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North County Sanitary Landfill -3— Module 3— Module 4-Liner Performance Demonstration San Joaquin County Section 3.2 Module 4 of NCSL—Liner System Design Basis The Report states that the leachate drainage layer can also be utilized to remove gas pressure over the landfill liner if such action is needed for gas control. In order to meet the minimum requirement that the liner prevents migration of pollutants to groundwater, the County should incorporate a landfill gas collection system into the design of Module 4. Additional comments on this issue are addressed in this letter. The leachate collection and removal system will include a granular drainage layer over liner base areas and a geocomposite drainage layer on liner side slopes. Please provide the details of the type of granular drainage layer particle size and angularity. Describe the specific design of the geocomposite drainage layer proposed for the side-slopes and whether there would be any slope stability issues for Module 4. Section 3.2.1 HDPE Geomembrane Bottom of page 3-2 is missing. Section 3.4 State-of-Practice Construction Quality Assurance Please provide details of what type of leak detection survey/testing protocols are proposed for the HDPE liner component of the system. We highly recommend the electrical leak detection survey be used to support the performance standard of the construction of the alternative liner. Section 3.5—Potential for Impact of Groundwater by LFG The report states that the liner system will be the primary means to contain and control landfill gas from reaching groundwater. As mentioned before, trace amounts of VOCs have been detected at the site. The County should discuss the attributes of the liner system to contain and control landfill gas and the likelihood that gas may migrate through the modeled 1 cm hole. The Report states that the County conducts a routine LFG program at the landfill. It includes three shallow monitoring probes located around the existing waste footprint and at no time has combustible gas been detected in these probes. The County must elaborate on how many LFG probes will be installed to monitor the new modules and supporting rationale. Provide information on whether other landfill gases, other than combustible, have been monitored and or detected in the vadose zone at the site. The Report states that, "The first step will be to remove the pressure from the landfill liner by venting or collecting LFG from the leachate collection system. Further actions will be taken based on an engineering evaluation of the situation." Please address what type of landfill gas control system will be incorporated into the design of Module 4 for capturing landfill gas during the operating phase and inactive phase. Provide information regarding LFG control at the site and how Module 4 will be incorporated into the system, as applicable. Section 4.2—Leachate Parameter Title 27 Section 20310(c) specifies that containment structuresrp event degradation. The performance demonstration report compares the modeled attenuated constituent concentrations to drinking water