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concentration at the perimeter (soil gas) wells falls below the regulatory limit of 5 percent, to zero if possible. However, excess withdrawal, as indicated by the reduction of methane content in an individual LFG well to below 35 percent, indicates the potential for a subsurface landfill fire. Note that LFG is approximately 55 percent methane at the point of production within the landfill. Methane content below this value indicates dilution by air drawn through the soil into the refuse by the negative pressure at the well. Maximizing LFG collection while minimizing air intrusion is a "balancing act" which requires continual adjustment of the operation of the LFG system. RESULTS � Corrective action is successful, indicated by the lack of continued increase in VOC concentration and by reduction in combustible gas in the soil gas wells since the installation of the LFG collection system in 2001. Groundwater VOCs Concentration of these compound appear to be stable. In October 2000, ten VOCs were detected above the PQL at well MW -5 immediately before the LFG system became operational. This quarter, four VOCs are reported above the PQL at MW -5, with two other compounds reported as trace concentrations (Table 1 Appendix E). No VOC was above the Maximum • Contaminant Levels (MCLs). Low concentrations of vinyl chloride, which is a degradation product of cis-1,2-dichloroethene, Trichloroethene, and Tetrachloroethene, is evidence of the biodegradation of VOCs in the groundwater at well MW -5. Three VOCs were reported above the PQL in MW -8 this quarter, similar to previous quarters, none above 6.3 ppb (Table 1). These VOCs are consistent with those found in MW -5, but are fewer in number and generally at lower concentrations. The direction of this concentration gradient indicates that the VOCs in both MW -5 and MW -8 emanate from a small area, possibly the refuse found closer to MW -5, rather than from the landfill as a whole. This significant finding allows us to focus on increasing LFG collection in that area. The County continues to apply vacuum to these wells such that the concentration of methane at these well is significantly reduced, and hopes that results will continue to be positive. Inorganic Compounds Sanitas continues to report decreasing trends in Chlorides, Sulfate, and total dissolved solids at MW -4 continuing this quarter, with no increasing trends. The County interprets this as the positive effects of improving grading and preventing ponding along the eastern drainage ditch in late 2003. Corral Hollow Landfill Department of Public Works/Solid Waste 3rd Quarter 2012 Groundwater Monitoring County of San Joaquin — October 30, 2012