The proliferation of mine pits that intersect the groundwater table has engendered interest in environmental consequences of the lakes that form after cessation of dewatering. The Getchell Main Pit (GMP) in Nevada hosts arsenic sulfide (AsxSy) mineralization (e.g., orpiment and realar) and ambient groundwater As up to 1.8 mg L-1 , making groundwater inflow a potentially significant As source to the future pit lake. Predictive simulations using MODFLOW-SURFACT show that the GMP lake water level will recover to within 99% of the pseudo-equilibrium stage within 100 years after the end of dewatering, resulting in a 75-m deep, terminal pit lake. The juvenile GMP lake (after 5 years) will be calcium sulfate, pH 7.8 water body containing 920 mg L-1 TDS and 0.6 MG L-1 As evolving towards ph 7.9, 1580 mgL-1 TDS and 0.9 mg L-1 As water body after 100 years. The predicted pit lake chemistry is consistent with earlier pit lake water quality after 16 years when the South and Center Pits, precursors to the Main pit, were allowed to fill during a mining hiatus (1968-1984). The GMP mature pit lake chemistry was used to assess ecological risk to potential local receptors, i.e., mallard duck, cliff swallow, golden eagle, little brown bat, spotted sandpiper, deer mouse, mule deer and cattle. Arsenic does not strongly bioaccumulate through the food chain at Getchell; hence, pit lake As will not pose an unacceptable risk.
Davis, A., T. Bellehumeur, P. Hunter, B. Hanna, C. G. Fennemore, C. Moomaw and S. Schoen. “The Nexus Between Groundwater Modeling, Pit Lake Chemogenesis and Ecological Risk from Arsenic in the Getchell Main Pit, Nevada, U.S.A.,” Chem. Geol., 228, 175-196 (2006).
Braden Hanna
Pit-Lake Chemistry and Ecological Risk.pdf