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I composed of a wide variety of rock types, including granitic, metamorphic, and volcanic rocks West of the San Joaquin Valley are the Coast Ranges, which are composed of metamorphic rocks of the Franciscan Formation A structurally significant feature in the Site area is the Stockton IFault, which trends northeast-southwest and is approximately eight miles northwest of the Site The soils beneath the Site characterized during the February 1993 drilling (Wallace-Kuhl) I 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 removal of the USTs in 1990 1.3 Hydrology 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 Surface drainage is to Little John Creek, a water course oriented east-west, approximately 1 5 miles north of the Site Little John Creek flows into Camp Slough and eventually into the San Joaquin River IGroundwater was encountered during drilling in February 1993 at depths ranging from 46 to 49 feet bgs Groundwater measurements made in the wells on February 24, 1993 indicated a static groundwater surface of approximately 45 feet bgs The groundwater flow direction beneath the Site on February 24, 1993 was calculated to be to the northeast with a hydraulic gradient of 0 001 feet/foot (ft/ft) The depth to groundwater in the three monitoring wells, MW1, MW2, MW3, on December 31, 1995 was 42 87, 42 87, and 42 55 feet bgs, respectively The hydraulic gradient I was calculated at 0 002 ft/ft flowing to the east-northeast The gradient is flatter than the calculated gradient for the previous quarter and the direction has changed slightly from southeasterly to east-northeasterly Versar believes this reflects seasonal fluctuations in the Igroundwater flow Groundwater elevations and gradient are included in Table 1 and shown on Figure 3 I 2 a SITE BACKGROUND The area surrounding the Site has been characterized by a mix of airport and agricultural usage for over 40 years The Site 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 fuel storage for the Site was originally located in three 5,000-gallon steel USTs installed in the early 1950s These USTs were removed in 1990 The USTs were oriented east-west and rested on a six-inch thick concrete slab The slab remains in place at an estimated depth of 11 feet bgs The H 1PROJECTSIDSAISTOCKTON13RD\REPORT FL 2