High mountain streams not always as pristine as they appear
By Summit Voice
SUMMIT COUNTY — The Snake River, flowing from headwaters in the mountains high above Keystone, is one of several streams in Colorado that’s been identified as being affected by naturally occurring acid rock drainage.
The new report from the Colorado Geological Survey looks at 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.
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.
State geologists collected 101 water samples from headwater areas and identified specific streams in the following areas as being affected by natural acid rock drainage: Silverton area, Lake City area, Platoro-Summitville area, Kite Lake area and East Trout Creek in the San Juan Mountains, the La Plata Mountains, Rico Mountains, headwaters of Lake Creek south of Independence Pass, the Ruby Range near Crested Butte, Red Amphitheatre near Alma, headwaters of the Snake River in eastern Summit County, and the Rabbit Ears Range.
Through detailed geologic mapping, the study characterized the type and intensity of hydrothermal alteration and correlated the geology with surface-water chemistry. Many of the areas exhibiting intense hydrothermal alteration also contain historic mine sites.
To order the Natural Acid Rock Drainage: Associated with Hydrothermally Altered Terrane in Colorado please call 303-866-2611 Option 0, or visit our online book store and search for NARD. Price is $30.00 plus shipping.
Filed under: Colorado, Environment, rivers, Summit County Colorado, Summit County news, water Tagged: | acid mine drainage, acid rock drainage, Colorado Geological Survey, Environment, Peru Creek, Snake River, Summit County Colorado, Summit County News, water quality