Betze Pit Lake
The Barrick Goldstrike Mine is located in the Carlin Trend, about 20 miles northwest of Carlin, Nevada. The majority of the ore mined at Goldstrike is from the Betze-Post open pit, the largest open pit operation in Nevada. Nevada law requires that the water quality of temporary or permanent reservoirs that may develop in open pits be periodically tested. Pit water quality prediction is conducted to monitor water quality and the possible impact that it may have on human, terrestrial, or avian life.
The geochemical effects of placing 570 million tons of backfilled waste rock into the open pit were examined in this study. This evaluation of pit lake water quality differs from previous studies in that it utilized laboratory studies where natural groundwater from the site was mixed with water that had contacted weathered mine rock as a means of simulating the pit lake geochemical reaction path. Calibration of PHREEQC using batch test results enabled refinement of solubility data obtained from the thermodynamic mineral equilibrium database employed by PHREEQC. The prediction of pit lake hydrology and water quality involved six key elements including 1) summarizing the mine plan, and geochemistry of exposed rocks and backfill, 2) evaluating mine filling using a regional groundwater flow model, 3) conducting eight large-diameter column studies to generate representative rock contact waters, 4) performing batch mixing tests to simulate final pit water quality at various stages of filling, 5) calibrating a geochemical model based on the batch tests, and 6) and using the calibrated model to predict water quality at various stages of pit lake recharge.
The PHREEQC model accurately predicted the common ion concentrations and pH of the batch tests, and correctly simulated the precipitation of large amounts of calcite when a mixture of various mine waters, groundwater and meteoric water was evaporated (although aragonite was actually the dominant solid that formed). Agreement between the model and batch tests was poorer for some metals, however. The PHREEQC model over-predicted zinc, nickel and antimony concentrations in the pit lake while under-predicting concentrations of barium, copper and arsenic.
Download a paper from the 7th ICARD in St Louis