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El assessment was performed by Geologic Associates (GLA) and summarized in the Low -Level VOC Detections in Monitoring Wells letter report dated February, 2001. In this report, GLA concluded that landfill gas production is the source of sporadic low-level VOCs in groundwater wells near the older unlined Forward Landfill and that the absence of VOCs at offsite monitoring locations indicate that VOCs degrade or disperse within a relatively short distance of the landfill boundary. At the existing Austin Road Landfill, a Solid Waste Assessment Test in 1989 identified several VOCs in the downgradient monitoring wells and an evaluation monitoring program was implemented. A convective action program (CAP) was then proposed in 1991 to address the chlorinated hydrocarbon impacts to groundwater and this plan was approved by the RWQCB in June, 1991. In January, 1999, Camp Dresser & McKee (CDM) prepared the "Austin Road Landfill Contaminant Plume Report" and the "Groundwater Corrective Action Feasibility Study Report". These reports indicate that VOC concentrations have been trending downward and may be attributable to landfill gas. Landfill gas and groundwater issues at the existing Austin Road Landfill were then assessed and a revised Engineering Feasibility Study (EFS) has recently been prepared (HA/AEE, 2001). In this report it was concluded that the most effective response to groundwater impacts at the site would include: • Enhanced landfill gas extraction. • Continued groundwater pumping from the two groundwater extraction wells along the north side of the facility. • Continued groundwater treatment using the facility's air -stripper unit. • Artificial recharge of the aquifer by treated groundwater. With these actions, existing VOC impacts are expected to decline below Maximum Contaminant Levels (MCLs) (drinking water standards) within five years and below the Water Standard (1 part per billion) within nine years. As noted above, impacts at the existing Forward and Austin Road Landfills have both been attributed to landfill gas migration and no evidence of a significant liquid phase component has been presented. Since the primary purpose of a double composite liner is to provide enhanced liquids containment, the fact that liquid migration (even from the unlined Austin Road site) appears to contribute little if any to the existing identified releases suggests that little benefit will be gained from a double composite liner system. The general lack of liquid phase impact is further validation of the attenuation characteristics of the underlying clays, silts and sands that was discussed previously. RWQCB staff have indicated their belief that a well designed, constructed and maintained single composite liner can be effective. They have however also indicated their concern that this theoretical performance can be put at significant risk by poor site operations and/or liner maintenance. As a result, it is considered important to also C:U000-049Torward Composite Liner V3 .doe\09/19/02 10 GeoLogic Associates