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s Ldo ATC Associates Inc. 1117 Lone Palm Avenue, Suite 201 Modesto,California 95351 LM (:V9%TC209-579-2221 A s 6 0 C I A T E S I IY C. Fax;209-579-2225 4.2 Vadose Zone Soil ti•• Laboratory analytical data from previous soil investigations indicated petroleum hydrocarbon constituents were detected in soil samples collected from soil borings advanced in the vicinity of the former UST ' location (borings SB3 and MW1) and in borings MW2 and VEW3 advanced south of the former UST La location. The highest concentration of TPHg, 1,900 milligrams per kilogram (mg/Kg), was reported in the soil sample collected from SB3 at a depth of 20 feet bgs. TPHg and BTEX concentrations detected in some of the soil samples exceeded current Regional Water Quality Control Board Environmental Screening Levels (ESLs). A soil sample collected from 13 feet below ground surface (bgs) during the UST removal activities was reported to contain elevated petroleum hydrocarbons. No other soil laboratory analytical data for depths less than 15 feet bgs exists. The absence of shallow soil data represents a potential data gap. However, PID measurements of soil samples at these depths during L,,, boring advancement were either zero or at background levels and no odors were noted on the boring logs (Appendix A). Based on the laboratory analytical data, petroleum impacted soil extended from 15 to 45.5 feet bgs in the vicinity of the former UST location and at VEW3. Low concentrations of petroleum hydrocarbons were detected in soil samples collected from MW2 at depths of 40.5 and 50.5 feet bgs. The t' petroleum hydrocarbon concentrations appeared to attenuate with depth and did not appear to extend to the water table. Soil mass balance calculations were presented in ATC's Addendum to the Site Conceptual Model and Corrective Action Placa dated March 5, 2004. At the time it was estimated that a>L,, approximately 200 pounds of TPHg and BTEX were present in the soil. In subsequent correspondence, SJCEHD indicated they were not in agreement with ATC's calculation and this mass was probably underestimated. Analytical data from soil samples collected at the site are summarized in Table 3. The estimated extent } of petroleum impacted soil prior to operation of the SVE system is shown on Figure 5. A generalized L. cross section is provided as Figure 6. As previously indicated in Section 1.2, an SVE system was operated at the site for approximately 16,500 hours from January 17, 2007 to February 13, 2009. Operation of the SVE system was terminated when extracted volatile organic vapor concentrations became asymptotic and too low to justify sustained continuous operation. Results of a rebound test conducted on July 14, 2009, after 151 days of non- operation, indicated the extracted VOC concentrations were similar to those at the termination of the SVE system in February of 2009. In order to estimate the contaminant mass remaining in the vadose zone following the SVE effort, laboratory analyses of soil samples collected subsequent to cessation of the SVE activities would be necessary. Since confirmation soil sampling and analyses post SVE activities have not been conducted, calculations regarding contaminant mass remaining in vadose zone soil were ■10 not performed. s 4.3 Saturated Zone Soil Subsurface soils within the saturated zone do not appear to be impacted. One soil sample was collected from the saturated zone at a depth of approximately 65.5 feet bgs in each of the boreholes associated with ' MWI, MW2, and MW4. An additional saturated zone soil sample was collected from the borehole of LW MW3 at a depth of approximately 60.5 feet bgs. Saturated zone soil samples were collected from the I S:Environmenta1162577aeports12010 closure summary.doc 5 LO �r