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Methylmercury Production and Degradation under Light and Dark Conditions in the Water Column of the Hells Canyon Reservoirs, USA
Citation:
Eckley, Chris S., Todd P. Luxton, Chris D. Knightes, AND V. Shah. Methylmercury Production and Degradation under Light and Dark Conditions in the Water Column of the Hells Canyon Reservoirs, USA. ENVIRONMENTAL TOXICOLOGY AND CHEMISTRY. Society of Environmental Toxicology and Chemistry, Pensacola, FL, 40(7):1827-1837, (2021). https://doi.org/10.1002/etc.5041
Impact/Purpose:
Statement of Problem: Reservoirs are a ubiquitous feature of the American landscape and are constructed for a variety of beneficial purposes including electricity production, flood control, water supply, and recreation. However, reservoirs can have negative impacts to the water quality both within and downstream of the impoundments. For example, the Hells Canyon complex (HCC) of three hydroelectric dams along the Idaho-Oregon border has resulted in low dissolved oxygen levels and elevated temperature and methylmercury (MeHg) in water. Identifying opportunities to improve the water quality associated with impounded river systems is an important task for environmental management agencies. The use of selective withdrawal technology to release cold bottom water has been discussed as a potential mechanisms to meet downstream temperature standards associated with Brownlee reservoir, which is part of the HCC; however, the bottom water also contains elevated MeHg concentrations. As such, the EPA needs to identify management solutions that can optimize conditions for multiple water quality endpoints simultaneously. Research Approach: The objective of this RARE project is to identify the potential water quality benefits within and downstream of Brownlee reservoir (the deepest reservoir in the HCC) by modifying the dam inlet structure to allow for selective water withdrawal at a different elevations. Currently, there is only single-point withdrawal capability. Specifically, we will focus on the collection of field data aimed at understanding: 1) the fate of MeHg released from the hypolimnion to identify the conditions and timing of downstream MeHg degradation (i.e. demethylation); and 2) the production of MeHg within the hypolimnetic waters of the reservoir. Our proposal will piggy-back on an existing USGS mercury (Hg) research effort currently underway within the HCC. Our research is designed to fill remaining knowledge gaps within this effort, specifically as they pertain to understanding reservoir management scenarios to improve water quality. Anticipated Results & Regional Impact: Our results, along with those of the pre-existing USGS study, can be used to support a modelling effort that can simulate the effect of different selective withdrawal scenarios on MeHg concentrations within the Hells Canyon reservoirs. This can be achieved by coupling a hydro-dynamic model (e.g. CE-QUAL-W2) to a water quality model (e.g. D-MCM, Water quality Analysis Simulation Program (WASP) model. These results can guide regulatory decisions aimed at optimizing reservoir management for multiple water quality parameters, including temperature, oxygen, and MeHg. While this information will be utilized within R10, the results will also help advance our understanding of technological solutions to elevated MeHg concentrations in reservoirs throughout the US.
Description:
Methylmercury (MeHg) is a highly toxic form of mercury that can bioaccumulate in fish tissue. Methylmercury is produced by anaerobic bacteria, many of which are also capable of MeHg degradation. In addition, demethylation in surface waters can occur via abiotic sunlight-mediated processes. The goal of the present study was to understand the relative importance of microbial Hg methylation/demethylation and abiotic photodemethylation that govern the mass of MeHg within an aquatic system. The study location was the Hells Canyon complex of 3 reservoirs on the Idaho-Oregon border, USA, that has fish consumption advisories as a result of elevated MeHg concentrations. Our study utilized stable isotope addition experiments to trace MeHg formation and degradation within the water column of the reservoirs to understand the relative importance of these processes on the mass of MeHg using the Water Quality Analysis Simulation Program.
URLs/Downloads:
DOI: Methylmercury Production and Degradation under Light and Dark Conditions in the Water Column of the Hells Canyon Reservoirs, USA
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