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' 2.3 Regional Hydrogeology GEOMATRIX ' The Charter Way facility is located in the northern portion of the San Joaquin Valley, a broad structural trough situated between the Coast Ranges on the west and the Sierra Nevada on the east. The facility is located on the lower flood plain of the San Joaquin River near the San Joaquin River Delta. Sediments beneath the facility consist of unconsolidated, interbedded clays, silts, sands and gravels designated as Quaternary Flood Basin Deposits. These sediments reportedly extend to a depth of about 1,000 feet in ' the Stockton area(DWR, 1967). ' According to the San Joaquin County Flood Control and Water Conservation District (SJCFCWCD), the depth to groundwater is highly variable in the Stockton area, ranging from less than 10 feet to more than 190 feet bgs. Groundwater flow is predominantly eastward from the San Joaquin River Delta area ' towards depressions formed by regional pumping centers north, east, and southeast of the City of Stockton. The largest groundwater depression is located east of Stockton along State Highway 4 ' (SJCFCWCD, 1992, 1995, 1996, 1997). 2.4 Local Hydrogeology ' The results of the previous investigations indicate that shallow sediments beneath and in the vicinity of the Facility consist of relatively continuous,thin layers or beds of sand, silt, and clay to a depth of about ' 150 feet bgs. These beds appear to dip slightly from east to west and from north to south In addition, the beds appear to become thinner and finer grained from north to south (Geomatrix, 1997b). The lithologic data indicate that the upper 16 to 20 feet of sediments beneath and in the vicinity of the Facility consist of relatively fine-grained materials. Between about 20 and 40 feet bgs is a relatively continuous fine- to medium-grained sand interval (Figure 3). This sand interval bifurcates into two sand layers separated by a clay layer towards the south and west. This sand interval has been desig- nated the shallow zone. The sediments to approximately 30 feet bgs appear light brown in color, suggesting an oxidizing depositional environment. These sediments are interpreted to be fluvial flood basin deposits from the nearby San Joaquin River. The shallow zone is currently monitored by 14 wells ' including MW-1 through MW-8, MW-10 through MW-14, and upgradient well MW-16(Figure 2). ' The results of previous investigations indicated that the sediments beneath the shallow zone to a depth of about 74 feet bgs comprised primarily low-permeability silty clay with occasional discontinuous permeable lenses (Figure 3). This fine grained interval appears to be an aquitard between the shallow ' zone and deeper water bearing intervals. The sediments are generally blue-green in color, suggesting a reducing depositional environment. These sediments are interpreted to be estuarine deposits of the San ' Joaquin River Delta basin. ' Between about 40 and 100 feet bgs is a sequence of discontinuous interbedded clay and sand layers. Relatively continuous sand intervals were identified at approximately 60, 80, and 100 feet bgs ' 1VROIECTCJ859DOCIIMENNBf9J).tlec 4