MERCURY METHYLATION AND DEMETHYLATION WATERS

Potential rates of methylmercury formation and demethylation were stud ied in runoff waters, sediment trap material, and aerobic water layers of five small forest lakes by radioisotope methods. In addition, the roles of microbes and particulate matter in methylation and demethylat ion processes were examined. Methylation rates were low (less than or equal to 0.12% d(-1)) in all sites studied. Water pH (4.9-6.9) had no clear effect on methylmercury formation. Methylation took place in fil tered water (nominal pore size 0.2 mu m) and in formaldehyde-treated o r autoclaved samples, but was strongly suppressed by both dissolved or ganic matter (color) and particulate matter. Demethylation rates in su rface waters (less than or equal to 13.2% d(-1)) were decreased by low temperatures and the process was blocked by sterile filtration, forma ldehyde treatment, and autoclaving. The results indicate that mercury methylation in aerobic humic waters is an accidental process caused by bacterial exoenzymes or other dissolved compounds. Methylmercury deme thylation was clearly microbial in nature.