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1.2 BACKGROUND <br /> The CHSL is a closed, Class III municipal solid waste disposal facility located southeast of the <br /> intersection of Corral Hollow Road and State Route 580 at 31130 South Corral Hollow Road <br /> in San Joaquin County, California (Figure 1). The site ceased operations in 1991, and was <br /> formally closed with construction of a prescriptive final cover system in 1995. <br /> Groundwater monitoring at the CHSL began in 1987 with the installation of wells MW-1 <br /> through MW-5 as part of the Solid Waste Assessment Test (SWAT) program. During <br /> construction of these wells, two water-bearing zones were identified: a shallow zone that is <br /> limited to the northern third of the site, and a deeper zone that underlies the entire site. <br /> In 1991, volatile organic compounds (VOCs), including trichlorofluoromethane (TCFM) and <br /> tetrachloroethene (PCE), were detected in samples from shallow groundwater monitoring <br /> well MW-5. The detections were verified during subsequent monitoring events, and the <br /> presence of VOCs has historically been attributed to landfill gas. In response to these <br /> detections, the County installed a landfill gas collection and destruction system at the site in <br /> 2001. <br /> Groundwater levels in the deeper zone decreased until wells MW-1, MW-2, and MW-3 went <br /> dry in 1990. In 2003, wells MW-6 and MW-7 were drilled to replace those, and well S13-1 <br /> was drilled to provide additional information regarding the shallow zone groundwater <br /> conditions. In 2007, well MW-8 was constructed to better assess the distribution of VOCs in <br /> the shallow zone along the point of compliance. <br /> Currently, groundwater at the site is monitored by network of nine monitoring wells <br /> (Figure 2). Wells MW-4, MW-5, MW-8, and SB-1 monitor a shallow aquifer. Wells MW-6 <br /> and MW-7 monitor a deeper aquifer. <br /> In 2012, three EMP wells, MW-9A, MW-10A, and MW-11A, were constructed beyond the <br /> landfill property to evaluate the nature and extent of VOC impacts to the shallow aquifer. <br /> 2.0 SITE SETTING <br /> The following sections provide a description of the site physiographic and climatic conditions <br /> in the vicinity of the CHSL, and the geology and hydrogeology underlying the CHSL based on <br /> existing subsurface investigations. <br /> Engineering Feasibility Study Geo-Lo Page ■ 2 <br /> Corral Hollow Sanitary Landfill A S S O C I AT E SS <br /> January 25,2013 <br />