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(bgs). Below this depth, a fine to coarse sand and silty clay are present. Similar soils were <br />reportedly observed to a depth of 15 feet bgs during the 1990 UST removal at the AASF. With <br />the exception of the former UST location adjacent to the OMS, Versar's PA/SI investigation <br />exploratory borings were advanced to a maximum depth of 15 feet, and were predominantly 5 <br />feet in depth or less. At the OMS former UST location, Versar's borings (B-26 and B-27) were <br />advanced to a maximum depth of 25 feet bgs. Soils encountered by Versar comprised clayey <br />silt, with localized areas of sandy silt, and silty clay. <br />1.3 HYDROLOGY AND HYDROGEOLOGY <br />The Site has been graded and modified to promote surface drainage. Catch basins collect <br />rainwater for flow north through underground pipes into the adjacent airport storm water system. <br />Eventually, surface drainage is to Little John Creek, a water course oriented east to west <br />approximately 1.5 miles north of the Site. Little John Creek flows into French Camp Slough and <br />eventually into the San Joaquin River. <br />Groundwater has been measured at the AASF at depths ranging from 37 to 44 feet bgs. Based <br />on water level measurements, the water table beneath the AASF has been determined to be <br />essentially flat with an average hydraulic gradient of 0.004 foot/foot (ft/ft) to the northeast. The <br />first groundwater encountered beneath the Site is considered by the State Regional Water Quality <br />Control Board (SRWQCB) to be an aquifer of beneficial use. <br />2 <br />3027-03/ 104700.4766.230/JUL23' 03 <br />