Human and Ecological Risk Assessment - Earthjustice

Section 3.0Analysiscontaminant mass in the landfill. CCW landfill area data were obtained from the EPRIcomanagment survey (EPRI, 1997). The landfill was assumed to be square.• Landfill Depth. Landfill depth is one of several parameters used by the landfill sourcetermmodel to calculate the contaminant mass in the landfill. For CCW landfills, averagewaste depth was estimated by dividing landfill capacity by landfill area. CCW landfillcapacity data were taken from the EPRI comanagement survey (EPRI, 1997).• Depth Below Grade. The depth of the bottom of the landfill below the surroundingground surface is used, along with depth to groundwater, to determine the thickness ofthe unsaturated zone. For CCW landfills, depth below grade was determined from anational distribution based on available measurements from a number of CCW landfills(see Appendix B).• Waste Fraction. The landfills were assumed to be CCW monofills, which corresponds toa waste fraction of 1.0.• Waste Density. The average waste bulk density, as disposed, is used to convert wastevolume to waste mass. The waste bulk density for all CCW waste types was assumed tobe 1.19 g/cm 3 (U.S. EPA, 1998b).• Leachate Concentration. The concentration of waste constituents in leachate wasassumed to be constant until all of the contaminant mass initially present in the landfillhas leached out, after which the leachate concentration was assumed to be zero. Theconstant value used for leachate concentration is from EPA’s CCW ConstituentDatabase, described in Appendix A.• Waste Concentration. In the finite-source scenario modeled, the total wasteconcentration is used, along with the waste bulk density and landfill area and depth, todetermine the total amount of a constituent available for leaching. Measured total CCWconcentrations were paired with leachate concentrations, as described in Appendix A andprovided in Attachment A-2.• Liner Type. The type of liner is used to determine the infiltration/leaching scenario usedto calculate leachate flux from the landfill. Table 3-8 shows the crosswalk used to assignone of the three liner scenarios to each facility based on the liner data in the 1995 EPRIsurvey (EPRI, 1997). Attachment B-2 to Appendix B provides these assignments, alongwith the original EPRI liner type, for each CCW landfill facility modeled. Oneuncertainty in these liner assumptions is how representative the EPRI survey data are ofcurrent conditions at coal combustion facilities.Table 3-8. Crosswalk Between EPRI and CCW Source Model Liner TypesEPRI Liner Type Model Liner Code DescriptionCompacted ash 0 no linerCompacted clay 1 clay(continued)April 2010–Draft EPA document. 3-25