Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 21, 1996
Publication Date: N/A
Interpretive Summary: Prediction of water and salt transport is essential to evaluating many environmental problems, including non point source pollutants such as irrigation return flows. Models are needed to predict the impact of management changes on drainage and return flow composition and volumes. Realistic modeling requires that we consider many processes which are usually omitted from predictive models. In this paper we review concepts of solute transport and evaluate the effects of chemical changes on hydraulic properties, assumptions regarding carbon dioxide concentrations and soil pH, and substitution of kinetic for equilibrium expressions for various chemical reactions. We demonstrate that inclusion of carbon dioxide production and transport is necessary, as is proper representation of the above-mentioned factors in prediction of solute transport for real world problems.
Technical Abstract: Proper representation of the important processes in the rootzone is critical to modeling the transport and chemical reactions of major solute species in the vadose zone. Realistic modeling of the vadose zone requires representation not only of physical and chemical processes but also biological processes such as microbial respiration and plant water uptake. We discuss traditional approaches for the modeling of reactive transport of single solute species including the concepts of both physical and chemical nonequilibrium. We include discussion of processes required for application of multicomponent models to field systems, and describe some of the features of the UNSATCHEM model which meets at least some of these requirements. Example simulations are given demonstrating the importance that chemical effects on hydraulic properties, root water uptake, carbon dioxide dynamics and kinetics of calcite precipitation have on prediction of solute concentration and distribution in the unsaturated zone. Consistent with field observations we predict that bentonite layers used as liners in waste ponds can fail catastrophically due to conversion of the clay from Na-saturated to Ca-saturated, causing a rapid infiltration of the waste.