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gk <br /> i - <br /> E <br /> ' Supplementary Site Assessment Report <br /> Former Mobil Bulk Plant 04-343 <br /> April 26, 1996 <br /> ' The scope of this investigation includes only the uppermost Tertiary and <br /> Quaternary age deposits which constitute the major aquifers of the Central <br /> ' Valley, and yield large quantities of water to wells. Lithologically these <br /> deposits are heterogeneous mixtures of poorly sorted clay, silt, sand, and <br /> gravel, and in places some beds of claystone, siltstone, sandstone, and <br /> conglomerate. The maximum thickness reported for these units is 3,000 <br /> feet (USGS 1986). <br /> s <br /> ' Geology: The predominant native soil types at the site are sandy silts and silty clays. <br /> Occasional gravel lenses, from a few inches to 3 feet in thickness, are <br /> interbedded with the silts and clays. Filled areas at the site generally <br /> consist of sandy soil. Saturated soils are generally encountered at a depth <br /> of between 10 and 11 fbg. <br /> ' Groundwater Quality <br /> and Usage: Groundwater in San Joaquin County is designated by the California <br /> Regional Water Quality Control Board in the Basin Plan to have beneficial <br /> or potentially beneficial use. Currently, however, the uppermost water- <br /> bearing zone is not used for domestic, industrial, or municipal purposes; <br /> ' this may be attributed to the degraded background water quality of this <br /> shallow aquifer. Background groundwater quality tests performed on <br /> groundwater from upgradient and downgradient wells in December 1992, <br /> ' indicates that EPA Secondary MCLS were exceeded by the following <br /> constituents: chloride, sulfate, aluminum, iron, and manganese. <br /> ' 3.0 BACKGROUND SITE CONDITIONS <br /> Prior to this investigation the following conditions existed at the site: <br /> • Average depth to water ranges between 9 and 11 fbg. The groundwater gradient direction <br /> ' is typically directed to the north. <br /> • 75 onsite and 12 offsite soil borings have been drilled at the site between 1988 and 1994, <br /> ' and 12 onsite test holes were excavated by backhoe in 1989. <br /> • Eight onsite and four offsite groundwater monitoring wells are present at the site. <br /> 2 <br />