Predicting Water Quality Trends in Lone Tree Pit Lake, Nevada (USA)

Lone Tree pit lake became acidic during groundwater recovery at a gold mine in Nevada. Alkalinity was added periodically to shift the lake to a neutral pH. Improved knowledge of the pit lake's sources of acidity was essential for water quality prediction. The primary source of acidity and metals at Lone Tree appears to be dissolution of chemical loads by groundwater flowing through sulfide-enriched rocks exposed in the highwall. About 60% of the highwall consisted of potentially acid-generating rock based on analysis of sulfur and carbon in blast-hole samples collected during mining. Analysis of acid ingress over time indicated that complete oxidation of about 2.3 m of highwall would account for the sulfate and acidity mass balance in the pit lake. In addition, the analysis showed that chemical loading stored in the submerged highwall was likely released over many years by influent groundwater. A synthetic rinsing curve was calibrated against measured water quality to simulate chemical releases from the weathered rock.