A numerical model for multicomponent reactive transport in variably saturated porous media

A general multicomponent reactive transport model (MIN3P) for variably-saturated porous media was developed. The model includes a simulator for geochemical batch problems as well as a variably-saturated flow module, and it solves the coupled reactive transport equations. The governing equations are discretized using a locally mass conservative finite volume method and linearized by a global implicit solution technique. Advective-diffusive transport of dissolved species and diffusive gas transport are included. The model is based on a partial equilibrium formulation. Reaction processes included are aqueous complexation, oxidation-reduction, ion-exchange, gas dissolution-exsolution and mineral dissolution-precipitation reactions. General rate expressions for kinetically-controlled intra-aqueous and dissolution-precipitation reactions were developed and allow the consideration of a large number of rate expressions reported in the literature. All reaction and rate parameters can be specified from a database, which allows utilization of the model for a wide range of reactive transport scenarios involving inorganic and organic chemicals. Several numerical techniques have been implemented and tailored towards reactive transport applications to increase the efficiency and robustness of the model. Verification examples involving the generation of acid mine drainage, reactive transport affected by ion exchange reactions and the mining of copper from a five-spot well pattern by acid leaching were conducted to prove the functionality and versatility of the model. Batch and one-dimensional simulations were conducted to demonstrate the model's capabilities for the investigation of the generation and fate of acid mine drainage. A final set of simulations demonstrates the versatility of the model as an analysis tool for the investigation of field data. The remediation of groundwater contaminated by hexavalent chromium and chlorinated organic chemicals by a reactive barrier composed of zero-valent iron is simulated and the results are compared to field observations. One and two-dimensional simulations were conducted to semi-quantitatively describe the processes defined in the conceptual model and to assess the possible effect of preferential flow and secondary mineral formation and on the treatment quality.