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t INTRODUCTION General This first quarter monitoring report for the Corral Hollow Sanitary Landfill (CHSL) is submitted in compliance with Waste Discharge Requirements (WDR) Order No. 5-01-176 issued by the California Regional Water Quality Control Board, Central Valley Region, on June 14, 2001. The CHSL is located at 31130 South Corral Hollow Road, in southern San Joaquin County, approximately 5 miles south of the City of Tracy, at the intersection of Interstate 580 and Corral Hollow Road(Figure 1). CHSL is located in Township 3 South, Range 5 East of Section 18 on the Mount Diablo Meridian. CHSL is a closed, unlined Class III landfill. The landfill property includes approximately 58 acres, 43 acres of which is permitted for refuse disposal. The remaining area is comprised of access roads, storm water retention ponds, setbacks for environmental monitoring, and firebreaks. Land within 1,000 feet of the facility is used for grazing and agriculture. CHSL ceased receiving waste in 1991. The final cover was constructed in the fall of 1995 in compliance with Order No. 94-259. The closure cover consists of a 2 to 6-foot foundation layer under a 1-foot minimum thick, low-permeability soil layer, with a 1-foot thick vegetative surface layer. A landfill gas collection system began operation in December 2001. Geology r Bolo Surface soils at the site are typically sands and gravels to a depth of about 70 feet, underlain by low permeability silts and clays to 150 feet. Sands and gravels predominate to a depth of approximately 430 feet. Highly impermeable sandy clays are present at 430 feet. The hydrogeology at the site is complex. The deep aquifer is found at an elevation of approximately -45 to -57 feet(MSL), at a depth from approximately 300 to 350 below the surface. Groundwater flows in the deep aquifer to the northeast. tA shallow perched aquifer is found beneath the northeast and east side of the landfill property. This aquifer is not found west of the landfill. This shallow aquifer is perched on the Corcoran Formation,which rises from the northeast, apparently intercepting the landfill base. The elevation of this shallow aquifer is approximately 230 feet(MSL). Depth to this shallow aquifer is between 12 and 65 feet below existing grade, these variations being the result of variations in the surface elevations and apparent recharge from off-site near SB-l. This perched aquifer is unconfined+and pinches out in the proximity of the landfill,with groundwater mounding near monitoring well SB-l. The Corcoran Formation and the associated shallow aquifer are not found beneath the southwest side of the landfill. Attempts to locate and monitor the shallow aquifer south and east of the landfill footprint in 2003 and 2007 indicated an inconsistent aquifer elevation and aquifer discontinuity within the landfill property. Groundwater flows in the shallow aquifer to the 1 Corral Hollow Sanitary Landfill Department of Public Works/Solid Waste 1S`Quarter 2012 Groundwater Monitoring County of San Joaquin—April 16,2012