Limnology, Arsenic Sorption, and Geochemical Modeling of Mine Pit Lakes
AuthorHershey, Ronald Lee
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Mining of disseminated low-grade ore requires the removal of hundreds of millions of kg of rock resulting in large open pits. This type of large-scale mining has increased substantially in the past several decades in the western U.S. with the development of very large open pits in Arizona, Montana, California, and Nevada. Of over 30 large mines in Nevada, eight will each excavate more than 600 billion kg of ore and waste rock.Studies of a large pit lake in Yerington, Nevada (Anaconda Mine), where the lake has been filling with groundwater for over 30 y, shows that the pit-lake limnology is similar to two nearby natural terminal lakes. The pit lake, even though it has a much smaller surface area-to-mean depth ratio than the natural lakes, is also monomictic and oligotrophic. The pit-lake's water chemistry has slightly alkaline pH at approximately 8.1 and total dissolved solids of approximately 600 mg L-1. Pit-lake ions are predominantly Ca2+, Na+, and SO42-, and concentrations for these ions are similar at different depths. In toxicity tests, the zooplankton Daphnia magna exposed to pit-lake water diluted in Walker River water had a 48 hr LC50 of 44.4 % with none of the animals surviving for 48 hr in 100 % pit-lake water. Elevated Cu and Se concentrations in the pit lake are well above aquatic life water-quality standards and may be adversely affecting phyto- and zooplankton populations.From laboratory batch sorption experiments, pit-lake sediments from a Carlin-type gold deposit pit lake had the greatest capacity to adsorb As(V) and As(III) under both oxidizing and slightly reducing conditions. Sediments from a porphyry copper pit lake had the least adsorption capacity while sediments from a quartz-adularia precious metal deposit pit lake had adsorption capacity between the other two sediments. Surface complication modeling suggests that the adsorption capacity of the sediments is controlled by the presence, and amount of, amorphous Fe hydroxides in the sediments.Modeling of the geochemical evolution of the Yerington pit-lake chemistry over a six-year period from 1994 to 2001 successfully reproduced the observed pit-lake chemistry and most of the observed changes in water chemistry except for Cu. Modeling demonstrated that the pit-lake water chemistry is dominated by the current pit-lake chemistry and that other water inputs to the lake including precipitation, spring flow, and deep groundwater are a small percentage (<10 %) of the water in storage in the pit lake. Evaporation, although a small component of the overall water balance for the pit lake (~ 3 % of the annual water input), is slowly increasing the TDS of the lake.