amount of recharge to the aquifer suggests a delicate balance exists between aquifer recharge and consumption in the more populous regions of Colorado’s high country. Discharge from crystalline rock aquifers occurs at natural springs, as baseflow in adjacent stream drainages, and by groundwater withdrawal from wells. Depth to water in Precambrian rocks varies with topographic position and the amount of fracturing, which permits recharge, but generally it is within 150 feet or less of the ground surface. Water levels can fluctuate up to ten feet or more depending on the season as well as on yearly variations in precipitation. In general, water levels are highest in the spring or early summer when runoff is high, and lowest in the winter when frozen ground and snow inhibit recharge. Regionally, the water table mimics the surface topography. The general flow direction is downslope and toward surface drainages. The State’s well permit database indicates that as of early 2001, approximately 36,000 water supply wells had been completed within Colorado’s Precambrian, fractured, crystalline-rock aquifers. Analysis of these well permit records indicates that 90 percent of the wells were completed at depths of less than 550 feet with mean and median depths of 275 feet and 245 feet, respectively. Wells exceeding 1,000 feet have also been drilled, producing adequate water supplies for domestic purposes. Well yields from Precambrian rocks are generally only a few gallons per minute (gpm), although wells that penetrate extensively fractured zones, fault zones, or shear zones may produce up to 50 gpm or more. Analysis of well permit records indicates that 90 percent of the wells completed in Precambrian rocks yield less than 15 gpm, with a median yield of only 4 gpm. Recent investigations suggest that, to some extent, well productivity may be related to rock type, amount and orientation of fracturing, and topographic position. As one might expect from a fractured system, the transmissivity of the aquifer is highly variable. Investigators have reported a range of transmissivity values from single digits to 13,000 gallons per day per foot (gpd/ft). Water Use/Withdrawals Population growth in the mountainous regions of Colorado, over the past decade, has placed tremendous demands upon the fractured, crystalline-rock aquifers. Tens of thousands of small-diameter wells have been drilled to accommodate domestic and some limited public water supply. Water availability and yield in this aquifer is highly dependent upon the density and orientation of joints and fractures with limited correlation to rock type. Due to low fracture porosities, crystalline rocks are incapable of storing or transmitting large quantities of water. As such, the principal use of water from the fractured, crystalline-rock aquifers is for domestic supply. No data are publicly available for specific groundwater withdrawals from the crystalline-rock aquifers as a whole. Countywide data indicate that annual groundwater withdrawals, in the counties dominated by crystalline rocks, do not exceed 1,000 acre-feet per year. These low groundwater withdrawal values are more reflective of the land use, availability of surface water resources, and low population densities than the amount of groundwater available. Larger developments proposed in the mountainous regions of Colorado must, however, consider the limitations of these aquifers in their decision making process if surface water resources are not available. Water Quality Water quality in Precambrian crystalline rock aquifers is generally good, except in areas of mineralization where acidic or metallic waters may be found. The water quality of wells in Precambrian rocks indicate that the majority have total dissolved solids concentrations of 500 mg/L or less. A large number of wells in the foothills west of metro-Denver have hard to very hard water. Individual well and septic disposal systems are common in the mountainous regions of Colorado. Due to the thin surficial soil cover and direct fracture connection to the water table, bacterial contamination from leach fields or other waste disposal facilities is a concern. Radon can also be a matter of concern in wells completed in Precambrian rocks because of the presence of naturally occurring uranium and radium, which decay into radon. Radon concentrations from published studies cite ranges from hundreds to hundreds of thousands picocuries per liter (pCi/L). The proposed U.S. Environmental Protection Agency drinking water standard for radon in groundwater is 300 pCi/L.
With the widespread extent of crystalline bedrock rocks across central Colorado, this hydrogeologic unit comes into contact with nearly all other aquifer types. Alluvial aquifers of each river basin cross over crystalline bedrock areas, particularly in basin watersheds. Hydrogeologic units of the Colorado Piedmont and Colorado Plateau, as well as in the Ancestral Rocky Mountains basins, may come into contact with the rocks in the uplifted ranges. Volcanic rocks often mantle areas of crystalline bedrock.
