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r VAPr NWW .. City of Stockton, Fire Station No. 12 A68I05.01 August 7, 1995 Page 4 • Field and analytical data were evaluated to assess the extent of PPC migration in soil and assess the need for additional site assessment; and .., • This technical report was prepared to document the investigative procedures, summarize the findings, and present conclusions and recommendations. 4.0 INVESTIGATIVE PROCEDURES •• This preliminary site characterization consisted of a field investigation, a laboratory testing program, and preparation of a technical report. These activities are summarized in the following subsections. Standard operating procedures (SOPs), relevant to the field and laboratory activities, are described in Appendix A. Any exceptions which were made to the SOPS are explained in the appropriate following section(s). raw 4.1 Field Investigation: The field investigation consisted of drilling soil borings and collecting soil samples for chemical analyses. am 4.1.1 Soil Boring Locations and Soil and Groundwater Sampling: On June 21, 1995, Twining drilled four soil borings (B-1 through B-4) at the locations shown on Drawing 3. VM The purpose of soil borings B-1, B-2, B-3, and B-4 was to assess the lateral and vertical extent of PPC migration in the vicinity of the former UST excavation. `' The first soil boring (B-1) was drilled approximately 17 feet south of the former UST excavation to a vertical depth of 45 feet BSG. The purpose of this soil boring was to determine that groundwater was not shallower than 45 feet BSG. This data was necessary owto preclude intercepting groundwater when drilling the soil boring through the PPC impacted soil. The second soil boring (B-2) was drilled in the center of the former UST excavation where PPC concentrations were expected to be greatest based upon previous bw sampling data. Because groundwater had not been encountered while drilling soil boring B-1, soil boring B-2 was advanced to a depth of 45 feet BSG. The third and fourth soil borings (B-3 and B-4)were drilled approximately 2 feet north and 4 feet south, respectively, am from the former UST excavation to a depth of 30 feet BSG. Soil samples were collected at 5 ft intervals from each boring and were observed in the field MW for visible staining, petroleum odor and field screened for volatile organic vapors using a photoionization detector (PID) consistent with our SOPS. The results of the field screening and lithologic descriptions of the subsurface soils are recorded in the soil boring logs IM presented in Appendix B. Laboratory analysis of the soil samples collected from soil boring B-1 were not considered MW necessary based on field observations and analytical results of soil samples collected from soil borings B-3 and B-4. Soil samples collected at 10, 15, 20, 25, 30, and 35 feet BSG in