Mercury Geochemistry in Wetland and its Implications for In Situ Remediation

Abstract

The objective of this study was to characterize the nature of Hg sorption on 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 Feoxides and/or with a precipitated phase, presumably a sulfide. Sediment Hg concentrations were significantly correlated (r = 0.94) to 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 (~4.2) and sulfate concentrations (0.14-mM) and high Eh levels at the study site. However, these calculations indicate that meta-cinnabar may have formed when the Hg first entered the wetland at elevated concentrations (~5-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.

Update to agency for toxic substances and disease registry 2012 report on assessment of biota exposure to mercury originating from Savannah River Site.

“…The minimum concentration of 0.11 µg/g was measured in 2013 while the maximum of 0.61 µg/g was measured in 2009 (SRNS 2013) (Figure 4-7). Beaver Dam Creek, but this value is within the range of background mercury levels measured in surface sediments that ranged as high as 1.00 µg/g (Kaplan et al 2002) (Table 4-3). On the SRS, alligators are abundant in the Savannah River, its swamp and tributaries, L-Lake, Par Pond, and other reservoirs on the site (Figure 4-8) (SREL 2012).…”

Mercury volatilization from salt marsh sediments

“…The sediment mercury concentration at the Great Bay site (0.5 mg kg −1 ) is at the high end of the range observed in nonpoint source impacted (inputs from atmospheric deposition only) sediments (0.007–1 mg kg −1 ) [ Gilmour et al , 1992; Cai et al , 1997; Leonard et al , 1998a, 1998b; Hung and Chmura , 2006]. In contrast, the concentration of total mercury in Secaucus High School Marsh sediments (7 mg kg −1 ) is greater than that in nontidal sediments of the New York/New Jersey Harbor and Newark Bay (0.3 to 2.0 mg kg −1 [ Hammerschmidt et al , 2008]), but similar to that of contaminated areas of the Savannah River in South Carolina where sediment total mercury concentrations are as high as 10 mg kg −1 [ Kaplan et al , 2002]. Although mercury concentrations were tenfold higher in Secaucus sediments than at the Great Bay site, another environmental factor, sunlight, appears to be a more important control of mercury volatilization from these sediments.…”

Role of Naturally Occurring Organic Matter on Thorium Transport in a Wetland

“…In fact the sediment samples from the two most distal locations in the contaminated site did not show any Th enrichment in the fine-grain particles. This in part can be attributed to the presence of Fe-oxide and organic matter coating on essentially all sediment particles and the important role these coatings play on metal sorption [10]. Sequential extraction data of the sediments show that a vast majority of the Th in the contaminated sediments, 81 ± 10 wt-%, was associated with the organic/sulfide fraction and very little was associated with the exchangeable fraction, 1 ± 1 wt-% (see table II).…”