Human and Ecological Risk Assessment - Earthjustice

Section 3.0Analysis3.4.4 Model OutputsCrosswalk Between EPRI and CCW Source ModelLiner Types (continued)EPRI Liner Type Model Liner Code DescriptionComposite clay/membrane 2 compositeDouble 2 compositeGeosynthetic membrane 2 compositeNone/natural soils 0 no linerFor each year in the simulation, the landfill source-term model uses the average annualleachate concentration and infiltration rate to calculate a constituent flux through the bottom ofthe landfill. This time series was used as an input for the EPACMTP unsaturated zone model.3.5 Surface Impoundment ModelReleases from surface impoundments were modeled using a surface impoundmentsource-term model contained in EPACMTP. An overview and statement of assumptions for thesurface impoundment model are presented here, followed by a listing of inputs to the surfaceimpoundment source-term model and a brief discussion of the output generated by the model.The primary differences between the treatment of landfills and surface impoundments are (1) theintegration of the surface impoundment source term into the unsaturated flow solution, and(2) clean closure of the impoundment after the operating period is over.3.5.1 Conceptual ModelThe surface impoundment model treats a surface impoundment as a temporary WMUwith a prescribed operational life. Unlike the landfill model, clean closure is assumed; that is, atthe end of the unit’s operational life, the model assumes that all wastes are removed and there isno further release of waste constituents to groundwater. Although this simplifying assumptionlimits the length of potential exposure, and is not consistent with the practice to close CCWsurface impoundments with these wastes in place, the peak annual leachate concentrations onwhich the CCW risk results are based are not likely to be affected, because they are highest whenthe surface impoundment is in operation due to the higher hydraulic head in an operatingimpoundment, which drives leachate into the underlying soil with greater force than infiltrationafter the impoundment is covered and closed. This higher head results in a greater flux ofcontaminants to groundwater during the active life of the surface impoundment, especially inunlined units. These assumptions are discussed further in Section 3.5.3.Following the unit’s closure, the surface impoundment model assumes that thecontaminated liquid and sediment in the surface impoundment are replaced by uncontaminatedliquid and sediment with otherwise identical configurations and properties. The contaminantsthat have migrated to the unsaturated zone during operation continue to migrate towards thewater table with the same infiltration rate as during operation. By continuing infiltration after thewastes are removed, the infiltration through the surface impoundment unit can be modeled as aApril 2010–Draft EPA document. 3-26