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Foothill Sanitary Landfill -4— Module 4_Module LF-2 -Liner Performance Demonstration San Joaquin County Section 2.5 Groundwater Monitoring System The Report states that the maximum historical groundwater elevation near proposed Module LF-2 ranges from 50 to 55 feet mean sea level (MSL). If a larger data set were used based on regional groundwater elevations prior to land filling at Foothill Landfill, the vadose zone depth would be thinner and a more conservative value would be computed. Therefore, historic elevation of the groundwater(before the landfill) should be evaluated and determined if it is possible, the water will rebound to these levels. Based on the determination the model should be run on the highest anticipated elevation. Please provide a discussion of any production, irrigation, and/or domestic wells in the vicinity. Include a map showing all wells within a one-mile radius of the site. Also,provide a discussion on historical regional high water levels in the Stockton area. This comment is also appropriate for Section 2.6 in Highest Anticipated Groundwater section. The San Joaquin County Flood Control District offices should have historical data referencing the highest groundwater elevations recorded. Section 2.6 Aquifer Characteristics Groundwater Quality: Based on historical analytical data, and the results reported in the quarterly groundwater and surface water monitoring reports, it appears that groundwater may be impacted from land filling activities. Specific VOCs found in the groundwater at Foothill Landfill should be considered when modeling leakage through the proposed single-composite liner system as mentioned above. Section 3.1 Liner System Properties Overview Paragraph 4 states the Geomembranes are essentially impermeable and can contain gases. Please provide documentation/information supporting this statement. 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 LF-2. 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 LF- 2. 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.