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accurately the long-term transport of contaminants from these sites. Long-term infiltration and drainage through unsaturated metal min-Heap leaching of ores mined from open pits is a teching waste poses the potential to transport toxic contaminants, such as arsenic, cyanide, and mercury, to the underlying groundwater. However, nique that allows gold and copper to be extracted from an almost complete lack of analysis of the long-term infiltration rates low-grade ores at low capital and operating costs (Bartthrough these wastes signficantly hampers quantitative assessment of lett, 1992). Heap leaching for copper (using acid as the the environmental impacts. This work synthesizes drainage data taken leach solution) has a long history, and now heap leaching from regulatory reporting of gold and copper heap leach structures in of low-grade gold ores (ෂ0.5 g gold Mg Ϫ1 ) is also comthe state of Nevada to investigate the magnitude of long-term infiltration mon. Such low-grade ores require large volumes of rock and the factors controlling infiltration. Because heap leach structures to be processed in a heap leach, and it is not uncommon are lined, infiltration and drainage rates can be directly measured to require 150 to 200 Mg of ore to produce 28.3 g (one at a downstream point. Drainage rates following rinsing showed an ounce) of gold. Ores used in heap leaching are either exponential decline, and in three of the eight sites examined in this taken directly from the mine pit (run-of-mine ores) or study, drainage reached a steady state derived from precipitation. The remaining five sites continued to show a very slow decline in drainage crushed to expose more surface area to the leaching after as much as 57 mo of drainage. Estimated precipitation-derived solution. Ores containing fine soil or rock particles are drainage ranged from 6 to 160 mm yr Ϫ1 , which constituted recharge often agglomerated with cement (agglomerated ores) to ranging from 2 to 23% of annual precipitation. At low precipitation allow adequate solution flow (Burkhalter et al., 1999). sites, estimated recharge through heaps was higher than predicted byAfter crushing and possible agglomeration, ores are models used to estimate recharge in semiarid regions, and the highest piled onto heap leach pads (Fig. 1) using haul trucks, recharge rates were calculated for heaps containing coarse textured front-end loaders, or mechanical conveyors. Successive ore. At higher precipitation sites, estimated recharge was lower than ore layers are called lifts (Bartlett, 1992), each ranging model predictions. Many of these sites had engineered soil and vegetain height from 7 to 16 m (Burkhalter et al., 1999). Some tion covers that successfully limited infiltration of moisture through heaps, known as expanding pads, are gradually exthe heaps. Water infiltration and drainage may be greater where the ratio of potential evapotranspiration to precipitation is low and where

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