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i Stantec �! > No Further Action Request Report Chevron and ARCO Commingled Plume No. CP0034 April 10, 2012 • A silty sand and poorly graded and well graded sand layer encountered from ten ft bgs to approximately 19 to 28 ft bgs. • A clay layer described as both a lean clay and -fat clay in various borings encountered from approximately 25 to 30 ft bgs to 45 to 50 ft bgs. • A poorly.graded sand to silty sand layer encountered between 45 to 50 ft bgs to 52 to 57 ft bgs. • The deepest layer encountered-to date is a clay layer below 57 ft bgs. • Depth to groundwater has varied from approximately six to.20 ft bgs and is in an unconfined aquifer. Historically, groundwater flow has been dominantly to the northeast at a gradient of 0.0001 to 0.0009 ft/ft; however, during the last four sampling events the groundwater direction has been to the north and north-northwest. A groundwater elevation contour map is included as Figure 3. Historical groundwater gradient data is included in Table 2 and is depicted on a groundwater direction rose diagram included as Figure 4. 2.4. SUMMARY OF PREVIOUS WORK 2.4.1. UST History i Chevron 94275 Facility The original service station was constructed in 1973. At that time,four single-walled, fiberglass USTs were installed. A gasoline release from a product line was detected in 1985. In January 1989, four single-walled USTs (three gasoline and one waste oil)were removed. No holes were found in any of the USTs. Soil samples were collected from beneath the tanks and beneath the product lines. Soil samples collected from below the excavated tanks contained up to 3,600 milligrams per kilogram (mg/kg) gasoline range organics (GRO) and 31 mg/kg of benzene. Soil samples from below the southern product line were non-detect for all analyzed analyses; however, samples from below the northern. product lines reported 310 mg/kg GRO and 0.63 mg/kg of benzene. Soil samples collected from the bottom of the waste oil UST pit were non-detect for GRO and benzene; however, diesel range organics (DRO)were reported at a concentration of 0.0011 mg/kg. Additionally, a soil sample from beneath the former waste oil j UST was analyzed for total oil and grease, halogenated volatile organic compounds, polychlorinated biphenyls, and pesticides. The constituents were not detected above the laboratory method reporting limits (MRL). A new UST pit was excavated and five double- walled, fiberglass USTs were installed (four gasoline and one methanol). Historical soil I 94275 NFAR 2012.doc 4