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Preliminary Investigations of Effluent Drainage from Mining Heap Leach Facilities

^a Graduate Program of Hydrologic Sciences, University of Nevada, Reno, NV
^b Department of Environment and Soil Science, Lleida University, Spain
^c Department of Environmental and Resource Sciences and Department of Geological Sciences, University of Nevada, Reno, NV
* Corresponding author (tylers{at}unr.edu)

Received 14 March 2002.

Long-term infiltration and drainage through unsaturated metal mining waste poses the potential to transport toxic contaminants, such as arsenic, cyanide, and mercury, to the underlying groundwater. However, an almost complete lack of analysis of the long-term infiltration rates through these wastes signficantly hampers quantitative assessment of the environmental impacts. This work synthesizes drainage data taken from regulatory reporting of gold and copper heap leach structures in the state of Nevada to investigate the magnitude of long-term infiltration and the factors controlling infiltration. Because heap leach structures are lined, infiltration and drainage rates can be directly measured at a downstream point. Drainage rates following rinsing showed an exponential decline, and in three of the eight sites examined in this study, drainage reached a steady state derived from precipitation. The remaining five sites continued to show a very slow decline in drainage after as much as 57 mo of drainage. Estimated precipitation-derived drainage ranged from 6 to 160 mm yr^-1, which constituted recharge ranging from 2 to 23% of annual precipitation. At low precipitation sites, estimated recharge through heaps was higher than predicted by models used to estimate recharge in semiarid regions, and the highest recharge rates were calculated for heaps containing coarse textured ore. At higher precipitation sites, estimated recharge was lower than model predictions. Many of these sites had engineered soil and vegetation covers that successfully limited infiltration of moisture through the heaps. Water infiltration and drainage may be greater where the ratio of potential evapotranspiration to precipitation is low and where a high proportion of slope surface area is exposed, but the relationship between these variables and drainage could not be clearly defined based on the data in this study.

Abbreviations: gpd, gallons per day • NDEP, Nevada Division of Environmental Protection • PET, potential evapotranspiration • PET/P, ratio of annual potential evapotranspiration to annual precipitation