Interpretation of Field-Barrel Reaction Rates : Can Fully Oxic Conditions be Assumed?

Field-barrel experiments are commonly used to evaluate the reactivity of mine waste rock. The barrels are generally 1 meter tall and filled with waste rock, with the largest size fractions screened out. Reaction rates are inferred by interpreting the drainage chemistry. Field barrels are often, but not always, constructed with holes drilled into their sides to ensure adequate air supply, allowing inferred reaction rates to be assumed representative of fully oxic conditions. We examine this assumption. We consider two sets of scenarios with MIN3P, a reactive transport code. In one set, we use high free-gas diffusion coefficients to mimic fully aerated conditions found in barrels with side holes. In another set, we simulate barrels without holes, using gas diffusion coefficients that represent waste rock under prevailing moisture-content conditions. The mineral reaction rates were calibrated to best match the effluent concentrations of sulfate (SO4) in the field barrels over a 10-year testing period. We find that the lower parts of field barrels without holes can become anoxic under some conditions, particularly with reactive minerals such as pyrrhotite and taller field barrels. If the entire mass of waste rock in these barrels is incorrectly assumed to be reacting with the oxygen present, the inferred oxic oxidation rates per mass of rock will be too slow and may not appropriately predict field-scale drainage.