COMPLIANCE INFO_1993-1994 - Laserfiche WebLink

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City of Stockton TL# 394-0027-02 March 10, 1994 Page 2 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 a'n 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 five miles northwest 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 80 to 90 feet BSG and generally flows to the southeast (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 Austin Road Landfill has been used as a municipal waste dump. Twining understands that SJCEHD issued a notice of violation to the City of Stockton regarding non-compliance with soil gas monitoring at this facility. An air solid waste assessment test (Air-SWAT) was performed by Mandeville & Associates, a division of Kleinfelder, Inc. in late 1987 . The Air-SWAT reported that methane was present at concentrations greater than the lower explosive level with significant 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 135 parts per million (ppm) was detected in one probe. Methane concentrations in the remaining probes were measured below 10 ppm.