As snowmelt and rain enter Colorado streams and percolate into the ground, the water picks up particles and dissolves components of 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 living organisms.
Metals 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 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.
Naturally occurring uranium deposits within the groundwater zone can cause contamination. In 1980, the Colorado Geological Survey 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, as can be seen on the map 90 of the wells did not meet EPA standards for uranium in drinking water.
High salinity concentrations can also be a concern for 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. The geology and water quality of an area should be assessed before any development takes place — this will alert planners to potential problems.
There are many stories in the media concerning potential degradation of water quality from hydraulic fracture stimulation (or ‘fracking’) used by the oil and gas industry to obtain commercial quantities of petroleum. The American Association of State Geologists (AASG) released a consensus statement on hydraulic fracturing in July 2012.