Trace Elements in Sediments of an Aging Reservoir in Rural Mississippi: Potential for Mobilization Following Dredging

Since 1948, the United States Department of Agriculture has assisted construction of over 10,000 flood control dams, many of which are reaching the end of their design life. Rehabilitation by dredging, or decommissioning by dam removal both have the potential to mobilize contaminants currently sequestered in the sediments. Plans to rehabilitate a small aging reservoir in a rural area of northern Mississippi led to a study characterizing the chemical composition of the reservoir sediments, and the potential for mobilizing contaminants if aerated. Sediment analyses included bulk and trace elements, and a suite of agrichemicals. Trace element concentrations are highly correlated with clay and Fe concentrations, with no elevated levels indicative of anthropogenic inputs. Batch leaching studies under reducing conditions simulating in situ conditions, and under mildly oxidizing conditions simulating disturbed sediments indicate that aeration should lead to a decrease in the aqueous concentration of trace metals currently found in sediment pore water. Decreased aqueous concentrations under oxidizing conditions are probably caused by precipitation of iron as amorphous oxides that scavenge released metals. Based on these results, dredging is a viable rehabilitation strategy for reservoir rehabilitation.