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Impact of Reservoir Water Level Management on Sediment Pore Water Chemistry and Methylmercury Production
Citation:
Luxton, T., C. Eckley, J. Goetz, AND J. McKernan. Impact of Reservoir Water Level Management on Sediment Pore Water Chemistry and Methylmercury Production. International Conference on Mercury as a Global Pollutant, Providence,RI, July 16 - 21, 2017.
Impact/Purpose:
The research presented will cover data collected at Cottage Grove Reservoir, OU 3 at the Black Butte Superfund site in OR. The research demonstrates that seasonal changes in water levels enhances the production of methyl Hg.
Description:
Reservoirs typically have elevated fish mercury (Hg) levels compared to natural lakes and rivers. A unique feature of reservoirs is water-level management which can results in sediment exposure to the air. The objective of this study is to identify how reservoir water-level fluctuations impact Hg cycling, particularly the formation of the more toxic and bioaccumulative methylmercury (MeHg). Total-Hg (THg), MeHg, stable isotope methylation rates and several ancillary parameters were measured in reservoir sediments (including some in porewater and overlying water) that are seasonally and permanently inundated. The results showed that sediment and porewater MeHg concentrations were over 3-times higher in areas experiencing water-level fluctuations compared to permanently inundated sediments. Sulfate cycling which is often associated with MeHg production was enhanced in the seasonally inundated sediments; however, statistical analysis showed that the main factors correlated with porewater MeHg concentrations were porewater THg, porewater DOC, and sediment-porewater THg partition coefficients (log Kd). The THg log Kd values showed distinct relationships with sediment organic carbon depending on whether the sediments were permanently are seasonally inundated. Overall, our results suggest that sediment exposure to the air increases organic matter breakdown which promotes the partitioning of THg and carbon into the porewater phase where it enhanced methylation.