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1 Boyett Petroleum January 15, 2001 ' 419 South Main Street, Manteca, California Page 7 Interim Report of Findings for Lateral Extent of Groundwater Contamination ' Ranges, and to the east by the foothills of the Sierra Nevada Range. The Manteca area is nearly devoid of structural geologic features. ' The Valley is floored by unconsolidated Quaternary sediments to depths of at least 400 or more feet in the Manteca area. All of these units can be considered soils in the engineering sense, because they are unconsolidated. Quaternary sediments in the Central Valley were ' deposited as a series of coalescing alluvial fans. The fans originated where valleys of the major streams which drained the Sierra Nevada Range emptied into the broad expanses of the valley. The coarser sediments which 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). 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 of the area. 5.1 Local Geology The geology underlying the subject site consists of alternating layers of silt and sand to an approximate depth of twenty-five feet (25'). The sand unit from twenty feet (20')to twenty-five feet (25') contains a groundwater aquifer. From twenty-five feet (25') to thirty feet(30') a unit of silty clay is present. The clay unit is underlain by a coarse sand and pee gravel unit,which contains a second groundwater aquifer. 1 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 known. ' This "aquifer" is considered to be of beneficial use and as a result, must be protected from contamination. Those identified uses include agricultural and industrial, as well as potential use for animal or human consumption. 1 , 1