Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk Characterizationassumption of the analysis. The lack of information to define and model actual exposureconditions also introduces uncertainty into this assessment, but EPA believes that the nationaldistribution of exposure factors used is appropriate for a national assessment.In addition, not all possible exposure pathways were evaluated. For example, the riskassessment did not consider potential indirect exposure to humans through game species thatmay have been exposed to surface impoundment waste (e.g., deer drinking surface impoundmentwater). This represents a potential uncertainty in the analysis.Additive Risks Across Pathways. The human receptors evaluated in the CCW riskassessment were assumed not to consume both contaminated fish and drinking water from thesame waterbody because untreated surface water is not considered potable water (municipalwater treatment facilities were assumed to reduce contaminant levels prior to consumption). EPAalso did not consider the potential cumulative exposure from contaminated fish and groundwaterin the CCW risk assessment, because the exposures are likely to occur over different timeframes(because of differences in transit time of the contaminant plume to wells versus surfacewaterbodies) and may involve different receptors (because a resident near a CCW surfaceimpoundment or landfill and exposed via groundwater may not be a recreational fisher).Although this could potentially miss some higher exposures for a maximally exposed individual,analysis of the individual pathway results does not indicate that adding such risks would changethe conclusions of this risk assessment in terms of the constituents exceeding the risk criteria.Also, risks were high enough for single chemicals for human exposure pathways (notablyarsenic) that this would not change the basic conclusion of the risk assessment that there arepotentially significant risks to human health from CCW disposal in landfills and surfaceimpoundments.Co-Occurrence of Ecological Receptors and Constituents. As a simplification fornational-scale analyses in the absence of site-based data, co-occurrence of the ecologicalreceptors and the constituents of concern is typically assumed. However, the prior probabilitythat a receptor will be found in waterbodies affected by constituent releases from CCW WMUsis not known, nor is it known whether a receptor will forage for food in contaminated areas or ifthose areas do, in fact, support the type of habitat needed by the receptor. Although theassumption of co-occurrence was necessary for this analysis, relatively few field studies areavailable to demonstrate the relationship between adverse ecological effects and constituentreleases from CCW as it is currently managed.Ecosystems and Receptors at Risk. One challenge in conducting a predictive ecologicalrisk assessment intended to reflect risks at a national scale is representing all of the receptors andecosystems at risk. In Wastes from the Combustion of Coal by Electric Utility Power Plants -Report to Congress (U.S. EPA, 1988b), the authors pointed out that plants or animals of concernwere located within a 5-km radius of the CCW WMUs at 12 to 32 percent of the sites. Althoughthese figures are of limited spatial resolution, they suggest the possibility that threatened andendangered species or critical habitats may be at risk from CCW constituents. Examples of othercritical assessment endpoints not evaluated in this analysis include the following:• Managed Lands: Because protected lands play a critical role in preserving plant andanimal species, managed areas in the United States represent well-recognized ecologicalApril 2010–Draft EPA document. 4-40