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x. City of Stockton TL# 394-0028-02 March 10, 1994 Page 2 An air solid waste assessment test (Air-SWAT) was performed by Mandeville & Associates, a division of Kleinfelder, Inc. in the summer of 1988. The Air-SWAT reported that methane was present at concentrations greater than the lower explosive level with trace concentrations of volatile organic compounds. Samples collected from probes driven around the perimeter of the landfill were analyzed for methane concentration. A maximum concentration of 120 parts per million (ppm) was detected in one probe. Methane concentrations in the remaining three probes were measured below 40 ppm. 3.2 Geologic and Hydrologic Characteristics 3.2 . 1 Geology The site is located in the northern portion of the San Joaquin Valley which comprises the southern segment of the larger Great Valley. The Great Valley is an asymmetrical synclinal trough interrupted by two major surface cross structures: the Stockton fault in the Stockton arch and the White Wolf fault in and south of the Bakersfield arch. The Stockton fault is located approximately 1/4 to 1/2 mile southeast of the site, trending northeast. The Great Valley has been filled with a sequence of older to younger alluvium of Pliocene to Holocene age which overlie sedimentary rocks of Cretaceous to Tertiary age. These sedimentary units, in turn, overlie a crystalline basement of Paleozoic and Mesozoic metamorphic and igneous rocks. The shallow subsurface geology in the site vicinity consists of a heterogeneous mix of gravel, sand, silt, and clay (Norris and Webb, 1990) . 3.2 .2 Hydrology Sediments containing fresh groundwater are largely unconsolidated silts and sands derived from river channel, flood plain, and alluvial fan deposits of Pliocene to Recent age (Davis and others, 1959) . First encountered groundwater in the site vicinity occurs under unconfined (water table) conditions at a depth of approximately 50 to 60 feet BSG and generally flows to the east (Personal Communication with Mr. John Fukumoto of the City of Stockton) . Local groundwater flow may be influenced by pumping of water wells and recharge from various sources . The effect of these influences on groundwater beneath , the site is beyond the scope of this investigation. 3.3 Project History The French Camp Landfill was used previously as a burn dump for agricultural vegetative matter. This project is the initial step toward compliance with Subtitle D.