Bioavailability of uranium and nickel to vegetation in a contaminated riparian ecosystem

The lower portion of Tims Branch (TB), a second-order stream system on the Savannah River site (SC, USA), receives influx of mixed waste-contaminated sediments from Steed Pond, a former settling basin for target processing wastes for over three decades. The magnitude and distribution of U, Ni, and other metals and the potential for trophic movement were studied to facilitate risk assessment and determine potential remedial action. Total and sequential extraction of TB soils demonstrated contaminant heterogeneity both spatially and between operationally defined fractions. Metal concentrations were elevated within riparian zone soils in contrast to stream sediments, suggesting off-site transport. Leaf tissue from TB contained an order of magnitude more Ni than tissue from reference sites. Leaves from streamside trees contained no U but elevated Ni up to 75.4 (+/-25) mg/kg dry weight (dry wt). Understory flora (Discanthelium sp. and Andropogon sp.) contained high concentrations of U associated with leaves up to 518 (+/-7.5) mg/kg dry weight U. The contrast in contaminant content and ratio of streamside and understory vegetation may result from resuspension of particulate U and Ni onto leaf surfaces and represents a potential pathway for trophic movement. The findings of this study have important ramifications for remediation of the ecosystem, suggesting that a strategy based on contaminant immobilization may be the most appropriate.