US EPA

EPA's Ecological Risk Assessment Support Center (ERASC) announced the release of the final report, Evaluating Potential Exposures to Ecological Receptors Due to Transport of Hydrophobic Organic Contaminants in Subsurface Systems. This technical paper recommends several types of screening assessments to evaluate site conditions for the potential to enhance transport of HOCs, as well as site artifacts that result from inadequate well installation and sampling procedures within a ground-water monitoring network. These assessments are incorporated into a suggested three-tiered decision analysis process that attempts to address all potential mechanisms that can result in HOC transport to surface water. For sites in which a complete exposure pathway exists from ground water to surface water, we summarize a process for using HOC exposure information to estimate risks to ecological receptors.

The Office of Research and Development’s Ecological Risk Assessment Support Center (ERASC) received a request from the Ecological Risk Assessment Forum (ERAF) relating to the mechanisms by which hydrophobic organic contaminants (HOCs) may transport via a ground water pathway to downgradient media, and how these mechanisms affect exposures, bioaccumulation and toxic effects on ecological receptors. Hydrophobic organic contaminants comprise a wide array of compounds, including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polychlorinated dibenzofurans, and polychlorinated dibenzo p-dioxins. These contaminants commonly enter into the environment in nonaqueous phase liquids (NAPLs), or more specifically in dense nonaqueous phase liquids (DNAPLs) such as creosote, coal tar, PCB oils and chlorinated solvents.

The occurrence of substantial concentrations of HOCs in ground water presents a conundrum in that HOCs are generally immobile in subsurface systems. For systems in which there is hydraulic connection between the contaminated aquifer and downgradient surface water bodies, assessment of ecological exposure to HOCs may be required. Mechanisms that may enhance subsurface HOC migration include increases in solubility due to cosolvency with other mobile organic constituents in the plume, and the association of HOCs with mobile colloids within the aquifer. HOC transport by either mechanism can be affected by the site-contaminant source composition, the geophysical characteristics of the hydrologic system, and the dynamics of subsurface chemistry, which can affect sorption of the HOC (chemical partitioning) to mobile (e.g., colloidal) or immobile subsurface solids. This technical paper recommends several types of screening assessments to evaluate site conditions for the potential to enhance transport of HOCs, as well as site artifacts that result from inadequate well installation and sampling procedures within a ground-water monitoring network. These assessments are incorporated into a suggested three-tiered decision analysis process that attempts to address all potential mechanisms that can result in HOC transport to surface water. The potential transport pathways (tiers) considered in the decision analysis process are 1) NAPL as primary transport medium, 2) ground water as primary transport medium, and 3) mobile colloids as primary transport medium. For sites in which a complete exposure pathway exists from ground water to surface water, the paper summarizes a process for using HOC exposure information to estimate risks to ecological receptors.

This report was written in response to a request from EPA’s Ecological Risk Assessment Forum (ERAF) relating to the mechanisms by which hydrophobic organic contaminants (HOCs) may be transported to groundwater and downgradient media, and how these mechanisms affect exposures, bioaccumulation and toxic effects on ecological receptors. This technical paper recommends several types of screening assessments to evaluate site conditions for the potential to enhance transport of HOCs, as well as site artifacts that result from inadequate well installation and sampling procedures within a ground-water monitoring network. These assessments are incorporated into a suggested three-tiered decision analysis process that attempts to address all potential mechanisms that can result in HOC transport to surface water. For sites in which a complete exposure pathway exists from ground water to surface water, we summarize a process for using HOC exposure information to estimate risks to ecological receptors.

Ford, R., M. Brooks, C. Enfield, AND M. Kravitz. Evaluating Potential Exposures to Ecological Receptors Due to Transport of Hydrophobic Organic Contaminants in Subsurface Systems (Final Report). U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-10/015, 2014.