Evaluation of Terminal Electron Accepting Processes for the Hells Canyon Complex: Gaining Information on Dominant Processes Controlling Mercury Methylation
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EXECUTIVE SUMMARY The biogeochemical processes associated with methylmercury (MeHg) production remain poorly understood. Biogeochemical conditions affecting the availability of terminal electron acceptors (TEAs) and electron donors for metabolism by heterotrophic methylating microorganisms vary from system to system. However, it is generally accepted that mercury methylation occurs under anaerobic conditions, with the highest methylation rates occurring in anoxic sediments and waters, often associated with sulfate reduction. This thesis aims to address the uncertainties surrounding water column mercury methylation and biogeochemical heterogeneities in the Hells Canyon Complex (HCC). Chapter 1 reviews existing laboratory and field investigations of the HCC, mercury and MeHg biogeochemistry, and terminal electron accepting processes (TEAPs). Additionally, Chapter 1 identifies the harmful effects of elevated MeHg concentrations to humans and the environment, potential sources of mercury to the HCC, and role that biogeochemistry plays on mercury methylation. A critical evaluation of mercury cycling in the environment indicates that the availability of TEAs in the water column dictates the dominant microbial populations. Microorganisms tend to favor redox processes that generate the maximum amount of available energy, although redox processes that are not the most energetically favorable can occur tangential to other TEAPs. For this reason, it is necessary to determine both the most energetically favorable TEAPs, as well as processes that are less favorable, but are possible, occurring near elevated concentrations of MeHg. In Chapter 2, the energetic favorability of TEAPs in the HCC is determined using geochemical data collected from the HCC segment of the Snake River. Using the Nernst equation, the theoretical redox potential of relevant redox processes is calculated for water samples collected from 2015 to 2017. Water column profiles of dissolved oxygen (DO), nitrogen, manganese, iron, and sulfur species are compared to theoretical redox potential values to identify the location of redox boundaries in the water column. The results of this study are consistent with the typical thermodynamic sequence of electron acceptors with the highest energy potential having the most favorable redox potential for microbial metabolism. Redox boundaries between TEAPs are not well defined and change with time and space due to redox disequilibrium. While sulfate reduction is correlated to elevated MeHg concentrations in other environmental systems, thermodynamic analysis of this site found that sulfate reduction is not a dominant redox process in zones of maximum MeHg concentration. Based only on a biogeochemical analysis of the system, this study concludes that mercury methylation is most likely occurring in the sediments and is transported vertically into the water column rather than in the water column. This study recommends that further studies analyze the system for evidence of physical transport of methylmercury from the benthos layer into the water column. The findings of this study provide insights into the biogeochemical processes supplying energy to methylating microorganisms, as well as narrow the scope of future microbial population analyses of MeHg contaminated systems.