Human and Ecological Risk Assessment - Earthjustice

Section 3.0Analysisused to determine infiltration rates for each CCW liner scenario were consistent with theapproach used in EPA’s Industrial D guidance, as described in Section 4.3 and Appendix A ofthe EPACMTP Parameter/Data Background Document (U.S. EPA, 2003a) and Section 4.2.2 ofthe Industrial Waste Evaluation Model (IWEM) Technical Background Document (U.S. EPA,2002b). EPA developed the IWEM model as part of a guide for managing nonhazardousindustrial wastes in landfills and surface impoundments (http://www.epa.gov/industrialwaste).To help ensure that it was technically sound, the model (including the liner scenarios andalgorithms used in the CCW risk assessment) was developed with a large stakeholder workinggroup, including representatives from industry. The model was also subjected to a peer review in1999 (64 FR 54889–54890, October 8, 1999, Peer Reviews Associated with the Guide forIndustrial Waste Management), and the model was updated and improved in response to thosecomments before its final release in 2003. That update included the addition of a more robustliner leakage database to support the existing algorithms for calculating infiltration rates throughcomposite liner systems.No-Liner (Unlined) Scenario. For the no-liner scenario, infiltration rates were selectedfrom a database in EPACMTP that contains 306 infiltration rates already calculated using EPA’sHydrologic Evaluation of Landfill Performance (HELP) water balance model (Schroeder, et al.,1994a, b). HELP is a product of an interagency agreement between EPA and the U.S. ArmyCorps of Engineers Waterways Experiment Station, and was subjected to the Agency’s peer andadministrative review. All of the infiltration rates were calculated based on the single typicallandfill design described in Section 3.4.1, with the only variables that changed between HELPsimulations being the meteorological data associated with 102 nationwide climate centers andthe type of cover soil applied at closure. Three cover soil categories representing coarse-grainedsoils, medium-grained soils, and fine-grained soils were used. The selection of an infiltration ratefrom the database depends on the type of cover soil selected for the landfill and the assignmentof the landfill to a HELP climate center. The unlined HELP-derived infiltration rates arepresented in U.S. EPA (2003a) by climate center. The assignment of HELP climate centers andsoil categories to each CCW site modeled is described in Appendix C.Clay Liner Scenario. The clay liner scenario is very similar to the unlined scenario inthat previously calculated HELP infiltration rates for a single clay-lined, clay-capped landfilldesign were used. The scenario was based on a typical engineered compacted clay liner that is 3feet thick with a design hydraulic conductivity of 1x10 -7 cm/sec. The one difference from theunlined case is that the clay liner and cover control the rate of water percolation through thelandfill and thus infiltration rate does not vary with cover soil (i.e., there is one clay linerinfiltration rate per climate center). The clay liner HELP-derived infiltration rates are provided inU.S. EPA (2003a).Composite Liner Scenario. Composite liner infiltration rates were compiled frommonthly average leak detection system (LDS) flow rates for industrial landfill cells reported byTetraTech (2001). The liner configurations are consistent with the composite liner designassumptions presented in Section 3.4.1 and are the same as those assumed for defaults in EPA’sIndustrial D landfill guidance (U.S. EPA, 2002b). The LDS flow rates were taken from 27municipal landfill cells and used in the IWEM model (U.S. EPA, 2002b). As shown inTable 3-7, these LDS flow rates included 22 operating landfill cells and 5 closed landfill cellsApril 2010–Draft EPA document. 3-22