TOTAL AND METHYL MERCURY IN SURFACE WATER AND SEDIMENTS FROM DIFFERING ECOREGIONS OF SOUTH CAROLINA

The discovery of elevated levels of Hg (>0.25 ppm) in fish from water bodies in South Carolina (SC) has resulted in the issuance of many fish consumption advisories throughout the state. The highest fish tissue concentrations have been found in unregulated blackwater rivers in the Middle Atlantic Coastal Plain ecoregion, with lower fish Hg concentrations found in the Southern Coastal Plain, and Piedmont ecoregions. The SC clean metals project was initiated in 2010 and included 4 quarterly sampling events. The primary objective of the study was to determine levels of total and methyl Hg in water and sediments from sites along a geographical gradient (west-east) within SC. Sites within differing ecoregions were chosen to identify potential factors governing the spatial variability of Hg levels in water and sediment throughout the state and how they may relate to the concentration of Hg in fish tissue. Twenty one sites were sampled in November 2010, February 2010, May 2011, and August 2011. Seven sites were located in the Middle Atlantic Coastal Plain (MACP), 4 sites were located in the Southeastern Plain (SEP), 3 sites were located in the Southern Coastal Plain (SCP), 5 sites were located in the Piedmont (P), and one site was located in the Blue Ridge (BR) ecoregion. Total Hg in the water samples ranged from below the analytical detection limit (0.1 ng/L) to 7.2 ng/L. Methyl Hg in the water samples ranged from below the analytical detection limit (0.0188 ng/L) to 2.03 ng/L. The percentages of Hg as methyl Hg in the water ranged from 0-27.6%. The MACP and SCP ecoregions contained the highest concentrations of total and methyl Hg and the highest percentages of methyl Hg in the water. Total Hg in the sediment samples ranged from 1.5-162 ng/g dry and the methyl Hg in the sediments ranged from below the analytical detection limit (0.0151 ng/g dry) to 0.57 ng/g dry. The percentages of Hg as methyl Hg in the sediments ranged from 0.27-2.6%. Typically the sediments are the primary site for Hg methylation and the percentages of methyl Hg in freshwater sediments usually range from 1-11%. The high percentages of methyl Hg in the water column and low percentages of methyl Hg in the sediments from the MACP and SCP suggest that the water column in these ecoregions may support Hg methylation to a higher degree than the sediments.