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Human Health Risk Assessment Former Mobil Oil Bulk Plant 04-343 500 East Grant Line Road Tracy, California not be consistent with the City of Tracy General Plan. Therefore, it is anticipated that any future development of the property will be consistent with a General Industrial land use. 1.2.4 Site Geology Since approximately 6,100 cubic yards of hydrocarbon-affected soil was removed from the site, fill material exists beneath most of the site. Sandy silts and sandy cla3 s are present across the site from grade to approximately 10 to 15 fbg. Underlying these units, fine-to medium-grained sand, ranging in thickness from less than I foot to approximately 3 feet is present in most of the ' northern portion of the site. If not manifested as a distinct unit, increasing sand content was noted in soil borings across the site in this stratigraphic horizon during drilling. A medium-stiff to stiff, silty clay underlies the sandy zone, and persists to the total depth of the borings (25 feet). Saturated soils were generally encountered at depths between -I- and I 1 fbg. ' 1.2.5 Local and Regional Hydrogeology Ground water in San Joaquin County is designated by the California Regional Water Quality Control Board to have beneficial or potentially beneficial use. Currerkty, however, the water- bearing zone, which has been impacted by residual petroleum hydrocarbons, does not yield water ' used for domestic, industrial, or municipal purposes. No ground water wells utilizing ground water from the near-surface aquifer were identified within a 1-mile radius of the site. 1.2,5.1 Local Ground Gr Water e Gradient and Seasonal Fluctuations Ground water elevations from Monitoring Wells AW-1 through AW-12, as measured on May 27, 1993 were used to develop the ground water elevation contour map shown in Figure 1-2. ' The average depth to ground water for these ground water monitoring wells is approximately 9.6 feet. The average ground water gradient direction is to the northeast, with a magnitude of ' approximately 0.01 foot-per-foot across the area of study. Ground water elevation, gradient direction, and gradient magnitude have remained relatively consistew since monitoring was ' initiated in 1990 (Alton Geoscience, 1992a). 30-0136-11 1-4 1 i