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Corral Hollow LFG System Audit and Upgrade Report ' Landfill Gas System History The LFG extraction system became operational in 2001. The system was originally comprised , of a series of 39 vertical extraction wells (designated EW-1 through EW-39) installed in refuse fill. The system has been expanded sequentially, as described below, and now consists of 51 ' vertical extraction wells (Figure 2). All gas collected by the LFG system is combusted in an enclosed ground flare. Steps Taken to Modify the Physical Components/Operating Elements to Provide VOC Source ' Control Subsequent to the initial well installation, the County has taken steps to modify the physical ' components of the system to prevent LFG, to the extent possible, from entering groundwater through the footprint of the landfill and surrounding vadose zone. In 2008, an additional five wells (EW-40 through EW-44), were installed in refuse fill in an , effort to enhance VOC source control. In 2013, after discovery that refuse was located outside of the initial refuse boundary, the County installed 7 vertical extraction wells (EW-45 through ' EW-51), concurrent with the extension of the closure cap. These new wells were installed at or near the edge of refuse fill with the intent to control VOC impacts to groundwater indicated at nearby groundwater monitoring wells MW-5 and MW-8 (Figure 2). As of this writing, the LFG ' extraction system upgrades undertaken as part of these projects have been fully implemented. LFG Extraction Devices Vertical LFG extraction devices are utilized in the collection system. There are 51 vertical extraction wells. Of these, 49 wells (EW-1 through -47, EW-50 and EW-51) were installed within the ' deeper refuse fill, and 2 wells (EW-48 and -49) were installed in shallow refuse fill at the perimeter in the north-northeastern area of the Site (Figure 2). It is likely that these shallow wells collect gas from the soil as well as LFG from the refuse in the area of these wells. ' Vertical extraction wells consist of a 24-inch boring backfilled with drain rock and four-or six-inch diameter HDPE or PVC extraction pipe. Wells installed in 2001 were screened over two thirds of , the depth of the installation and the upper portion of each boring is sealed with bentonite. Wells installed after 2001 were provided with a screened section of approximately half the bore depth, and a minimum 30-ft deep compacted soil over the rock section. Extraction well casings are 4- ' inch diameter. Vertical extraction wells are installed on approximately 200-foot centers, with closer spacing as ' needed for additional control. During SCS's October 2013 operations review, well depths were measured to range from 17-feet (EW-17) near the refuse perimeter to a depth of over 90-feet for extraction wells located on the top deck of the WMU. ' Vacuum is applied by blowers to all landfill gas wells and delivered to the flare for combustion. The current perimeter gas extraction well locations are shown in Figure 2. ' 4 December 201 3 1