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Modeling Watershed Mercury Response to Atmospheric Loadings: Response Time and Challenges
Citation:
GOLDEN, H. E. AND C. D. KNIGHTES. Modeling Watershed Mercury Response to Atmospheric Loadings: Response Time and Challenges. Presented at American Geophysical Union Fall Meeting, San Francisco, CA, December 15 - 19, 2008.
Impact/Purpose:
We present preliminary results suggesting that after more than 150 years, watershed response to atmospheric loading does not reach a steady-state condition, which is potentially attributed to lack of extreme overland runoff events during the simulation period and increased storage due to low soil mercury reduction rates.
Description:
The relationship between sources of mercury to watersheds and its fate in surface waters is invariably complex. Large scale monitoring studies, such as the METAALICUS project, have advanced current understanding of the links between atmospheric deposition of mercury and accumulation of methyl mercury in fish tissue. However, effective watershed-scale models simulating the effects of changes in mercury deposition on surface soil mercury concentrations and watershed mercury loadings are currently lacking. As a result, numerous default assumptions - such as steady state relationships between atmospheric loading of total mercury and methyl mercury concentrations in surface waters - are required for management purposes (e.g, total maximum daily loads (TMDLs)). We use a spatially distributed watershed fate and transport model to simulate historic and future patterns of watershed soil mercury concentrations and associated watershed loadings in two watersheds within the Cape Fear River Basin, North Carolina. Simulations were initiated using background soil concentrations and constant atmospheric mercury deposition rates. We also simulate watershed mercury response (soil concentrations and loadings) during a period of locally-increased emissions from a mercury cell chlor-alkali facility (in operation from 1963-1999 using the mercury cell chlorine production process) and estimate the length of time for watershed mercury loading to return to baseline conditions after plant closure. All model simulations are performed using a recently developed spatially distributed grid-based watershed mercury (Hg) model (GBMM v2.0, Tetra Tech, 2006) that computes daily mass balances for hydrology, sediment, and mercury within each GIS grid cell and produces daily flux estimates of each to a tributary network. We present preliminary results suggesting that after more than 150 years, watershed response to atmospheric loading does not reach a steady-state condition, which is potentially attributed to lack of extreme overland runoff events during the simulation period and increased storage due to low soil mercury reduction rates. We discuss the implications of this for managing mercury in aquatic ecosystems. Further, we explore the time lag between decreasing emissions near the chlor-alkali facility and return of soil mercury concentrations and watershed mercury loadings to baseline conditions.