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0 2.5.3 Leachate and Groundwater Mixing Analyses Since many inorganic constituents may be unaffected by biodegradation processes, calculations were completed to evaluate the magnitude of impacts that could be expected if undiluted leachate were to migrate vertically and mix with groundwater. HDPE liner defects were assumed to be consistent with the default criteria specified by the USEPA's HELP3 computer model, and one foot of leachate head was assumed to be applied on the liner. Attenuation processes in the clay liner and vadose zone were ignored, and leachate/groundwater mixing was assumed to occur at the same rate as saturated flow through the liner. Groundwater flow parameters were assumed to be consistent with those identified by Herst & Associates / Andrews Environmental Engineering ([HA/AEE], 2001). As noted by RWQCB staff, the mixing analyses included a calculation error (the number of defects per acre were unintentionally omitted), but, as detailed here in Section 3.1, the conclusion that the impacts on groundwater quality would be statistically insignificant remains true. 3.0 ADDITIONAL ANALYSES Recognizing RWQCB staff's recent concerns, additional analyses were completed that considered potential groundwater impacts associated with unattenuated leachate concentrations. As described below, new leachate/groundwater mixing analyses were completed using the historical average leachate concentrations measured at the Forward Landfill. Analyses were then completed that consider the more likely condition of unsaturated leachate flow through the vadose zone beneath the landfill. As requested, additional leachate / groundwater mixing analyses were completed to estimate the potential impacts to groundwater associated with undiluted leachate constituents leaking directly to groundwater from the proposed single composite liner at the Forward Landfill. These calculations conservatively assumed that no leachate would be entrained in the clay liner or vadose zone and that all leaked leachate mixes directly in groundwater. As indicated below, leachate leakage and migration to groundwater is calculated to result in a statistically insignificant increase in constituent concentrations in groundwater. In a real sense, the calculated changes in groundwater chemistry would be undetectable. 3.1.1 Liner Leakage Volume The USEPA's HELP3 computer model assumes that a typical high density polyethylene (HDPE) liner defect is approximately 1 square centimeter, and that approximately 4 defects occur over a given acre of liner. Assuming that the hydraulic conductivity of the 2 -foot thick clay section of the liner is equal to 1 E-07 cm/sec, the volume of leachate that will ultimately leak through an acre of lined area in saturated flow is: CA2002-002\Revised Appx A - Leakageldoc\09/19/02 Geologic Associates