Human and Ecological Risk Assessment - Earthjustice

Section 3.0AnalysisNote that although mercury was originally addressed in both the 2002 screening and2003 full-scale analyses, results were removed from the 2007 draft and this version of the riskassessment report because subsequent evaluation found that the very high proportion of mercurynondetects in the CCW constituent database, along with the use of one-half the detection limitfor the nondetect measurements, led to the results being driven by the detection limit, rather thanthe actual (but unknown) levels in CCW leachate and porewater. Therefore, the results were notmeaningful in terms of the actual risks mercury in CCW poses to human and ecological health.Similarly, a large number of nondetects (or a very small number of measurements) preventedaccurate screening or full-scale analysis for antimony, thallium, and cobalt in surfaceimpoundments. These uncertainties are discussed in Section 4.4.3.1.Full-scale modeling was not conducted for all 21 constituents that had 90th percentilerisks above the screening criteria for the groundwater pathways. Instead, those 21 constituentswere ranked and divided into two groups to focus the full-scale analysis on the CCWconstituents that were likely to pose relatively higher risks to human and ecological receptors.The ranking was based on the magnitude of the HQs and the number of HQs exceeding thescreening criteria, and was used to select chemicals for full-scale modeling. Constituents with atleast one human health HQ greater than 6 or with ecological HQs greater than 100 for bothlandfills and surface impoundments were modeled. Arsenic, with cancer risks greater than 1 in1,000, exceeded the cancer risk criterion by a factor of 100 and was also modeled in the fullscaleanalysis. Constituents with no human health HQs greater than 6 and only one or noecological HQs greater than 100 were not modeled, but were addressed in a separate analysisusing results from the modeled constituents.Table 3-5 shows the 21 constituents and which of these constituents exceeded thescreening criteria and thus were modeled in the full-scale analysis. As shown, 12 constituentswere subjected to the full-scale probabilistic risk assessment described in this document. Another9 constituents exceeded the screening criteria and were addressed using risk factors developedfrom comparing the screening and full-scale results for the modeled constituents, as described inSection 4.1.5 of this document.3.3 Full-Scale Modeling ApproachThis section describes the framework, general assumptions, and constraints for thefull-scale probabilistic analysis. Section 3.3.1 describes the temporal and spatial framework.Section 3.3.2 describes the probabilistic framework, and Section 3.3.3 describes how theassessment was implemented within the probabilistic framework.3.3.1 Spatial and Temporal FrameworkThe spatial framework for the analysis was determined by the geographic distribution ofCCW facilities modeled and by the site layout assumed as the conceptual site model for riskassessment. As described in Section 2.1.2, the geographic distribution of landfills and surfaceimpoundments managing wastes onsite at coal-fired utility power plants was determined fromthe 177 sites in the 1995 EPRI survey of the onsite management of CCW (EPRI, 1997). Theassessment assumes that these 177 sites and their locations were representative of theApril 2010–Draft EPA document. 3-13