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Foothill Sanitary Landfill -6— Module 6_Module LF-2 -Liner Performance Demonstration San Joaquin County Please address the issue of a thicker layer for interim/intermediate cover during the inactive life of the modules as part of the HELP simulation. Please justify the minimum 12"cover considering that heaviest leachate flows are before the final cover, and long length of time before this unit is closed. Maybe a thicker interim cover could be used to "minimize percolation of liquids"per Title 27 and then removed when new waste will be placed on top. Section 4.4.1 Sensitivity Analysis The performance demonstration report compares the modeled attenuated constituent concentrations to drinking water Maximum Contaminant Levels for the inorganic constituents chloride and barium to demonstrate that the liner design will prevent degradation of groundwater. However, the results of the performance demonstration modeling need to be compared with background concentrations of the constituents of concern as established in the Water Quality Protection Standard as discussed above. The Report mentions that a limited sensitivity analysis was performed on selected input parameters. However, the details of this analysis are not presented. The only detail provided is the outcome of increasing the leakage rate from 0.2 gal/ac-day to 2.0 gal/ac-day, in which it was reported that"all seven modeled constituents remained well below MCLs." The definition of degradation is any statistical or nonstatistical increase above the WQPS (that is, two or more constituents detected above the MDL or one or more constituent detected above the PQL in the case of non-naturally occurring organics, as per the Standard Provisions). The outcome of the sensitivity analysis for this particular input parameter is therefore evidence of a release. Please perform a sensitivity analysis on all input parameters and provide a detailed explanation of the analysis and impact of the results. Appendix A— Table A-1 Table A-1 shows computed leakage rates for composite liner designs. The alternative GCL liner was computed with"poor" and"good contact,"but the prescriptive liner was only computed with"poor contact." Please provide an analysis of the prescriptive liner with"good contact" for comparison. Appendix D— VLEACH Modeling Table 2 - Please provide an explanation of what these parameters were based on? The foc percentage appears to be high for soils on the eastern side of the San Joaquin Valley based on samples taken at other sites. Please provide the rationale for using the organic carbon fraction of 0.05%. We recommend the County measure the true organic carbon content of the representative soil underlying the site (i.e. at the proposed depth of the base of the landfill). The polygon are of 100 sq. feet should be reduced to one square foot to represent more closely preferred pathways. A sensitivity analysis should be performed on each of these parameters.