Over geologic time the landscape of Colorado has continuously evolved. Rocks and sediments in Colorado tell a complex tale of this evolution stretching back nearly three billion years. Geologic time is divided into Eons which are then subdivided into Periods as shown in Table 13-01. The order shown is from youngest on top to oldest on the bottom, just as the rocks are normally found layered with with youngest rocks at the surface. Even though much has happened through such vast amounts of time, it is possible to group the evolution of the geologic landscape into major events. Eight major geologic events have left characteristic marks on the bedrock stratigraphy of the region (see the primary stratigraphy chart for the state, copies of the chart are available). These major events, also listed in Table 13-01, are the basis for grouping rocks and sediments shown in a generalized geologic map (Figure 13-01). Each of the eight major events formed rocks and sediments with unique sets of characteristics that determine aquifer type and groundwater conditions. Events are based on periods of relative stability or great tectonic upheaval. Periods of relative stability are those where regional deformation by tectonic forces is minor. There may have been prolonged times when older rock formations were exposed for erosion and the development of deep weathered surfaces. At times there was widespread deposition of sediments with similar characteristics across the region. Environments could have been either marine, with widespread carbonate reef development, or non-marine with broad fluvial plains or vast dunefields of wind-blown sand. Periods of great tectonic upheaval were those when the landscape changed dramatically because of deep crustal downwarps or the development of deep basins separated by uplifts. Material eroding off of the active uplifts filled the adjacent deepening basins. Faults and folds deformed older rocks and features. Widespread volcanism often accompanied the deformation. Tectonic forces changed over time, both in intensity and direction. And through time newer features overlapped older. Features formed during episodes of tectonic upheaval from one event may have formed where features from an earlier event formed. The newer features may be similar to older; a new basin overprints an older one with the same basic geometry. Conversely younger features may cross-cut older features with a different geometry. In some cases a younger feature may have formed with a completely opposite sense of deformation than an older feature it overlapped; an uplift brings up what was once a basin. This progression of changing forces and environments created a very complex three-dimensional architecture of geologic formations: basins overlap basins, uplifts overlap basins, basins conceal uplifts, and so on (Figure 13-02). A brief summary of the major events follows in order from oldest to youngest to convey the progression through time. Back to Table of Contents
During the Precambrian, nearly 2 billion years ago, the North American Continent was expanding as plate tectonics merged slivers of island arc complexes with the main core. This process built a basement of igneous and metamorphic rocks upon which all younger sediments accumulated. There were likely many different periods of tectonic upheaval separated by periods of relative stability. However, because the rocks formed over this long period of time are generally similar in their physical characteristics, they are considered a product of one event. The Proterozoic crystalline rocks formed during this period of continental accretion underlie the entire state but rise to the surface only in the cores of mountain ranges that uplifted much later during subsequent tectonic events. In these uplifted areas, fractured and faulted igneous and metamorphic rocks form the crystalline bedrock aquifer that in many places is the sole groundwater source. The oldest known Precambrian rocks in Colorado are represented by meta-sediments exposed in the Uinta Mountains in the northwest corner of the state. Back to Table of Contents
During the Cambrian through Mississippian Periods, from approximately 540 to 320 million years ago, the region of the continent where Colorado now sits was relatively stable. Periodically, marine advances flooded the area with shallow seawater. This environment and the evolution of shellfish resulted in the deposition of sandstone, limestone, and dolomite. The resulting formations flank many of the younger uplifts and can be local aquifers. Primary porosity and permeability may be limited in many of these formations but secondary porosity and permeability from fracturing and dissolution of the carbonates can lead to very productive aquifers. Vast ancient cave systems in the Mississippian Leadville Limestone is an excellent example of this type of secondary porosity and permeability.
In early Pennsylvanian time, about 320 million years ago, this region underwent a period of tectonism that created a series of mountain uplifts referred to as the ancestral Rocky Mountains. The uplifts brought up the deep Proterozoic crystalline basement rocks while eroding sediments accumulated along their flanks. These sediments include sandstone, conglomerate, shale and limestone that can be thousands of feet thick in places. A deep trough between uplifts cut across Colorado that was frequently flooded with seawater. Restricted flow with open ocean lead to accumulation of thick deposits of evaporite minerals, chiefly halite (salt) and gypsum. The coarser-grained sediments that often flank modern mountain ranges, or span plateau regions, can be local sedimentary bedrock aquifers. However, where present near the surface, the thick accumulations of evaporite minerals can lead to dissolution features, convoluted geologic structure, and degradation of groundwater.
