Human and Ecological Risk Assessment - Earthjustice

Appendix BWaste Management Units• Clay Liner Scenario. For landfills, waste is placed directly on a 3-foot compacted clayliner, which is installed on the local soils, either on grade or excavated to some designdepth and without a leachate collection system. After the landfill has been filled tocapacity, a 3-foot clay cover is installed and covered with 1 foot of loam to supportvegetation and drainage. The hydraulic conductivity of both the liner and cover clays isassumed to be 1x10 -7 cm/sec. For surface impoundments, wastewater is placed on acompacted clay liner, which is installed on the local soils. The assumptions for an unlinedimpoundment also apply to the compacted clay liner scenario, except that a compactedclay liner filters out the sediments that clog the native soils in the unlined case, so theeffect of clogging the native materials is not included in the calculation of the infiltrationrate. The thickness of the compacted clay liner was assumed to be 3 feet and thehydraulic conductivity was assumed to be 1x10 -7 cm/sec.• Composite Liner Scenario. For landfills, wastes are placed on a liner system thatconsists of a 60 mil HDPE membrane with either an underlying geosynthetic clay linerwith a maximum hydraulic conductivity of 5x10 -9 cm/sec, or a 3-foot compacted clayliner with a maximum hydraulic conductivity of 1x10 -7 cm/sec. A leachate collectionsystem is also assumed to exist between the waste and the liner system. After the landfillhas been filled to capacity, a 3-foot clay cover is assumed to be installed and coveredwith 1 foot of loam to support vegetation and drainage. For surface impoundments,wastewater is placed on a synthetic membrane with an underlying geosynthetic or naturalcompacted clay liner with a hydraulic conductivity of 1x10 -7 cm/sec. The membrane linerwas assumed to have a number of pinhole leaks of uniform size (6 mm 2 ). The number ofthese leaks was based on an empirical distribution of membrane leak density valuesobtained from TetraTech (2001), as described in the IWEM Technical BackgroundDocument (U.S. EPA, 2002).Table B-2 shows the crosswalk used to assign one of the three liner scenarios to eachfacility based on the liner data in the EPRI survey data (EPRI, 1997a). Attachment B-2 providesthese assignments, along with the original EPRI liner type, for each CCW landfill facilitymodeled.Table B-2. Crosswalk Between EPRI andCCW Source Model Liner TypesEPRI Liner TypeModel LinerCodeDescriptionCompacted ash 0 no linerCompacted clay 1 clayComposite clay/membrane 2 compositeDouble 2 compositeGeosynthetic membrane 2 compositeNone/natural soils 0 no linerApril 2010–Draft EPA document. B-4