MODELING LONG-TERM SOLUTE TRANSPORT IN DRAINED UNSATURATED ZONES

Long-term assessment of solute transport in the unsaturated zone is an important consideration for irrigation management, pesticide management, and subsurface contaminant restoration analysis and design. Mathematical models are often used to perform such analyses. Modeling fluid flow and solute transport in the unsaturated zone typically requires solution of the nonlinear Richards equation and an advective-dispersive equation for contaminant transport as a function of time. Such solutions are possible but computationally expensive. A simplified water balance approach to solve fluid flow in shallow, drained unsaturated zones has been developed and refined over the last 15 years. The objectives of this study were to use results from a water balance model to obtain solutions for solute transport in drained, shallow water table soils, and to compare the results with solutions based upon Richards' equation. Transient soil water flux rates computed with a water balance model were used as input to a Petrov-Galerkin advective-dispersive transport model to simulate solute transport in unsaturated soils. The transport model was checked for consistency by comparison with an analytical solution. Sample simulations showed good agreement between a Richards' equation-based transport model and a water balance-based transport model. Simulations were performed to show predicted trends in water quality over 1-year periods.