Methylmercury Levels Spiked After Tennessee Coal Ash Spill

A large 2008 coal ash spill at a Tennessee power plant tripled the levels of potentially harmful mercury in parts of two of the state’s rivers, according to a new study (Environ. Sci. Technol., DOI: 10.1021/es303639d). Researchers found that bacteria converted mercury from the spill to methylmercury, a form that is biologically available to wildlife.

Earlier sediment studies conducted by the Environmental Protection Agency and the state focused mainly on total mercury and did not find levels of concern, leading authorities to conclude in part that elevated mercury in the systems was likely caused by previously recognized contamination from nuclear weapons processing in the 1900s. But a team led by Heileen Hsu-Kim, an aquatic chemist at Duke University, took a more detailed look. “EPA might have been a little too quick to judge that mercury was not a problem,” she says.

In an earlier study Hsu-Kim and her team established a mercury isotope signature for the spill that allowed them to map where mercury from the coal ash had accumulated (Environ. Sci. Technol., DOI: 10.1021/es303111). In analyzing samples from these contaminated spots in the Clinch and Emory Rivers, the researchers found that bacteria converted about 2% of spill mercury to methylmercury--enough to raise methylmercury levels as much as three times as high as baseline levels in places. Further work could establish the extent of the risk of toxicity to wildlife, if any, according to Hsu-Kim.

Future environmental studies of mercury pollution, performed for instance to assess the impact of wastewater treatment plants or coal ash storage facilities, should focus on the biologically troubling, though more difficult to measure, methylmercury, Hsu-Kim says. Current methods focus on total mercury. “If the test is not really meaningful environmentally,” she asks, “then what’s the point of it in the first place?”