Wetlands as Sources of Total and Methyl Hg to Coastal Floodplain/Estuarine Systems in South Carolina

Concentrations of total Hg in water were determined from sites along a geographical gradient within South Carolina. Sites with differing watershed characteristics were chosen to identify potential factors governing the spatial variability of Hg levels throughout the state Overall, there is a spatial west to east gradient in the state, with water column concentrations of total Hg (10-55 pM) and total organic carbon (10-2500 uM) increasing as one moves from the western upstate piedmont region to the eastern coastal floodplain region (r2 = 0.78; p<0.001). The percentage of wetland area in these watersheds increases from <1% in the upstate piedmont region to 14-40% in the organic rich coastal flood plain systems. Correspondingly, 89% of the SC fish consumption advisories are located within these coastal flood plain regions. There is a significant correlation between increasing fish Hg concentrations and increasing percent wetland area across the state (r2 = 0.62; p<0.001). A time series study of mercury speciation within a coastal flood plain river (25% wetland area) indicates that 70-90% of the total and methyl Hg are found in the <0.45 micron size class, and of this size class, 75% of the total Hg is <0.2 micron. The dissolved methyl Hg concentrations (1-7 pM) range from 3-26% of the total Hg in the river and from 1-2 % in the seawater end member. Watershed transport efficiencies for these coastal floodplain systems, during moderate to high flow discharge rates (200-8000 m3/sec), range from 32-100% for total Hg and 78-477% for methyl Hg. This suggests that watersheds containing significant percentages of wetlands within floodplain regions can be sources of total and methyl Hg to coastal estuarine systems.