Congratulations to Spencer Washburn who defended his dissertation on April 26, 2018.
Advisor: Joel Blum
Mercury (Hg) is a neurotoxic trace metal pollutant with a global distribution and a complex biogeochemical cycle. Gaining a better understanding of the behavior of Hg in the environment has implications for both environmental and human health. Anthropogenic activity has directly altered the biogeochemical cycling of Hg, both by the direct release of Hg to the environment related to historic use (i.e. mining, industrial activity) as well as ongoing emissions of Hg as a byproduct of energy production (i.e. coal and natural gas combustion). There are still significant uncertainties in the understanding of how anthropogenic Hg sources, both legacy and modern, affect global Hg cycling and environmental health. The developing study of Hg stable isotope ratios in environmental samples has presented a new tool for understanding the processes that control Hg biogeochemistry. Throughout this dissertation, we have applied measurements of Hg stable isotope ratios in samples from sites affected by anthropogenic Hg contamination to enhance understanding of the biogeochemical behavior of Hg. In Chapter 2 and 3, we focused on understanding the Hg cycling within freshwater aquatic ecosystems by studying the South River, VA, which is the site of historic industrial Hg contamination. We developed a source end-member mixing model to describe the observed Hg isotopic variation within the channel environment, identifying a regional background end-member and two end-members deriving from the historic industrial activities. We described the discharge-dependent isotopic partitioning of Hg between the dissolved and suspended particulate phase and proposed a fractionation mechanism to explain this observation. We also explored the temporal variability of the isotopic composition of past releases of Hg into the South River by examining the sediments of a floodplain profile, and discovered evidence in the floodplain record of brief excursions in isotopic composition in the past.In Chapter 4, we examined for the first time the isotopic composition of Hg within natural gas, analyzing catalysts from mercury removal units at gas processing facilities that served to concentrate Hg for isotopic analysis. We observed significant variation in the isotopic composition of Hg within natural gas on a global scale, as well as the regional scale. With further work these results could be used to investigate the impacts of natural gas processing at a local scale and could be included in Hg emissions models that incorporate Hg isotope mass balances. Altogether, this dissertation has expanded the use of stable Hg isotope ratios as tracers of anthropogenic Hg releases to the environment.