As snowmelt and rain enter Colorado streams and percolate into the ground, the water picks up particles and dissolves some components of the rocks. Ordinarily, this natural process does not affect the water quality enough to be of concern. However, in some areas elements in the rocks are dissolved in high enough concentrations to adversely affect the bio-system.
Metal deposits in Colorado’s mountains sometimes seep into streams from abandoned mines and waste rock piles. In other streams, high concentrations of metals occur naturally as a result of the particular geology of the area. Acid rock drainage occurs when water and oxygen interact with metal-sulfide minerals, such as pyrite, producing sulfuric acid that dissolves metals and carries them into groundwater and streams. The award-winning 2011 CGS Bulletin B-54 details an investigation of natural acid rock drainage (NARD) in Colorado mountain streams associated with hydrothermally altered terrane.
Naturally occurring uranium deposits within the groundwater zone can also cause contamination. In 1980, the CGS studied natural groundwater contamination in the Cheyenne Basin of Colorado as a baseline for potential in-situ uranium mining. The study of 104 water wells showed significant levels of existing contaminants. Indeed, many of the wells did not meet EPA standards for uranium in drinking water!
High salinity concentrations may also be a cause of degraded water quality. Groundwater and surface water may dissolve ancient sea salt deposits that underlie many parts of Colorado. High selenium concentrations may occur when water dissolves selenium-rich rocks, such as the Upper Cretaceous marine shales that are found in the Gunnison River Basin/Grand Valley area, the Pine River Basin, and the Middle Arkansas River Basin. Although the South Platte River basin is also underlain by these Upper Cretaceous marine shales, selenium is not as significant there. Rather, high salinity and uranium contamination are more significant. Salinity concentrations are high enough to negatively impact agriculture in the region, which has major economic implications. Following up on a prior 2018 study, a 2021-22 salinity study of the South Platte River and primary tributaries east of the Front Range to the State Line is underway by Neirbo Hydrogeology and the CGS. So important is the issue that representatives of a number of agencies, including the CGS, formed the South Platte Salinity Stakeholders Group: to identify and quantify salinity sources, to identify potential solutions, and to recommend best water management practices.
As a general rule—to alert planners to potential problems—the geology, hydrogeology, and water quality of an area should always be assessed before any development takes place.
Colorado Abandoned Mine Land (AML) Information map — May also be accessed through the CDPHE at the Mine Impacted Streams TaskForce. The user interface is a GIS platform and includes AML information layers which can be projected on road maps, satellite imagery, property ownership maps, watershed maps, and so on.
Colorado’s Decision Support Systems — A water management system developed by the Colorado Water Conservation Board (CWCB) and the Colorado Division of Water Resources (DWR) for each of Colorado’s major water basins. Includes numerous online tools related to water resources.
Colorado Department of Public Health and Environment (CDPHE) – Water Quality — Statewide monitoring and reporting entity on surface and groundwater resources.
Colorado Water Conservation Board (CWCB) – Technical Tool Dashboards — Many resources dealing with surface water issues including snowpack and instream flow assessments, best-practices manuals, and more.
CGS AML Project information — Abandoned mine lands (AML) include lands, waters, and surrounding watersheds contaminated or scarred by the extraction, beneficiation or processing of coal, ores, and minerals. This project included surface water testing.
National Water Quality Monitoring Council – Water Quality Portal — A cooperative service that integrates publicly available water quality data from the USGS National Water Information System (NWIS), the EPA Water Quality Exchange (WQX) Data Warehouse, the USDA ARS Sustaining The Earth’s Watersheds – Agricultural Research Database System (STEWARDS) and more than 400 state, federal, tribal, and local agencies.
Neirbo Hydrogeology – South Platte River salinity sources, trends, and concerns – 1995-2018 — A detailed examination of salinity within the South Platte River Basin drainages.
US Environmental Protection Agency (EPA) – Nonpoint Source (NPS) Pollution Resources — Toolbox for students and educations dealing with this important issue.
US Environmental Protection Agency (EPA) – Water Topics — Many water quality resource links.