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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.

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At many sites, long-term monitoring (LTM) programs include metals as chemicals of concern, although they may not be site-related contaminants and their detected concentrations may be natural. At other sites, active remediation of organic contaminants in groundwater results in changes to local geochemical conditions that affect metal concentrations. Metals should be carefully considered at both types of sites, even if they are not primary contaminants of concern. Geochemical evaluation can be performed at LTM sites to determine if the monitored metals reflect naturally high background and, hence, can be removed from the analytical program. Geochemical evaluation can also be performed pre-and post-treatment at active remediation sites to document the effects of organics remediation on metals and identify the processes controlling metal concentrations.Examples from both types of sites are presented in this article. O c 2008 Wiley Periodicals, Inc. INTRODUCTIONMetal concentrations in groundwater samples can be natural, reflect anthropogenic input, or be indirectly affected by organic contaminants and/or their treatment. The mechanisms controlling metal concentrations under natural conditions are well established (Appelo & Postma, 2005;Drever, 1997;Hem, 1985;Stumm & Morgan, 1996). One mechanism of particular importance is trace-element adsorption on suspended minerals. In neutral-range pH waters lacking high concentrations of complexing anions (such as fluoride), detections of aluminum above approximately 1,000 micrograms per liter (µg/L) indicate the presence of suspended clay minerals; similarly, under neutral-range pH and oxic conditions, detections of iron above approximately 1,000 µg/L indicate the presence of suspended iron oxide minerals (Hem, 1985). Trace elements have affinities to adsorb on the surfaces of these suspended clay and oxide minerals, depending in part on the charges of the element species and sorptive surfaces.In the absence of contamination, the concentrations of the trace element versus aluminum (representing clays) or trace element versus iron (representing iron oxides) usually covary in a given data set, when adsorption of the trace element on suspended particulates is the dominant process. Under reducing conditions, however, such covariance is not always observed, at least for iron and associated trace elements that are 99Geochemical Evaluation of Metals in Groundwater at Long-Term Monitoring Sites and Active Remediation Sites redox active. Iron oxides become soluble as the redox potential drops below a threshold value, and this increases the dissolved concentrations of iron and the dissolved concentrations of trace elements that were adsorbed on the iron oxide surfaces. Detailed discussion of the effects of suspended particulates and reductive dissolution on elemental concentrations, as well as the methodology for evaluating these effects, is provided in Thorbjornsen and Myers (2007).Iron oxides become soluble as the redox potential drops below a threshold value, and this increases the di...