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southwest, which is an opposite direction to the flow in the deep aquifer. The sediments below the Corcoran Formation are dry until the deep aquifer is encountered at depths of over 300 feet. COMPLIANCE HISTORY SUMMARY Groundwater monitoring is in accordance with WDRs issued in 1989, 1994 and 2001. These WDRs require monitoring the shallow and deep aquifer on the east and west sides of the landfill, and monitoring the shallow aquifer on the east side of the landfill. The shallow aquifer is not monitored on the west side because it is found only on the east side. In 1990, deep aquifer wells MW-1, MW-2, and MW-3 went dry due to falling water levels. In the fourth quarter of 1991,volatile organic compounds(VOCs) (Trichlorofluoromethane and Tetrachloroethene)were first detected in groundwater samples collected from MW-5 (shallow aquifer), and were confirmed in February 1992. The County believes that the presence of VOCs in MW-5 was due to landfill gas (LFG) influence. In 1995, a corrective action plan(CAP) was implemented which included the closure and final cover construction for the landfill. In 1996, closure was complete, including the installation of LFG migration monitoring wells. Monitoring of these wells indicated combustible gas above the regulatory limit of 5%. In 2001, a LFG collection system was installed as a corrective action(source removal) against VOC impacts, as well as to prevent offsite migration of LFG. Effectiveness of the LFG system is indicated by the absence of landfill gas at the perimeter monitoring wells and prevention of further VOC impacts indicated by a reduction of the number of individual constituent VOC detections (Appendix E, Tabular Summary of Historical Monitoring Data). This system is presently extracting as much LFG as possible, as indicated by the low methane concentration of the gas reaching the LFG flare station. tThe WDRs of 2001 required the installation of additional deep-aquifer monitoring wells because the deep aquifer level had lowered such that the original deep aquifer wells (MW-1, -2, and-3) no longer reached that aquifer. In 2003 deep-aquifer monitoring wells MW-6 and MW-7 were installed. Knowledge of the direction of the flow in this aquifer by monitoring the three previous deep-aquifer wells allowed these wells to be located directly up gradient and down gradient of the landfill. In 2003, an investigation of off-site contamination and the effect on water quality parameters in MW-4 was initiated due to an increasing trend in Nitrate as Nitrogen and a decreasing trend in pH. This investigation was inconclusive. In late 2003 the drainage ditch adjacent to the eastern access road was improved to reduce ponding and minimize infiltration along the ditch. Corral Hollow Sanitary Landfill 2 Department of Public Works/Solid Waste 1 s,Quarter 2012 Groundwater Monitoring County of San Joaquin—April 16,2012