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Influence of Reservoir Water Level Fluctuations on Sediment Methylmercury Concentrations Downstream of the Historical Black Butte Mercury Mine, OR
Eckley, C., T. Luxton, J. McKernan, J. Goetz, AND J. Goulet. Influence of Reservoir Water Level Fluctuations on Sediment Methylmercury Concentrations Downstream of the Historical Black Butte Mercury Mine, OR. Michael Kersten (ed.), APPLIED GEOCHEMISTRY. Elsevier Science Ltd, New York, NY, 61:284-293, (2015).
This marks the first work conducted by ORD (ETSC) and Region 10 into the impacts of reservoir management on the production of methyl mercury. The research was conducted on the Cottage Grove Reservoir (Black Butte Mine Superfund Site OU-3). The topic of reservoir management and the enhancement of methyl mercury production is currently an important issue facing Region 9 and 10. This work begin to fill in necessary information required to develop optimum reservoir management practices that fullfill the needs of the immediate community (flood water management, irrigation, drinking water source) while minimizing negative environmental impacts associated with historical inputs of mercury. The objective of this study was to identify the role of ongoing water level fluctuations on MeHg production within the sediment of an old reservoir. Our objective was assessed by collecting seasonal measurements of total-Hg (THg) and MeHg and several ancillary parameters from the reservoir sediment and water-column, as well as water samples upstream and at the reservoir outflow.
Mercury (Hg) is a pollutant of global concern due to its ability to accumulate as methylmercury (MeHg) in biota. Mercury is methylated by anaerobic microorganisms such as sulfate reducing bacteria (SRB) in water and sediment. Throughout North America, reservoirs tend to have elevated methylmercury (MeHg) concentrations compared to natural lakes and rivers. This impact is most pronounced in newly created reservoirs where methylation is fueled by the decomposition of flooded organic material, which can release Hg and enhance microbial activity. Much less is known about the longer-term water-level management impacts on Hg cycling in older reservoirs. The objective of our study was to understand the role of on-going water-level fluctuations on sediment MeHg concentrations and sulfur speciation within a reservoir 75 years after initial impoundment. The study was performed at the Cottage Grove Reservoir located 15 km downstream of the historical Black Butte Hg mine. For 8 months each year, the water level is lowered resulting in roughly half of the reservoir’s sediment being exposed to the atmosphere. Water samples from the inflow, water-column, outflow, and sediment were collected seasonally over a year for total-Hg, MeHg, and several ancillary parameters. The results showed that conditions in the reservoir were favorable to methylation with a much higher %MeHg observed in the outflowing water (34%) compared to the inflow (7%) during the late-summer. An anoxic hypolimnion did not develop in the reservoir indicating that methylation was predominantly occurring in the sediments. In the sediments subjected to seasonal inundation, MeHg production was highest in the top 2 cm of the sediments and declined with depth. The seasonally inundated sediments also had significantly higher methylation activity than the permanently inundated area of the reservoir. Oxidizing conditions in the sediments during periods of exposure to air resulted in an increase in sulfate concentrations which likely stimulated SRB methylation following the raising of the water levels. In contrast, the sulfur in the permanently inundated sediments was all in a reduced form (sulfide) and sulfate remained below detection throughout the year. Overall, our results indicate that reservoir water level fluctuations can affect sediment redox conditions and enhance MeHg production. This process can result in a continued elevation of MeHg concentrations in older reservoir after the initial impact of landscape flooding has subsided.