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MONITORING RESULTS Sampling and analysis procedures are in Appendix A; 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, with an upgradient well providing background data (Table 2). MW-4 is now known to provide background water quality information, but the quantity of data from this well is insufficient to establish concentration limits until at least eight samples are collected. Therefore, data from MW-1 will be used to calculate concentration limits on a provisional basis. Additional data from new background well MW-4 will provide new tolerance limits for all parameters, including sulfate and bicarbonate. Until sufficient data from MW-4 is available for that analysis, conclusions regarding possible groundwater impacts from Sanitas software will be tentative, and will be tempered with review of the data. The other limits shown were taken from the WDRs (Appendix D). Groundwater Monitoring and Field Parameters A review of the third quarter general water quality parameters (Table 2) indicates compliance with concentration limits that can be validly derived. Using data from MW-1 to calculate concentration limits has caused false exceedences of concentration limits for two parameters in several of the site's monitoring wells including new background well MW-4, as follows: • Sulfate in both the new wells MW-4 and MW-5 exceeds concentration limits calculated from MW-1 data. In fact, sulfate in the background well MW-4 slightly exceeds that of the downgradient wells, removing the concern of contamination by the landfill. It should be noted that the secondary maximum contaminant level (MCL) for sulfate is 250 mg/L. Sulfate concentrations in all groundwater monitoring wells at the Foothill Landfill are very low (10 mg/L or less), indicating good water quality. • Bicarbonate alkalinityt a both new wells and in MW-3 exceeds concentration limits calculated from MW-1 data. However, data from MW-1 includes a bicarbonate reading of 130 mg/L in first quarter 2008. Therefore, bicarbonate concentrations of 130 mg/L for wells MW-3, MW-4, and MW-5 are within that data set. Note that there is no MCL for bicarbonate alkalinity. Foothill Sanitary Landfill 7 Department of Public Works/Solid Waste 2nd Semester and Annual 2008 Groundwater Monitoring County of San Joaquin—January 31,2009