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MODELING TOOLS USED FOR MERCURY TMDLS IN GEORGIA RIVERS
Ambrose Jr., R B. AND T. A. Wool. MODELING TOOLS USED FOR MERCURY TMDLS IN GEORGIA RIVERS. Presented at Georgia Water Resources Conference, Athens, GA, March 26-27, 2001.
Develop, test, and refine models to evaluate sub-basins to determine whether local water quality problems due to excessive nutrient loading exist, and if so, to characterize them and determine their relationships to nutrient loading. Develop models to simulate overland flow and non-point source pollutant loads to track and assess nutrient loadings across watersheds and provide approaches for estimating nutrient budgets within sub-basins and for predicting changes in nutrient budgets in response to changes in watershed activities/land use/land cover. Demonstrate the application of the recommended approach/models for predicting changes in nutrient budgets in response to changes in proposed watershed activities/land use/land cover, resulting in specific recommendations for reducing the nutrient loads to a basin. For coherence, cooperation, and economics, these models will be housed in a unified, consistent, computational environment for environmental analyses that allows teaching (i.e., technology transfer) to multiple users (users concentrate on problem, not model input/output); that appeals to multi-disciplinary groups for distribution and use as a consistent assessment methodology (includes models, tools, modular design and facilitated updates of science/engineering); that includes resident visualization, animation tools, documentation and tutorials on-line, hooks to GIS and environmental databases; and is executable on UNIX, personal computers, and HPC resources.
Objective # 2.2 Conserve and enhance nation's waters: By 2005, conserve and enhance the ecological health of the nation's (state, interstate, and tribal) waters and aquatic ecosystems-rivers and streams, lakes, wetlands, estuaries, coastal areas, oceans, and groundwater-so that 75% of waters will support healthy aquatic communities.
The Clean Water Act and associated regulations require each State to identify waters not meeting water quality standards applicable to their designated uses. Total maximum daily loads (TMDLs) are required for pollutants violating these standards. The Consent Decree in the Georgia TMDL lawsuit required TMDLs for mercury in six south Georgia rivers by August 2000. This paper summarizes the scientific models and associated procedures used to develop these mercury TMDLs, with example calculations from the Upper Ochlockonee River.
Three separate software tools were used to produce the mercury TMDLs ? the Watershed Characterization System (WCS), a mercury delivery spreadsheet, and the water pollutant fate model Wasp5. The WCS takes wet and dry atmospheric deposition and calculates mercury concentrations in soil as well as runoff and erosion fluxes to the stream system. Processes simulated include reduction and volatilization from the soil, runoff, and erosion of solids and associated mercury. The mercury delivery spreadsheet calculates the fraction of mercury from the landscape that is lost in the watershed's tributary system due to reduction and volatilization. Speciation of the watershed loadings between divalent and methyl mercury is based on site-specific data. The Wasp5 model takes the speciated loadings delivered from the watershed and from point sources, and calculates total and methyl mercury concentrations in the water column and sediments of the river channel. Processes simulated include advection, sediment exchange, reduction, volatilization, methylation, and demethylation in the water column, and methylation and demethylation in the sediments.
The WCS model gives loadings by several categories. Total watershed loadings and delivery ratios using average tributary flows were used in Wasp5 to calculate expected average mercury concentrations. These concentrations were used as initial conditions in a short (6-month) simulation of recent mercury dynamics in the recent drought conditions. Direct dry deposition loadings to water surfaces were used with drought tributary flows to give estimated loadings delivered to the Ochlokonee River during summer 2000. Wasp5 calculations of mercury concentrations under these conditions compared reasonably well with observed data from surveys in July 2000. This offers some degree of confidence in the TMDL relating atmospheric nonpoint source and point source loadings to river mercury concentrations under average conditions.