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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, with an upgradient well providing background data(Tables 2 & 3). MW-4 was located directly up-gradient of the landfill based on groundwater elevations from the previously-installed wells. This well has been sampled six times since installation. Because the quantity of data from this well is insufficient to establish concentration limits until at least eight samples are collected, data from MW-1 is used to calculate concentration limits on a provisional basis. 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. However, it should be noted that MW-1 was dry and could not be sampled in the second semester of 2009. Therefore, the reliance on MW-1 as an indicator of background water quality conditions could lead to false impressions of landfill impact. Groundwater Monitoring and Field Parameters A review of the third and fourth quarter general water quality parameters (Tables 2 & 3) indicates compliance with concentration limits that were previously derived from MW-1. Using data from MW-1 to calculate concentration limits can cause false exceedences of concentration limits for parameters in the site's monitoring wells including new background well MW-4. A list of provisional exceedences is as follows: • Sodium detected at a concentration of 17 mg/L during the 3'd quarter sampling event in well MW-3 slightly exceeded concentration limit of 16 mg/L calculated from MW-1 data. This provisional exceedence may be false, as MW-1 could not be sampled during this monitoring period(due to low groundwater elevation) and insufficient data exists for MW-4 to develop a more appropriate sodium concentration limit. Note that there is no MCL for sodium. A concentration of 17 mg/L of sodium is considered good water quality. • Since MW-4 is hydraulically upgradient from the landfill, concentration limits calculated at side gradient well MW-1 would not apply to this well. Therefore, bicarbonate concentrations of 140 mg/L at well MW-4 during the P and 4th quarter monitoring are not a result of impact from the landfill. Note that there is no MCL for bicarbonate alkalinity. VOCs Two VOCs were detected in MW-3 during the August 2009 sampling event including a trace concentration of 0.42 ug/L 1,1-Dicloroethene and 1.0 ug/L Trichloroethene(TCE). None of these detections exceeded their respective MCLS. 1,1-Dicloroethene was last detected in this well in 1 sc Foothill Sanitary Landfill 8 Department of Public Works/Solid Waste 2nd Semester and Annual 2009 Groundwater Monitoring County of San Joaquin—January 31,2010