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i y � ATC Associates Inc. Iww ' � 1117 Lone Palin Avenue,Suite 201 Modesto,California 95351 209-579-2221 Lop %T 4: Fax:209-579-2225 ASS O C 1 ATE S I N C. 2.0 GEOLOGIC SETTING Stockton, California is located in the San Joaquin Valley, the southern extension of the California Great Central Valley. The Great Central Valley is a deep alluvial plain extending nearly 500 miles from the Siskiyou Range of the Cascade Mountains in the north to the Tehachapi Range to the south. At Stockton, the San Joaquin Valley is approximately 40 miles wide. The western Valley boundary consists of the low, rolling foothills of the California Coast Range Mountains (maximum elevation due west is less than L& 3,000 feet) and the eastern boundary consists of the more rugged foothills of the Sierra Nevada (maximum elevation due east is over 10,000 feet). The Great Central Valley contains a thick sequence of sediment, which, in places, reaches a depth of ten miles. These sediments range in geologic age from Jurassic (205 million years before present) to Recent (present time) and include both marine and continental deposits. The site's immediate subsurface geology consists of Pleistocene and Recent alluvial deposits. These alluvial deposits consist of heterogeneous sequences of sand and gravel originating from active stream channels, and silt and clay originating from overbank and marsh depositional environments. 1W Recent alluvium consists of sediments dep( sited on top of the erosional surface that was formed near the end of the last glacial period. Recent deposits are generally composed of alluvial deposits consisting of unconsolidated gravel, sand, silt, and clays. Subsurface soils encountered during drilling at the site consisted of poorly graded fine to gravelly sands with layers of clay and silt to a depth of approximately 65 feet. Soil boring/well construction logs from previous site activities are contained in Appendix A. 3.0 HYDROLOGIC SETTING The site is located within the Great Central Valley geomorphic province. Groundwater in the Stockton area is present in recent alluvium. Groundwater enters the Great Central Valley via surface and subsurface flow and by direct percolation of precipitation, stream flow, and applied water. Replenishment of the groundwater occurs by percolation of precipitation, stream flow, and applied water Y occurs in the forebay areas where permeable sediments are exposed at ground surface. According to the United States Geological Survey (USGS) 7.5-Minute Series Topographic Map of the Stockton West, California Quadrangle, the site is located at an approximate elevation of 24 feet above mean sea level. The regional surface topographic gradient in the vicinity of the site is towards the west. The groundwater flow direction beneath the site is generally towards the south and the gradient has historically been very flat. Since 1998, the depth to groundwater beneath the site has varied from approximately 45 to 58 feet bgs. The groundwater table has recently been encountered beneath the site at depths of approximately 52 to 53 feet bgs. According to the San Joaquin County Flood Control and Water V Conservation District's Spring 1997 Groundwater Report, the regional groundwater gradient is to the northeast. Historical groundwater elevation data collected at the site indicate the hydraulic gradient beneath the site is very flat with variable directional flow. The predominant groundwater flow direction at 1.. the site appears to be south, southeasterly. Historical groundwater monitoring data are included in Table 1. A groundwater contour map associated with data collected on February 26, 2010, is provided as Figure 4. ISO ` S Environmenta€1625771Reports12010 closure summary.doc 3 ir