Human and Ecological Risk Assessment - Earthjustice

Section 2.0Problem Formulation2.3 Screening AnalysisTo assist in selecting constituents for full-scale modeling, a screening analysis wasconducted in 2002 (U.S. EPA, 2002a) that compared very conservative estimates of exposureconcentrations (e.g., leachate concentrations) to health-based concentration benchmarks toquickly and simply identify constituents with risks that clearly do not exceed the risk criteria sothat these could be eliminated from further analysis. For example, for the groundwater-todrinking-waterpathway, leachate concentrations were compared directly to drinking waterstandards, which is equivalent to assuming that human receptors are drinking leachate. 5 Similarlyconservative estimates were used for ecological receptors (e.g., fish swimming directly inleachate). EPA made use of those screening results in this risk assessment, which was conductedin 2003 and documented in the August 6, 2007, draft report and its subsequent revisions,including the current document. Section 3.2 provides further detail on how the CCW screeninganalysis was conducted to develop the list of CCW constituents modeled in the full-scaleanalysis.2.4 Full-Scale Risk AssessmentAlthough the screening analysis identified the potential for risk for a subset of theconstituents reported in CCW, the conservative assumptions used precluded an accuratequantitative estimate of these risks. The screening results were not intended to, and do not,characterize the risks that we expect would actually occur, because the purpose is not tocharacterize risks but rather to identify those constituent/pathway/receptor combinations that areunlikely to be problematic versus those that are most likely to be problematic. To gain a betterunderstanding of the risks that may be posed by the constituents identified as likely to beproblematic, EPA conducted a full-scale probabilistic (Monte Carlo) risk assessment to estimatethe national distribution of the risks to human health and the environment posed by CCWdisposal, and to provide the information needed to assess future management options for thesewastes in the context of their risks to human health and the environment. The full-scale CCWMonte Carlo risk assessment was designed to characterize the national CCW risk profile in termsof WMU type, waste type, and constituent, and to use distributions in a probabilistic modelingframework to incorporate variability and uncertainty into the analysis.The full-scale modeling approach used data about waste management practices andenvironmental conditions at 181 utility CCW disposal sites across the United States. 6 These siteswere assumed to represent the universe of CCW onsite waste disposal sites at the time of theEPRI survey (1995) and defined the national framework for the risk assessment. One questionrelated to this risk assessment is how CCW facilities may have changed since the 1995 EPRIsurvey. Although the DOE/EPA survey did not include all of the data needed to conduct a riskassessment (WMU area and capacity data were not collected), liner conditions were addressed,and by comparing the DOE/EPA survey results to the EPRI data, it is possible to assess how5 Note that RCRA waste disposal risk assessments do not address direct discharges from impoundments to surfacewaters because they are regulated as permitted point source discharges under the Clean Water Act by EPA’sOffice of Water.6 These 181 sites include177 sites from the EPRI survey and 4 additional CCW sites added by EPA to betterrepresent FBC waste disposal facilities; see Section 2.1.2.April 2010–Draft EPA document. 2-12