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The 4 to 5 year response time between surface actions and groundwater impact is consistent with the relatively deep (200 feet) vadose zone at the site. Therefore, it is our professional opinion that VOCs driven out of the base of Area"I" are trapped in the vadose zone. This is because the increased LFG generated by an increase in refuse moisture due to ponding was corrected in 1998. The new driving force for VOCs must be the entrapment of LFG beneath the impermeable HDPE barrier. ENGINEERING FEAST ILI STUDY The current groundwater well network consists of five wells; upgradient well MW-4 and downgradient wells MW-IA, MW-2R, MW-3, and MW-5 (Figure 2). The current vadose zone monitoring network consists of eleven single-probe interior wells SG-1 through SG-11, and 25 triple-probe perimeter wells, SG-12 and SG-102 through SG-125 (Figure 2). Wells SGA through SG-I 1 were installed in 2003. Well SG-8, was later abandoned and removed for the expansion into Area 2. The NOV required that the EFS shall include the following: I. A LFG containment plan that provides a time schedule such that landfill gas is no longer detected migrating beyond the footprint of Area"I", ' 2. An engineering design for the system enhancements necessary for the ability to prevent the detections of LFG outside Area"I", 3. A work plan to reinstall all current soil gas probes (SG-1 through SG-11) such that they extend throughout the entire vadose zone and monitor, independently the zones of highest permeability. The Discharger must evaluate the need for multiple probes at different depths within the same borehole, ' 4. A work plan to re-install SG-8 as a nested soil gas probe south of future Area 2, 5. A work plan to install one groundwater monitoring well south-southeast of future Area 2, and, 6. A proposed monitoring plan for the landfill gas wells, and collection system. Each of these requirements are discussed below: Landfill Gas Containment Schedule As previously discussed, based on the previous hydrogeological response time between surface correction actions to LFG migration and the elimination of groundwater impacts, it is our professional opinion that the current detections of VOCs in groundwater at MW-3 can be eliminated in 4 to 5 years,provided that the trapped LFG under the impermeable liner is addressed. Landfill Gas Containment Design The following section describes the enhancements to the LFG extraction system that will be installed in Area"I" as shown on Figure 3. Three (3) additional LFG extraction wells are proposed. Two LFG extraction wells will be installed in the waste mass adjacent to both the HDPE liner and MW-3. A third LFG extraction well will be installed through the HDPE liner. The first two wells will be used to provide an Foothill Sanitary Landfill 4 Department of Public Works/Solid Waste Engineering Feasibility Study County of San Joaquin—August 2014