Use of Stable Mercury Isotopes to Assess Mercury and Methylmercury Transformation and Transport across Critical Interfaces from the Molecular to the Watershed Scale

The over-arching goal of this effort is to use natural mercury (Hg) stable isotope signatures, imparted by molecular-scale reactions, to gain a more comprehensive quantitative and mechanistic understanding of the processes that supply dissolved Hg to surface water and thus drive observations of watershed-scale mercury fluxes in the East Fork Poplar Creek (EFPC) ecosystem near Oak Ridge, Tennessee. We also aim to elucidate the sources of methylmercury and the balance of reactions that govern its production and fate in this Hg-contaminated stream ecosystem. To achieve these goals, we will integrate mercury isotopic analysis with ongoing efforts of the field-based SBR SFA at ORNL to provide a synergistic multiple-lines-of-evidence approach for assessment of Hg transformation and transport across critical interfaces within the EFPC ecosystem.

Much has been learned about Hg cycling in lotic systems, and East Fork Poplar Creek in particular, through decades of previous research. Nevertheless, some of the most fundamental questions regarding the source(s) of bioavailable Hg and its transformation to toxic methylmercury (MeHg) remain unanswered. These fundamental questions include: 1) the source(s) and biogeochemical processes that lead to the input of dissolved Hg to EFPC stream water, and 2) the source(s) and biogeochemical processes that control the production and fate of bioaccumulative MeHg in the EFPC ecosystem. To address these fundamental questions, this project aims to couple laboratory experiments and field observations, and use natural abundance Hg stable isotope techniques to identify the processes responsible for generating bioavailable dissolved Hg from relatively recalcitrant legacy sources of Hg within critical subsurface zones (e.g., hyporheic zone, riparian wetlands, groundwater, and streambed periphyton). Moreover, we propose to use the isotopic signature of this bioavailable dissolved Hg to track its mobilization across critical interfaces in order to directly link diffuse subsurface sources of dissolved Hg with increases in surface water dissolved Hg flux measured at the watershed scale. Furthermore, we will directly determine the isotopic composition of MeHg within these same critical subsurface zones. Using natural Hg isotope signatures, we will assess the direct linkage between the dissolved Hg supply and the MeHg generated within and released across these critical interfaces, and we will evaluate the in situ balance of methylation and demethylation reactions that lead to net production of MeHg in EFPC.

The Department of Energy (DOE) oversees many large environmental remediation operations, and the biogeochemical transformation of mercury (and other contaminants) has been identified as a key contaminant of concern. In order to manage Hg contaminated sites and make informed decisions about Hg remediation strategies, additional tools are needed to trace the sources, pathways, and bioavailability of Hg. Mercury isotopes have been demonstrated to be an important new tool in Hg research and have provided important new insights into mercury biogeochemistry in EFPC and other lotic ecosystems. We propose to further develop and apply this rapidly evolving tool to trace the location of inorganic Hg methylation and the pathways for the transport and transformation of mercury within the EFPC ecosystem. By identifying the dominant reaction pathways and transport processes underlying watershed scale phenomena, this approach will provide a logical guide for the development of process-rich mechanistic models that will help predict the outcome of remediation efforts, and anticipate the effects of global environmental change on the hydrobiogeochemical cycling of Hg in stream ecosystems. Moreover, such insight will aid in the development of an approach that integrates stable Hg isotope techniques with more traditional watershed-scale approaches, thus serving as a model for future study of other point-source and non-point-source Hg-contaminated watersheds.