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Title: Mercury in Louisiana Freshwater Lakes: The effect of anaerobic conditions on methylation and demethylation of mercury

Focus Categories: TS, SED, NPP

Keywords: Water quality, fish advisories, food chain, health effects, biogeochemistry.

Duration: March 1, 1999 through February 28, 2000.

Federal funds requested: $18,000.

Non-Federal (matching) funds pledged: $36,081.

Principal investigators names, university city, and water resource institute

W. H. Patrick, R. D. DeLaune and R. P. Gambrell, Wetland Biogeochemistry Institute, Louisiana State University, Baton Rouge, LA 70803. Louisiana Water Resources Research Institute.

Congressional district: Sixth Congressional District.

Statement of critical regional water problems

Mercury contamination of fish in Louisiana lakes has become a serious water quality problem with advisories on comsumption of fish issued for several important lakes in the state. The accumulation of unacceptable concentrations of mercury in fish is thought to occur largely as the result of bioaccumulation of methymercury up the food chain to the top predator fish, which are also the most desired species. Methylmercury is thought to form largely in the anaerobic sediments of lakes and then move up through several trophic levels. Information is needed on the first step in this process which is the formation of methymercury by anaerobic bacteria in the oxygen free sediment of lakes. Information is needed on the extent of anaerobic conditions responsible for methylation in order to develop treatments that will prevent or correct this condition. Our preliminary research indicate that mercury contamination of fish is greatest in lakes receiving organic runoff from the watershed. If this relationship also holds for methylation of mercury in these lakes it might be possible to control the amount of organic matter going into the lakes.

Statement of the results, benefits, and/or information expected to be gained

This research will show the optimum redox potential and pH for the formation of methylmercury and the optimum redox potential-pH for the demethylation of mercury in freshwater lakes in Louisiana. Three lakes where mercury contamination of fish is a problem due to methymercury moving up through the food chain and three lakes where the mercury contamination of fish is below the advisory level will be selected for this study. The knowledge of the sediment conditions where methylmercury is formed and the conditions where methylmercury is destroyed should lead to the development of treatment techniques for the contaminated lakes. Based on the critical redox -pH for formation of methylmercury the affected lakes may be corrected by drawdowns to oxidize the sediment or addition of chemical amendments.

Nature, scope and objectives of the research

Mercury contamination of lakes and streams has taken on new importance worldwide since it was discovered several decades ago that inorganic mercury compounds can undergo methylation in sediments under proper anaerobic conditions with a resulting increase in solubility, mobility and bioaccumulation in the food chain (Jernolov and Lann, 1971). Many waterbodies worldwide have elevated mercury levels in top predator fish that has prompted the issuance of advisories on fish consumption by various public health agencies. Lakes and waterbodies in the Southern U. S. including Louisiana have been impacted by mercury contamination. Recent finding of elevated mercury concentrations in fish and wildlife from fresh water lakes throughout much of the Southern United States including Louisiana has caused great concern for persons for whom fish are a significant part of their diet and for pregnant women and nursing mothers. Levels up to 3 ppm have been found in largemouth bass in some lakes, which is considerably above the maximum permissible level (1 ppm) for edible fish tissue. The Louisiana Department of Health and the Department of Environmental Quality have issued mercury advisories for a number of lakes in the state with the result that the desirability of these lakes for fishing has decreased. This proposal describes a study of the conditions in freshwater lakes in Louisiana that govern the formation and removal of methylmercury, the form of mercury that bioaccumulates in the food chain of lakes.

The Louisiana State University Wetland Biogeochemistry Institute has carried out research studies for several decades on various aspects of mercury contamination in soils and sediments and water bodies (Gambrell et al 1976, Khalid et al 1977, Gambrell et al 1980, Patrick, et al 1994 , Patrick & Verloo, 1998). The study proposed here will allow us to study the biogeochemistry conditions in these lakes that result in accumulation of mercury in the top predator fish in some of these lakes but not in others.

No definite point-sources for mercury in these lakes have been identified. It is assumed that the mercury content in fish is related to soil or sediment Hg originating from the increase in atmospheric fallout of mercury during the past century as a result of coal burning and smelting operations and from runoff into the lakes from watersheds receiving this fallout mercury.

The approach we will take in this research project is to use microcosms to duplicate the range of substrate conditions that exist in the freshwater lakes and to determine the biogeochemical conditions in the lakes that both favor methlylation of mercury and favor demethylation. The overall objective is to determine the combination of substract biogeochemical conditions that will minimize methylmercury formation and maximize demethylation. This objective is achievable because the microcosms we use allow for natural adjustment of redox potential and pH over the entire range of conditions encountered in the lakes. The microcosm system is thus an effective, low cost method of duplicating a wide range of substrate conditions without having to carry out expensive field manipulations.

Examples of studies that have been carried out in the Wetland Biogeochemistry Institute using microcosms to measure critical redox potentials for changes in the form or activity of toxic substamces are lead, manganese, iron: Patrick and Verloo (1998); sulfide: Connell and Patrick (1968); manganese: Patrick and Turner (1968); Gotoh and Patrick (1972); iron: Patrick, et al. (1973), Gotoh and Patrick (1974); Cadium, lead, zinc and copper: Reddy and Patrick 1977a, Reddy and Patrick 1977b; selenium: Masscheleyn et al. (1990); arsenic: Masscheleyn et al. (1991); chromium: Masscheleyn et al. (1992). Examples of studies that have been carried out using the controlled redox-pH system to study degradation rates of toxic organic compounds are: hydrocarbons: Hambrick, et al. (1980), DeLaune et al. (1980), various pesticides: Gambrell et al. (1984), PCBs: Pardue, et al. (1988), atrazine: DeLaune, et al. (1997).

U.S. Department of the Interior, U.S. Geological Survey
Maintained by: John Schefter
Last Updated: Wednesday March 23, 2005 9:17 AM
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