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ASSOCIATESTit I N C 3.0 PREVIOUS ENVIRONMENTAL WORK Previous subsurface investigations performed included drilling seventeen soil borings; installing five groundwater monitoring wells (MW-1 through W-5), seven vapor extraction wells (VW-I, VW-2s, VW-2d, VGA-3, TGV-4, BTW-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 Industries Inc. 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 in 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. Copies of vapor extraction test monitoring data are included in Appendix Br 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, PHS1EHD 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 PHSIEHD which was subsequently approved with minor modifications. On November 23, 1998, a vapor extraction test was performed by ATC using vapor well V -1 as the vapor extraction point while monitoring vacuum response in the surrounding wells. The w:162574.011CAY,report.doc 4