Geochemical modeling approach to predicting arsenic concentrations in a mine pit lake

Between 1968 and 1983, the North pit at the Getchell Mine, Humboldt County, NV, filled with water to form a lake. In 1983, water quality data were col lected with the following results: As concentrations of 0.29 to 0.59 mg/L, pH of 7.1 to 7.9, SO4- concentrations of 1490 to 1640 mg/L, and TDS of 2394 to 2500 mg/L. Using geochemical modeling techniques presented here, pit la ke waters have been theoretically allowed to react for 8.5 a, the approxima te time that the North pit had been completely full by 1983. Modeling resul ts predict pH of 7.9 to 8.2, SO4 concentrations of 1503 to 1644 mg/L, TDS o f 2054 to 2365 mg/L, and As concentrations ranging from 0.57 in the hypolim nion to 96 mg/L in the epilimnion. In the epilimnion, model results do not match observed As concentrations, suggesting that mechanisms, such as preci pitation of arsenate salts or adsorption to mineral surfaces, may control A s levels in an actual pit lake system. Adsorption to Fe oxyhydroxide surfac es is questioned by the authors because of the low Fe content in the Getche ll system, but adsorption to Al(OH)(3) (gibbsite) and clay mineral surfaces may be important in controlling natural As concentrations. (C) 2000 Elsevi er Science Ltd. All rights reserved.