Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk Characterizationunder the Clean Water Act, and although they pose an uncertainty in the analysis, they areoutside the scope of the risk assessment (see Section 4.4.1 for further discussion).4.1.2.2 FBC WastesTables 4-12 and 4-13 show the 90th and 50th percentile risk results for FBC landfills byliner type. These results are much lower than those for conventional CCW and comanaged CCWlandfills seen above, and suggest that releases from FBC landfills do not present a hazard tosurface waters. This difference may be attributed to lower FBC leachate concentrations and thealkaline nature of FBC wastes. However, as with the FBC results reported for drinking water, theresults here are strongly influenced by the small sample size of site data available. Thus, thelimitation of only having seven sites may present an uncertainty in the analysis.Table 4-12. 90th Percentile Risk Results for FBC Wastes: Landfills,Groundwater-to-Surface-Water Pathway90th Percentile HQ or Cancer Risk Value aChemical Unlined Units Clay-Lined UnitsFBC Waste – 7 landfillsCancerComposite-LinedUnitsArsenic III 4E-12 7E-08 0Arsenic V 3E-12 3E-08 0NoncancerCadmium 7E-07 0.02 0Selenium IV 3E-17 8E-03 0Selenium VI 5E-06 0.1 0Thallium 5E-06 0.2 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.Table 4-13. 50th Percentile Risk Results for FBC Wastes: Landfills,Groundwater-to-Surface-Water Pathway50th Percentile HQ or Cancer Risk Value aChemical Unlined Units Clay-Lined UnitsFBC Waste – 7 landfillsCancerComposite-LinedUnitsArsenic III 0 3E-13 0Arsenic V 0 6E-14 0(continued)April 2010–Draft EPA document. 4-17