Mercury isotope ratios as a tool to trace and quantify Hg species transformations in contaminated soils and aquifers (MIRACOSA)

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Mercury (Hg) is a toxic pollutant element of great environmental concern. Different Hg species (e.g., Hg(II)Cl2, Hg(0), HgS) exhibit a wide range of physico-chemical properties. Thus, Hg species transformations exert a major influence on the environmental mobility and bioavailability of Hg. However, recognizing such transformations and their significance for Hg in the environment has been challenging. However, in the proposed work we explore the use of Hg isotope fractionation as a novel tool to address this problem. The stable isotope distribution of Hg is altered to a measurable extent during Hg species transformations (e.g., reduction/oxidation, sorption, precipitation, volatilization) may be used to trace Hg dynamics in environmental systems using Hg isotope ratios. Different processes cause characteristic mass-dependent and mass-independent fractionation signatures of Hg isotopes. This allows obtaining new insights into biogeochemical controls of Hg species transformations and potentially quantifying the relative importance of different processes in natural samples. We have selected two field sites in SW-Germany (both Hg-contaminated due to the use of Hg(II)Cl2 in wood impregnation) serving as natural laboratories with a single Hg point source to investigate Hg species transformations and associated Hg isotope fractionation in soil and groundwater systems. We have collected a set of preliminary data identifying the presence of different Hg species and revealing significant Hg isotope variations in soil and groundwater samples. We hypothesize that Hg isotope signatures in soil and groundwater will provide an integrated picture of past and present Hg species transformations and allow tracing processes at solid-solution interfaces and between different solid phase Hg pools. In addition to collecting a large set of field samples, laboratory experiments investigating Hg isotope fractionation during relevant individual processes will be conducted to obtain fractionation factors and mechanisms needed for the interpretation and quantitative description of the field data. Furthermore, a reactive transport model incorporating Hg isotope fractionation between different Hg species will be developed to provide a predictive understanding of Hg plumes in groundwater systems over time. The novel combination of Hg concentration, Hg speciation (using pyrolytic-thermodesorption), and Hg isotope (using cold vapor-MC-ICP-MS) analysis of liquid, solid, and gaseous samples will significantly improve our capability to predict the behavior and fate of Hg not only for the studied field sites but also for other contaminated as well as pristine aquifer and soil systems in general. In contrast to the already documented use source tracer, the application of Hg isotope signatures as process tracer at contaminated sites has not been described yet and no Hg isotope data for groundwater are currently available in the literature.