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Soil Gas The soil is monitored for VOCs per the WDRs Order No. R5-2003-0020. Ten wells, SG-2 through SG-11, were installed surrounding modules "I" and 1 (Figure 5). Well depths range from 25 feet to 68 feet. Soil gas trench, designated as SG-T, is also used to monitor VOCs. Anew soil gas monitoring well, SG-12,was installed in June 2004 because the combustible concentration measured at SG-11 was above the lower explosive limit(Figure 5). This three- probe well will be sampled for the first time in July 2004. The Monitoring and Reporting Program requires that soil gas be monitored semi-annually. Presently, soil gas samples are retrieved quarterly. Samples are and analyzed by certified laboratory per EPA test method TO-14. MONITORING RESULTS Groundwater Monitoring and Field Parameters The concentration of Chloride and Nitrate(as N)at MW-3 slightly exceeded the concentration limit(Table 2). A release from the landfill is unlikely due to these indications. Although these concentrations are above the concentration limit,we note that they are well below the drinking water standard(California DHS). The general quality of the groundwater at the landfill is considered good. It should be noted that the concentration of Sulfate exceeded the concentration limit at background well MW-1. This is an indication that regional changes in general water quality conditions have occurred at the landfill. Because the concentrations of many of the general water quality parameters are low, slight regional changes in the water quality could be the cause for the apparent concentration exceedences observed at MW-3. Stiff Diagram Analysis Stiff diagrams are submitted annually and will be included with the second semester and annual 2004 report. The stiff diagrams that were submitted the previous semester indicate a calcium- bicarbonate water type since these ions have the highest concentrations in the four wells. The water type is similar in all wells as indicated by similarity in concentrations of the various constituents and the similarity in diagram shapes. This similarity indicates that the groundwater is not impacted as it moves under the landfill. Foothill Sanitary Landfill 6 Department of Public Works/Solid Waste yIST Semester 2004 Groundwater Monitoring County of San Joaquin-July 27,2004