Groundwater is simply water that occupies the pore spaces, crevices, or fractures within soil or rock. Some materials have a greater ability to store and transmit water than others. An aquifer is a groundwater reservoir composed of geologic units that are saturated with water and sufficiently permeable to yield water in a usable quantity from wells and springs. Sand and gravel deposits, sandstone, limestone, and fractured crystalline rocks are examples of geologic units that form aquifers. The USGS identifies seven principal aquifers or aquifer systems in Colorado: South Platte Aquifer, Arkansas Aquifer, High Plains Aquifer, San Luis Valley Aquifer System, Denver Basin Aquifer System, Piceance Creek Basin Aquifer, and the Leadville Limestone Aquifer of west-central Colorado.

Groundwater use in Colorado dates back to before the turn of the 20th century augmented by the advent of the deep-well turbine pump. Nineteen of Colorado’s 63 counties rely solely on this important water resource for potable supplies and domestic uses. Groundwater withdrawals by private wells and public water supply systems serve an estimated twenty percent of the state’s population. Agriculture is the largest user of groundwater, primarily for irrigation, however, it is also used to meet nearly all livestock and rural domestic water needs as well.

Sufficient water for a single household may be relatively easily obtained from wells in the bedrock aquifers, even the crystalline Precambrian rocks of the mountains. However, as households multiply, creating increased demands from many wells, a local aquifer with very limited storage that was sufficient for a few users, may be quickly depleted or polluted. Through wise water-management policies, protective regulations, and conservation activities, we can assure both the availability and the suitability of groundwater for future use.

The recently published ON-010 Colorado Groundwater Atlas has comprehensive information about every aspect of groundwater across the state.

The CGS is also continuing its program of detailed reporting on the groundwater resource status for individual counties across the state:

In 2015 the CGS implemented a program for generating individual county-based geology and groundwater surveys as both publications and web-based GIS applications. The surveys are tailored to inform the public, planners, and policy-makers about local geology and groundwater resources. They address all aquifers that are utilized in each county including those in alluvial, sedimentary, and crystalline geological formations. Patterned after county-wide groundwater resource series produced in the 1960s through mid-1970s, this current effort is intended to address counties where development pressures are straining current water resources and where no previous comprehensive surveys were done. The more recent surveys support and augment existing decision support and monitoring systems.

The work is also derivative of our ongoing participation in the USGS STATEMAP program which is part of the National Cooperative Mapping program focused on detailed 1:24,000 scale 7.5 quadrangle geologic mapping. The county surveys also build on other work by the CGS, including the 2003 Ground Water Atlas of Colorado which used Operational Account funding. The importance of groundwater is clearly recognized in Colorado’s Water Plan and these county-wide data-sets will assist in sound management of existing resources.

Each study takes between one and two years to complete. Counties completed to date include Chaffee, Douglas, Elbert, Mesa, and Park. La Plata County is in progress, Bent County is next in line.