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Site History 14000 Highway 88, Lockeford, CA Time Oil Property No.04-049 Site Assessment-August 1992 On August 26, 1992, a soil vapor survey was performed to assist in determining the extent of impacted soils. A pneumatic hammer was used to drive 13 soil vapor probes to depths of up to 13 feet in the vicinity of the former tanks. Vacuum pumps and charcoal tubes were used to collect vapor samples from each probe at various depths. Immediately after collection, each sample was submitted to an onsite mobile lab where it was extracted and injected into a gas chromatograph. Due to the presence of hardpan just below the ground surface, the results of the soil vapor survey were limited, as no probes could be advanced deeper than 13 feet. During the week of September 28, a drill rig was used to advance 4 soil borings and install 3 vapor extraction wells and 3 vapor inlet wells. During field activities, soils were found to consist of hardpan to approximately 15 feet; this strata was underlain by loose fine to medium sands. Dense silty sands and sandy silts were generally encountered at depths of 30 to 35 feet to the extent of the borings. Groundwater was not encountered during field activities. Drilling at each location was continued until hydrocarbons were no longer indicated by field instrumentation. Accordingly, borings SB-1, SB-2, and SB-4 were terminated at a depth of 40 feet while SB-2 (W1) was advanced to 45 feet. Wells W3, W5, and W6 were installed to 40 feet, while W4 was installed to 30 feet and W2 was installed to 45 feet. Wells W1, W2, W3 were installed as 4" diameter vapor extraction wells, and wells W4, W5, and W6 were installed as 2" diameter vapor inlet wells. Although each well was constructed using 0.020 slotted casing, screened intervals varied depending on well depth and field observations. Three soil samples from each boring were submitted for analysis based on field screening and visual observation. (Only 2 samples were submitted from W3.) Each sample was analyzed for TPH, BTEX, `..• and ethylene dibromide. Results indicated that soil contamination was generally limited to the area of the former tanks and appeared to exist at a depth of 30 feet to 35 feet. The highest concentrations of contaminants were indicated in 30-foot samples collected from W1(SB-2), W2, and SB-2. Samples collected from a depth of 15 feet in SB-1 and W3 contained 200 - 300 ppm gasoline; however, samples collected at 30 feet contained less than 10 ppm gasoline. Samples collected from SB-4, W4, W5, and W6 contained trace to "non-detect" concentrations of contaminants. None of the samples collected from depths of 40 feet to 45 feet contained detectable concentrations of gasoline or its constituents. After the installation of the wells, a vapor extraction pilot test was performed on W1, W2, and W3 to evaluate the feasibility of in-situ remediation by vapor extraction. The results of the study indicate that soil vapors can be easily extracted from the existing wells and that vapor extraction appears to be appropriate for site remediation. Based on the results of the site investigation, it appeared that impacted soils were limited to the former tank excavation and the more dense soils encountered at a depth of 30 feet. Although several of the soil samples submitted for analyses contained trace concentrations of hydrocarbons, SB-4, W4, W5, and W6 generally defined the extent of soil contamination to the west, southwest, north, and south of the tanks. A copy of Canonie's "Site Assessment Report" was forwarded to the appropriate regulatory agencies on May 25, 1993. In their reply dated June 18, 1993, the PHSIEHD requested that a workplan for an additional investigation to verify the extent of soil contamination be submitted. Workplan for Additional Site Activities- October 1993 A copy of Science and Engineering Analysis Corporation's (SECOR) "Workplan, Additional Environmental Investigation" was forwarded to the appropriate regulatory agencies on October 20, 1993. The goal of the proposed investigation was to completely define the lateral extent of impacted soils at the subject site. Site assessment activities proposed to meet the project goals included: 2