A coupled systems approach to solute transport within a heterogeneous vadose zone-groundwater environment

A coupled systems approach is presented for the simulation of fluid flow and solute transport within a heterogeneous vadose zone-groundwater environment. Separate model domains were developed for the vadose and saturated zones. The numerical model used is FEMWATER, a three-dimensional (3D) finite element model for simulating flow and transport in variably saturated media. A control plane (CP), normal to the mean flow direction, couples the two systems. The fluid flux and solute mass passing through the CP during a vadose zone simulation is used to construct the recharge boundary condition in the saturated system. Coupling the two systems allows for the accurate representation of flow and transport within the vadose zone while maintaining the ability to simulate regional plume migration in the saturated zone. The coupling approach is referred to as the Recharge Boundary Construction Algorithm (RBCA). The RBCA adaptively reconciles both the spatial and temporal discretization differences between the vadose zone and saturated zone models.; The accuracy of the RBCA is determined by a set of user-defined parameters controlling the algorithm's adaptive spatial mesh and adaptive temporal time step routines. The routines are designed to capture the greatest resolution of spatial and temporal changes in fluid flux across the CP during a vadose zone simulation. A sensitivity analysis performed on the control parameters for the construction of the recharge boundary shows the maximum percent difference in total mass passing through the CP during a vadose zone simulation as having the greatest effect on the boundary's level of resolution. Numerical simulations are made investigating the impact of effluent recharge at the ground surface on the solute temporal (breakthrough) behavior and spatial distribution in the two systems.; The usefulness of the RBCA method is in its ability to accurately describe the groundwater fluxes across the vadose zone-saturated zone interface (a.k.a. the recharge boundary condition). The method is most applicable for modeling the transport of contaminants from sources originating at the ground surface or within the vadose zone, where the potential for aquifer contamination exists. Specific systems that could benefit from the decoupled systems approach include those associated with artificial recharge, aquifer storage and recovery, and disposal of wastes by land or pond application.