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Remedial Action Plan RE: 425 W. Larch Street, Tracy, CA October 1 , 2008 W&A Project No. FTIM-08-001 Page 3 of 11 3.2 Site Investigations 3 .2 . 1 Soil Investigations In August of 1996, Twinning Laboratories, Inc. (Twinning) drilled and sampled six boreholes (13- 1 , 13-2, B-3 , MW- 111A, MW4RB, and MW-2RA) and one angle borehole (B-4) to depths between 10. 5 feet and 12.5 feet below surface grade (bsg) at the site. The boreholes were drilled and sampled to assess subsurface conditions near the northeastern excavation (Figure 4, Borehole and Well Locations) . In addition, two boreholes were drilled with hand augers (S- 1 and S-2) to 3 feet bsg to assess subsurface conditions beneath the dispenser and product lines. Three groundwater monitoring wells, MW- 1 through MW-3 , were constructed, each to approximately 25 feet bsg to assess the groundwater beneath the site. Initial groundwater was encountered at 10.5 feet bsg (Appendix 1 , Borehole Logs) . Gasoline and related compounds were reported at concentrations exceeding 1 ,000 mg/kg of TPH in all of the soil samples collected from the capillary fringe in boreholes within 20 feet of the former UST farm Concentrations generally in the 100-500 mg/kg range were reported in those samples collected from the capillary fringe at distances greater than 20 feet from the former UST farm. The samples collected from beneath the product lines were relatively clean (Table 1 ). In January of 1998, Blakely Environmental Investigations, Inc. (BEH) constructed four groundwater monitoring wells (MW-4 through MW-7) to assess the extent of subsurface contamination beneath the site. No concentrations of TPHg or related compounds were reported in the soil samples collected (Table 1 ). In January 2000, BEH drilled and sampled two boreholes (B-5 and B-6), constructed four additional groundwater monitoring wells (MW-8 through MW- 11), and collected nine soil vapor samples as part of an additional site assessment (Figure 4) . The laboratory reported no concentrations of TPHg or related compounds in any of the soil samples except those at 10 and 15 feet below ground surface (bgs) in borehole B-6. B-6 was drilled near the former UST farm and the TPHg was reported at insignificant concentrations. The soil vapor results are discussed below. In May 2002, BEH constructed two deeper groundwater monitoring wells (MW42 and MW- 13) and abandoned well MW-3 . BEII later constructed a new well one to replace MW-3 (MW-3R) . The deeper wells were screened from 30 feet bsg to 40 feet bsg. The laboratory reported small concentrations of TPHg and related compounds (including MtBE for the first time) in all of the soil samples collected from MW- 13 , to a depth of 40 feet bgs. MW- 13 is located just northwest of the former UST farm. The laboratory reported no detection of TPHg or related compounds in the samples collected from MW- 12 . In June 2003, BEII constructed an additional deeper groundwater monitoring well (MW- 14) at the site. MW44 was screened from 30 feet bsg to 40 feet bsg. The laboratory reported small amounts of MtBE in the samples collected at 5 and 10 feet bgs, but no other TPHg or related products in any of the samples. VY ELU E ASSOCIATES,AfV CSRN