Identifying Likely Sources of Mercury and Methylmercury to the South River, Virginia, USA

There exists a strong spatial concentration gradient in mercury and methylmercury concentrations in various environmental compartments in the South River, VA. The sources of mercury vary with flow regime, mercury form and storage compartment (e.g., sediment, soil, water, biota). During high flow rates, mercury concentrations on suspended solids resemble both sediment and soil concentrations, suggesting eroding soils as a key source contributing to sediment and suspended mercury and methylmercury concentrations. During low flow rates, methylmercury concentrations on suspended solids are generally similar to periphyton methylmercury concentrations when normalized to organic carbon. This has implications with respect to the bioavailability of methylmercury for bioaccumulation, because sediment-attached periphyton and/or detritus serve as the basis of riverine foodchains. Methylmercury production varies by season with peak concentrations in the Spring and lowest concentrations in the late Fall and Winter. Mercury methylation occurs throughout the South River with no apparent “hot spots” of methylation activity. Controls on mercury methylation are not entirely clear. Detailed incremental and integrated loading analyses under different flow conditions suggest that the majority of mercury and methylmercury loadings occur within a 10 mile section downstream of the historic source of mercury to the South River. Loading analyses and sediment flux calculations also suggest that porewater mercury is only a small component to the overall mercury loading in the South River.