Modelling of sulfide oxidation with reactive transport at a mine drainage site

A plume of groundwater affected by sulfide oxidation extends 160 m downgradient from the Nickel Rim Mine tailings impoundment and discharges to a small lake. The plume water has near neutral pH, but contains high concentrations of Fe and SO4. Field observations indicate that the release of SO4, H+, Fe2+ and other metals and their distribution in the aquifer is controlled by the rate of sulfide oxidation in the vadose zone of the tailings and by subsequent dissolution and precipitation of carbonate and oxyhydroxide mineral phases in the tailings and aquifer. Using field-determined aqueous and solid geochemistry, a conceptual geochemical model for the site was developed. This model accounts for trends in pH and in the concentrations of Fe and SO4. The numerical model MINTOX was used in conjunction with this conceptual model to simulate sulfide oxidation and two-dimensional reactive transport of dissolved sulfide oxidation products. Simulated sulfide oxidation and transport are compared to trends observed in the field after 35 years of oxidation. Similarity between observed and simulated concentrations demonstrates that, when used with a well-developed conceptual geochemical model, MINTOX is capable of simulating sulfide oxidation and plume evolution at mine drainage sites.