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Human Health Risk Assessment Former Mobil Oil Bulk Plant 04-343 500 East Grant Line Road Tracy, California On April 3, 1992, six additional soil borings (B-10 through B-15) wer<< installed. Boring B-10 was located near the former aboveground diesel storage tanks and was subsequently converted to ground water Monitoring Well AW-9. The other five borings (B-11, B-12, B-13, B-14, and 13-15) were located across Grant Line Road in the Leprino Foods parking lot. Three soil samples were collected from Boring B-10 at depths of 9.5, 11.5, and 211 fbg. Two soil samples ' were taken from each of the borings B-11, B-12, B-13, B-14, and B-t5 at depths of 7.5 and 9.5 fbg. No detectable concentrations of TPH or BTEX were detected iti samples collected from ' Borings B-11, B-12, B-13, B-14, or B-15. TPH-G (480 ppm), TPB-D (510 ppm), benzene (4.4 ppm) , toluene (0.37 ppm), ethylbenzene (9.5 ppm), and xylenes (3.2 ppm) were detected in Boring B-10 at a depth of 11.5 fbg. No other detectable concentrations of TPH or BTEX were observed in any other samples. On March 25, 1993, three supplementary borings (AW-10, AW-11, „ad AW-12) were drilled onsite within the former aboveground diesel storage tank area, and subsequently were converted ' to ground water monitoring wells. Two soil samples were obtained from Borings AW-10 and AW-12 at 9.5 and 10.5 fbg, and from Boring AW-11 at 9.5 and 11.5 fbg. Detectable concentrations of TPH-G, TPH-D, and BTEX were observed in soil samples from AW-10, AW-11, and AW-12. A comprehensive summary of the analytical results for soil is presented in Table 1-2. 1.2.6.2 Ground Water Investigations ' A total of eight onsite (AW-1 through AW-4 and AW-9 through AW-12' and four g g , ou offsite (AW-5 ' through AW-8) ground water monitoring wells have been installed since 1990. Static ground water is present beneath the site at an average depth of approximately 10.5 fbg. A summary of the ground water monitoring well installation activities is provided below. In September 1990, four ground water Monitoring Wells (AW-1 through AW-4) were installed. As part of a supplemental site investigation, Soil Borings 13-1 through B-9 were drilled. Soil Barings B-2, B-3, B-4, and B-9 were completed as ground water Monitoring Wells AW-5, ' AW-7, AW-6, and AW-8, respectively. Ground water Monitoring Well AW-9 was installed in April 1992 and ground water Monitoring Wells AW 10, AW-11, and AW-12 were installed in March 1993. 30-0136-11 1-7 1