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Understanding future pit lake water quality has become an important element in permitting open pit mines, due to the potential long-term water quality impacts on ecological receptors. Quantifying pyrite oxidation kinetics is an integral part of this analysis. To date, constraints resulting from low moisture content in the arid regions of the southwestern U.S. that host many large, disseminated deposits have not been considered. In this study, laboratory humidity cells routinely used to simulate pyrite reactivity were found to overestimate pyrite oxidation rates in arid environments by a factor of >2 compared to the same material emplaced in the field. Increasing particle size also resulted in decreasing reactivity, with 16-64 mm diameter grains <50% as reactive as 4-16 mm diameter grains. A pyrite oxidation model that incorporates both fracture and porous media pyrite oxidation using site-specific data, e.g., wall rock geometry, moisture content, etc., was developed to simulate wall rock reactivity in arid environments. The results demonstrated a 3-fold reduction in oxidized wall rock thickness in arid environments compared to assumptions of constant 100% moisture, with a concomitant reduction in solute loading to the incipient pit lake.

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