CORRESPONDENCE_2007 (1/07 - 6/07)

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The hydrogeology at the site is complex. A deep aquifer is found at an elevation of approximately-45 to -57 feet(MSL), at a depth of between approximately 299 and 357 below the surface. Groundwater flows in the deep aquifer to the northeast. A shallow aquifer is found beneath the northeast and east side of the landfill property due to the presence of the Corcoran Formation, which rises from the northeast, apparently intercepting the landfill base. This aquifer is unconfined, and mounds near monitoring well SB-1. The elevation of this shallow aquifer is approximately 230 feet MSL. Depth to this shallow aquifer varies between 12 and 65 feet below existing grade (depth variations result from variations in the surface elevations). Attempts to locate and monitor the shallow aquifer south and east of the landfill footprint in 2003 indicated that this aquifer is inconsistent in elevation, and discontinuous within the landfill property. The Corcoran Formation, and the associated shallow aquifer, is not found beneath the southwest side of the landfill. Groundwater monitoring proceeded on this basis; monitoring the deep aquifer beneath the western portion of the property, and both deep and shallow aquifers in the northern and eastern portion of the property (Figure 2). This monitoring program was in accordance with the WDRs first issued in 1989, 1994, and 2001. Compliance History Summary Groundwater quality at MW-5 has improved since corrective action was taken. Combustible gas at the perimeter monitoring wells has decreased to near zero at all wells, and the number and concentration of individual constituent VOC detections has been reduced. In the quarterly groundwater monitoring at MW-5 immediately before the operation of the LFG system (October 2001), 10 VOCs were seen with a maximum concentration of 9.2 ppb. In the most recent quarterly monitoring(October 2006), five VOCs were reported,with a maximum concentration of 7.4 ppb. VOCs (Trichlorofluoromethane and Tetrachloroethene)were first detected in groundwater samples collected from MW-5 (shallow aquifer) during the fourth quarter of 1991. These compounds have been found repeatedly and consistently, indicating the presence of VOCs in MW-5 due to landfill gas influence. A corrective action plan (CAP) included the closure and final cover construction for the landfill implemented in 1995, and the LFG control and collection system in operation since December 2001. The LFG system also prevents offsite migration of LFG. In 2003, in response to an increasing trend in Nitrate as Nitrogen and a decreasing trend in pH at shallow aquifer monitoring well MW-4,the County investigated off-site contamination and the effect on water quality parameters. The results of this study were inconclusive. The shallow aquifer is apparently inconsistent in elevation, and discontinuous within the landfill property. Increase in Corrective Action 2 Corral Hollow Sanitary Landfill January 23,2007 San Joaquin County Public Works/Solid Waste