Estimating Vadose Zone Drainage From a Capped Seepage Basin, F Area, Savannah River Site

Large volumes of waste solutions were commonly discharged into unlined seepage basins at many different facilities in the past. Plutonium was extracted from depleted uranium from 1955 to 1988 at the F-Area within the Savannah River Site, with contaminated process waters disposed of in permeable seepage basins. The primarily acidic solutions contained radioactive components (including tritium, 129I, and multiple isotopes of U, Pu, Sr, and Cs), elevated nitrate, and some metals (Hg, Pb, Cd). Basin 3 was the largest F-Area seepage basin, covering 2.0 hectare, with the water table typically at about 20 m below the soil surface. The local groundwater flows at an average velocity of 200 m/y in the approximately 10 m thick shallow aquifer, and is underlain by the low permeability Tan Clay. We used nearly 20 years of groundwater quality data from a monitoring well immediately downstream of Basin 3 to estimate the post-closure drainage of waste solutions through its underlying vadose zone, into the shallow aquifer. The measurements of tritium, nitrate, and specific conductance, were used as plume tracers in our estimates of vadose zone drainage. These calculations indicate that early stages of post-closure waste drainage occurred with high fluxes (≈ 1 m/y), and quickly declined. However, even after 20 years, drainage continues at a low but significant rate of several cm/y. These estimated drainage fluxes can help constrain predictions on the waste plume behavior, especially with respect to its emerging trailing gradient and anticipated time scales suitable for monitored natural attenuation.