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.M C�colagtcalTtcltxfcslrc� Page 5 Revised Site Conceptual Model Report Project No.507.2 April 3,2008 2.4 Geological/Hydrogeologic Site Characteristics Geology The site is in southwest portion the City of Stockton, California, which is located in the San �.. Joaquin Valley. The Valley is underlain by a series of Quaternary and Tertiary marine and non-marine sediments. The sediments investigated in the project are Pleistocene and Holocene non-marine sediments consisting of basin deposits: major stream flood-deposits in +. basins lying between natural levees and alluvial fans [source: Geologic Map of California, San Jose Sheet, Division of Mines and Geology, 1966 (truncated geologic map copy located in Appendix E)]. The subject property is at an elevation of approximately 5 feet above mean sea level based on a December 20, 2001 survey conducted by Thompson-Hysell Engineers of Modesto, California (survey data included in Appendix F). Regionally, the surrounding area gently slopes to the southwest [source: USGS, Stockton West Quadrangle, 15 Minute Series Topographic Map, 1987 photo-revision (truncated topographic map copy located in Appendix E)]. The following section discusses the subjective field observations and geology documented during this investigation based on the interpretations of various field geologists: Borings logged by Wm.J Hunter &Associates (WJH): • In boring MW-8, WJH described the soils as predominantly clayey with some silty clays, silts, and 1" inch fine grained sand layers. The clays range in color from tan, brown to dark brown, and some grey hues. Blue to blue-grey units were noted starting at 20 feet and extend to 30 feet. Black mineral inclusions were present at 5 feet and white minerals veins were noted at 10 feet bgs. • Boring MW-9 was logged by WJH as containing predominantly clays similar to MW-8. The clays range in color from black, tan, It. brown to dark brown, and some grey and green hues. Grey, clayey sand was noted at 20 feet and blue-grey silt was noted at 30 feet. GTI logged the remainder of the borings: BoringsSB-i through SB-5, SB-109, and MW4 through MW-6: • The geology of the site is predominately clays and sandy clays with lesser amounts of silts and sands. • For most of the boreholes, except for those drilled very close to the old tank pit, gasoline odor was not observed in the soil samples until a depth of 20 feet bgs. It appears that the gasoline spread out on top of the old groundwater table. • The deeper soil encountered in SB-109 was a bluelgrey to blue/green color so it was +� difficult to visually determine when drilling had progressed past the contamination. v t..