Controls on pit lake water quality at sixteen open-pit mines in Nevada

Thirty-five mines in Nevada currently have, or will likely have, a pit lake . The large bulk mineable deposits in Nevada mined below the water table ar e of several types, including Carlin-type Au, quartz-adularia precious meta l, quartz-alunite precious metal and porphyry-Cu (-Mo) deposits. Of the 16 past or existing pit lakes at 12 different Nevada mines, most had near neut ral pH and low metal concentrations, yet most had at least one constituent (e.g., SO4) which exceeded drinking water standards for at least one sampli ng event. Water quality data indicate that, in general, poor water quality will not develop in Carlin-type Au deposits. Wall rocks in the geologic env ironment typical of these deposits, and in the specific pits sampled, conta in substantial amounts of carbonate, which buffers the pH at slightly basic conditions and thereby limits the solubility of most metals. Similarly, th e quartz-adularia precious metal deposits generally have geologic condition s that buffer pH and naturally prevent the development of poor water qualit y. In both of these deposit types, certain elements such as As and Se that are mobile in neutral to basic waters may accumulate to levels near or exce eding drinking water standards. Pit lakes forming in quartz-alunite preciou s metal deposits hosted in volcanic rocks or in porphyry-Cu. (-Mo) deposits in plutonic rocks are of greatest environmental concern in Nevada, as both deposit types have relatively high acid-generating potential and low buffe ring capacity. However, the sampled Nevada pits in these deposit types indi cate that the water may not be of poor quality. In addition, water quality in some pits may actually improve with time due to the increased water-rock ratio as the pit fills with water, as suggested by pit waters at one mine in a Carlin-type deposit (Getchell) that improved between 1968 and 1982. Al though water quality in pits in each deposit type is generally good, local, site specific conditions (e.g., surface water inflow) and variations (e.g. , evaporation rates) result in some pit lakes (e.g., Boss) in the quartz-ad ularia deposit type being of substantially poorer water quality than other lakes (e.g., Tuscarora) in the same deposit type. Despite underlying geolog ic controls based on deposit type, site specific variations in hydrogeologi c conditions and surface geologic features can result in differing water qu ality in pit lakes in the same deposit types, and these factors may, in som e cases, provide an overriding control on the geochemical evolution of spec ific pit lakes. (C) 1999 Elsevier Science Ltd. All rights reserved.