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11 integrated conservative assumptions regarding the hydraulic conductivity of the compacted clay liner, the leachate head that would be applied on the liner, and integrated conservative estimates regarding the potential for biodegradation concurrent with leachate leak migration. The results of these analyses indicate that, even under assumed saturated now conditions, VOCs would likely be degraded between 80% and 100%. Recognizing the low VOC concentrations in leachate samples from Forward Landfill, this level of degradation would likely result in non -detection of VOCs at the base of the liner section. However, in the absence of peer-reviewed field demonstrations, CVRWQCB staff requested the additional analyses that are presented in Section 3.0. 2.5.2 Unsaturated Flow Analyses Owing to the low hydraulic conductivity and high matric potential of clays, leachate will be drawn through small defects in the HDPE liner section in radial fashion, both vertically and horizontally within the clay liner and away from the membrane hole. Because of the low hydraulic conductivity of the clay, only limited volumes of leachate may flow through the small defects, and leachate migration will take place as unsaturated flow through the two -foot thick compacted clay liner. The fate of leachate constituents migrating in an unsaturated condition was evaluated in two parts using the SEEP/W and CT programs (Geo -Slope Ltd., 1992) to determine the likely unsaturated flow rates and the initial concentrations of leachate constituents in unsaturated flow. The BIOCHLOR and BIOSCREEN programs were also used to evaluate leachate degradation at synthetically reduced steady-state flow rates. The SEEP/W simulations indicate that unsaturated flow through the two -foot -thick liner takes approximately 8.8 years, compared with about 5.1 years for saturated flow. The CTRAN/W analyses indicate that the concentration of leachate constituents at the end of 8.8 years of unsaturated flow will be less than 1% of their initial concentrations at the top of the clay liner section. The difference between the saturated time of travel (5.1 years) and the 8.8 year unsaturated flow time is caused by the lateral migration of leachate within the clay which results in "wicking" of leachate is a "tear -drop" shaped wetting front. In fact, owing to continued wicking in the vadose zone, moisture conditions within the clay liner would likely always remain unsaturated. Since the concentrations of leachate constituents at the base of the liner would gradually increase, the BIOCHLOR and BIOSCREEN analyses evaluated VOC attenuation within the clay liner under steady-state conditions and at the reduced unsaturated flow velocity. Again, 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 results of these analyses indicate that VOC degradation will likely be approximately 99 to 100%. If biodegradation processes are ignored, VOC attenuation within the liner was calculated to be 100% for all constituents evaluated except for vinyl chloride, which was degraded by 90%. C:t2002-0021Revised Appx A - Leakageldoc109/19102 Geologic Associates