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22 October 1996 <br /> AGE-NC Project No. 95-0100 <br /> ' Page 2 of 9 <br /> California. The valley is a nearly flat lying, elongate trough trending northwest and southeast for <br /> approximately 4750 miles. The valley is enclosed by the granitic Sierra Nevada Mountains on the <br /> east and the sedimentary and metamorphic Franciscan Coast Range on the west. The surfcial and <br /> ' upper several thousand feet of subsurface layers consist primarily of unconsolidated and <br /> consolidated alluvial and floodplain sheet deposits(predominantly sand, silt and clay)of Quaternary <br /> age,which were derived from the Sierra Nevada Mountains. Beneath the upper sedimentary deposits <br /> ' lies a thick sequence of marine deposits of Mesozoic age,which in tutu is underlain by a pre-Jurassic <br /> complex of igneous and metamorphic rock. <br /> ' The Modesto, Riverbank and Turlock Lake Formations and overlying modern alluvium are the <br /> principal source of domestic ground water in the 13,500 square-mile San Joaquin Valley Ground <br /> Water Basin (Basin 5-22). This basin is drained primarily by the San Joaquin River <br /> 1 <br /> ' 2.3. GROUND WATER DEPTH AND GRADIENT <br /> Ground water depth north the site is approximately 45 feet bsg, based on information from Defense <br /> Distribution Region West - Tracy site. Ground water at the site was observed to be approximately <br /> 54 feet bsg during the installation of a well on site. Regional ground water flow direction in the area <br /> is easterly. However, flow direction can vary locally and seasonally. <br /> 2.4. UNDERGROUND STORAGE TANK REMOVAL <br /> ' Two 1,500-gallon USTs,piping and dispensers were removed from <br /> the property to September 1993. <br /> Analysis of soil samples collected from beneath the dispenser at a depth of six feet below surface <br />' grade(bsg) indicated that total petroleum'hydrocarbons quantified_as gasoline (TPH-g) and volatile <br /> aromatics (benzene, toluene, ethylbenzene and xylenes: BTE&X) were present at concentrations as <br /> high as 48,000 parts per million (ppm, reported as milligrams per kilogram). Total petroleum <br />' hydrocarbons quantified as diesel were not detected in the soil sample analyzed. Total lead was <br /> detected in the sample collected from the dispenser area at 270 ppm. Ethyl ene-dibromide was also <br /> detected in the same sample at 70 parts per billion(ppb,reported as micrograms per kilogram). The <br />' analytical results of the soil samples from the UST removal are summarized on Table L 'The <br /> laboratory report is included in-Appendix A. <br /> 2.5. INITIAL SITE ASSESSMENT <br /> 1 On 12 August 1994, two borings (boring B l and boring 132) were drilled under the supervision of <br /> Geological Audit Services, Inc. (GAS) in the area of the former fuel dispenser island. TPH-g was <br /> t - <br />