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•,L rs it#INC 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 Fault, 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), consist 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 northward flow 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 ' Groundwater 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 (MWI,1, MW2, and MW3) on April 3, 1997, was 37 31, 37 33, and 37 05 feet bgs, respectively Based on well construction diagrams developed by Wallace-Kuhl, the top of the screen in monitoring wells MW1, MW2, and MW3, is approximately 38 00, 37 00, and 35 00 feet bgs, respectively Although the groundwater level was above the screening in monitoring wells MWl and MW3, Versar does not believe it has had serious impact on the integrity of the samples The groundwater in MW 1 and MW3 was above the screen by approximately 0 69 feet and 2 05 feet, respectively However, since samples collected from MW3 have historically never contained contaminants of concern above the laboratory detection limits, it should not be regarded as a significant concern The hydraulic gradient was calculated at 0 003 ft/ft flowing to the northeast, a slight increase from the ' 0 002 ft/ft which was reported during the last sampling event and is consistent with previous data Groundwater elevations and gradient are included in Table 1 and shown on Figure 3 i H 1PROJECTSIDSAISTOCKTON14TUTINALIREPORT FL 2 t