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City of Stockton - French Camp Landfill TL 394-0028-02 April 28, 1994 Page 2 2.0 BACKGROUND A description of the site, the geologic and hydrologic characteristics, and the project history are summarized in the following subsections. 2.1 Site Description. The French Camp Landfill (site), a former burn dump for municipal waste, is located south of the City of Stockton, California, west of Manthey Road and between Walker and French Camp Sloughs (Drawing 1). An air solid waste assessment test(Air-SWAT) was performed by Mandeville & Associates (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 tested 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. 2.2 Geologic and Hydrologic Characteristics 2.2.1 G=. 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). 2.2.2 Hvdroloay. 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).