Tim A. Wool

Modeling Mercury Fluxes and Concentrations in a Georgia Watershed Receiving Atmospheric Deposition Load from Direct and Indirect Sources

Abstract This paper presents a modeling analysis of airborne mercury (Hg) deposited on the Ochlockonee River watershed located in Georgia. Atmospheric deposition monitoring and source attribution data were used along with simulation models to calculate Hg buildup in the subwatershed soils, its subsequent runoff loading and delivery through the tributaries, and its ultimate fate in the mainstem river. The terrestrial model calculated annual watershed yields for total Hg ranging from 0.7 to 1.1 μg/m2. Results suggest that approximately two-thirds of the atmospherically deposited Hg to the watershed is returned to the atmosphere, 10% is delivered to the river, and the rest is retained in the watershed. A check of the aquatic model results against survey data showed a reasonable agreement. Comparing observed and simulated total and methylmercury concentrations gave root mean square error values of 0.26 and 0.10 ng/L, respectively, in the water column, and 5.9 and 1 ng/g, respectively, in the upper sediment layer. Sensitivity analysis results imply that mercury in the Ochlockonee River is dominated by watershed runoff inputs and not by direct atmospheric deposition, and that methylmercury concentrations in the river are determined mainly by net methylation rates in the watershed, presumably in wetted soils and in the wetlands feeding the river.

Characterizing Spatial and Temporal Dynamics: Development of a Grid-Based Watershed Mercury Loading Model

A distributed grid-based watershed mercury loading model has been developed to characterize the spatial and temporal dynamics of mercury from both point and nonpoint sources. The model simulates flow, sediment transport, and mercury dynamics on a daily time step across a diverse landscape. The model is composed of six major components: (1) an ArcGIS interface for processing spatial input data; (2) a basic hydrologic module; (3) a sediment transport module; (4) a mercury transport and transformation module; (5) a spreadsheet-based model post-processor; and (6) links to other models such as WASP and WhAEM 2000 developed by the U.S. Environmental Protection Agency (EPA). The model fully uses the grid processing capacity of the latest ArcGIS technology. The water balance, sediment generation and transport, and mercury dynamics are calculated for every grid within a watershed. Water and pollutants are routed daily throughout the watershed based on a unique and flexible algorithm that characterizes a watershed into many runoff travel-time zones. The mercury transport and transformation module simulates the following key processes: (1) mercury input from atmospheric deposition; (2) mercury assimilation and accumulation in forest canopy and release from forest litter; (3) mercury input from bedrock weathering; (4) mercury transformation in soils; (5) mercury transformation in lakes and wetlands, including reduction and net methylation; (6) mercury transport through sediment and runoff; and (7) mercury transport in stream channels. By using the grid-based technology, flow, sediment, and mercury dynamics can be examined at any of several points in the watershed. The model is capable of supporting large-scale watershed modeling with high-resolution raster data sets and will be used in mercury research projects sponsored by EPA. The model is programmed in Visual Basic and requires two ArcGIS (version 9.0) components—ArcView 9 and the Spatial Analyst extension.