Relationship of Sediment Redox Conditions to Methyl Mercury in Surface Sediment of Louisiana Lakes

Abstract

Surface sediment from three Louisiana Lakes containing overlying water layer spiked with 2 µg/g (2 ppm) mercury were incubated under oxygenated (air) and nonoxygenated (N₂) conditions for determining the impact of oxygen status of overlying water on methylation of Hg in surface sediment from these lakes. The added mercury resulted in a greater than ten fold increase in methyl mercury (MeHg) as compared to native concentration of MeHg. The increase in methyl Hg production was less in sediment in which overlying water was exposed to oxygen rather than nitrogen. Results suggest that methyl Hg production would be less in lakes containing an oxygenated water column. In parallel microcosm studies without added mercury, MeHg decreased in sediment when redox potential of sediment suspension was increased from −200 mV to + 50 mV. Results of these studies demonstrate the importance of oxygenation or redox condition of surface sediment on mercury methylation and demethylation. Sediment conditions, which either reduce methylation or enhance demethylation in surface sediment, will limit the bioavailability of MeHg to the aquatic environment.

Keywords:

• Mercury
• Methylation of Hg
• MeHg
• Redox potential (Eh)

Acknowledgments

This work was supported by Louisiana Department of Environment Quality.