Preliminary Investigations of Effluent D

Long-term infiltration and drainage through unsaturated metal mining waste poses the potential to transport toxic contaminants, such as arsenic, cyanide, and mercury, and can severely impact groundwater. However, an almost complete lack of analysis of the long-term infiltration rates through these wastes significantly hampers quantitative assessment of the environmental impacts. This work synthesizes drainage data taken from the regulatory reporting of open pit and heap leach structures in the state of Nevada to investigate the magnitude of long-term infiltration and the factors contributing 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 m of drainage. Estimated precipitation-derived drainage ranged from 6 to 160 mg L−1, which constituted recharge ranging from 2 to 23% of annual precipitation. At low precipitation sites, estimates of 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 rates were lower than model predictions. Many of these sites had engineered and vegetated covers that successfully limited infiltration of moisture through the heaps. Water infiltration and drainage may be greater where the heap has a high potential for aggradation in places where environmental disturbance variables are coupled, and both the factors between these variables and drainage could be closely linked.