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ATC Associates Inc. 1 117 Lone Palm Avenue,Suite B ("t Modesto,California 95351 209-579-2221 TC Fax:209-579-2225 A S S O C I A.T,E S t N C. collected in the pursuing 2 to 3 quarters. The EHD directed that quarterly groundwater monitoring be reinitiated. REGIONAL GEOLOGY Tracy, California, is located in San Joaquin County and, lies on the west flank of the large northwest trending asymmetrical structural trough called the Central Valley, adjacent to the Sierra-Nevada geomorphic provinces. The Central Valley contains a .thick .sequence of sediment, which, in places, reaches a depth of 10 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 near surface of the. geology underlying the. site is comprised of unconsolidated alluvial. deposits of Pleistocene to Holocene age. These deposits sometimes referred to as "older alluvium," consist of intercalated beds of gravel, sand, silt, and clay. Underlying the older alluvium are Plio/Pleistocene continental deposits of similar derivation and lithology. The older alluvium functions as the most important in the site area (USGS Professional Paper 1401-C, 1986). Soils developed on the alluvium are generally well drained, differing from the parent material only in the increased volume of organic matter(DWR Bulletin No. 146, 1967). The lateral.extent of TPHg and benzene, and the presence of these contaminants in the capillary fringe are indicative. of groundwater impact and transportation. As stated in Smith. Technology. Corporation's Additional Soil and Groundwater Report, Fayette Manufacturing Corporation, 7675 West Eleventh Street, Tracy, California, dated January 31, 1997, the lithology of the capillary fringe is primarily silty. clay, with clayey sand in some areas. . The capillary. fringe observed in borings completed as MW1, MW2, and MW3 was observed to be. silty clay.. Petroleum hydrocarbons in groundwater would readily adsorb.to the clayey soil in the capillary fringe. REGIONAL AND SITE HYDROLOGY Regional groundwater depth is reported to be. approximately 8. feet below grade. (San Joaquin County Flood Control and. Water District, Spring 1995) and the regional inferred groundwater gradient is northwest. This is consistent the results of quarterly monitoring events with a depth to water ranging from approximately 6.25 to 9.00 feet bgs and with a northwesterly.groundwater flow direction. Page 6 � ! w:\17861\WP for Additional Subsurface Investigation V. Soil Vapor Study,and Modeling ••'•o'o,..e. c.