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1.0 INTRODUCTION This Evaluation Monitoring Report has been prepared on behalf of the County of San Joaquin, Public Works Department, Solid Waste Division (County) by Geo -Logic Associates (GLA) to fulfill a reporting requirement contained in Item 3, Page 4 of the Order for Technical Reports submitted by the California Regional Water Quality Control Board — Central Valley Region (RWQCB) on July 7, 2011. Work described herein has been conducted in accordance with the Evaluation Monitoring Program (EMP) workplan that was approved by the RWQCB on August 25, 2011, as part of an ongoing EMP at the Corral Hollow Sanitary Landfill (CHSL). 2.0 BACKGROUND The CHSL is a closed, Class III municipal solid waste disposal facility located southeast of the intersection of Corral Hollow Road and State Route 580 at 31130 South Corral Hollow Road in San Joaquin County, California (Figure 1). The site ceased operations in 1991, and was formally closed with construction of a prescriptive final cover system in 1995. Groundwater monitoring at the CHSL began in 1987 with the installation of wells MW -1 through MW -5 as part of the Solid Waste Assessment Test (SWAT) program. Through construction of these wells, two water -bearing zones were identified: a shallow zone that is limited to the northern third of the site, and a deeper zone that underlies the entire site. In 1991, volatile organic compounds (VOCs), including trichlorofluoromethane and tetrachloroethene, were detected in samples from shallow groundwater monitoring well MW -5. The detections were verified during subsequent monitoring events, and the presence of VOCs has historically been attributed to minor landfill gas migration. In response to these detections, the County installed a landfill gas collection and destruction system at the site in 2001. Groundwater levels in the deeper zone decreased until wells MW -1, MW -2, and MW -3 went dry. In 2003, wells MW -6 and MW -7 were drilled to replace the deeper zone monitoring wells that had gone dry, and well SB -1 was drilled to provide additional information regarding the shallow zone groundwater conditions. In 2007, well MW -8 was constructed to better assess the distribution of VOCs in the shallow zone. Currently, groundwater at the site is monitored by network of six monitoring wells: Wells MW -4, MW -5, MW -8, and SB -1 monitor a shallow aquifer. Wells MW -6 and MW -7 monitor a deeper aquifer. EMP Report— Phase 1G@O-Log0 Page ■ 1 Corral Hollow Sanitary Landfill ASSOCIATES February 1, 2012