June 7, 2012--Geological water survey wins kudos (Pagosa Post)

Is high, pristine mountain water always clean and pure? Can streams unaffected by human activities and livestock influences be unfit for human consumption, or fish? A study by the Colorado Geological Survey (CGS) has some surprising answers. The study examines areas in Colorado that have naturally poor, surface-water quality due to the area’s geology. The pioneering report, titled Natural Acid Rock Drainage Associated with Hydrothermally Altered Terrain in Colorado, was recently recognized by the Geological Society of America (John C. Frye Memorial Award) as the best environmental publication of 2011. The report identifies a number of streams in Colorado where surface water is acidic and has high concentrations of metals upstream of historic mining. Hot water circulating in the earth’s crust can “hydrothermally alter” rock composition by dissolving some minerals and depositing others. In affected areas, the hydrothermal-alteration process deposited metal-sulfide minerals, commonly pyrite (fool’s gold), in the rocks. When these rocks interact with oxygen, 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 combines with water to form weak sulfuric acid. The acid then dissolves minerals in rock, often adding significant amounts of dissolved metals to streams. Natural acid rock drainage has been active in Colorado for thousands, possibly millions of years.

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