A phytoremediation project was established at the U.S. Department of Energy's Savannah River Site (South Carolina) to reduce fluxes of tritium-contaminated groundwater to surface waters. Contaminated groundwater was collected in a pond and applied by spray irrigation to a catchment of mixed forest on Atlantic Coastal Plain soils. The objectives of this research project were to simulate tritium uptake by the vegetation and to determine if subsurface lateral flow at the sand-clay interface impacts tritium uptake by forest vegetation. To simulate water and tritium fluxes within the catchment, we developed a spatially explicit water and solute transport model. Vertical water flow was simulated within a grid using a layered capacitance model with lateral flow between cells dependent on head development and the local slope of the impeding clay layer. Tritium movement was simulated on a daily basis assuming complete mixing within a cell. The model was evaluated by comparing biweekly measurements of soil tritium activity and soil water content in 18 measurement clusters distributed across the irrigated portion of the catchment. Although lateral flow was predicted to occur locally, after 3 yr of tritium irrigation, the model predicted that lateral transfer of tritium was <70 m downslope of the irrigation area. Transient saturated conditions were observed and simulated to occur within parts of the catchment. Including the simulation of subsurface lateral flow of water and tritium under transient saturated conditions, however, did not significantly alter average tritium uptake predictions for the catchment.