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- ----- ----- <br /> C X? <br /> f <br /> .- A <br /> r� A S S O C TAT E S I N C . <br /> increasing in the wells. A more detailed discussion of the natural attenuation at this site is <br /> contained in Smith Technology Corporation's Additional Soil and Groundwater Report, Fayette <br /> Manufacturing Corporation, 7675 West Eleventh Street, Tracy, California, dated January 31, <br /> 1997. <br /> 3.0 SUBSURFACE CONDITIONS E <br /> -_� 3_1 Regional Geology <br /> Tracy, California, is located in San Joaquin County and lies on the west flank of the large F <br /> northwest trending asymmetrical structural trough called the Central Valley, adjacent to the <br /> geomorphic provinces. The Central Valley contains a thick sequence of sediment <br /> �-�� Sierra-Nevadag p <br /> which, in places, reaches a depth of 10 miles. These sediments range in geologic age from <br /> _{ Jurassic (205 million years before present) to Recent (present time) and include both marine and <br /> continental deposits. <br /> The near surface of the geology underlying the site is comprised of unconsolidated alluvial <br /> =� deposits of Pleistocene to Holocene age. These deposits, sometimes referred to as "older <br /> alluvium", consist of intercalated beds of gravel, sand, silt, and clay. Underlying the older <br /> alluvium are P1io/Pleistocene continental deposits of similar derivation and lithology. The older <br /> = r alluvium functions as the most important in the site area (USGS Professional Paper 1401-C, <br /> 1986). Soils developed on the alluvium are generally well drained, differing from the parent <br /> material only in the increased volume of organic matter (D)ArR Bulletin No. 146, 1967). <br /> _ The lateral extent of TPHg and benzene, and the presence of these contaminants in the capillary <br /> fringe are indicative of groundwater impact and transportation. As stated in Smith Technology <br /> Corporation's Additional Soil and Groundwater Report, Fayette Manufacturing Corporation, <br /> 7675 West Eleventh Street, Tracy, California, dated January 31, 1997, the lithology of the <br /> capillary fringe is primarily silty clay, with clayey sand in some areas. The capillary fringe int e <br /> vicinity of wells.MW1, 1\4W2, and MW3 was observed to be silty clay. Petroleum hydrocarbons <br /> in groundwater would readily adsorb to the clayey soil in the capillary fringe. <br /> 3.2 Hydrogeology <br /> Regional groundwater depth is reported to be approximately 8 feet below grade (San Joaquin <br /> .; <br /> County Flood Control and Water District, Spring 1995) and the regional inferred groundwater <br /> gradient is northwest. This is consistent the results of quarterly monitoring events with a dept to <br /> =' water ranging from 6.34 to 8.99 feet bgs and with a northwesterly groundwater flow direction. <br /> Fr <br /> <J <br /> w:1951221reportalparcap.doc 4 <br />