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Water Board Question: What is the quality of gas going into the gas to energy plant? County Response: The current quality of LFG going into the gas to energy plant is about approximately 48.7% methane based on September 2014 data. Existing Soil Gas Wells The existing single -probe soil gas monitoring wells SG -2 through SG -11 were installed in a pea - gravel -filled boring that extends to depths of approximately 65 feet. A single probe extends into the top of the pea gravel, allowing the collection of a sample representative of the gas in the soil adjacent to the pea -gravel -filled boring. These wells do not allow the County to determine at what depth (or interval) any methane or volatile organic compounds may be detected. Although these wells are still monitored, the County has installed new, tri -level wells, in compliance with Title 27 standards, around the perimeter of the landfill. These three -probe wells are drilled to the elevation of the base of the landfill. Water Board Question: How and when will the existing landfill gas wells (SG -1 through SG - 11) be redesigned and replaced such that they monitor the presence of landfill gas, independently, within the intervals of highest permeability. The distance from the new well to the foot print of the landfill shall not exceed the distance to the nearest SG well. County Response: As part of the Phase 1 corrective action plan, in an effort to enhance the monitoring of the vadose zone in the vicinity of Module "I" and to monitor the performance of the existing and proposed GCCS wells in controlling LFG migration at depth, the County is proposing to replace five (5) of the existing single -probe soil gas wells (SG- 2, SG -3, SG -4, SG -5, and SG -11) with triple -probe monitoring wells similar in configuration to the existing Title 27 wells on site. Figure 3, in Appendix B illustrates the proposed location of these new wells and the table below presents the proposed depths of the probes at each location. These proposed probe depths are be based on the existing geology of the site recorded during the previous installation of groundwater monitoring wells at the site. The depths will be finalized in the field based on sections with the greatest chance of conveying LFG through the site (i.e., zones of highest permeability). A registered geologist will be on site during the installation of the wells to observe each boring for the presence of sandy lenses and to determine where each monitoring interval (screened interval) will be installed or set. Foothill Sanitary Landfill 11 Department of Public Works, Solid Waste Division Revised Engineering Feasibility Study County of San Joaquin — December 19, 2014