ENVIRONMENTAL IMPACTS OF THE TENNESSEE VALLEY AUTHORITY KINGSTON COAL ASH SPILL. 2. METHYLMERCURY IN HISTORICALLY CONTAMINATED RIVER SEDIMENTS

The Tennessee Valley Authority Kingston coal ash spill in December 2008 deposited approximately 4.1 million m³ of fly ash and bottom ash into the Emory and Clinch River system. Here, we investigated the impact of the ash on surface water and sediment quality over an eighteen month period after the spill, with a specific focus on mercury and methylmercury production in sediments. Surface water quality was not impaired with respect to total mercury concentrations. In the sediments of the Emory River, total mercury concentrations in areas impacted by the ash spill increased by a factor of 3 to 4 compared to upstream sediments. Methylmercury concentrations in the Emory and Clinch River sediments near the ash spill were slightly elevated (up to a factor of 3) at certain locations compared to upstream sediments. Up to 2% of the total mercury in sediments containing coal ash was present as methylmercury. Mercury isotope fractionation and biogeochemical data suggested that elevated methylmercury production occurred in regions where native sediments were mixed with coal ash (between 4 % to 25% as coal ash). This coal ash may have stimulated methylation of mercury due to the sulfate that leached from the ash. The production of methylmercury in these areas is a concern because this neurotoxic organomercury compound can be highly bioaccumulative in the food web. Future regulations that seek to minimize the risk of coal combustion products with respect to mercury should consider not only the leaching potential of the metal, but also the potential for methylmercury production in receiving waters.