Approaches for Multicomponent Reactive Transport Modeling in the Vadose Zone.

In soils and aquifers, dissolved inorganic and organic chemicals are commonly affected by a variety of physical and biogeochemical processes, which influence their mobility, but also alter the composition of the pore water and sediments. This is particularly true in the vadose zone, where the exchange of gases with the atmosphere can enhance the progress of geochemical reactions. Reactions of importance include aqueous complexation, microbially mediated redox reactions, mineral dissolution precipitation, and competitive adsorption processes such as ion exchange and surface complexation. In addition, plant-soil interactions play an important role near the ground surface. Due to the complexity of these systems and the strong non-linear coupling between the processes, existing conceptual models are often incomplete and data interpretation from field and laboratory studies is not intuitive. This provides the motivation for the development of and application of multicomponent reactive transport models that explicitly include as suite of geochemical reactions and implicitly account for the coupling between transport and reaction processes.

This presentation will describe the formulation of the multicomponent reactive transport code MIN3P, which was specifically designed for applications in variably saturated media. The model solves Richard’s equation for unsaturated flow and employs the global implicit method (GIM) for the solution of the reactive transport equations. The GIM is particularly well suited for simulating reactive transport in the vadose zone, because it deals effectively with the short time scales of gas migration. Recently the code has been enhanced to include vadose zone gas advection, multicomponent gas diffusion based on the Dusty Gas Model, and gas exchange processes across the capillary fringe. In addition, modules that account for plant-soil interactions are under development. Examples that highlight the complexity and non-linear interactions of the coupling between transport and reaction processes will be presented.