The Effect of Mine-Related Groundwater Use and Management on Surface Water in the Middle Humboldt River Basin

The Humboldt River, in Northern Nevada, USA, is a source of water for agricultural, municipal, and industrial uses. Mining has been active in Nevada since the 1800s. The recognition that groundwater withdrawal for many purposes-including mining-has the potential to deplete streamflow has prompted studies of the Humboldt River Basin. Nevada Gold Mines (NGM) operates several mine complexes in the Middle Humboldt River Basin. Although dewatering rates have reached about 3.9 cubic meters per second or greater (m³/s: 100,000 acre-feet per year [afy]), most water was not consumed by mine operations. Rather, most water was returned to groundwater, used to augment surface water, or used to offset other groundwater withdrawals. The objective of this study is to evaluate the effect of groundwater use and management by NGM on surface water in the Middle Humboldt River Basin, emphasizing the Humboldt River and spanning mine development through post-closure and pit lake formation. The study proceeds by first estimating historical and potential future net groundwater uses; then identifying features that control the propagation of pumping effects toward surface water bodies; and then using expedient calculations to estimate the locations, timing, and quantities of surface-water depletion. The study yields a best estimate and an ensemble of results illustrating a range of potential outcomes. As a result of water management practices and identified factors that delay and diminish stream depletion versus the mine dewatering, the study finds that the peak stream depletion is likely to be on the order of 0.25 m³/s (about 6000 afy), a rate which may be challenging to discern for much of the stream hydrograph.