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S.00f Closure Report Former Mobil Bulk Plant 04-343, Tracy, California April 14, 1997 siltstone, sandstone, and conglomerate. The maximum thickness reported for these units is 3,000 feet(USGS, 1986). Geology: The predominant native soil types beneath the site are sandy silts and silty clays, with the exception of the silty, sandy gravel backfill. Sandy silts and sandy +— clays are present across the site to approximately 10 to 15 feet below grade (fbg). Occasional gravel lenses, from a few inches to 3 feet in thickness, are interbedded with the silts and clays. A medium-stiff to stiff, silty clay underlies '� the sandy zone, and persists to a depth of at least 32 fbg. Saturated soils are generally encountered at a depth of between 10 and 11 fbg. Groundwater Quality and Usage: Groundwater in San Joaquin County is designated by the California Regional Water Quality Control Board's Basin Plan to have beneficial or potentially 'r beneficial use. Currently, however, the uppermost water-bearing zone in the vicinity of the site is not used for domestic, municipal, or industrial purposes. LW This may be attributed to the degraded background water quality of this shallow aquifer. Background shallow groundwater quality tests performed on groundwater from upgradient and downgradient wells in December 1992 La w indicate that EPA Maximum Concentrations Limits (MCL) for Secondary Drinking Water Standards were exceeded by the following constituents: chloride, sulfate, aluminum, iron, and manganese. 3.0 PREVIOUS SITE ASSESSMENT ACTIVITIES L. • Plant operations were discontinued at the site in 1988. All structures were removed from the property in 1988, including one 1,000-gallon gasoline UST, four 10,000-gallon diesel ASTs, �- and the associated piping. To the knowledge of Alton Geoscience,there were no recorded fuel leaks at the site. L' • Initial soil samples collected beneath the gasoline UST at depths of 10 and 12 fbg yielded total petroleum hydrocarbon (TPH) concentrations of 240 and 200 parts per million (ppm), respectively, and benzene concentrations of 0.46 and 2.2 ppm, respectively. A grab groundwater sample collected beneath the former UST contained concentrations of 1,700 parts per billion(ppb)TPH and 130 ppb benzene(KEI, 1988). LW • Extensive soil samples have been collected from over 80 onsite and offsite borings at the site between 1988 and 1995. Soil boring locations are shown in Figure 3, and the analytical results �.. are presented in Table 1. Soil hydrocarbons are characterized vertically and laterally beneath ,y. the site. Residual soil hydrocarbons are present from approximately 10.5 to 12.5 fbg. These residual concentrations are adsorbed to clayey soils present below the water table at the base of 2