Mercury Methylation in Denitrifying Bioreactors: An Investigation in Pollution Swapping

Woodchip and corn cob filled bioreactors have been shown repeatedly to be effective at reducing nitrate pollution from agricultural fields by supporting denitrifying bacteria. Little attention has been paid, however, to other microorganisms that may also proliferate in these environments. Of particular concern are sulfate reducing bacteria and other organisms known to convert mercury to highly toxic methylmercury, since supporting such organisms could lead to increased levels of methylmercury in downstream rivers and lakes. To investigate this concern, we conducted two studies. We measured total and methylmercury concentrations and related parameters in upflow column bioreactors filled with either woodchips or corn cobs. The temperature of the water pumped into the column bioreactors was in the range of 1.8°C to 18.6°C to simulate cooler autumn and winter weather. There was no significant mercury methylation detected at these temperatures. The concentration of methylmercury flowing out of the column bioreactors filled with corn cobs showed greater variability than that of woodchip filled bioreactors. The second study was set up to pump water with low concentrations of nitrate into four edge-of-field woodchip bioreactors to produce a residence time of 24 hours or more. We measured total and methylmercury concentrations along with other indicators of water chemistry and biological activity. While the conditions monitored in the bioreactors were consistent with documented conditions that support mercury methylating bacteria, we did not find evidence of methylmercury being generated within the bioreactors. These studies indicate that the production of methylmercury is not likely in Minnesota edge-of-field woodchip bioreactors or in woodchip or corn cob bioreactors where water temperatures are below 18.6°C and nitrates are not completely reduced.