Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk Characterization• Ecosystems and receptors at risk. Certain critical assessment endpoints were notevaluated in this analysis, including impacts on managed lands, critical habitats, andthreatened and endangered species.• Synergistic risk. The impact of exposures of multiple contaminants to human andecological risks was not evaluated in this analysis. EPA recognizes that a singleconstituentanalysis may underestimate risks associated with multiple chemicalexposures.These are potentially the more significant uncertainties associated with the CCW riskassessment. Other uncertainties are discussed in Section 4.4.Given the results and characterization above, composite liners, as modeled in this riskassessment, effectively reduce risks from all pathways and constituents to levels below an excesscancer risk of 1 in 100,000 or an HQ of 1 for both landfills and surface impoundments. TheCCW risk assessment suggests that the management of CCW in unlined landfills and unlinedsurface impoundments may present risks to human health and the environment. From theperspective of what is known about toxic effects in humans, arsenic, nitrates, cadmium, andselenium appear to be among the constituents that may present risks of concern depending on thespecific waste management practices employed. From the perspective of what is known abouttoxic effects in ecological receptors, arsenic, boron, lead, and selenium emerge as havingdocumented adverse effects on ecological receptors.The estimated human health arsenic risks from clay-lined units are lower than the risks ofunlined units, but are still above a 1 in 100,000 excess cancer risk or an HQ of 1. In addition,surface impoundments typically showed higher risks than landfills, regardless of liner type.These risk results are largely consistent with damage cases compiled by EPA (U.S. EPA, 2000,2003e, 2007) and others (Lang and Schlictmann, 2004; Zillmer and Fauble, 2004; Carlson andAdriano, 1993; Rowe et al., 2002; Hopkins et al., 2006). These results suggest that with a higherprevalence of composite liners in new CCW disposal facilities, future national risks from onsiteCCW disposal are likely to be lower than those presented in this risk assessment (which is basedon 1995 CCW WMUs).April 2010–Draft EPA document. 4-61