METHYLMERCURY LEVELS IN THE EDISTO RIVER BASIN OF THE COASTAL PLAIN PHYSIOGRAPHIC PROVINCE, SOUTH CAROLINA

Accumulation of mercury in fish and wildlife is a widespread problem that is being evaluated on global, national, regional, and local scales. Methylated forms of mercury serve as potent neurotoxins that affect the health of aquatic biota. In some areas of South Carolina (SC), this problem also has produced human health issues. To protect the public, the South Carolina Department of Health and Environmental Control (SCDHEC) issued fish-consumption advisories on 36 rivers, creeks and streams and 17 lakes. In 1998, the U.S. Geological Survey (USGS) national assessment of mercury speciation in sediment, water, and fish was conducted at 22 participating river basins throughout the United States. The Edisto River Basin in SC had the highest methylation efficiency and resulted in one of the highest bioaccumulation rates in fish tissue of the participating units. The high methylation efficiency reported in the Edisto indicated an imbalance in the methylation and demethylation processes.

To address the mercury issue in SC, an investigation by the USGS has targeted two objectives: (1) describe the total and methylmercury cycle (atmosphere, sediment, water, biota) in a mercury-sensitive stream ecosystem; and (2) evaluate the bioavailability of the methylmercury. This presentation will provide the preliminary findings of the investigation. In 2006, thirteen water samples were collected and analyzed for dissolved total and methylmercury and particulate total and methylmercury at the parts-per-trillion level (nanograms per liter) in the Edisto. Ultra-clean sampling techniques were used to prevent contamination. Streamflow, dissolved organic carbon (DOC), suspended sediment, ultraviolet absorbance at 254 nanometers (UVA), pH, and sulfate also were measured. Atmospheric deposition of mercury also was monitored during that sampling period. Relation between these parameters and mercury were evaluated. During the period of sampling, UVA was observed to be a strong surrogate for DOC concentration in the Edisto River Basin. Both UVA and DOC concentration were related significantly to the dissolved total mercury concentration in the water column. Fish tissue samples also were collected in 1998 and 2005. Levels of mercury in fish exceeded the SCDHEC consumption advisory levels.