EPA Region 10 Mercury Strategy Framework - Environmental ...

EPA Region 102. Assess Factors ControllingMercury MethylationMercury is usually released into the environment in aninorganic form, which is generally considered to have lowtoxicity if ingested. Approximately ninety percent of themercury found in fish tissue is methyl mercury, which istoxic when ingested. For mercury to be assimilated into thefood chain, it must first be converted into the organic methylform, generally through the action of sulfate reducingbacteria (SRB).USGS is the leader in research on methylation in the EasternUnited States (US), where it was noted that some waters haddisproportionately high fish tissue mercury relative to theirproximity to known mercury sources. After 15 years ofintensive study, factors responsible for these highconcentrations are well enough understood in the East thatthey can be predicted from data on the pH, sulfate levelsand dissolved organic carbon characteristic of thewaterbody.Conditions in the West differ substantially enough from theEast that research done to date on mercury methylation cannot be reliably applied to predict methylmercury abundanceand distributions in Region 10.For example:Mercury Strategy Framework• There are a considerable number of reservoirs in theWest – drying and rewetting of sediment has beendemonstrated to increase methylation rates, resulting inmethylmercury concentration spikes in water, sedimentand food webs.• Sulfate levels are a major factor influencing mercurymethylation. Low sulfate abundance in the East cansometimes limit methylation, and in general there aregreater amounts of sulfate in soils of the West. Howeverthe role sulfate plays in methylation in Region 10 isunclear and more data is needed in order to informresource managers.• Waters in the West generally have higher pH than Easternwaters and low pH tends to accelerate methylation• Selenium has a complex, multi-modal affect on mercury.At the right levels, selenium can effectively “tie up”mercury and render it unavailable for methylation. Onthe other hand, selenium itself is toxic at elevated levelsso attributing toxicological effects to a single co-factorcan be difficult where both selenium and mercury vary inspace. In general, low levels of native selenium in theEast have not stimulated much research in the past.However, the generally much higher levels in the Westwarrant more attention.• Conditions in Alaska may be substantially different thanin the rest of the Region and the U.S., given the abundantwetlands and extreme northern latitudes. In addition,some parts of Alaska contain many metal deposits,including mercury mines, therefore geologic sources mayplay an important role in the occurrence and distributionof mercury in local ecosystems.Understanding methylation factors that are important in theRegion will provide more information on alternative meansto reduce fish tissue levels in waters where the primarysources of mercury input are not immediately controllable,such as global mercury deposition or historic mining.Region 10 efforts to evaluate methylation will need to becoordinated closely with activities by other agencies. USGSis beginning to conduct research in the West under theirown initiative, although their limited budget has slowed thepace of this work. In addition, as mentioned above underthe Monitoring Support activity, several Federal agenciesare proceeding to develop a national monitoring planhoping that funds will be available in the future.Mercury Strategy Framework - Page 5