In a new report that has implications for many Western states, the Colorado Geological Survey found that the natural geology in the headwaters of numerous streams is the source of poor surface water quality downstream. [Right, this stream, in the East Mancos River headwaters in the La Plata Mountains of southwest Colorado, is naturally acidic with high concentrations of metals because of the surrounding geology. Credit, Colorado Geological Survey]
The report, titled “Natural Acid Rock Drainage Associated with Hydrothermally Altered Terrane in Colorado,” identifies a number of streams in eleven different headwater areas of Colorado where surface water is acidic and has high concentrations of metals upstream of any significant human impacts.Hydrothermally altered rocks occur across many of the geologically active states in the Western U.S., including Arizona.
Rocks in these areas were altered by intensely hot water circulating in the earth’s crust, often associated with volcanic activity during Colorado’s geologic past. The “hydrothermal alteration” of the rocks changed their composition by dissolving some minerals and depositing others. In the affected areas, the hydrothermal-alteration process deposited metal-sulfide minerals, commonly pyrite (fool’s gold), in the rocks.
When these rocks are exposed at the surface, they interact with oxygen and the iron sulfide "rusts" to form iron oxide minerals, creating striking yellow, orange, and red colors – similar to the oxidation of metal in an old rusty car. “Acid rock drainage” occurs when the sulfur that is displaced by the oxygen combines with water to form weak sulfuric acid. The acidic water then dissolves minerals from the bedrock, often adding significant amounts of dissolved metals to these headwater streams. Natural acid rock drainage has been active in Colorado for thousands, possibly millions of years.
[Quotes taken from CGS announcement.]