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ENSR ASE CC3M 3.2 Contaminant distribution in soil Soil and groundwater investigations conducted on the site indicate that the main source area is located in the vicinity of the former 1,000-gallon USTs near the southeastern corner of the property and has been adequately assessed. Residual source material in soil extends from former dispenser island soil boring D-4 to the southeastern corner of the site property and north to just short of former dispenser island soil boring D-3. A review of field screening and analytical data within this area indicate that soil concentrations were present above default cleanup criteria at the following depth intervals: Table 3-2 Depths of source area impacts above cleanup criteria Soil Boring Identifier Depth of Impacts(feet bgs) U-C <5-n32 U-D <5-X32 U-K 41.5,not above or below U-L 15-X50 U-M 30-X45 U-PIUV-1 10-X40 U-3 40-50 Notes: Depths indicated with<or 7 indicate that no samples were collected for screening or analysis respectively above or below the given depth. Because the historically low water table was measured at approximately 48 feet bgs, soil concentrations are understood to diminish at depths below those investigated, since any light non-aqueous phase liquid (LNAPL) that was previously on site would not penetrate below the water table. Groundwater concentrations present below the water table are understood to be a function of the strong downward hydraulic gradient in the area (see Section 3.1.2). The extent of the source area(i.e., the area where petroleum constituent concentrations detected in soil were above the default cleanup criteria specified in Section 4.0) is depicted on Figure 3-7. This source area surrounds USTs that were removed in 1967, indicating that this is a very old release. Analytical data demonstrate a near absence of MTBE or MTBE metabolites, which also supports this finding. 3.3 Contaminant distribution in groundwater Figures 3-8 through 3-10 depict MTBE,TPHg, and benzene analytical results from the February 2005 groundwater sampling event. Groundwater concentrations above MCLs have been in Zone A and Zone B on site and in Zone B along the east side of California Street. Beyond California Street, groundwater concentrations in excess of MCLs have been detected in Zone B and Zone C. In this latter area, the horizontal extent of benzene concentrations exceeding MCLs is greatest in Zone C; however, the highest concentrations have been detected along the plume centerline in Zone B. This pattern of impacts is indicative of a groundwater plume that migrated vertically on site, due to the strong downward component to the hydraulic gradient(i.e., approximately two orders of magnitude stronger than the horizontal gradient), and subsequently migrated horizontally off site. The downgradient extent of petroleum impacts extends to the Vintage Car Wash property, where a separate release of petroleum hydrocarbons is under investigation. The data from the Vintage Car Wash property site JARem_EngTrolect Files1P60(6000- 6999)16940-UNOCAL.Portfolio Closure16940- 3-2 January 2006 UNOCAL_Portfolio CiosurelSite 0187-Stockton East MlnoACSM ReporINCSM Report Final.doc