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C A M B R I A two were for industrial use, one was for municipal use, and two wells were of unknown use. The closest well is a municipal well located approximately 1,100 feet northeast(cross-gradient) of the Shell-branded station. The approximate well locations are plotted on Figure 1. No additional wells were identified by Cambria's 500-foot door-to-door survey. Two surface water bodies were also identified within the study area. Unnamed ponds are located at the San Joaquin Storm Water Facility, which is located approximately 250 feet north of the site, and a diverting canal is approximately 1,900 feet southwest of the site. The surface water bodies are also shown on Figure 1. Complete survey results are included in Cambria's February 13, 2002 Fourth Quarter 2001 Groundwater Monitoring/Site Investigation Report and Receptor Survey. Groundwater Monitoring Program: There are currently four groundwater monitoring wells (MW-1 through MW-4) onsite that are sampled quarterly. Historically, groundwater flow beneath the site is predominantly northerly. During the second quarter 2004 sampling event, TPHg was detected in wells MW-1, MW-2, and MW-4 at concentrations ranging from 69 to 230 parts per billion (ppb). The laboratory noted that the hydrocarbons reported as TPHg do not match the laboratory's gasoline standard. MTBE was detected in wells MW-1 and MW-3 at 130 and 2.9 ppb,respectively, and 340 ppb TBA were detected in well MW-1. Based on the quarterly groundwater monitoring and CPT boring results, SJCEHD requested additional investigation. Technical Rationale for Proposed Scope of Work • The lateral extents of impacted soil and groundwater beneath the site have not been fully assessed north of the USTs and dispenser islands. • The vertical extent of MTBE in groundwater has not been defined. Note that groundwater analytical results from the samples collected from the CPT boring drilled in 2001 delineate the vertical extent of TPHg, benzene, fuel oxygenates other than MTBE, and lead scavengers. • Cambria proposes to install two groundwater monitoring wells north of the USTs and dispensers to further investigate the lateral extent of impacted soil and groundwater beneath the site in the downgradient direction. • Cambria proposes to install one deep groundwater monitoring well to assess the vertical extent of MTBE in groundwater beneath the site. 1359 3 i i