This paper discusses the use of innovative investigation and analytical techniques to differentiate among mercury (Hg) forms present in soil/sediment samples to allow for the evaluation of overall environmental mobility of mercury and provide the basis for establishing technically-sound cleanup objectives. Mercury is an environmental contaminant that exists in several forms, including elemental Hg, inorganic Hg compounds, and organic Hg complexes. Typically, remedial evaluations are based on total Hg concentrations in soils and do not differentiate among the various Hg forms, and remediation standards have been developed assuming toxicological effects from one or more of the more toxic forms of Hg. In actuality, the potential human and environmental risks and environmental mobility associated with Hg contamination are highly dependent on the form of Hg present. Some forms of Hg, particularly Hg-sulfide complexes, are highly immobile, chemically stable, and generally have extremely low bioavailability in the subsurface. Hg-speciation data can be used to develop a technical basis for the application of alternate remediation standards when Hg contamination is present as less toxic or less mobile forms. At a former manufacturing facility, soil samples with total Hg concentrations ranging from 0.7 to 1795 mg/kg were analyzed using Sequential Selective Extraction (SSE) methods to speciate Hg. Initial SSE results demonstrate the absence of elemental Hg along with an average of 99.8% of Hg primarily biased to the later extracting fractions, indicating low environmental mobility. The bioavailable fractions represented on average 0.2% of the total Hg present and a very low portion of the total Hg (on average 0.01%) was found to be methyl Hg. Additional sampling and SSE testing are planned for this site to confirm the initial testing results and support development of cleanup objectives in consultation with regulators. Hg speciation also provides data that allow remediation efforts to be focused in areas where more toxic forms of Hg are present, and to support the selection of appropriate remedial approaches such as containment and/or long-term monitoring. At a former manufacturing facility known to be contaminated with elemental Hg, SSE of soil samples verified the presence of elemental Hg, speciated other Hg forms, and showed that two thirds of the samples contained over 90% of Hg in the later–extracting fractions. The remaining samples were characterized with the more soluble and bioavailable fractions. Sediment sample SSE analyses showed 89% to greater than 99% of the total Hg was in the later-extracting fractions. This result indicated that the majority of Hg present in the aquatic system, which is the key exposure area at the site, was less bioavailable and less mobile. These results provide key lines of evidence of reduced risk where consideration of literature information and other site chemical data (such as sulfide, carbon, and redox) are not sufficient to influence remedial decision making. The use of Hg speciation during site investigation provides a more accurate assessment of potential risks than does relying upon total Hg, resulting in more cost effective remedial action.
Use of Mercury Speciation to Support Alternate Remediation Standards and Cost-Effective Remediation
Authors: Jeffrey L. Pintenich, Adam R. Sherman and Tamara L. Sorell