Historical deposition and cycling of Mercury in lakes and reservoirs, Central Region USA

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Task Contact: John Gray

Task Objectives

This task will focus on collecting landscape geochemical data to understand processes related to Hg contamination of reservoirs in the western USA. Mercury contamination of lakes, reservoirs, and other water bodies is a major environmental problem that affects a large portion of the population in the USA. There are over 1,700 aquatic systems in the USA with state or tribal advisories recommending against consumption of Hg contaminated fish. These aquatic advisories are now posted on more than 15 percent of all water bodies in the USA. Recognition of the problem continues to grow as more aquatic systems and fish populations are surveyed. Aquatic Hg advisories in the USA are more numerous than all other contaminants combined (including PCBs, DDT, dioxins, chlordane, and all other toxic metals). Many states are presently monitoring Hg contamination in fish, which is the primary source of methyl-Hg in the diet of humans and wildlife; however, little research is on-going to evaluate sources of Hg contamination, especially in the central region of the USA. In this study, we will investigate reservoirs with Hg advisories and focus primarily on the conversion from inorganic forms of Hg to methyl-Hg, which is the most toxic Hg compound that severely damages the central nervous system in humans and all organisms, especially embryos. The source(s) of such Hg contamination may be natural (due to erosion from surrounding rocks and soils), anthropogenic (mining related or atmospheric due to burning of fossil fuels), or a combination of sources.

In this task, we have collected geochemical data to evaluate geological and mineral-deposit related sources of Hg that potentially contaminate reserovirs and lakes. We have completed mercury studies at Narraguinnep Reservoir in Colorado, which has a fish-consumption advisory for Hg. During this part of the project, we evaluated whether Hg contamination of Narraguinnep Reservoir was from geologic or anthropogenic sources and we collected geochemical data from reservoir sediment cores to evaluate natural-geologic and anthropogenic-related sources of Hg that potentially contributed Hg to this reservoir. Narraguinnep Reservoir is fed by the Dolores River, which cuts the Mancos shale, and the historical Rico mining district is also located upstream. These possible geologic and miningrelated sources of Hg were found to be insignificant to Narraguinnep Reservoir. We determined that atmospheric Hg deposition from local coal-fired electric power plants was the dominant source of Hg in this area and to Narraguinnep Reservoir.

Future studies in this task will examine potential sources of the Hg, investigate Hg cycling, and evaluate effects of Hg to ecosystems surrounding other reservoirs in the USA. Results of the study will help to understand the cause of Hg and methyl-Hg contamination of biological resources in various water bodies, and also to help formulate remediation strategies for the cooperating state and federal agencies.

Statement of Work

This year we will be carrying out additional studies to evaluate mercury cycling and methylation of mercury in Narraguinnep Reservoir, Colorado. Reservoir sediment samples will be collected at the sediment-water interface from several localities in the reservoir to extract sediment pore water for determination of mercury, methylmercury, and measurement of other various geochemical parameters. Sediments will also be collected to measure rates of mercury methylation and methylmercury demethylation. The objective of these studies will be to understand mercury speciation in this reservoir, which appears to have a high rate of mercury methylation.

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