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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

View larger version (21K): [in a new window]   Fig. 1. Heap leach process diagram. Mined ore is crushed and/or agglomerated and placed onto an impermeable liner. During leaching, cyanide solution is applied to the top of the heap of ore with sprinklers or drippers. Solution passes through the heap and is collected in a drainage ditch at the bottom of the heap. This solution containing a gold–cyanide complex is piped to a "pregnant" pond, and gold is subsequently extracted from the pregnant solution. The remaining solution then flows into a barren pond and may be recycled back to the top of the heap for additional rinsing.

View larger version (20K): [in a new window]   Fig. 2. Locations of Nevada mine sites examined in this study. Black dots mark mine sites, and dashed lines indicate county boundaries.

View larger version (21K): [in a new window]   Fig. 3. Draindown as a function of time since the last rinse for low drainage heaps. Curves are somewhat variable, but generally, heap draindown decreases rapidly just after rinsing stops and drops to <1 m3 ha-1 d-1 within a year after the last rinsing. Small spikes in draindown are apparent, particularly in the Preble curve, indicating probable response to climate forcing.

View larger version (24K): [in a new window]   Fig. 4. Draindown as a function of time since the last rinse for higher drainage heaps. Curves are variable, but generally the sharpest decrease in draindown occurs soon after the last rinsing followed by a gradual decrease to steady-state drainage rates between 0 and 2 m3 ha-1 d-1. Spikes in curves indicate probable response to climate forcing, particularly in the Wood Gulch curve, which shows spikes in drainage more than 5 yr after the last rinsing. Note that the gradual increases in draindown suggested by parts of the White Pine and Wood Gulch curves represent discontinuities in the data set and should not be considered representative of overall drainage patterns during those time periods.

View larger version (28K): [in a new window]   Fig. 5. Estimated recharge percentage as a function of available moisture. Recharge percentage represents the best estimate of long-term steady-state drainage as a percentage of annual precipitation. PET/P values represent estimated annual potential evapotranspiration divided by average annual precipitation. Bubble size is 1/N, with N being the number of data points used to calculate recharge percentage. Large bubbles represent fewer data points. No definitive relationship between recharge and available moisture is apparent, perhaps due to variations in vegetation cover.

View larger version (28K): [in a new window]   Fig. 6. Estimated recharge percentage as a function of average heap height. Recharge percentage represents the best estimate of long-term steady-state drainage as a percentage of annual precipitation. Bubble size is 1/N, with N being the number of data points used to calculate recharge percentage. Large bubbles represent fewer data points. The correlation is not definitive, but the data suggest that higher heaps may be more likely to have higher recharge rates.

View larger version (25K): [in a new window]   Fig. 7. Estimated recharge percentage as a function of ore texture. Recharge percentage represents the best estimate of long-term steady-state drainage as a percentage of annual precipitation. Crush size represents the estimated average particle size in the heaps. For heaps containing run-of-mine ore, this size was assumed to be 15 cm. Bubble size is 1/N, with N being the number of data points used to calculate recharge percentage. Large bubbles represent fewer data points. The plot suggests that heaps with coarser material allow greater moisture flow and a higher percentage recharge.