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Groundwater monitoring at the site began in 1988 with the construction of monitoring wells as part of the Solid Waste Assessment Test (SWAT) program, and as outlined in Section 3.3, six monitoring wells comprise the current groundwater monitoring network. Groundwater is sampled on a quarterly basis and reported semiannually. Historical groundwater equipotential contours provided in quarterly monitoring reports suggest that groundwater in the shallow perched aquifer flows to the south at a gradient 0.0375 feet per foot (ft/ft). The available data suggest that recharge occurs near well SB-1; however, infiltration from a sand and gravel quarry or leakage from the Delta-Mendota Canal and/or California aqueduct to the northeast of the site may also affect the groundwater flow direction and gradient in the shallow aquifer. Historical groundwater equipotential data suggest that groundwater in the deeper aquifer flows to the northeast at a gradient of about 0.004 ft/ft, generally parallel to the regional topographic gradient toward the floor of the Central Valley. 3.4 GROUNDWATER CHEMISTRY Groundwater in the shallow aquifer has been impacted by VOCs; whereas no VOCs have been identified in the deeper aquifer. VOC impacts to the shallow aquifer were first identified in 1991 in samples from well MW-5, and have also been detected in samples from well MW-8. Constituents in MW-8 are similar to those detected in MW-5, although generally at lower concentrations. VOCs historically detected in samples from the shallow aquifer wells have typically included dichlorodifluoromethane, trichlorofluoromethane, tetrachloroethene (PCE), trichloroethene (TCE), cis-1,2-dichloroethene, and 1,1-dichloroethane. The total concentration of all detectable VOCs has been less than 30 micrograms per liter (µg/L) during the last five years, with concentrations of PCE periodically exceeding the State of California and Federal Maximum Contaminant Levels (MCL; 5 µg/L) in samples from well MW-5. Inorganic constituent concentrations have remained relatively static over time in samples from the shallow aquifer. The VOCs detected in groundwater samples are i similar to those detected in adjacent soil-pore gas monitoring probes, further suggesting landfill-gas related impacts. In response to historical detections of VOCs in samples from wells MW-5 and MW-8, the County installed additional landfill gas extraction wells near the northeastern portion of the landfill in 2007. However, the effect of this additional LFG collection is not yet apparent in the water quality at wells MW-5 and MW-8. 4.0 EVALUATION MONITORING PROGRAM FIELD WORK In August 2011, the County submitted an EMP Workplan to the RWQCB to further characterize the nature and extent of VOCs migrating from the CHSL. The workplan EMP Report—Phase II Geo-Logic Page ■ 5 Corral Hollow Sanitary Landfill ASSOCIATES August 21,2012