Tools for Assessing the Role of Particle Association in Point Source Hg and Pb Contamination at a Superfund Site
The Berlin Superfund site, in northeastern New Hampshire, is a former chlor-alkali facility on the banks of the Androscoggin River, where liquid Hg has been observed seeping from rock fractures along the river bank. The site is currently undergoing an EPA Site Assessment to determine the extent of Hg contamination in the river. In this pilot project, Dartmouth researchers collaborated with USGS and the USEPA, to further understanding of the Hg mobility, methylation and bioavailability downstream of a point source of contamination.
A confounding feature to the Berlin site is the presence of 7 dams within 12 miles downstream of the Superfund site. Dam reservoirs can act as both deposition zones for contaminated sediment, as well as good environments for Hg methylation. The pilot project focused on assessing Hg and MeHg profiles in sediment and porewaters in these reservoirs. Concentrations of Hg and MeHg were elevated in reservoirs downstream relative to upstream of the Superfund site, although levels were highly variable within each site. Porewaters are considered an important biogeochemical compartment for assessing metal contamination and are used to predict the bioavailable and mobile fractions of contaminants in sediment. Yet, concentrations of mercury species in porewaters can be difficult to relate to MeHg concentrations in other environmental compartments, in part because porewaters are generally considered as a single dissolved fraction, which ignores the role of colloidal and organic ligand binding on geochemical behavior. Using three different techniques to size fractionate Hg in porewaters, inorganic Hg was found to be colloid-bound in porewaters, whereas the more toxic species, MeHg was truly dissolved or bound to low molecular weight organic particles, suggesting it is more mobile.
Sampling for this study was co-ordinated with the EPA site assessment, and findings of this study were communicated with the USEPA during meetings at Dartmouth in 2011 and at the EPA office Chelmsford MA in 2012. The techniques developed for assessing particle binding in this study were included in a successful RO1 grant application in 2012.