APPLICATION OF A SEDIMENT-BUDGET APPROACH TO EVALUATE SOURCES AND SINKS OF HG-CONTAMINATED SEDIMENT IN A GRAVEL-BED RIVER, S. RIVER, VIRGINIA

The purpose of this study is to construct an annual sediment budget for Hg-contaminated silt and clay for South River, a steep gravel-bedded stream in western Virginia. The significant sediment-related processes in the study area are suspended sediment transport, bank erosion, levee deposition, and storage of fine-grained sediment trapped by large woody debris (LWD) in the channel margins (“channel margin storage”). Suspended sediment fluxes for the study reach were determined using a dimensionless regional sediment-rating curve. Bank erosion rates were measured using aerial photographs from 1937 and 2005. Floodplain deposition rates were estimated using dendrochronology. The unique contribution to the sediment budget is channel margin storage. Field observations indicate that silt and clay is stored in the near-bank regions of the channel. The volume of storage in these deposits is equivalent to less than 5% of the river's annual suspended sediment load and annual deposition in these deposits is less than 1% of the total annual load. Sediment storage in the near-bank regions is a result of reduced velocity caused by the bank obstructions. Storage occurs in four different geomorphic settings: 1) long pooled sections caused by bedrock or old mill dams, 2) the upstream areas of pools in pool/riffle reaches with abundant LWD, 3) bank obstructions usually caused by trees, 4) side channel backwaters where flow separates around islands. These deposits have been cored and analyzed for Hg, grain size, loss-on-ignition, and they have been dated using post 1950 “bomb radiocarbon”. The sediment budget, when combined with Hg concentration data, suggests that Hg is produced from bank erosion, stored in levees and channel margin deposits, and exported from the system downstream.