Mine Adits, Mine-Waste Dumps, and Mill Tailings as Sources of Contamination

The long history of exploration and mining in the upper Animas River basin has produced thousands of prospects and now-inactive mines that range in size from small exploration pits to large mines with tens of miles of workings and mine waste with a corresponding range in size. The recovery of metals required the milling of 18.6 million tons of ore that created large volumes of mill tailings, part of which were placed in streams, or placed in impoundments after 1935. The high precipitation—particularly snowfall—in this area infiltrates mine workings, mine-waste dumps, and mill tailings to create acidic drainage that in places carries high concentrations of metals. To characterize the magnitude of acidic drainage and the concentrations of contaminants, we sampled 97 mine-waste sites, 18 mill-tailings sites, and 60 mine-adit drainages—the most significant ones that we could identify on public lands. Detailed descriptions of the geochemical determinations made on mine water and mine waste enabled us to rank sites for their potential to contaminate the watershed. We utilized three scores—acid generation, metal release in leach tests, and size— to rank waste dumps and mill tailings. For mine-adit water, we utilized metal-loading scores for copper and zinc to rank the potential to contaminate water and degrade aquatic habitat. Our geochemical characterization results and rankings of sites suggest a wide range in potential to contaminate the watershed. Out of the more than 500 identified sites on public land, only about 40 sites have sufficient size, adit discharge, or waste reactivity to be significant sources of contamination. Sites that have high rankings as sources and those identified by tracer studies of the main streams deserve attention for remediation work.