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i Annual Summary Groundwater General Water Quality Parameters Monitoring data shows that all constituents were within interwell tolerance limits except for: • Well MW-3, at which Chloride concentration limits were slightly exceeded in the second semester. However, if the concentration limit is rounded to one significant figure (10 mg/L) no exceedence would have occurred. VOCs Two VOCs (Chloromethane and Chloroethane) were detected in trace concentrations during the first semester 2006 sampling event. Chloromethane was detected in MW-1 and Chloroethane was detected in MW-3 at concentrations of 0.36 [tg/L, and 0.52 µg/L,respectively. One VOC (trichloroethene) was detected in MW-3 during the second semester 2006 at a trace concentration of 0.18 [tg/L. No further VOCs were detected in any of the remaining wells during first and second semester 2006. Surface Water Surface water samples were collected on February 27, 2006. Monitoring results indicate that the concentrations of field and monitoring parameters were below established concentration limits. Acetone and 2-Butanone were detected at trace concentrations in both the upstream (SW-1) and downstream(SW-2) samples. However, because these VOCs were detected at higher concentrations in the upstream sample, the landfill could not have contributed these compounds to the surface water. HISTORICAL TIME SERIES PLOTS AND STIFF DIAGRAMS Historical Time Series Plots are presented in Appendix F. Note that constituents not detected are plotted at a value of half their respective detection limits. Outliers were included in the plots as directed by RWQCB staff. Stiff diagrams are submitted annually with the second semester and annual report. The stiff diagrams submitted for the first and second semesters indicate a calcium-bicarbonate water type since these ions have the highest concentrations in the four wells (Appendix F). 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. The shapes of the diagrams between the first and second semesters are also very similar indicating that the general groundwater quality has not changed throughout the year,, which is also an indication of no impact from the landfill. Foothill Sanitary Landfill 10 Department of Public Works/Solid Waste 2nd Semester Groundwater Monitoring County of San Joaquin—January 31,2007