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EXECUTIVE SUMMARY <br /> Supplemental Site Assessment Report <br /> Former Mobil Bulk Plant 04-343 <br /> 500 East Grantline Road <br /> Tracy, California <br /> This report presents the findings of a supplemental site investigation, including the installation <br /> of additional monitoring wells and an aquifer pumping test at former Mobil Bulk Plant 04-343, <br /> located at 500 East Grantline Road, in Tracy, California (Figure 1). This work was performed <br /> in accordance with the San Joaquin County Public Health Services, Environmental Health <br /> Division (PHS/EHD) requirements (SITE CODE 1968) and the Alton Geoscience work plan <br /> dated March 22, 1993. <br /> FINDINGS <br /> The following is a summary of the findings and conclusions of this investigation: <br /> • The site is located approximately 3,500 feet south of Highway 205 at an elevation of <br /> approximately 30 feet above mean sea level (NGVD-1929). Local topography is relatively <br /> flat, dipping slightly to the north. The site is bound by vacant lots to the east and south. <br /> Industrial properties are located to the north and west. <br /> • Soil types encountered consist of silty, sandy gravel in the backfilled area and predominantly <br /> clays and silts under the remainder of the site. Sandy silts and sandy clays are present <br /> across the site from grade to between approximately 10 and 15 fbg. Underlying these units, <br /> a fine- to medium-grained sand is present in all soil borings. In the northern portion of the <br /> site, the sand appears as a distinct lithologic unit, attaining a thickness of approximately 3 <br /> feet (AW-10). Silty clay underlies the sandy zone, and persists to the total depth of the <br /> borings. Saturated soils are generally encountered at a depth of between 10 and 11 fbg. <br /> • 4n May 27, 1993, static water levels averaged 10.5 fbg (an average elevation of 21.0 feet <br /> above mean sea level [NGVD-19291). The calculated ground water gradient is <br /> approximately 0.01 foot-per-foot toward the north/northeast. <br /> • No dissolved-phase TPH-G concentrations were detected in onsite or offsite monitoring <br /> wells. Dissolved-phase TPH-D was present in detectable concentrations only in AW-3 (210 <br /> parts per billion [ppbl). Dissolved benzene was detected in AW-9 and AW-12 at <br /> concentrations of 3.4 and 9.6 ppb, respectively. <br /> • Adsorbed-phase hydrocarbons were detected in Monitoring Wells AW-10, AW-I1, and AW- <br /> 12 at the capillary fringe and within the uppermost saturated zone. Soil samples were <br />