Water quality impacts from waste rock at a Carlin-type gold mine, Elko County, Nevada

Changes in water quality in the North Fork of the Humboldt River, Nevada are caused by weathering of waste rock from an inactive Carlin-type gold mine. Review of historical water-quality data, monthly water sampling, and continuous monitoring of water-quality parameters were used to quantify these impacts. River water pH, which ranged between 7 and 8, did not show statistically significant variation from upstream of the mine to downstream. Several constituents, most notably sulfate, calcium, and magnesium, showed statistically significant increases in dissolved-ion concentrations. These data, along with geochemical modeling, suggest that oxidation of sulfide minerals and in situ acid neutralization by carbonate host rocks are occurring. Large increases in dissolved-ion concentrations were observed twice a year--during spring snow melt and the onset of the winter precipitation season. These spikes are likely caused by flushing of pore waters that have reacted with waste rock during months-long periods when shallow groundwater recharge is not occurring.[PUBLICATION ABSTRACT]