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4.1 NO. 1: EXTENSION OF THE FINAL COVER SYSTEM As indicated herein, in 2005, the County determined that a small area of waste, approximately 1.3 acres in size, extends to the north of the current landfill closure area. Uncovered, surface water run-on and precipitation can infiltrate the waste to a greater degree than the covered wastes. The increase in moisture content allows for increased rate of microbial decomposition of the waste. As a result, landfill gas generation rates are expected to be higher as long as the waste remains uncovered, increasing the risk to water quality. The County is planning to cover the small area of waste outside the present closure area in 2013 (Figure 7). This corrective action will include a geosynthetic final cover system that consists of a prepared subgrade, a linear low-density polyethylene (LLDPE) barrier layer, a geocomposite drainage layer, and a vegetative soil layer. The cover system will tie into the existing landfill final cover system, and a perimeter drainage system will be constructed to divert surface water around the new closure area (Table 8A). The proposed final construction project includes installation of additional landfill gas collection wells in the newly-covered area to mitigate gas accumulation beneath the geosynthetic cover system. These wells will collect LFG from the area now owned by Caltrans, where LFG has not been collected previously. Design and cost for the landfill gas system components are included in Section 4.2. Capital cost to construct the final cover system extension are approximately $508,000 (Table 8B), and the annual operations and maintenance (O&M) costs are expected to be $22,300 (Table 8C). 4.2 NO. 2: LANDFILL GAS EXTRACTION SYSTEM ENHANCEMENTS Chlorinated aliphatic compounds; namely, DCDFM, TCFM, and PCE, typically comprise the highest concentrations of VOCs in groundwater samples collected at the CHSL. These VOCs are also typically found at high concentrations in landfill gas, suggesting that VOC impacts to groundwater are largely attributable to landfill gas migration. As a result, enhancing the landfill gas extraction system could provide a means to reduce migration of VOC-impacted landfill gas to groundwater. Review of the existing landfill gas extraction system (Figure 6) indicates relatively uniform distribution of landfill gas extraction wells across the site. Wells are connected to a header Engineering Feasibility Study Geo-Lo Page ■ 22 Corral Hollow Sanitary Landfill A S S O C I AT E S January 25,2013