Human and Ecological Risk Assessment - Earthjustice

Section 3.0Analysisstream (surface impoundment porewater and landfill leachate) for comparison with health-basednumbers (HBNs) and CSCLs. Data processing to create these analyte concentrations involvedtwo steps:• Calculation of average constituent concentrations by site for landfill leachate,surface impoundment porewater, and total ash concentrations. Site averaging avoidspotential bias toward sites with many analyses per analyte. During site averaging,separate waste disposal scenarios at a site (e.g., non-FBC and FBC ash; FGD sludge andash) were treated as separate “sites” and averaged independently. Nondetects wereaveraged at one-half the reported detection limit. 6• Selection of screening concentrations from site-averaged values. For the screeningcalculations, the analysis used the 90th percentile of the site-averaged concentrationsacross all sites for landfill leachate and surface impoundment porewater.Appendix A describes the CCW constituent database and how the waste constituentconcentrations were selected and processed for the screening analysis and full-scale riskassessment.3.2.2 Media-Specific Exposure Concentrations for ScreeningThe screening analysis required media concentrations for groundwater, surface water,and sediment to compare with the HBNs and CSCLs. As a simple first screen of risk, theanalysis used waste concentrations as protective estimates of offsite groundwater and surfacewater concentrations.For groundwater-to-drinking-water exposures, the analysis used the 90th percentile wasteporewater 7 and leachate concentrations to represent groundwater contamination from the surfaceimpoundment and landfill, respectively. No dilution or attenuation was assumed between theWMU and the drinking water well because the large size range of CCW units precluded the useof a dilution attenuation factor (DAF) 8 for a nearby well. Similarly, surface water concentrationswere assumed to be equivalent to waste leachate and porewater concentrations.3.2.3 Screening MethodologyThe CCW screening approach compared protective health-based concentrations in eachmedium of concern with estimated offsite media concentrations of CCW constituents describedin Section 3.2.2. Both human and ecological receptors were addressed. HBNs are mediaconcentrations developed to protect human health, and CSCLs are media concentrationsdeveloped to protect ecological receptors. HBNs were calculated based on the target risk criteria6 Appendix A contains figures showing how site-averaged 90th percentile concentrations and 90th percentileconcentrations taken across all analyses (nonaveraged concentrations) compare with HBNs for surfaceimpoundment porewater, toxicity characteristic leaching procedure (TCLP) leachate, and whole wasteconcentrations.7 Although the 95th percentile was used in 1998, the 90th percentile was used in this analysis as a reasonablyconservative value considering the protective screening analysis assumptions and the larger 2002 constituent dataset.8 A DAF is the waste concentration divided by the media concentration at the point of exposure.April 2010–Draft EPA document. 3-9