Understanding future pit lake water quality has become an important el
ement in permitting open pit mines, due to the potential long-term wat
er quality impacts on ecological receptors. Quantifying pyrite oxidati
on kinetics is an integral part of this analysis. To date, constraints
resulting from low moisture content in the arid regions of the southw
estern 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 oxidatio
n rates in arid environments by a factor of >2 compared to the same ma
terial emplaced in the field. Increasing particle size also resulted i
n decreasing reactivity, with 16-64 mm diameter grains <50% as reactiv
e as 4-16 mm diameter grains. A pyrite oxidation model that incorporat
es 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 de
monstrated 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.