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Human Health Risk Assessment <br /> Former Mobil Oil Bulk Plant 04-343 <br /> 500 East Grant Line Road <br /> Tracy, California <br /> have been drilled at the site between 1988 and 1993. In addition, a total of 12 ground water <br /> monitoring wells (8 onsite and 4 offsite) have been installed since 1990 Additional information <br /> regarding soil investigations, ground water sampling results, and compleLCd remediation activities <br /> is presented in the following sections. A site plan showing the localrons of soil borings not <br /> affected by the soil excavation, and ground water monitoring wells is presented as Figure 1-3. <br /> 1.2.6.1 Soil Investigations <br /> Site investigations conducted prior to July 1990 provided info <br /> rnation regarding the nature and <br /> ' extent of hydrocarbon-affected soil in the subsurface. This information was used to design a <br /> remediation plan for the site. The remediation activities were init'ated in July 1990 and <br /> consisted of excavation and disposal of approximately 6,100 cubic yard, of hydrocarbon-affected <br /> ' soil. Characterization activities completed after July 1990 provided additional information <br /> regarding the relative effectiveness of the remedial action taken. <br /> ' On September 26, 1990, Alton Geoscienceersonnel supervised tht: <br /> p p drilling of four ground <br /> water Monitoring Wells (AW-1 through AW-4) to a total depth of 24 feet below ground level. <br /> One soil sample was extracted from the boring for AW-2 (6.0 fbg) a• c- two soil samples were <br /> extracted from the boring for AW-4 (6.0 and 9.5 fbg); these samples were analyzed for TPH <br /> ' as gasoline (TPH-G) and diesel (TPH-D), and BTEX. Monitoring Wei ,s AW-1 and AW-3 were <br /> installed in the backfill material and, therefore, analytical results were not obtained. No <br />' detectable concentrations of TPH-G, TPH-D, or BTEX were observers in any soil samples. <br />' On April 30 and May 1, 1991, nine additional soil borings (B-1 through B-9) were installed at <br /> the site. Soil Borings B-2, B-3, B-4, and B-9 were completed as ground water Monitoring Wells <br /> AW-5, AW-7, AW-6, and AW-8, respectively. Soil samples were collected and analyzed for <br /> TPH-G, TPH-D, and BTEX in the borings that were converted to i,-Bund water monitoring <br /> wells. Two soil samples were analyzed from Borings AW-5 and AW-6 at depths of 6 and <br /> 11 fbg. Two soil samples were analyzed from Boring AW-8 (at 5 and .0 fbg). One soil sample <br /> was analyzed from Boring AW-7 (6 fbg). TPH-D concentrations w" 180 and 5.4 parts per <br /> million (ppm) were detected in Boring AW-6 at 6 and 11 fbg, respec,..vely. No other samples <br />' contained detectable concentrations of TPH or BTEX. <br /> 30-0136-11 <br /> 1-6 <br />