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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 4 of 11 In January 2004, BEII constructed four additional groundwater monitoring wells (MW- 15 through MW- 18) . MWs- 15, 16, and 18 were constructed as deep monitoring wells and screened from 30 feet bsg to 40 feet bsg. Shallow monitoring well MW- 17 was screened from 5 feet bsg to 20 feet bsg. The laboratory reported no detection of TPHg or related compounds, except MtBE in any of the samples collected. MtBE was reported in small quantities in samples from each of the boreholes and generally in the samples collected at 10 and 15 feet bgs (Table 1 ) . In August 2005, BEA constructed three shallow groundwater wells. Monitoring wells MW- 19, MW-20 and MW-21 were screened from 5 feet bsg to 20 feet bsg. The laboratory reported small concentrations of TPHg, BTEX and MtBE in the samples collected at 10 feet bsg from wells MW- 19 and MW-21 . The laboratory reported only small amounts of MtBE ( 15 feet bsg) and no TPHg or BTEX in the samples from MW-20. In September 2006, two additional wells, MW-22 and MW-23 , were constructed in preparation for an aquifer pump test at MW-3R. The laboratory reported TPHg, BTEX, and MtBE in the soil samples collected from the boreholes (close proximity to the former UST farm). In February 2007 additional sentinel wells MW-24, MW-25 and MW-26 were constructed to determine the extent of contaminant migration. The laboratory reported TPHg in only one sample, and in a small amount. The laboratory reported MtBE in samples collected from MW- 24, but not MW-25 or 26 (Table 1 ) . 3 .2 .2 Groundwater Investigations Groundwater investigations at the site have been performed through the construction of groundwater monitoring wells and then through quarterly groundwater monitoring. The description of the construction of the wells has been discussed in Section 3 .2. 1 . There are two water-bearing zones in which groundwater monitoring wells have been constructed. Wells MW- 12, 13 , 14, 150 16, and 18 have all been constructed in the deeper zone with screened intervals from 30-40 feet bgs. The remainder of the wells have been constructed in the shallow water-bearing zone and have screened intervals from 5020 feet bgs. Groundwater monitoring has been performed at the site since 1995 . Groundwater flows generally westerly beneath the site. According to data from the latest groundwater monitoring report (Appendix 2), there is a large plume of MtBE extending from the easterly property line westerly to likely just off the westerly edge of the site. Concentrations of up to almost 5 ,000 µg/1 of MtBE have been reported in the middle of the plume. There is a smaller plume of BTEX that has generally the same shape, is centered around the same area (the former UST farm), but is just smaller extending westerly across most of the eastern half of the site. Diagrams taken from the 1 st Quarter 2008 Groundwater Monitoring Report illustrating these plumes are provided in Appendix 2 . The report also provides diagrams showing water depths in the wells and various contaminant concentrations over time in the wells. 3 .2 .3 Soil Vapor Study In January 2000, BEII drilled 9 shallow boreholes and from each collected soil vapor samples as part of an additional site assessment. Vapor was collected from each location at a depth of 4 feet WINE FIEIO E RSS*FfTy CO S, INC.