Mercury Stable Isotopes as Tracers in the Environment: Applications to Aquatic and Natural Gas Systems

Abstract

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. To describe the large range of observed Hg isotopic variation within the channel environment, a source end-member mixing model was proposed, identifying a regional background end-member and two end-members deriving from the historic industrial activities. We observed for the first time a discharge-dependent isotopic partitioning of Hg between the dissolved and suspended particulate phase of surface waters and proposed a fractionation mechanism to explain this observation. Examination of sediments of a floodplain profile provided evidence that there was significant temporal variability of the isotopic composition of past releases of Hg into the South River, with brief excursions in isotopic composition in the past recorded in the floodplain profile. In Chapter 4, we present the first measurements of the isotopic composition of Hg within natural gas. To obtain these measurements, we analyzed catalysts from mercury removal units at gas processing facilities that served to concentrate Hg for isotopic analysis. Significant variation in the isotopic composition of Hg within natural gas on a global scale was observed, 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.