Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk Characterizationfacilities, including liner leakage rate for landfills and the number of liner perforations forsurface impoundments (TetraTech, 2001). Because data on CCW liner leakage rates were notavailable, there is some uncertainty in applying these Industrial D Guidance liner performancedata to CCW disposal units. Still, these rates do reflect actual performance data from liners underreal WMUs. They demonstrate that composite liners can be effective in reducing leaching fromCCW WMUs and suggest that there will be a decrease in risk from CCW disposal if morefacilities line their WMUs with composite or clay liners. Information from the more recentDOE/EPA study (U.S. DOE, 2006) indicates that composite liners are becoming more prevalentin newly constructed facilities, so the risks from CCW disposal should be lower for newer CCWlandfills and surface impoundments.4.1.4 Constituents Not Modeled in the Full-Scale AssessmentAs described in Section 3.2.4, full-scale modeling was not conducted for all 21constituents that were above the screening criteria in the initial screening analysis; onlyconstituents that were judged likely to have generally higher risks to human health andecological health were modeled in the full-scale risk assessment. 8 Five chemicals (chromium,fluoride, manganese, vanadium, and nickel) had drinking water pathway HQs in the screeninganalysis ranging from 1 to less than 6 for surface impoundments, and three (chromium, fluoride,and vanadium) had screening HQs of 2 for landfills.To address these unmodeled constituents, EPA developed surrogate risk attenuationfactors by dividing the screening risk results by the full-scale risk results, across all unit (liner)types combined, for the constituents modeled in the full-scale assessment. This comparison wasdone only for the drinking water exposure pathway, the only human health exposure pathway forwhich the risks for these constituents were above the screening criteria. Table 4-15 shows therisk attenuation factor statistics for the modeled constituents, and Table 4-16 shows the resultsof applying the median and 10th percentile attenuation factors to the screening risk results for themarginal constituents. Differences in attenuation among the modeled constituents reflectdifferences in contaminant sorption and mobility. To be conservative, the 10th percentileattenuation factor was selected as a high-end value representing the more mobile constituents,such as arsenic, selenium, and molybdenum. The 50th percentile (or median) risk represents acentral tendency value.Table 4-15. Risk Attenuation Factor a Statistics for Modeled Constituents—Groundwater to Drinking Water PathwayStatistic Landfill Surface Impoundment10th percentile 7 1.650th percentile 12 2.6Average 16 3.3Maximum 40 9.3(continued)8 These constituents of concern had human health HQs greater than 6 or both ecological HQs greater than 100 at the90th percentile.April 2010–Draft EPA document. 4-20