Uranium partitioning from contaminated wetland soil to aqueous and suspended iron-floc phases: Implications of dynamic hydrologic conditions on contaminant release

Environmental mobility of anthropogenic heavy metal releases is governed by site-specific hydrobiogeochemical factors. Uranium (U) releases from 1965 to 1988 resulted in contamination of the Tims Branch wetland at the United States Department of Energy Savannah River Site (SRS) near Aiken, South Carolina. Today, 80% of the 43,000 kg released remain sequestered in soils of two former ponds, Beaver Pond and Steed Pond. The objective of this study was to understand U partitioning from contaminated wetland sediments to aqueous and particulate iron-floc phases. The approach was to correlate U, iron (Fe), and organic matter (OM) in soil cores at key depositional areas; determine U and Fe concentrations in aqueous- and solid-phases in the stream; and compare these stream water data following two storm events. Uranium was released into an outfall canal leading away from the SRS M-Area and first deposited in a topographically low area where a beaver pond is frequently in place. Uranium soil concentrations in Beaver Pond exceed 14,000 mu g/g and a second deposition area 1 km downgradient in a former farm pond (Steed Pond) has U soil concentrations near 300 mu g/g. Uranium releases to the stream are minimal at Beaver Pond, but U incorporation into Fe-OM colloids, or flocs, at Steed Pond results in appreciable fluxes of U from the wetland. This work yields four key conclusions. At the kilometer and meter scales, geochemistry cannot be the sole predictor of U mobility, as the anthropogenic U deposition is geologically young relative to Fe and OM. Storm intensity and duration do not appear to be directly proportional to U releases from the wetland, as flooding decreases U inclusion in mobile flocs. The mass of suspended solids present in a stream reach often governs how much U moves. The solid-bound U inventory at Beaver Pond does not enter the mobile aqueous phase as easily as Steed Pond U, resulting in U sequestration amid wetland metal cycling in Tims Branch. This work expands upon studies conducted in the 1990's and early 2000's, improving our understanding of the geochemical controls of uranium migration through Tims Branch, particularly with respect to the changes over the last 30+ years. Current wetland conditions do not favor significant U movement, but long-term changes in local and regional hydrology may alter this delicate biogeochemical balance. (C) 2021 Published by Elsevier Ltd.