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I;0� A S 5 0 C I A T E S I N C T-1 550-gallon Diesel Removed 10/28/93 T-2 1,000-gallon Regular Unleaded Removed 10/28/93 ' T-3 1,000-gallon Unleaded Removed 04/01/87 T4 350-gallon Waste Oil Removed 04/23/92 T-5 550-gallon Diesel Removed 10/28/93 T-6 1,000-gallon Gasohne/Water Removed 08/07/87 The locations of the former tanks T-1 through T-5 are shown on Figure 2 In 1988, the City of 1 Stockton contracted RESNA Industries Inc to conduct a subsurface investigation to assess the extent of residual hydrocarbons in soil 1 The investigation included drilling seventeen soil borings, installing five groundwater monitoring wells (MW-1 through MW-5), seven vapor extraction wells (VW-1, VW-2s, VW-2d, VW-3, ' VW-4, VW-5s, and VW-5d), one piezometer (PZ-1), and one recovery well (RW-1), collecting and analyzing soil and groundwater samples, and monitoring fluid levels in the wells Quarterly groundwater monitoring was initiated at the site in December 1988 The locations of monitoring wells and pertinent site features are shown on Figure 2 A summary of well construction details is included in Table 1, a summary of monitoring data is included in Table 2, and a summary of laboratory analytical results is included in Table 3 In 1992, RESNA performed a groundwater aquifer test and performed a soil vapor extraction test (VET) A second VET was performed in January 1994 A complete summary of the pilot testing results and previous environmental work performed on site is contained m RESNA Industries Inc 's Final Remediation Plan for Soil and Groundwater Remediation, dated November 17, 1994 RESNA recommended groundwater extraction and vapor extraction as a final remediation ' plan for impacted soil and groundwater at the site In January 1996, at the request of the City of Stockton, the site was reevaluated based on the ' release of a Lawrence Livermore report which recommended natural attenuation as a remedial alternative at petroleum hydrocarbon sites and the Final Remediation Plan was never implemented In May 1998, PHS/EHD requested that an air sparge/vapor extraction pilot test be performed at the site to evaluate the feasibility of using these methods to remove dissolved gasoline hydrocarbons from groundwater beneath the site In October 1998, ATC submitted a ' workplan to PHS/EHD which was subsequently approved with minor modifications 2�2 Regional Geology and Hydrogeolo ' The site is underlain by the Modesto Formation, which is divided into nonmarine basin deposits, fan deposits, and stream channel deposits consisting of clay, sdt, and sand (Rogers, 1966) Basin deposits ' are dominant at this site The Modesto Formation dips gently to the west(Wagner, et al, 198 1) The site is in the Eastern San Joaquin County Groundwater Basin (California Department of Water Resources, 1980) According to the San Joaquin Flood Control and Water Conservation District, the ' W 162574 Mpilot-fa report doc 2