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Scientific Investigations Report 2009-5021

Toxic Substances Hydrology Program and National Water-Quality Assessment Program

Prepared in cooperation with the National Park Service

Comparison of Methylmercury Production and Accumulation in Sediments of the Congaree and Edisto River Basins, South Carolina, 2004-06

By Paul M. Bradley, Francis H. Chapelle, and Celeste A. Journey


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Fish-tissue mercury concentrations (approximately 2 micrograms per gram) in the Edisto River basin of South Carolina are among the highest recorded in the United States. Substantially lower mercury concentrations (approximately 0.2 microgram per gram) are reported in fish from the adjacent (about 30 kilometer) Congaree River basin and the Congaree National Park. In contrast, concentrations of total mercury were statistically higher in sediments from the Congaree River compared with those in sediments from the Edisto River. Furthermore, no statistically significant difference was observed in concentrations of methylmercury or net methylation potential in sediments collected from various Edisto and Congaree hydrologic settings. In both systems, the net methylation potential was low (0-0.17 nanogram per gram per day) for in-stream sediments exposed to continuously flowing water but substantially higher (about 1.8 nanograms per gram per day) in wetland sediments exposed to standing water. These results are not consistent with the hypothesis that differences in fish-tissue mercury between the Edisto and Congaree basins reflect fundamental differences in the potential for each system to methylate mercury. Rather, the significantly higher ratios of methylmercury to total mercury observed in the Edisto system suggest that the net accumulation and(or) preservation of methylmercury are greater in the Edisto system. The marked differences in net methylation potential observed between the wetland and in-stream settings suggest the hypothesis that methylmercury transport from zones of production (wetlands) to points of entry into the food chain (channels) may contribute to the observed differences in fish-tissue mercury concentrations between the two river systems.

First posted March 10, 2009

For additional information contact:
USGS South Carolina Water Science Center
Stephenson Center, Suite 129
720 Gracern Road
Columbia, SC 29210-7651

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Suggested citation:

Bradley, P.M., Chapelle, F.H., and Journey, C.A., 2009, Comparison of methylmercury production and accumulation in sediments of the Congaree and Edisto River basins, South Carolina, 2004-06: U.S. Geological Survey Scientific Investigations Report 2009-5021, 9 p. [Available only online at]




Methods and Materials

Hydrologic Settings

Total Mercury and Methylmercury Concentrations in Sediments

Sedimentary Organic Matter and Redox Conditions

Mercury Methylation and Demethylation Potential

Statistical Tests

Comparison of Methylmercury Characteristics in Edisto and Congaree River Basin Sediments



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