DEFINING ARSENIC-RELATED ECOLOGICAL RISKS IN A PIT LAKE

At the Getchell Main Pit in Nevada, potential ecological risks related to a future pit lake were assessed by using a groundwater model to determine infilling rates, coupled with a geochemical model to predict the mature pit lake chemistry. Both models were verified using data from 1963-1984, when the early pits reached steady state groundwater levels and chemistry and from 1984-2002 during dewatering activities. The results of the pit lake chemistry model were used in a screening ecological risk assessment.

Based upon site characterization, future habitats expected at the pit lake include the lake, a minimal littoral and riparian zone, and the upland habitat along the rim of the pit. Characteristic indicator species included: the mallard duck, cliff swallow, golden eagle, little brown bat, spotted sandpiper, deer mouse, mule deer and cattle that could be exposed to the identified Compounds of Potential Concern (COPCs) Sb, As, Cd, Ni and Zn. Arsenic food web bioconcentration data and the results of bioavailability testing were used in developing uptake factors.

The potential dose for each receptor-analyte combination were compared to Toxicity Reference Values to generate Hazard Quotients. If the HQLOAEL is >1, risk is likely to exist while HQNOAEL <1 indicates unlikely risk. HQNOAELs were <1 for Sb, As, Cd and Ni, indicating that the reproduction, growth and survival of populations of avian and mammalian species that may use the future pit lake habitat are protected from adverse effects resulting from exposure to concentrations of these metals predicted for the pit lake system. HQNOAELs were at or slightly above one for zinc (max: 1.47), but HQLOAELs were <1. Because several conservative assumptions were made about receptor exposure, including bioavailability assumptions, resource availability, and likely receptor area use, adverse ecological effects to receptors in the pit lake system are unlikely.