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Stockton Taxi Cab/Sunland Gasoline September 16,2003 2085 Fremont Street,Stockton, California Page 7 Report of Findings. Lateral and Vertical Extent of Soil and Groundwater Contamination Stockton lies in the Central Valley Physiographic Province of California. The Valley is taproximately 50 miles wide. The foothills of the Coast Ranges, and to the east by the foothills of the Sierra Nevand Range bound it to the west. The Stockton area is nearly devoid of structural geologic features. The Valley is floored by unconsolidated Quarternary sediments to depths of at least 400 I or more feet in the Stockton area. All of these units can be considered soils in the engineering sense, because they are unconsolidated. Quarternary sediments in the Central Valley were deposited as a series of coalescing alluvial fans. The fans originated where ' valleys of the major streams that drained the Sierra Nevada Range emptied into the broad expanses of the valley. The coarser sediments that comprise the fans are mainly arkosic in composition and were derived from erosion associated with glacial stages in the ' mountains. The finer grained sediments are predominantly composed of rock flour washed out of the former extensive glaciers in the Sierras (Ackley, 1964). I The axis of the Central Valley Trough was a marshy, wet area throughout much of the Pleistocene Epoch. Lacustrine, flood plain and marshy depositional environments predominated along the axial portions of the trough, in the toe areas of the fans, throughout much of this time period. Coarser grained sediments of the upper portions of the fans occasionally prograded over the axial area of the trough in response to major climatic changes in the Sierras. This has resulted in a predominance of fine-grained silts and clays in the subsurface area. ' 5.1 Local Geology The geology underlying the subject site consists of alternating layers of silt and sand to an approximate depth of seventy-five feet (75'). Clayey silt layers predominate the subsurface. The boring logs can be found in Appendix B. Lithological cross-sections can be found in Figures 4a and 4b. 6.0 Hydrology ' California and Federal Governments have mandated that virtually all groundwater is beneficial for current or future use, unless it can be demonstrated that no beneficial use is iknown. This "aquifer" is considered to be of beneficial use and as a result, must be protected from ' contamination. Those identified uses include agriculture and industrial, as well as potential use for animal or human consumption. 1