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Expansion of the Final Cover System: Viable — The County recently identified a 1.3-acre segment of the waste that was not covered as part of the 1995 site closure. Waste that has not been covered with a formal final cover system may allow more surface water and precipitation to infiltrate wastes when compared to wastes that have an engineered final cover system, thereby potentially increasing the production of landfill gas and the mechanisms by which VOCs are carried to groundwater. Expansion of the final cover system over the 1.3-acre segment is planned using a geosynthetic-soil composite design or monolithic soil (evaportranspirative) design. Construction of an engineered final cover system is a proven means, of reducing moisture infiltration into a waste, thereby reducing the waste decomposition and associated landfill gas and leachate formation that can impact the underlying groundwater. Expanding the engineered final cover system over the 1.3-acre waste prism segment is a viable (and required) alternative for reducing VOCs in groundwater at the CHSL. Landfill Gas Extraction and Treatment: Viable— Results of the CHSL EMP (GLA, 2012) suggest a landfill gas source for VOC impacts based on the type of VOCs detected in groundwater samples, the lack of inorganic impacts, and the knowledge that a portion of waste at the north end of the site does not now have landfill gas collection. Landfill gas extraction is a common and proven technology employed at most landfills to mitigate gas migration and reduce VOC concentrations in groundwater, and gas collection systems are already in place at the CHSL. Expansion of the landfill gas extraction system would include drilling new landfill gas extraction wells and connecting the wells to the existing header pipes. Gas would be conveyed to the existing flare system for destruction. The LFG collection system was expanded in 2010 with the addition of five wells. A second expansion is planned for 2013 with the addition of two wells in the Caltrans property. Limiting factors and design considerations for enhancing the landfill gas extraction system include identifying areas for well construction and header pipelines that will not be affected by final cover construction. The existing blowers, flare, and condensate collection system are sufficient to treat the added volumes expected to be produced by two new extraction wells. Poor quality (low methane) gas is expected, but will be compensated by adjusting the LFG field. Engineering Feasibility Study Geo-Logic Page ■ 14 Corral Hollow Sanitary Landfill ASSOCIATES January 25,2013