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1 1 Closure Report Former Mobil Bulk Plant 04-343, Tracy, California r Aril 14, 1997 the 1990 remedial excavation. The adsorbed-phase hydrocarbon concentrations attenuate rapidly with depth and do not appear to be migrating downward. • Maximum post remediation soil hydrocarbon concentrations detected at the site are as follows: +- TPH-D(ppm) TPH-G(ppm) Benzene(ppm) Max.' max: Max., +� 6,700 1,900 8.4 (5/94) (4/90) (5194) • A total of twelve groundwater monitoring wells (eight onsite and four offsite) have been installed at the site(Figure 2). No free product has ever been recorded in any well at the site. • The sampling results for the first quarter of 1997 are presented in Figure 8. Historical groundwater levels and chemical analysis results are included in Table 2. • Dissolved-phase diesel-range hydrocarbons have historically been detected only in Monitoring Well AW-3, along the northern margin of the site; however, levels have significantly decreased over time. Significant concentrations of dissolved-phase benzene have historically not been present at the site. Low-levels of BTEX detected in the offsite wells (AW-5 through AW-8) during the January 14, 1997 sampling event were likely due to equipment cross-contamination- The wells were resampled on February 27, 1997, and no TPH-G, TPH-D, or benzene was detected. The sampling results of the first quarter of 1997 are presented in Figure 8. Although isolated occurrences of low levels of dissolved-phase hydrocarbons have occasionally been detected in the offsite wells, it appears that the hydrocarbon plume has not migrated offsite. The lack of dissolved-phase hydrocarbon concentrations at the site suggests that adsorbed- phase hydrocarbons present in the saturated zone are not readily dissolving into groundwater. • In 1993, a human health risk assessment determined that that no potential human health risks r= were associated with the site(Alton Geoscience, 1993). 3.1 REMEDIAL ACTIVITIES LO • Remedial soil excavation was conducted in 1990. Approximately 6,080 cubic yards of hydrocarbon-impacted soil were excavated to groundwater(10 fbg) at the site. Approximately LO 3,060 cubic yards of Class II soil was transported to Liquid Waste Management in McKittrick, California, and approximately 3,020 cubic yards of Class III soil was transported to Durham LM Road Landfill in Fremont, California. Approximately 7,300 cubic yards of clean fill were imported and compacted to grade(Alton Geoscience, 1990). L. 3