Redox-Controlled Multiple-Species Reactive Chemical Transport 2. Verification and Application

A redox-controlled multiple-species, multidimensional reactive chemical transport model, DYNAMIX, has been presented in part 1 of this paper. In part 2 the model is checked against two other independently developed reactive chemical transport codes. Simulated results from DYNAMIX agree reasonably well with those obtained from the other two models. Two approaches for simulating redox reactions, an external approach (based on hypothetical electron activity) and an effective internal approach (based on conservation of electrons), have been examined. The results show that in the external approach the simulated redox front is smeared out by diffusion and dispersion, whereas in the effective internal approach the redox front is sharp and is controlled by the redox reactions. The evolution of solid phases from the two approaches give markedly different mineral assemblages. It is suggested that the external approach is of relevance in industrial processes such as electrometallurgy, in which mineral dissolution is driven by externally supplied electric power. The effective internal approach is applicable to hydrogeochemical systems (groundwater contamination, diagenesis, ore formation, rock weathering, and soil genesis) in which redox potential is dictated by the states of the redox species. The results also suggest that in the presence of precipitation (and consequent retardation of the concentration front), spreading due to hydrodynamic dispersion may be significantly inhibited. To demonstrate the applicability of DYNAMIX for realistic field problems, two field cases are simulated by this model. In the first case, the supergene enrichment of copper at Butte, Montana, the simulated mineral assemblages and their distributions agree reasonably with the mineral assemblages and the ore grade observed in the field. In the second case, involving two-dimensional contaminant transport in a hypothetic aquifer, the simulated results suggest that DYNAMIX is capable of handling a realistic multi-dimensional field problem with several hundred grid blocks and time scales of practical interest.

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