METHYLMERCURY OXIDATIVE-DEGRADATION POTENTIALS IN CONTAMINATED AND PRISTINE SEDIMENTS OF THE CARSON RIVER, NEVADA

Sediments from mercury-contaminated and uncontaminated reaches of the Carson River, Nevada, were assayed for sulfate reduction, methanogenes is, denitrification, and monomethylmercury (MeHg) degradation. Demethy lation of [C-14]MeHg was detected at all sites as indicated by the for mation of (CO2)-C-14 and (CH4)-C-14. Oxidative demethylation was indic ated by the formation of (CO2)-C-14 and was present at significant lev els in all samples. Oxidized/reduced demethylation product ratios (i.e ., (CO2)-C-14/(CH4)-C-14 ratios) generally ranged from 4.0 in surface layers to as low as 0.5 at depth. Production of (CO2)-C-14 was most pr onounced at sediment surfaces which were zones of active denitrificati on and sulfate reduction but was also significant within zones of meth anogenesis. In a core taken from an uncontaminated site having a high proportion of oxidized, coarse-grain sediments, sulfate reduction and methanogenic activity levels were very low and (CO2)-C-14 accounted fo r 98% of the product formed from [C-14]MeHg. There was no apparent rel ationship between the degree of mercury contamination of the sediments and the occurrence of oxidative demethylation. However, sediments fro m Fort Churchill, the most contaminated site, were most active in term s of demethylation potentials. Inhibition of sulfate reduction with mo lybdate resulted in significantly depressed oxidized/reduced demethyla tion product ratios, but overall demethylation rates of inhibited and uninhibited samples were comparable. Addition of sulfate to sediment s lurries stimulated production of (CO2)-C-14 from [C-14]MeHg, while 2-b romoethanesulfonic acid blocked production of (CH4)-C-14. These result s reveal the importance of sulfate-reducing and methanogenic bacteria in oxidative demethylation of MeHg in anoxic environments.