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Sanitary -5— Foothill tary L n S a dfill Module LF-2 -Liner Performance Demonstration San Joaquin County 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 LF-2 for capturing landfill gas during the operating phase and inactive phase. 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 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 and/or Water Quality Protection Standards of the constituents of concern. Section 4.3.2—Leakage Zone For a one centimeter hole a 10-foot by 10-foot polygon was used. Please provide an explanation of why this size polygon was chosen. To be more conservative and possibly more realistic with regards to preferred pathways, we recommend that a 1-foot by 1-foot polygon be modeled. Section 4.3.4—Model Simulation Period We are concerned that additional infiltration will be occurring through the modeled 12 inch interim layer cover (HELP model) over the life of the entire landfill (multiple modules). Most of the rainfall in the valley comes in large storm events rather than slow rainfall over time. These storms erode the cover and expose the waste to infiltration on the steep side-slopes. We propose that the interim cover be thicker than 12 inches in order to minimize infiltration throughout the lifetime of the facility. This cover material may be excavated upon final waste disposal to reach final elevation/capacity. We expect that additional soil cover(i.e., 2, 3, or 4 feet)may aid in the settlement/compaction of the waste over time and possibly provide additional landfill space at the end of the landfill's life. Alternative interim cover materials may be acceptable. Title 27, Section 20705 (b) Minimize Percolation states that, "Interim cover over wastes discharged to a landfill shall be designed and constructed to minimize percolation of liquids through wastes."