Spatial and temporal variation in the isotopic composition of mercury in the South River, VA

Historic point source mercury (Hg) inputs from industrial processes on the South River (Waynesboro, Virginia) ended many decades ago, but sediment and surface water Hg concentrations remain elevated relative to the regional background. To understand Hg sources, mobility, and fate in the South River, we analyzed total Hg concentrations and Hg stable isotope compositions of streambed sediments, bank soils, suspended particles, filtered surface waters, and channel margin hyporheic zone pore waters. Hg isotopes allow for the identification of three distinct Hg end-member inputs to the South River, consisting of a regional background source and two isotopically distinct contaminant sources. Hydrologic conditions are demonstrated to have an influence on within-channel Hg isotope fractionation and Hg partitioning, with no observed isotopic discrimination between suspended particulate Hg and filtered surface water Hg during elevated flow conditions. Channel margin hyporheic zone porewaters had significantly higher δ202Hg values than surface waters (δ202Hg = −0.52 ± 0.44‰ and δ202Hg = −0.89 ± 0.20‰ respectively [mean ± 1SD]). A subset of porewaters exhibited mass independent fractionation signatures (Δ199Hg = 0.33 ± 0.06‰; Δ200Hg = 0.19 ± 0.03‰ [mean ± 1SD]) that are suggestive of a precipitation-derived origin for the dissolved Hg pool. Sediments from a floodplain profile were analyzed to explore the temporal variation in Hg isotopic composition within the South River, indicating brief excursions (up to δ202Hg = +0.61‰) from the average composition observed in modern samples (δ202Hg = −0.52 ± 0.09‰). By improving understanding of the spatial, temporal, and hydrologic conditions that contribute to variations in Hg isotopic composition, this study provides insights into the processes that control Hg isotopic end-member sources, Hg-loading to the channel during elevated flows, and Hg fate in the South River.