Density-dependent, multi-species transport in saturated porous media

A three-dimensional, reactive numerical flow model is developed that couples chemical reactions with density-dependent mass transport and fluid flow. The model includes equilibrium reactions for the aqueous species, kinetic reactions between the solid and aqueous phases, and full coupling of porosity and permeability changes that result from precipitation and dissolution reactions in the porous media. Two sets of chemical transport simulations are performed. The first investigates coupled processes of reactive chemical transport and density-dependent fluid flow and the subsequent impact on the development of flow instabilities created by heterogeneities in the porous media. Results demonstrate that instability development is sensitive to the initial perturbation due to density differences between the solute plume and the ambient ground water. If the initial perturbation is large, then it acts as a “trigger” in the flow system that enhances instability development in a planar reaction front. When permeability changes occur due to dissolution reactions occurring in the porous media, a reactive feedback loop is created by calcite dissolution and the mixed convective transport of the system. Although the feedback loop does not have a significant impact on plume shape, complex concentration distributions develop as a result of the instabilities generated in the flow system. In the second set of chemical transport simulations, the numerical flow model is used to demonstrate that model verification can be performed despite differences in hydrogeochemical transport code formulations. One of the test problems involves uranium transport under conditions of varying pH and oxidation potential, with reversible precipitation of calcium uranate and coffinite. Despite differences in the oxidation potentials, results show similarities in mineral assemblages and aqueous transport patterns. Because model verification can be further complicated by differences in the approach for solving redox problems, a comparison of the oxygen fugacity approach (based on O₂ partial pressure) to both the external approach (based on hypothetical electron activity) and effective internal approach (based on conservation of electrons) is performed. The comparison demonstrates that the oxygen fugacity approach produces different redox potentials and mineral assemblages than both the effective internal and external approaches.