Waste disposal into seepage basins has generated groundwater contaminant plumes at many locations. At the F Area within the Savannah River Site, Pu was extracted from depleted U from 1955 to 1988, with wastewater discharged into seepage basins. Basin 3 was the largest F-Area seepage basin, receiving acidic wastewater containing radionuclides (including 3H, 129I, and multiple isotopes of U, Pu, Sr, and Cs), elevated NO3, and some metals. Contaminants transported into the groundwater migrate toward Fourmile Branch, a tributary to the Savannah River. We developed a two-compartment model and used 20 yr of groundwater quality data to estimate the post-closure drainage of waste solutions through its vadose zone into the aquifer. Tritium, NO3−, and specific conductance were used as tracers in the model to estimate drainage rates. Our calculations indicate that early stages of post-closure waste drainage occurred with high water fluxes (~0.5 m yr−1) and quickly declined. Even 20 yr after basin closure, however, drainage continues at several centimeters per year. While the magnitude of this late-stage drainage rate is low, its impact is large because of the high concentrations of contaminants it continues to supply to the groundwater. These estimated drainage fluxes constrain predictions on the waste plume behavior, especially with respect to its trailing gradient and time scales suitable for monitored natural attenuation. Our methodology requires only groundwater monitoring data and a small number of well-constrained input quantities. This approach can be useful for understanding contaminant dissipation at other locations as well, especially where the hydrogeological setting is relatively simple.