Several open pit mines in Nevada and other western states lower groundwater, typically by aggressive pumping, to mine ore below the water table (Miller et al. 1996).  After mining, the remaining pits partially fill with groundwater to form pit lakes.  Water quality in the pit lakes is affected by the quality of inflowing ground water, outflow of groundwater, precipitation, sulfide oxidation in surrounding rock, dissolution of metals, precipitation of metals, and evaporation.  In many cases, water contained in pits is of poor quality and may contain concentrations of metals or other inorganic constituents that greatly exceed water quality standards.  Wildlife use and the degree of threat presented by inorganic contaminants in pit lakes is uncertain.  It is assumed that riparian and aquatic communities will become established in most pit lakes. However, water contained in mine pits may be nutrient-poor or may contain elevated concentrations of contaminants which may restrict productivity. Mine pit lakes will typically be deep and steep sided, thereby limiting riparian and shallow lentic habitat.  However, benches and ramps in the mine pit, along with erosion of pit walls, may provide limited areas where shallow lentic or riparian communities may become established.  Wildlife using pit lakes may be exposed to hazardous levels of environmental contaminants.

The first phase of this investigation would provide information on habitat and community development, habitat quality, wildlife use, and inorganic contaminant behavior and partitioning, and the potential for wildlife exposure to contaminants in mine pit lakes in Nevada.  If it is determined during the first phase of the investigation that pit lakes provide suitable wildlife habitats and food resources, the second phase of the investigation would directly assess contaminant uptake and biological effects at one representative pit lake.  To accomplish this task, captive waterfowl and fish would be introduced to the pit lake.

In 2000, the USFWS identified 18 existing pit lakes in Nevada. Water quality data was obtained for 12 of the existing lakes.  Of the pit lakes for which data was available, four were slightly acidic. Water quality modeling predicted at least two of the pit lakes will not remain acidic over the long-term (David Gaskin, Nevada Division of Environmental Protection, pers. comm., 2000).   All pit lakes for which water quality data was obtained contained at least one trace element at concentrations that are potentially toxic to aquatic life or wildlife.  Aquatic life effect concentrations were exceeded for arsenic, cadmium, and chromium in 2 of the 12 pit lakes for which water quality data were available. Copper concentrations exceeded an aquatic life effect level in at least six pit lakes. Mercury was detected in four pit lakes.  All concentrations exceeded aquatic life and wildlife effect concentrations.  However, detection level used for mercury in the remaining pit lakes were greater than wildlife effect concentrations. Selenium exceeded a wildlife effect concentration in six pit lakes.  Zinc exceeded an aquatic life effect concentration in six pit lakes.

Learn More by Reading the Full Report:Higgins, D.K. And Wiemeyer, S.N., Assessment of Wildlife Hazards Associated with Mine Pit Lakes, USFWS. Reno FWO. 2001.

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