From the Summit County Citizens Voice (Bob Berwyn):
Frequently, acid rock drainage from natural sources and mine sites combine to cause severe downstream water quality problems. In these situations it is important to distinguish the natural, or background, water quality so that realistic clean- up goals for water quality can be set.
Peru Creek and the Snake River are a perfect example of this combination. The abandoned Pennsylvania Mine is thought to contribute a significant amount of acid mine drainage to water that is already tainted. As a result, the water downstream is toxic to trout and other aquatic organisms. Various agencies and groups have been wrestling with cleanup scenarios for decades.
The research explains that rocks in parts of Colorado’s mineral belt were altered by intensely hot water circulating in the earth’s crust, often associated with volcanic activity during Colorado’s geologic past.
These hydrothermal alteration changed the composition of the rocks by dissolving some minerals and depositing others.
In the affected areas, the 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.
More water pollution coverage here.