Characterizing mercury concentrations and fluxes in a Coastal Plain watershed: Insights from dynamic modeling and data

Mercury (Hg) is one of the leading water quality concerns in surface waters of the United States. Although watershed-scale Hg cycling research has increased in the past two decades, advances in modeling watershed Hg processes in diverse physiographic regions, spatial scales, and land cover types are needed. The goal of this study was to assess Hg cycling in a Coastal Plain system using concentrations and fluxes estimated by multiple watershed-scale models with distinct mathematical frameworks reflecting different system dynamics. We simulated total mercury (Hg_T, the sum of filtered and particulate forms) concentrations and fluxes from a Coastal Plain watershed (McTier Creek) using three watershed Hg models and an empirical load model. Model output was compared with observed in-stream Hg_T. We found that shallow subsurface flow is a potentially important transport mechanism of particulate Hg_T during periods when connectivity between the uplands and surface waters is maximized. Other processes (e.g., stream bank erosion, sediment re-suspension) may increase particulate Hg_T in the water column. Simulations and data suggest that variable source area (VSA) flow and lack of rainfall interactions with surface soil horizons result in increased dissolved Hg_T concentrations unrelated to DOC mobilization following precipitation events. Although flushing of DOC-Hg_T complexes from surface soils can also occur during this period, DOC-complexed Hg_T becomes more important during base flow conditions. TOPLOAD simulations highlight saturated subsurface flow as a primary driver of daily Hg_T loadings, but shallow subsurface flow is important for Hg_T loads during high-flow events. Results suggest limited seasonal trends in Hg_T dynamics.