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r 2 GEOLOGY . The site is situated in the northeastern portion of the San Joaquin Valley, in the southern Great Valley physiograpluc province The Great Valley is an elongated northwest trending asymmetric structural basin that has a wide flank and a narrow western flank expressed by the upturned edge of the sediments that have accumulated in the basin The San Joaquin Valley is filled with a duck sequence of marine and continental sediments ranging in age from Jurassic to Recent The thickest accumulation underlies the western portion of the valley The Sierra Nevada Range is the primary source for this sequence of sedimentary rocks The youngest formation beneath the site is mapped as Quaternary ( Recent ) alluvial fan deposits of the Modesto Formation as indicated on the Geological Map of the San Jose Quadrangle USGS- 1981 ( 1 75,000) SITE STRATIGRAPHY Soils encountered beneath the site consist of an accumulation of silty sand grading at depth to poorly graded, coarse grained sand These sands are interbedded with thin to moderately thick accumulation of sand, silty sand , silty clay, or clay Clay interbeds do not appear to be laterally continuous across the site as shown in the cross sections (Figures 6 and 7). The orientation of subsurface cross sections are indicated on Figure 3. GROUNDWATER Groundwater was first encountered during drilling in poorly graded sand and silty sand at a depth of 30 feet below the ground surface Flowing silty sands were encountered at depths greater than 35 feet below grade Table 1 lists the measured groundwater surface elevations Groundwater stabilized at 26 feet Based on the depth of water measurements collected on December 9, 1998, as shown on Table 1, the direction of groundwater flow was determined to be flowing North/Northeast with a hydraulic gradient of 0 065 ft/ft Figure 8 illustrates the configuration of the groundwater surface beneath the site on December 9, 1998 Surface drainage is to the southeast towards the Stanislaus River, located approximately 750 feet south of the site Table 1 Water Table Data 23569 South Santa Fe Road Riverbank, California Well No 1 2 3 4 TOC 9894 9850 9900 10000 (MSL) Date 12/9/98 Depth to Water 2636 25 90 2646 2738 (FBGS) Groundwater Elevation 72. 58 7260 7254 7262 (MSL) TOC top of well casing FBGS feet below ground surface 09 MSL mean sea level