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Vadose Zone Soil Gas Soil gas samples were collected on September 29 and 30, 2005. Methane concentrations were zero percent in all but two of the wells. The concentration varied from 2.3 percent in SG-7 to 50.3 percent in SG-6. VOCs were detected in ten of the seventeen air samples. Of the ten samples containing VOCs, most samples contained two or less VOC compounds. The highest VOC concentration was found in well SG-6 (Table 6 and Appendix I). This is consistent with the high methane content at that well. It is interesting to note that the groundwater appears not to be impacted by the VOCs in the Vadose zone. It is also interesting to note that eight VOCs were found in an air sample taken upwind of the site. Although VOCs are not naturally occurring and therefore considered indication of groundwater contamination by landfill gas, this air sample represents ambient VOC concentration above the ground surface at the time of sampling. Most interesting is that the majority of the soil gas samples were less contaminated than the ambient air. The County is now installing a LFG extraction system in accordance with the Engineering Feasibility Study. Leachate Waste Discharge Requirements require monthly monitoring of leachate flow rate, pH, and Electrical Conductivity (Table 2). A comparison of the leachate monitoring data to groundwater monitoring data shows that leachate generally contains significantly higher TDS, Electrical Conductivity, and Chloride than groundwater. Leachate is also slightly more acidic than groundwater. Leachate sampling occurred in the fourth quarter of 2005 at collection point L-1,L-3, and L-4 (Table 7). The next Leachate sampling event is scheduled for the third quarter of 2006 and will be included in the second semester 2006 report. Seventeen different VOCs were reported in the fourth quarter 2005 sampling event. The Highest VOC concentrations reported were the Xylenes, MtBE, the benzene compounds, and Toluene. Of the dissolved metals, Iron and Manganese concentrations were the highest. Results are consistent with past leachate monitoring events. Leachate is removed from the base of the landfill by a Leachate Collection and Recovery System as approved in WDR Order No. R5-2002-0219. North County RCSL 8 Public Works/Solid Waste 1 st Semester 2006 Groundwater Monitoring County of San Joaquin—July 31,2006