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.  In some areas, however, 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. 

High salinity concentrations can also be a concern for water quality.  Groundwater and surface water may dissolve ancient sea salt deposits that underlie 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, Pine River Basin and 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.  Naturally occurring uranium deposits within the groundwater zone can cause contamination.

WATER QUALITY PROJECTS
Abandoned Mines on US Forest Service Lands
Directory of Colorado Water Quality
El Paso County Groundwater Quality Study - Phase 1
Geology, Mining and Water Quality PowerPoint
Groundwater in Colorado: RockTalk Article
Water Quality: RockTalk Article
Turkey Creek Watershed