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RESULTS OF PHASE 1 INVESTIGATION TO EVALUATE POTENTIAL IMPACTS TO GROUNDWATER As Part of Corrective Action At the Corral Hollow Sanitary Landfill INTRODUCTION Purpose The purpose of this document is to present the results of investigation at the Corral Hollow Sanitary Landfill in September 2005. The purpose of this investigation was to determine if refuse had been placed beneath the drainage ditch adjacent to the eastern landfill road, and if this refuse was covered by the closure barrier. If refuse is beneath the drainage ditch but the ditch is not provided with closure cap, then water flowing in the ditch could be a likely source of apparent leachate impact to the shallow aquifer, as indicated by data collected at monitoring wells MW-4 and MW-5. Background The Corral Hollow Sanitary Landfill (CHSL) is an unlined, municipal solid waste landfill, located in southern San Joaquin County at 31130 South Corral Hollow Road at the intersection with Interstate 580 (Figures 1 and 2). The CHSL was closed in 1996 by the installation of a prescriptive closure cap in accordance with Title 27 CCR, and is currently monitored through a Corrective Action Program under Waste Discharge Requirements (WDRs) Order No. 5-01-176. A landfill road was constructed along the eastern edge of the landfill. A high-pressure petroleum pipeline, owned by Shell Oil, is located near the center of the road in the vicinity of the investigation(Figure 3). Two aquifers exist at the eastern edge of the site. The shallow aquifer is monitored by one up- gradient well (SB-1) and two down-gradient wells (MW-4 and MW-5). The deep aquifer is monitored by one up-gradient well (MW-7) and one down-gradient well (MW-6) (Figure 1). Groundwater monitoring data from the shallow aquifer indicated the possible impact by leachate. If the closure cap did not extend to the drainage ditch west of the eastern landfill road, then water could percolate through the refuse, causing such impact.. To avoid digging and disturbing the closure cap, the.County engaged Shaw Environmental, Inc. to perform a geophysical investigation using non-invasive magnetic and density differential methods in 2004. This investigation did not conclusively reveal the extent of refuse; however, a possible interpretation of the geophysical drawings is that refuse underlies the drainage ditch and the road west of the ditch. Investigation of Groundwater Impact San Joaquin County Public Works/Solid Waste Corral Hollow Sanitary Landfill March 3,2006