Surface Water Quality

Intro

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.

Naturally Degraded Water

Naturally degraded water may occur where surface water or shallow groundwater flows through geologic deposits with particular characteristics. These can include hydrothermally altered areas, evaporite deposits, metal-rich deposits, and others.

Evaluating natural sources of water degradation is important, as these sources can combine with anthropogenic ones (such as acid mine drainage) to impact surface water quality more severely. It is important to understand all the sources of poor surface water quality when making remediation decisions.

Below, we discuss three different types of natural water degradation that occur in Colorado: acid rock drainage (ARD), uranium contamination, and high salinity.

Acid Rock Drainage

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.

Uranium

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!

Salinity

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.