ARCHIVED REPORTS_PRELIMINARY ASSESSMENT_SITE INVESTIGATION REPORT

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• volcanic rocks, which range considerably in age and composition. To the west of the Sacramento Valley are the Coast Ranges which are composed of sedimentary and metamorphic rocks comprising, to a large extent, the Franciscan Complex. Structurally significant is the Stockton Fault which trends northeast-southwest and is located within Stockton city limits, and the north trending Midland Fault Zone which is located approximately 25 miles northwest of the Site. The San Andreas fault zone is also present approximately 50 miles west of the Site. The soils beneath the Site characterized during drilling in February 1993 (Wallace-Kuhl) consisted of fine sands and silt/sand mixtures to approximately 50 feet below ground surface (bgs). Below this depth, a fine to coarse sand and silty clay are present. Similar soils were reportedly observed to a depth of 15 feet bgs during the 1990 underground storage tanks (USTs) removal at the AASF. Versar encountered soils similar to those described by Wallace-Kuhl (1993). With the exception of the former UST location adjacent to the OMS, Versar's PA/SI investigation exploratory borings were advanced to a maximum depth of 15 feet, and were predominantly 5 feet in depth or less. At the OMS former UST location, Versar's borings (B-26 and B-27) were advanced to a maximum depth of 25 feet bgs. Soils encountered by Versar typically comprised clayey silt, with localized areas of sandy silt, and silty clay. Versar's drilling logs are presented in Appendix A. 1.3 Hydrology and Hydrogeology Groundwater has been measured at the AASF at depths ranging from 37 to 44 feet bgs. Based on • water level measurements, the water table beneath the AASF has been determined to be essentially flat with an average hydraulic gradient of 0.004 foot/foot (ft/ft) to the northeast. The first groundwater encountered beneath the Site is considered by the State Regional Water Quality Control Board (SRWQCB) to be an aquifer of beneficial use. The Site has been graded and modified to promote surface drainage. Catch basins collect rainwater for flow north through underground pipes into the adjacent airport storm water system. Eventually, surface drainage is to Little John Creek, a water course oriented east to west approximately 1.5 miles north of the Site. Little John Creek flows into French Camp Slough and eventually into the San Joaquin River. 2.0 BACKGROUND The area surrounding the Stockton CA ARNG facility has been a mix of airport and agricultural usage for over 40 years. The AASF has been in existence since the early 1950s as an airplane fueling station and was expanded to its present size of approximately 25 acres in 1974. The Armory , CSMS, and OMS appear from aerial photographs to have been constructed in 1963. • 2187-01/3134-100/MAY26'00 2