Mercury geochemistry in wetland and its implications for in situ remediation

The objective of this study was to characterize the nature of Hg sorption to a wetland sediment with the intent of providing guidance for the selection of an appropriate in situ remediation strategy. Total Hg concentrations in the sediments were as high as 10 mg/kg, whereas associated pore water Hg concentrations were below detection, <0.010 mg/L. Sediment Hg was not in an exchangeable form, and < 8 % of it was associated with organic matter. The remainder of the Hg was strongly associated with Fe oxides and/or with a precipitated phase, presumably a sulfide. Sediment Hg concentrations were significantly correlated (r = 0.94) with Fe oxide concentrations. Thermodynamic calculations based on field Eh/pH measurements and laboratory results suggest that under present field conditions metacinnabar (HgS) would not be stable due to the relatively low pH (similar to4.2) and sulfate concentrations (0.14 mM) and high Eh levels at the study site. However, these calculations indicate that metacinnabar may have formed when the Hg first entered the wetland at elevated concentrations (similar to5 mg/L). Given the ecologically sensitive nature of the wetland and the fact that the Hg is strongly bound to the sediment, it was concluded that a monitored natural attenuation approach for site remediation may be appropriate.