MODELING A CENTURY OF MERCURY POLLUTION IN THE SOUTH RIVER, VIRGINIA: A GEOLOGICAL APPROACH

Effective remediation strategies for restoring contaminated rivers must acknowledge ongoing natural processes of contaminant transport, storage, and remobilization, requiring quantitative models that integrate fluvial processes operating over timescales from days to millennia. We present a method for modeling long-term mercury (Hg) transport and storage within South River, VA, a gravel bed river with a history of contamination caused by an industrial release from 1929-1950. Our model has useful advantages over other available methods. Empirical or statistical models require long-term monitoring data that are rarely available. Process-based models provide useful understanding of how contaminants are cycled, but many transport processes are poorly understood, resulting in models with parameters that must be calibrated, also requiring monitoring data. We calibrate our model by age dating the contaminated sediment in storage. Because these data provide a record of historical processes, monitoring data are unnecessary. We use a distribution of sediment ages, a function specifying the concentration of Hg on suspended particles through time, and the mass of sediment stored to generate a model that predicts the accumulation of mercury mass in the FGCM deposits from the water column through time from the release period to the present. We use the model to document almost 80 years of mercury pollution in South River, a period which monitoring data are unavailable. The model predicts 1) a peak concentration of mercury on sediment of 1200 ppm during the release period which decays to currently-observed values of 10 ppm, and 2) that about 1300 kg of mercury was deposited within the active margins of the 20-km reach during the height of the industrial release. Once the release from the plant ceased, mercury stored in the channel was very gradually reworked and concentrations were greatly reduced from those of the original release period. The slow rate of removal since the 1970s provides at least one hypothesis to explain the lack of improvement in fish tissue mercury concentrations. We evaluate a remediation scenario which suggests that it will take several decades for mercury to be removed by natural processes. These results would be difficult to obtain with process-based models, because the processes are complex and poorly understood.