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Stantec No Further Action Request Report Chevron and ARCO Commingled Plume No. CP0034 April 10, 2012 2. BACKGROUND 2.1. SITE DESCRIPTION The Chevron facility No. 94275 and ARCO Service Station No. 2133 are located at the intersection of West Benjamin Holt Drive and Plymouth Road in Stockton, California. The site is located in a light commercial and residential area of Stockton in a relatively flat lying area. ARCO 2133 is currently comprised of an AM/PM mini-mart that retails gasoline fuel with four underground storage tanks (USTs), dispenser islands, and associated product lines. Chevron 94275 features include two double-walled, fiberglass USTs, a station building, six product dispensers under a common canopy, and a car wash. The site is bound by retail shopping to the north, residential homes to the south, and Interstate Highway 5 to the west. The site is bound by Plymouth Road to the east, beyond which are a golf course and residential homes. According to a United States Geological Survey Map dated 1976, the elevation of the site is at approximately 3 feet (ft) above mean sea level. The nearest open body of water is the Fourteen Mile Slough, located approximately 2,000 ft south of the site. A site location map and a site plan are included as Figures 1 and 2. 2.2. REGIONAL GEOLOGY AND HYDROLOGY The City of Stockton is located in the San Joaquin Valley which is underlain by alluvial sediments up to 1,000 ft thick. The principal source of recharge to the groundwater in the valley is the Sierra Nevada Mountains to the east. Groundwater historically flowed from the base of the Sierra Nevada Mountains westward towards the San Joaquin-Sacramento River delta. However, increased municipal use of groundwater in the Stockton area has developed a groundwater depression changing the regional gradient away from the delta and towards the municipal wells. This has caused easterly migration of poor quality saline groundwater from the delta. Most water wells in the Stockton area draw groundwater sandstone beds at depths greater than 150 ft bgs (EMCON, 1986). 2.3. SITE GEOLOGY AND HYDROGEOLOGY Geologic cross-sections for the site are presented in Appendix B. Historical boring logs are provided in Appendix C. Soil boring and well construction details are presented in Table 1. The cross sections and boring logs depict the following conditions: • A predominantly clay and silt layer encountered from the ground surface to approximately ten ft below ground surface (bgs). 94275 NFAR 2012.doc 3