Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk Characterization4.2 Ecological RisksEPA defines ecological risk characterization in terms of (1) the risk estimation, whichintegrates the exposure and stressor-response profile to estimate the likelihood of adverseecological effects and (2) the risk description, which synthesizes the overall conclusion of theassessment and addresses assumptions, uncertainty, and limitations.For assessments that are based on a HQ approach, as this one was, the comparison ofmodeled exposure concentrations to CSCLs to estimate risk has a binary outcome: either theconstituent concentration is above the concentration corresponding to an HQ of 1 or theconcentration is less than or equal to the concentration corresponding to an HQ of 1. For the fullscaleanalysis, an ecological HQ greater than 1 was selected by EPA as a criterion for decisionmaking. Because the CSCLs were based on de minimis ecological effects, it is generallypresumed that an HQ at or below 1 indicates a low potential for adverse ecological effects forthose receptors included in the analysis for which data are available. However, it is important torecognize that although this method provides important insight into the potential for adverseecological effects, the results are relevant only to those receptors that were included in theassessment and for which data were available. The results have limited utility in interpreting theecological significance of predicted effects, and caution should be exercised in extrapolating toecosystems (e.g., wetlands) and receptors (e.g., threatened and endangered species) not explicitlymodeled.This section presents risk results for direct surface impoundment exposure (as evaluatedin the 1998 CCW risk assessment, U.S. EPA, 1998a,b), screening results for boron that indicaterisks to plants from aboveground exposure, and the two groundwater-to-surface-water ecologicalexposure pathways investigated in the full-scale analysis: (1) receptors exposed to CCWconstituents in the water column (surface water receptors) and (2) receptors exposed to CCWconstituents in bed sediment (sediment receptors). Results are presented for the two WMU typesaddressed in the analysis: landfills and surface impoundments, and are broken out separately forthe different unit (liner) types. Finally, ecological damage case reports from U.S. EPA (2007)and from the published literature are summarized as field evidence supporting the conclusions ofthis risk assessment.The ecological risk results and damage cases suggest the potential for adverse ecologicaleffects to plants, terrestrial organisms, and aquatic systems from CCW releases into thesubsurface and subsequent connection with surface waters, particularly for CCW managed inunlined surface impoundments. As with human health risks, the higher prevalence of liners innewer facilities should result in lower risks in current and future CCW disposal facilities thanthose presented in this risk assessment.4.2.1 Direct Surface Impoundment ExposureThe current risk assessment addresses exposure to receptors in offsite surfacewaterbodies impacted by groundwater, where both the aquatic communities and upper trophiclevel terrestrial receptors would need to be protected. 7 The 2003 CCW constituent database used7The 2002 CCW constituent database does not include impoundment water samples, and the direct exposurepathway was not addressed.April 2010–Draft EPA document. 4-26