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3.2 Previous Site Assessment Activities GeaHydrologic Consultants, Inc Hydrotech Consultants, Inc performed the initial investigation at the Site in December of 1987 The details of this investigation are presented in the Hydrotech report titled ' "Limited Subsurface Investigation, Thrifty Oil Company Station No 171, 1250 N Wilson Way, Stockton, California", dated January 7, 1988 The investigation consisted of advancing four exploratory borings (B1, B2, B3, and B4) to depths of 20 feet bgs near the former USTs Laboratory results indicated the presence of a maximum of 1,983 mg/kg total petroleum hydrocarbons (TPH) in one soil sample at 10 feet bgs A second assessment of the Site was conducted by Woodward-Clyde Consultants (WCC) in August 1988 The details of this investigation are presented in the WCC report titled "Subsurface Assessment Report, Thrifty Oil Service Station 171, Stockton, California", ' dated October 6, 1988 The investigation consisted of advancing three exploratory borings at the Site A 4-inch diameter PVC monitoring well was installed in each of the borings (MW-1 through MW-3) to depths of about 50-60 feet bgs Soil sample MW2 at ' 36 feet bgs indicated the only significant TPH and benzene, toluene, ethylbenzene, and xylene (BTEX) concentrations (2,200 mg/kg TPH and less than detectable to 235 mg/kg BTEX) ' In November 1992, Thrifty conducted an additional assessment that included installation of four groundwater monitoring wells (MW-3R, MW-4, MW-5, and MW-6) at the Site Because the water table had dropped about 30 feet since the initial wells were installed at lie the Site, these deeper wells were required to monitor the groundwater quality These wells were installed to 90 feet bgs and were constructed of 2-mch diameter PVC In ' addition, as part of the investigation, two vapor wells (VW-1 and VW-2) were installed near the former tank pit Results of the investigation showed that soils near the western boundary of the Site contained significant hydrocarbon concentrations Groundwater in ' each of the deep wells had high contamination levels Results of the investigation are presented in Thrifty's "Site Assessment Report", dated January 17, 1988 ' In November 1994, Thrifty removed the former tanks and replaced them with the new tanks in the northeast corner of the Site These construction activities resulted in the loss of MW-1, VW-1, and VW-2, which were located in close proximity to the former tanks ' In addition, well MW-3R was damaged during this installation of the new tanks During site reconstruction, some of the contaminated soil that was encountered during excavation was reportedly placed in the former tank pit cavity as fill material In December 1994, Thrifty installed two vadose zone wells (TB-4 and TB-6), and three soil borings (TB-3, TB-13, and TB-14) at the Site TB-4 was installed to a depth of ' approximately 50 feet bgs, TB-6 to a depth of 25 feet bgs, and TB-3, TB-13, and TB-14 to 15 feet bgs Remediation piping was installed to many of the wells at this time Proposed well locations that could not be drilled were set with surface vaults to allow access for future drilling • In June 1995, groundwater monitoring wells TB-I and TB-10 were installed and constructed as 2-inch diameter monitoring wells Wells TB-7 and TB-8 were also ' TOC 171 Pilot study Page 3