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migration from the base of the landfill, the new extraction wells will be preferentially screened near the bottom of the landfilled waste to exert a greater vacuum at the base of refuse and minimizing air intrusion from the surface of the landfill. Similar to the existing LFG extraction wells, the new wells will be constructed of SCH40 PVC pipe, however, the upper well section comprised of solid pipe will be lengthened from the previous design to 35 to 50 feet. The lower section will still be perforated pipe, but will be installed to a depth closer to the bottom of the waste mass. The construction details of a typical LFG well are shown in Appendix C. The actual depth of the wells and length of solid and perforated sections will be determined in the final design phase. Should field conditions present an unexpected site condition (e.g. waste is shallower or deeper than anticipated) the final well installation may also be adjusted in the field. Water Board Question: How will these new and existing gas extraction wells be operated, maintained and monitored? County Response: Vacuum will be applied to the new extraction wells gradually, slowly increasing the vacuum at each well by adjusting each well head control valve based on the quality of the gas collected at each well. These new wells will be monitored weekly until each well and the wellfield reaches steady state operation, then monthly thereafter as part of the existing operations and monitoring program. Routine operations, monitoring, and maintenance are described below. The extraction wells will be operated to control landfill gas emissions and migration, to meet the LFG quality goals of the onsite landfill -gas -to -energy (LFGTE) power plant while minimizing air intrusion. Vacuum is increased or decreased using the well head control valve based on the quality of the collected LFG. Wells are monitoring monthly for applied vacuum, operation temperature, and concentrations of methane, carbon dioxide, oxygen, and nitrogen (balance gas). Wells are inspected monthly for air leaks or any visible damage to the well head or casing. Periodically, wells are "sounded" to check for the presence of water, casing damage from settlement, or other blockages that would impede LFG recovery. Water Board Question: How will these landfill gas wells be installed? County Response: The LFG extraction wells will be installed vertically using a bucket auger or similar drill rig. Bucket augers are typically 24 -inch, 30 -inch, or 36 -inch in diameter. The wells will be installed to a maximum depth possible without penetrating through the refuse mass. State of Engineering Practice's call for installation depths of wells to be defined by a certain percentage of the refuse thickness, usually between 70 percent and no more than 80 percent, allowing the LFG generated at the base of the landfill mass to be collected. However, because the purpose of these wells is to prevent LFG from impacting groundwater, we proposed to drill to within ten (10) feet of the base of the landfill. Water Board Question: What is the time schedule for installation, and for proposed phases, if necessary? What criteria will be used to determine moving from Phase I to Phase Il? Foothill Sanitary Landfill 9 Department of Public Works, Solid Waste Division Revised Engineering Feasibility Study County of San Joaquin — December 19, 2014