Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk Characterization50th Percentile Risk Results for FBC Wastes: Landfills,Groundwater-to-Surface-Water Pathway (continued)50th Percentile HQ or Cancer Risk Value aChemical Unlined Units Clay-Lined UnitsNoncancerComposite-LinedUnitsCadmium 0 2E-05 0Selenium IV 0 8E-16 0Selenium VI 0 1E-03 0Thallium 0 1E-03 0aValues are HQs for all chemicals except arsenic; arsenic values are cancer risk. Zero resultsindicate that contaminant infiltration rates were too small for the contaminant plume to reachthe receptor during the 10,000 year period of the analysis.b Note that only the chemicals with adequate data that were identified in the screening analysisas needing further assessment (see Section 3.2.4) were modeled.4.1.3 Results by Liner TypeThe effect of liner type on human health risk for the groundwater-to-drinking-waterpathways can be seen in Tables 4-1 through 4-6 and for the groundwater-to-surface waterpathway in Tables 4-8 through 4-13, which present risks for WMUs that are unlined, clay lined,and lined with composite liners from the 1995 EPRI survey data (EPRI, 1997). At the 90thpercentile, lined units produced lower risk estimates than unlined units for all constituentsmodeled. Composite liners produced very low to zero risk estimates as compared to clay linersfor all constituents modeled for both landfills and surface impoundments. For surfaceimpoundments, clay liners produced higher risk estimates for all constituents as compared toclay liners in landfills. Similar trends are evident at the 50th percentile, where composite linersproduced risk estimates of zero or near zero for all constituents for surface impoundments.Table 4-14 shows how frequent each of the liner types is in the 1995 EPRI survey datamodeled in this analysis, and it compares these data with the liner type frequency in the morerecent DOE/EPA study (U.S. DOE, 2006). The 56 WMUs surveyed in the U.S. DOE 2006 studywere commissioned between 1994 and 2004. Although the actual number of WMUs that wereestablished in that timeframe cannot be verified, based on proxy data (i.e., CCW available fordisposal in those states with identified, new WMUs and coal-fired power plant generatingcapacity), the sample coverage is estimated to be at least 61–63 percent of the total population ofthe newly commissioned WMUs. 6 With the exception of one landfill, the newly constructedfacilities are all lined, with either clay, synthetic, or composite liners. The single unlined landfillidentified in the recent DOE report receives bottom ash, which is characterized as an inert wasteby the state, and therefore, a liner is not required.6For additional details as to how these estimates were derived, the reader is referred to the DOE study, pages S-2– S-3 of the Summary Section and Section 3.1.2..April 2010–Draft EPA document. 4-18