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Environmental Geosciences, V.
Reconstructing suspended sediment mercury contamination of a steep, gravel-bed river using reservoir theory
Katherine J. Skalak has Ph.D. from the University of Delaware in Geological Sciences. She is a research hydrologist at the USGS National Research Program in Reston, VA. Her research focuses on understanding and predicting geomorphic change in response to anthropogenic disturbances such as transport and storage of fine sediment and particle-associated nutrients and contaminants and geomorphic response of rivers impacted by dams.
James Pizzuto has Ph.D. from the University of Minnesota in Geology. He is a Professor at the University of Delaware in the Department of Geological Sciences and is presently the Acting Chair. His research has spanned a variety of topics in fluvial geomorphology including riparian vegetation controls on channel width, climate and land use impacts on river adjustment, floodplain dynamics, downstream impacts of dams, and fine sediment routing and implications for fate and transport of contaminants and nutrients.
Financial support was provided by the DuPont Company. Many useful ideas originated from discussions with The South River Science Team. J. K. Bohlke, Nancy Grosso, and anonymous referees provided thoughtful comments during the review. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
We use sediment ages and mercury (Hg) concentrations to estimate past and future concentrations in the South River, Virginia, where Hg was released between 1930 and 1950 from a manufacturing process related to nylon production. In a previous study, along a 40 km (25 mi) reach, samples were collected from 26 of 54 fine-grained deposits that formed in the lee of large wood obstructions in the channel and analyzed for grain size, Hg concentration, and organic content. We also obtained radiometric dates from six deposits. To create a history that reflects the full concentration distribution (which contains concentrations as high as 900 mg/kg [900 ppm]), here, we treat the deposits as a single reservoir exchanging contaminated sediments with the overlying water column, and assume that the total sediment mass in storage and the distribution of sediment ages are time invariant. We use reservoir theory to reconstruct the annual history of Hg concentration on suspended sediment using data from our previous study and new results presented here. Many different reconstructed histories fit our data. To constrain results, we use information from a well-preserved core (and our estimate of the total mass of Hg stored in 2007) to specify the years associated with the peak concentration of 900 mg/kg. Our results indicate that around 850 kg (1874 lb) of Hg was stored in the deposits between 1955 and 1961, compared to only 80 kg (176 lb) today. Simulations of future Hg remediation suggest that 100-yr timescales will be needed for the South River to remove Hg-contaminated sediments from the channel perimeter through natural processes.
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