SIMULATING MULTI-SCALE FATE AND TRANSPORT OF MERCURY FROM ATMOSPHERIC DEPOSITION TO RECEIVING SURFACE WATERS ACROSS AND THROUGH THE TERRESTRIAL LANDSCAPE IN A COASTAL PLAIN WATERSHED (MCTIER CREEK, SC)

Mercury (Hg) is the toxicant responsible for the largest number of fish advisories across the United States, with 1.1 million river miles under advisory. The processes governing fate and transport of Hg in streams and rivers are not well understood, in large part, because these systems are intimately linked with their surrounding watersheds and are often highly spatially variable. In this study, we apply a linked watershed hydrology and biogeochemical cycling (N, C, and Hg) model (VELMA, Visualizing Ecosystems for Land Management Assessment) to simulate daily flow, mass fluxes, and soil and stream concentrations of total mercury (THg) and methylmercury (MeHg) at multiple spatial scales in McTier Creek, a Coastal Plain watershed within the Edisto River basin of South Carolina, USA. VELMA is a spatially discrete, four soil layer subsurface model simulating the movement of water and the mercury cycle at a daily time step. Our goals were to (1) calibrate and simulate Hg fate and transport processes at a focused reach scale (0.1 km²) from hilltop to hillslope to riparian zone to stream and (2) assess how representative the reach-scale parameterization and processes are when multi-scale watershed information is included in Hg cycling simulations. Thus, reach-scale parameterization was applied to multi-scaled watersheds, including two headwater sub-watersheds (28 km² and 25 km²) nested within the McTier Creek watershed (79 km²), to evaluate model performance and how well reach-scale parameterization and processes characterize nested watersheds with increasing drainage areas. The current VELMA simulations suggest that water column stream THg concentration predictions perform reasonably well at different scales based on reach-scale calibrations, but the model simulations of MeHg stream concentrations at reach, sub-watershed, and watershed pour points are out-of-phase with observed MeHg concentrations. This result suggests that processes governing MeHg loading to the main channel may be under-represented in the current model structure and underscores the complexity of simulating MeHg dynamics in watershed models. This work supports the importance of hydrology in understanding transport and transformation of Hg in watersheds and streams, as well as the influence of out-of-channel versus in-channel processes.