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line network that conveys gas to the flare stations at the south end of the site for incineration. As currently proposed, enhancements to the landfill gas extraction system would include construction of seven additional vertical extraction wells, including two that will be located in the waste in Caltrans property. These wells will be connected to the existing header network. According to the County, the current blower and flare station have sufficient capacity to accommodate additional gas extracted from these wells. These wells will be drilled to depths ranging from 25 to 40 feet (Table 9A). Approximately 400 feet of additional landfill gas pipe would be added to existing system. Wells and header pipelines will be buried to avoid damage by vehicular traffic and post-closure maintenance activities (Figure 8) The cost of enhancing the LFG collection system is estimated to be $97,776 (Table 9B). Operations and maintenance (O&M) costs associated with this system are approximately $29,484 (Table 9C). Note that the following alternatives would be considered if the extension of the cover and enhancement of the landfill gas system are not successful at reducing VOC concentrations in the shallow aquifer (see Figure 12). 4.3 NO. 3: IN-SITU AIR SPARGING/VAPOR EXTRACTION SYSTEM An in-situ air-sparging/vapor extraction system (IAS/VES) relies on injected air to strip VOCs from groundwater, and an applied vacuum within the vadose zone to remove the stripped VOCs. Literature suggests that in relatively permeable soils the radius of influence of air- injection wells is approximately equal to the thickness of the aquifer. For the CHSL, the approximate saturated thickness of the shallow aquifer is 25 feet. Depending on the effective porosity of the Corcoran Clay, this efficiency could be greatly limited at the CHSL. Although actual implementation of an IAS system would require pilot testing, for purposes of this EFS, a 25-foot spacing was assumed to be sufficient to ensure adequate overlap between the injection wells (Table 10A; Figure 9). IAS/VES is a viable technology for VOC reduction in the plume area. A conceptual IAS/VES well array is proposed along the point-of-compliance to intercept VOC-impacted groundwater and minimize the potential for offsite migration. Based on this distribution of VOCs and the required spacing of IAS/VES wells necessary to provide overlap, a total of approximately 37 IAS and 37 VES wells would be required. IAS and VES wells will be two Engineering Feasibility Study Geo-Logic Page ■ 23 Corral Hollow Sanitary landfill ASSOCIATE S January 25,2013