Tectonic stability returned to the region in the Permian Period, about 270 million years ago, and the ancestral Rocky Mountain landscape was reduced to a nearly flat surface. The Paleozoic era began with an explosion of life and ended with a dramatic mass extinction of marine invertebrates. It is estimated that 85 percent of all marine species and 70 percent of all terrestrial species went extinct in less than 1 million years (Bowring and others, 1998). At the end of this era, Colorado was a relatively flat, low-lying region with an arid or semi-arid climate. From late Permian through Early Cretaceous time, alluvial, lacustrine and aeolian sediments intermittently accumulated on this surface. The coarser-grained sediments that often flank modern mountain ranges, or span plateau regions, can be local sedimentary bedrock aquifers. Back to Table of Contents
Tectonic forces at the western edge of the continent beginning about 100 million years ago in the Cretaceous caused the entire region where Colorado sits to subside. This sag allowed seawater to inundate the region to form what is called the Cretaceous Interior Seaway. The Cretaceous sea shoreline moved westward over the entire state depositing shoreline beach, barrier island, and deltaic sediments. As the shoreline continued to transgress westward, thick layers of marine shales were deposited. Marine shale and limestone, some rich in organic material, accumulated to thousands of feet. Sandstone, shale and coal accumulated in the bounding shoreline and delta settings. Not only do the sediments that accumulated during this event account for most of the state’s fossil fuel resources, but they also form regional sedimentary bedrock aquifers. Since marine shale makes up a large portion of these sediments permeability can be very low and water quality can be very poor. Groundwater constituents of concern frequently found in these sediments include sodium, chloride, sulfate, selenium, and uranium. Back to Table of Contents
Late in the Cretaceous, or about 70 million years ago, the style of deformation in the region changed. This became one of the most significant events in Colorado’s geologic history, the Laramide mountain-building event and the rise of the modern day Rocky Mountains. The seas of the western interior seaway retreated for the last time with emerging mountain ranges and terrestrial basins dominating the Colorado landscape through the ensuing Cenozoic era. As with the Paleozoic, the Mesozoic era ended with a mass extinction, this time of the dinosaurs. What was once a broad downwarp began to fragment into a series of basement-cored uplifts separated by deep subsiding basins driven by compressive tectonic forces. The Laramide mountain-building event continued into the Paleogene period and produced tens of thousands of feet of uplift with associated downwarp creating ranges and basins throughout Colorado. The subsiding basins pulled the older formations to great depths while sediments derived from erosion of the uplifts filled in above. Aerial extent of the Laramide basins in Colorado is expansive across seven major structural features. These large basins hold energy resources in the deeply buried older formations and groundwater resources in the coarse-grained sediments that accumulated while the basins formed. Back to Table of Contents
At the end of the Laramide Orogeny, a period of erosion is recorded in the rock strata by thick sequences of alluvial and colluvial sediments in the valleys and basins between mountain ranges. Tectonic forces slowly changed from compressive to extensional late in the Paleogene Period, about 30 to 25 million years ago. Widespread volcanism accompanied the transition as ash, lava flows, and debris from multiple volcanic centers blanketed much of the state. The San Juan Mountains represent one of the larger volcanic centers, containing several large calderas or basin-shaped volcanic depressions. Flows, welded tuffs, and intrusive bodies from this episode form part of the crystalline bedrock aquifer system. Gradually, extensional forces uplifted and deformed the region to produce Colorado’s present topography of block-faulted mountains and basins, plateaus, and high plains. Fault-bound sediment-filled grabens formed during this event drop the older geologic formations to great depths. These grabens have different characteristics from the earlier Laramide basins in both structural architecture and sediment type. Sediments filling the basins includes sand and gravel along with volcanic ash-rich mudstones. At the same time blankets of sediments similar to those filling the grabens, spread downslope from the central highlands. Back to Table of Contents
The landscape as we know it today evolved during this last phase of tectonic activity as surface drainage developed into the major river basins of the Platte River, Colorado River, and Rio Grande with their many tributaries. The Quaternary period, representing the last 2 million years of geologic history, has been dubbed the “Ice Age,” as great continental glaciers covered most of Canada and much of the northern United States during that time. The ice sheets did not extend into Colorado, but the cooler climate produced alpine glaciers in many of the mountain ranges. The glacial meltwater accelerated ongoing erosional downcutting leaving terrace deposits in several mountain valleys. With the end of the glaciation, about 10,000 years ago, wind-related (or eolian) deposits covered large parts of the plains of eastern Colorado. Sediments carried by these rivers form the unconsolidated alluvial aquifers. Back to Table of Contents