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• 12 Geology and Hydrology The Site is located in the central portion of the Sacramento Valley The Sacramento Valley is composed of unconsolidated Quaternary sediments from alluvial and Iake deposits This alluvium is the result of the erosion of the foothills of the Sierra Nevada Mountains to the east The Sienas are composed of a tAide variety of rocks, including granitic, metaimoiphic, and volcanic rocks, which range considerably in age and composition To the west of the Sacramento Valley are the Coast Ranges which consist of the Fianciscan Complex of sedimentary and metamorphic rocks Structurally significant is the Stockton Fault wfach 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 soils beneath the Site, characterized during the February 1994 investigation, consisted of fine sands, silts, and silt/sand mixtures to approximately 50 feet below ground surface (bgs) Similar soils were obseived during the 2000 PA/SI investigation (Versar 2003) Versai's 2004 investigation exploratory borings were advanced to a maximum depth of approxunately 50 feet, where groundwater was encountered Versar's boring logs are presented in Appendix A The static groundwater level has been measured at the AASF at depths ranging from 37 to 44 feet bgs Based on water level measurements, the grater table beneath the A.4SF has been deterinined to be essentially flat, with an average hydraulic gradient of 0 004 foot/foot (ft/ft) to the northeast The frist groundwater encountered beneath the Site is considered by the State Regional eater Quality Control Board (SR'WQCB) to be an aquifer of beneficial use The Site has been graded and modified to promote surface diamage Catch basins collect rainwatei for flow north through underground pipes into the adjacent airport storm water system EN entually, surface drainage is to Little John Creek, a water course oriented east to west apprommateI3 1 5 miles north of the Site Little John Creek flours into French Camp Slough and eventually into the San Joaquin River 13 Scope of Work The objective of the work is to collect Site-specific information necessary for the evaluation of the residual and new areas of concerti at the AASF, CSMS, and OMS To meet this objective, soil borings were drilled, and soil and grab-groundwater samples will be collected and analyzed for identified constituents of concern in addition, two areas of pavement upwelling were excavated and the apparent cause of the upwelling identified A detailed discussion of the nrvestication activities is included in Section 2 0, Site Ing estigations 104700 4766 230/08sEPT2004 4