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completed for the California Leaking Underground Fuel Tank program (Rice et al, 1995) and for the Texas Bureau of Economic Geology (Mace et al., 1997) have found no statistical difference between sites where groundwater remediation activities were implemented and sites where fuel hydrocarbon plumes were allowed to naturally attenuate. Newell et al. (1990) reported similarly stabilized plume configurations for many chlorinated hydrocarbon sites that were allowed to attenuate naturally. Non-destructive processes that result only in the reduction of VOC contaminant concentrations, but not of the total contaminant mass, include dispersion, sorption, dilution, and volatilization. Destructive processes that result in the reduction of contaminant concentrations (and mass) include biodegradation and abiotic degradation mechanisms. As shown in Table 3, the concentrations of nitrate and sulfate in the leachate collected at the Forward Landfill are relatively low compared to the concentrations of these constituents in groundwater near the landfill. Together with the presence of high free iron concentrations and low VOC concentrations measured in leachate collected at the site (Tables 1-3), the low nitrate and sulfate concentrations suggest that anaerobic biodegradation processes favorable for VOC degradation actively occur within the landfill. As detailed in the February and May 2002 report submittals and summarized here in Section 3, the attenuation analyses that were originally completed for the project ® conservatively ignored potential sorption, dilution, volatilization and abiotic degradation processes, and focused on dispersion and biodegradation of VOCs within the clay liner. As detailed in the May 30, 2002 report submittal, three sets of attenuation analyses were previously submitted to the RWQCB. These analyses considered 1) saturated leachate leakage conditions (unlikely considering the size of membrane leaks and the hydraulic conductivity of compacted clays); 2) unsaturated leachate leakage conditions; and 3) direct mixing of leaked leachate with groundwater (unlikely considering the attenuation processes likely to occur within both the clay liner and vadose zone). 2.5.1 Saturated Flow Analyses The first set of analyses were completed using the U.S. Environmental Protection Agency's (USEPA'S) BIOCHLOR and BIOSCREEN programs (Azia and Newell, 1999; Newell et al., 1996, respectively) to evaluate the fate of select VOCs associated with a hypothetical leachate leak within the proposed Forward Landfill liner. These analyses conservatively ignored potential sorption, dilution, volatilization and abiotic degradation processes, and focused on dispersion and biodegradation of VOCs within the clay liner. The analyses also assumed that saturated flow conditions would exist within the clay liner and that VOC concentrations would be significantly higher than the concentrations historically measured in Forward Landfill leachate samples. These analyses also C:\2002-002\Revised Appx A - Leakageldoc\09/19/02 Geologic Associates