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Seepage from an historical mine tailing deposition areas. Whitewood
Valley, South Dakota. USFWS photo by Joy Gober and Scott Larson.
Mining has played an important economic role throughout the history of the United States. After the precious metals were depleted, many mines were abandoned without any environmental consideration. Today heavy metals remain in the tailings piles and in the abandoned mines. Groundwater exiting the abandoned mine portal can eventually enter nearby streams and contribute high concentrations of heavy metals such as copper, lead, zinc, and mercury. Precipitation percolating through mine tailings can react with rocks containing sulfur, leaches out heavy metals and significantly lowers the pH of the water. Water draining from these tailings piles is referred to as acid mine drainage. The acidity of the water plus the high metal concentrations can be deadly to animals and plants inhabiting the receiving streams. Additionally, acid mine drainage coats stream bottoms with iron hydroxide, giving the subtrates in impacted streams an orange color.
Because acid mine drainage can destroy a stream's aquatic community for miles, many of the old mining sites are being reclaimed. Reclamation can include plugging old mine addits and removal, isolation, or burial of tailings piles. Streams are sometimes rerouted to avoid contact with tailings piles. New technologies include passive treatment systems that use microscopic organisms to bind metals to a removable substrate.