280 - Tracing Geochemical Processes Governing the Stationary behavior of a Hg Plume in Groundwater Using Hg Isotopes

Wood impregnation with mercury chloride (HgCl2) was used until the 1970s and often resulted in Hg contamination of soils, surface- and groundwater. Predicting the fate of legacy Hg in environmental systems is important for risk assessment and remediation, but our current knowledge on the geochemical behavior and transformation processes of highly soluble HgCl2 in groundwater systems is still limited. We report total Hg, aqueous Hg species and Hg isotope ratios of groundwater samples collected at a former wood impregnation facility. Additionally, data from batch and column leaching experiments with aquifer material from this site are discussed. A narrow contamination plume with remarkable stability over the past decades exists in the groundwater downstream of the site. Maximum Hg concentrations varied between 72 and 209 µg L-1 during our sampling campaigns. Regardless of the concentration fluctuations, Hg isotope ratios showed a very consistent trend. Samples from the most contaminated well had slightly negative δ202Hg values (-0.05 ± 0.02 ‰; 1SD of replicates). As Hg concentrations decreased, the isotope ratios became progressively heavier reaching δ202Hg values of 0.63 ± 0.01 ‰ (1SD of replicates) towards the end of the plume, presumably caused primarily by preferential sorption of light Hg isotopes to the aquifer solids. Experiments showed that the aquifer has a very high Hg sorption capacity despite its low organic content (0.1%). In both sets of experiments, we observed an initial phase of fast Hg release followed by slow continuous release. Most leachates were enriched in heavy isotopes compared with aquifer solids. However, the presence of Hg(0) in solution had a large effect on δ202Hg values, presumably controlled by isotope exchange and equilibrium fractionation between Hg(0) and Hg(II). Further movement of the Hg groundwater plume is hindered by strong retardation to the aquifer solids leaving an imprint in the groundwater Hg isotope signatures.

Authors: Lorenz Schwab, Alina Kleindienst, Carina Esser, David S. McLagan, Harald Biester, Stephan M. Kraemer, Jan G. Wiederhold