[1] A generalized formulation for kinetically controlled reactions has been developed and incorporated into a multicomponent reactive transport model to facilitate the investigation of a large variety of problems involving inorganic and organic chemicals in variably saturated media. The general kinetic formulation includes intra-aqueous and dissolution-precipitation reactions in addition to geochemical equilibrium expressions for hydrolysis, aqueous complexation, oxidation-reduction, ion exchange, surface complexation, and gas dissolution-exsolution reactions. The generalized approach allows consideration of fractional order terms with respect to any dissolved species in terms of species activities or in terms of total concentrations, which facilitates the incorporation of a variety of experimentally derived rate expressions. Monod and inhibition terms can be used to describe microbially mediated reactions or to limit the reaction progress of inorganic reactions. Dissolution-precipitation reactions can be described as surface-controlled or transport-controlled reactions. The formulation also facilitates the consideration of any number of parallel reaction pathways, and reactions can be treated as irreversible or reversible processes. Two groundwater contamination scenarios, both set in variably saturated media but with significantly different geochemical reaction networks, are investigated and demonstrate the advantage of the generalized approach. The first problem focuses on a hypothetical case study of the natural attenuation of organic contaminants undergoing dissolution, volatilization, and biodegradation in an unconfined aquifer overlaid by unsaturated sediments. The second problem addresses the generation of acid mine drainage in the unsaturated zone of a tailings impoundment at the Nickel Rim Mine Site near Sudbury, Ontario, and subsequent reactive transport in the saturated portion of the tailings.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001WR000862

Multicomponent reactive transport modeling in variably saturated porous media using a generalized formulation for kinetically controlled reactions

Mining and mineral processing generates large volumes of waste, including waste rock, mill tailings, and mineral refinery wastes. The oxidation of sulfide minerals in the materials can result in the release of acidic water containing high concentrations of dissolved metals. Recent studies have determined the mechanisms of abiotic sulfide-mineral oxidation. Within mine wastes, the oxidation of sulfide minerals is catalyzed by microorganisms. Molecular tools have been developed and applied to determine the activity and role of these organisms in sulfide-mineral-bearing systems. Novel tools have been developed for assessing the toxicity of mine-waste effluent. Dissolved constituents released by sulfide oxidation may be attenuated through the precipitation of secondary minerals, including metal sulfate, oxyhydroxide, and basic sulfate minerals. Geochemical models have been developed to provide improved predictions of the magnitude and duration of environmental concerns. Novel techniques have been developed to prevent and remediate environmental problems associated with these materials.

R.H Johnson

Papers

The hydrogeochemistry of the Nickel Rim mine tailings impoundment, Sudbury, Ontario