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Leachate samples were last analyzed for COCs during the first semester 2005 monitoring event. The next monitoring event for leachate COCs will occur during the first semester of 2010. Vadose Zone Soil Gas The soil is monitored for VOCsp er the WDRs Order No. R5-2003-0020. Eleven wells, SG-1 through SG-11, are located around modules "I" and 1 (Figure 5). Well depths range from 25 feet to 68 feet. Gas from the soil gas trench under Module 1, designated SG-T, is also analyzed for VOCs. The Monitoring and Reporting Program requires that soil gas be monitored semi-annually. Samples are analyzed by a certified laboratory per EPA test method TO-14A. MONITORING RESULTS Sampling and analysis procedures are in Appendix A and the field data sheets are in Appendix B. The laboratory reports are in Appendix C. Statistical Analysis The FSL has been approved to use interwell tolerance limits methods (up-gradient well provides the background data). Analytical results from this monitoring period were compared to the concentration limits (Table 2). Tolerance limits for five of the general water quality parameters were determined using statistical analysis. The other limits shown were taken from the WDRs. The statistical analysis is described in Appendix D. Groundwater e Monitoring and Field Parameters The concentration of Sulfate slightly exceeded the concentration limit in wells MW-4 and MW-5 this semester; however, MW-4 is the upgradient well, and represents un-impacted conditions. Both of these wells are a considerable distance from the active module and have only been sampled twice for sulfate. The sulfate tolerance limit exceedences in MW-4 and MW-5 may not be due to landfill activities, but rather to the small timber of samples available for analysys. The concentration limit of Bicarbonate was exceeded in wells MW-1, MW-2R, MW-4, and MW-5 this semester. As noted above, MW-1 is a background well, these high values are not likely a result of landfill activities. The bicarbonate values for second quarter sampling events are usually higher compared to historical levels. VOCs VOCs were not detected in any wells this semester. Organosphorus Compounds Azinop os methyl (Guthion) was reported in well MW-5 in the second quarter's data, but was not found in the first quarter's results (the well has been sampled twice since installation). No other Organophosphate Pesticides were reported at this well during this sampling event and Organophosphate Pesticides have not Foothill Sanitary Landfill 8 Department of Public Works/Solid Waste I"Semester 2008 Groundwater Monitoring County of San Joaquin—July 31,2008