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Closure Report Former Mobil Bulk Plant 04-343, Tracy,' California': gril 14, 1997 4.0 ASSESSMENT OF REMAINING HYDROCARBONS 4.1 HYDROCARBONS IN Soil] A review of soil sample analytical results,;collected from borings after completion of soil L characterization and excavation activities at the site indicates that residual TPH-D soil .1 hydrocarbon concentrations remain in the.IcaPillarY fringe zone beneath the site (10.5 to 12.5 fbg). Soil borings; drilled"in 1995 :indicate that residual hydrocarbons exist above L groundwater northwest of the`site:! 3. LDegradation of the residual hydrocarbons is �kely taking place at the site. This is evident in the areas of SB-2 and SB-4 'where soil with'concentrations of up to 2,700 ppm TPH-D in 1989 contained no detectable hydrocarbons-by May 1994. An estimate of the area of remain* hydrocarbons in soil (above and below water) is presented in Figure 5. These from Borings SB-1 through SB- r contours are based on the most recent soil'analytical data f q 11, and AB-1 through AB-4, .and applied approximate! concentrations. Residual soil hydrocarbons above'and below groundwater are estimated to be equivalent to approximately 647 gallons of diesel. The mass balance calculations are presented in Appendix C. % 4.2 HYDROCARBONS IN GROUNDWATER On January 14, 1997, fluid levels were measured in eight onsite and three offsite groundwater monitoring wells. The hydrAulic gradient on January 14, 1997 was to the northeast at 0.012 foot per foot (Figure 7)1 Groundwater elevation has remained relatively constant at approximately 20 feet above mean sea level [NGVD-1 929]; approximately 11 fbg since the site was first sampled in 1990. The gradient is generally to the north at 0.01 foot-per- foot. A summary of groundwater elevations"is provided in Table 2. On January 14, 1997, samples were collected from nine wells. Groundwater samples:were submitted to state-certified laboratories and analyzed for total petroleum hydrocarbons as gasoline (TPH-G); benzene,'; toluene, etliylbenzene, and total xylenes (BTEX); total petroleum hydrocarbons as diesel '(TPH-D), and methyl-tertiary-butyl-ether (MTBE). Due to possible equipment cross-contamination, 'Wells AW-5 through AW-8 were resampled on February 27, 1997. Dissolved-phaise 4ydro&arbon concentrations for this quarter are shown L on Table 2 and Figure 8. Recent monitoring and sampling data are consistent with historical data. Residual diesel-range hydrocarbons are present in the vicinity of M8nitoring Well AW-3, Significant dissolved-phase benzene is not present beneatl the site. It appears the hydrocarbon plume has not migrated offsite. The lack of dissolved-phise hydr6carbon concentrations at the site suggests that 4