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i Using data from MW-1 to calculate concentration limits has caused false exceedences of concentration limits for three parameters in several of the site's monitoring wells including new background well MW-4, as follows: • Sulfate in well MW-5 exceeds concentration limits calculated from MW-1 data; however, 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 (11 mg/L or less), indicating good water quality. • Bicarbonate alkalinity at new well MW-4 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 140 mg/L for wells MW-3, and MW-4 are within that data set. Note that there is no MCL for bicarbonate alkalinity. VOCs Two VOCs were detected at trace concentrations in MW-3 during the January 2009 sampling event,but were not found in a subsequent confirmation sampling in February. Therefore, other than false positives, no VOCs were found in any wells this semester. No VOCs have been confirmed in any groundwater well at the Foothill Landfill since October 2001, over seven years afro. Surface Water Surface water samples were taken from the creek flowing along the eastern perimeter of the property this semester. The results of analyses of the upstream and downstream samples were similar(Appendix Q. The downstream sample was less in both total dissolved solids and electrical conductivity than the upstream sample. Note that the creek receives contribution from the ground surface and several smaller creeks on the property east of the creek. The County and landfill operator cannot control constituents that may be carried into the creek from the other property. Leachate Waste Discharge Requirements require monitoring leachate flow rate, pH, and Electrical Conductivity on a monthly basis (Table 4). Leachate volumetric data for several months indicates reasonable average flow rates. Vadose Zone Soil Gas First Semester Results Soil gasses were sampled on January 22 and 23, 2009. Samples were collected by first measuring the methane content using a CES-LANDTEC GEMTM 500 portable gas analyzer. The GEMTM 500 was operated until a steady-state gas composition indicated that the well was purged. After Foothill Sanitary Landfill 8 Department of Public Works/Solid Waste Vt Semester 2009 Groundwater Monitoring County of San Joaquin—July 31,2009