CONCEPTUAL MODELS OF PARTICULATE AND MERCURY TRANSPORT AND STORAGE FOR SOUTH RIVER, VA

An annual sediment budget was constructed in a 21 km reach of South River, a steep gravel-bedded stream in western Virginia, which has a history of mercury (Hg) contamination. Important components of the sediment budget for South River are suspended sediment transport into and out of the reach, bank erosion, deposition on natural levees, and storage of fine-grained sediment trapped by large woody debris (LWD) in the channel margins (FGCM deposits). Average Hg concentrations for these processes are as follows: suspended sediment into the reach, 0.1 ppm; eroding banks, 10 ppm; natural levees, 13 ppm; and FGCM deposits, 30 ppm. We used the average concentrations and the volumes calculated from the annual sediment budget to predict an annual averaged concentration of Hg on suspended matter in the water column of our 21 km study reach. We also assume that suspended sediment Hg concentrations should be equal to average sediment concentration of suspended sediment stored in the wetted perimeter of the channel. This analysis predicts Hg concentrations on the order of 1 ppm, while concentrations of particle associated Hg in suspension and in storage are typically an order of magnitude higher. This suggests that the discharges which transport the annual sediment load are not the ones redistributing Hg-contaminated sediments in the study reach. Hg analyses during storm and base flows support this interpretation: particulate Hg concentrations are typically about 10 ppm during low flows, but concentrations during peak storm flows are reduced by an order of magnitude because of dilution from “clean” sediment supplied from upstream. During lower, relatively frequent discharges, Hg contaminated sediments are locally supplied from bank erosion and other processes and are stored in the river channel. Thus, suspended sediments in storage along the South River are deposited during relatively frequent low discharges, while the bulk of the suspended sediment transported by high discharges is transported through the study area without significant storage.