Hydrogeochemical Modelling of Variably Saturated Soils : Model Development and Application to Bauxite Refining Residue

This thesis is about the simulation of flow, transport and geochemical reactions in variably saturated soils. Motivated by the problematic disposal and revegetation of residue from aluminium refining, novel modelling tools for general geochemical reactions in variably saturated flow conditions were developed. In addition, a new model for the effect of mineral reactions on unsaturated hydraulic properties was established. The most sophisticated of the presented series of models was applied in a comprehensive simulation of the geochemical evolution of residue from aluminium refining in response to rainfall- and evaporation-induced leaching. Model development commenced with the direct implementation of the one-dimensional moisture-based form of Richards' equation into the geochemical modelling framework PHREEQC for the simulation of biogeochemical reactions in the vadose zone. The implementation gives access to the full suite of reactions in PHREEQC with activity corrected solution speciation, advanced adsorption models and the flexible definition of kinetic mineral reactions. The numerical approach is based on the conceptual treatment of liquid phase flow and solute transport as time- and space-dependent reactions. The accuracy of the applied finite difference scheme profits from the Kirchhoff transformation on the diffusivity term. In case of the van Genuchten hydraulic model, the integration of soil moisture diffusivity led to analytical series solutions that necessitate numerical approximation. The model's novel capabilities for the simulation of unsaturated flow and ion complexation with variable charge surface sites as well as water content modifying mineral reactions, such as dissolution of hydrated minerals were demonstrated in examples. In a second step, the applicability of this simulation tool was extended to soil profiles with heterogeneous hydraulic properties and variably saturated conditions where parts of the domain may be fully saturated. For this, the head-based form of Richards' equation was implemented using a mass balancing, implicit finite difference scheme with Picard iteration. The use of a total variation-diminishing scheme for solute transport yields second order accurate yet oscillation free solutions for the advective transport of sharp concentration fronts. Applications of the scheme were illustrated for (i) infiltration with saturation-dependent cation exchange capacity, (ii) changes in hydraulic properties due to mineral reactions and (iii) transport of mobile organic substances with complexation of heavy metals. Using this extended simulation tool, an advanced model for the effect of mineral reactions on unsaturated hydraulic properties was developed. The selective radius shift model accounts for the fact that mineral reactions in unsaturated conditions only affect the water filled pore size fraction. In addition, the volume change in each pore due to dissolution/precipitation is proportional to the pore volume. Chang