MOBILIZATION AND METHYLATION OF MERCURY DERIVED FROM COAL ASH HOLDING POND EFFLUENTS

Coal combustion products (CCPs) represent the largest industrial waste stream in the United States. Coal ash, in particular, contains elevated levels of toxic elements such as mercury (Hg). Much of this waste is stored in unlined holding ponds and landfills that are not always monitored for their discharge to adjacent waters. Moreover, these holding ponds are susceptible to failures such as the disaster at the Tennessee Valley Authority Kingston Fossil Plant in 2008 that caused more than 1 billion gallons of coal ash slurry to spill into the adjacent Emory River. In this presentation, we summarize findings from field studies to assess the release and methylation of mercury originating from coal ash. We performed a two year survey of the rivers surrounding the TVA Kingston fossil plant and documented elevated levels of methylmercury (MeHg) in the river sediments near the coal ash spill site. Moreover, we used stable isotopes of Hg to differentiate between mercury originating from the coal ash and mercury originating from historical contamination sources to this ecosystem. These results suggested that the coal ash was stimulating microbial methylation of Hg in the river sediments near the TVA site (either by providing Hg or other substrates for sediment bacteria). We also performed a one-year survey of surface waters near CCP holding ponds in North Carolina, including Hyco and Mayo Lakes near Roxboro, NC. At surface water sites near the coal ash holding pond discharge points, we observed mercury that was more than 100 ng/L, a concentration range that is greater than typical for N.C. lakes and streams (<10 ng/L). The sediments of Hyco and Mayo Lakes contained 0.10 to 0.25 mg/kg mercury, which is slightly greater than typical background sediment and soils (<0.05 mg/kg) but consistent with coal ash and soils from urban settings. Our research highlights the possibility for the release of mercury from CCP holding ponds. This mercury would be most hazardous if it was converted to methylmercury, the bioaccumulative form of the metal, in the sediments of receiving waters. Therefore, we are now performing laboratory studies to determine the speciation of mercury associated with various CCP materials and sediment microcosm experiments to determine the methylation potential of mercury derived from coal ash.