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' Site Background r S ackg ound info matron GUARDING AND CRAWFORD 517 West Fremont Street, Stockton, California ' The site is located on Fremont Street in an area of low topographic relief in west Stockton, south of the site is the Stockton Deep Water Channel Local Iand use is the vicinity of the site is industrial ' and commercial Residential areas are Iocated adjacent to the site REGIONAL GEOLOGIC/HYDROGEOLOGIC SETTING The site is situated within the southem portion of the Great Valley Geomorphic Province of i California, a large, elongate, northwest trending, asymmetric structural trough, the northern and southern portions of the Province have been designated the Sacramento and San Joaquin Valleys, respectively The Province is bordered by the Coast Ranges to the west,the Klamath Mountains and Cascade Range to the north, and the Sierra Nevada to the east The Great Valley has been filled with sediments derived from both marine and continental sources ' Thickness of the sedimentary fill ranges from thin veneers along the valley edges to more than 20,000 feet in the south central portion of the valley The sedimentary formations range in age from , . Jurassic to Recent,with the older deposits being primarily marine in origin and the younger deposits being primarily continental Continental-denved sediments were primarily deposited in lacustnne, fluvial,and alluvial environments with sediment sources being the mountain ranges surrounding the valley(Olmsted and Davis, 1961),the site itself is located on unconsolidated and semi-consolidated alluvium, Iake, playa and terrace deposits of Quaternary age (California Division of Mines and Geology, 1977) Rocks composing the basement complex of the Province have not been completely ' defined but are believed to be metamorphic and igneous in ongin The Modesto, Riverbank and Turlock Lake Formations and overlying Recent alluvium are the ' principal sources of domestic ground water in the 13,500-square mile San Joaquin Valley Ground Water Basin(Basin 5-22) The groundwater at the site generally flows in a northwesterly direction and is found at a depth of 13 feet to 16 feet below surface grade The prevailing ground water flow ' direction in central Stockton is generally toward the northeast,however,localized ground water flow direction may vary during winter/summer cycles and tidal fluctuations Ground water is considered to be of beneficial use and is utilized for domestic, industrial and commercial purposes UNDERGROUND STORAGE TANK REMOVAL ' In September 1989, one 1,000-gallon underground storage tank(UST), piping dis enser and were p removed from the site Petroleum hydrocarbons were detected in soil samples collected from the ' UST excavation and dispenser areas at concentrations up to 700 parts per million(ppm, reported as . milligrams per kilograms) Advanced GeoEnvaron mental,inc