USING GEOMORPHIC MAPPING AND GIS TO LOCATE MERCURY CONTAMINATED FLOODPLAIN DEPOSITS, SOUTH RIVER, VIRGINIA

Mercury was introduced into the South River from the 1930s to the early 1950s from an industrial plant in Waynesboro, Virginia. Locating contaminated floodplain deposits dating from this period has been an important management goal. Mercury concentrations in fish tissue continue to exceed acceptable levels, and contaminated sediments in the river's floodplains are probably the present source of mercury to the South River ecosystem. The South River is a sinuous, single thread alluvial river with frequent bedrock exposures along its bed and banks. Overbank deposits are discontinuous and thin. Using studies of historical aerial photographs in a GIS framework, field mapping, dendro- and radionuclide dating, and Hg analyses, we have identified distinctive floodplain deposits along South River dating from 1937- 2005. Rates of lateral migration by the South River are extremely low, averaging 0.02 m/yr, and the river has been influenced by 13 historic mill dams, now all breached, along our 37 km study reach. Not surprisingly, traditional depositional models of meandering rivers do not apply. Floodplain depositional units include small point bars, concave bank benches, mill dam deposits, bench deposits in straight reaches, deposits created by cattle, and deposits related to tributary confluences. The most important deposits for sequestering historic mercury are those that also store the most silt and clay. These include mill dam deposits, concave bank benches, and point bars.