SITE INFORMATION AND CORRESPONDENCE_2001-2018

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• Mr. Jeff Goold - 2 - 10 October 2008 • Shell Stockton Terminal f' data indicated that maximum contamination was at a depth of 15 feet bgs in borings SB-23 and SB-24. Samples obtained from below 86 feet bgs in hot spot borings SB-22 through SB-24 contained TPHg and TPHd concentrations exceeding their respective water quality objectives (WQOs) of 5 micrograms per liter (Ng/L) and 100 pg/L. SB-24 contained the highest concentrations of TPHg and TPHd at 1,400 and 290 pg/L, respectively. However, the MIP data were inconsistent with these laboratory results and showed no responses at 86 feet bgs. SB-25 through SB-27 were advanced in the vicinity of deep zone monitoring well DW-2 to maximum depths that varied between 79 and 90 feet bgs. Groundwater samples from SB-25 and SB-26 were non-detect at their maximum depths of 79 and 90 feet bgs, respectively. Tertiary butyl ether (TBA) was detected at 25 pg/L in boring SB-27, located about 80 feet downgradient of DW-2, in the deepest sample which was collected at 86 feet bgs. California does not have a WQO for TBA. Shell concluded that the elevated TPH detections at depth are the result of onsite migration from upgradient. Shell also concluded that the low level oxygenates observed in the downgradient groundwater samples migrated from the adjacent fuel terminal. The Soil Vapor Report was prepared to assess the distribution of contaminants in the vadose zone in anticipation of applying dual-phase extraction (DPE) technology and to assess the potential for vapor intrusion into indoor air at the onsite commercial buildings. Shell sampled eight vapor wells (V-1, V-2, V-4, V-5, V-6, V-7, V-8, and V-10) and two soil borings (SB-20 and SB-21) for this investigation. Soil samples were collected from depths of one and five feet bgs in all vapor wells except V-7 where shallow groundwater prevented collection of a soil sample from five feet bgs. Soil samples were collected from depths of three, five and 10 feet bgs in the two soil borings. The laboratory analytical results showed that TPHg and TPHd were the most frequently detected constituents. With the exceptions of V-1 and V-5, TPHg and TPHd detections were below 100 mg/kg. In boring SB-21, located in the MW-1 hot spot, maximum TPHg and TPHd concentrations were 4,700 and 1,200 mg/kg, respectively, in shallow soils (10 feet bgs or less). With the exception of SB-21, the soil samples from all borings were generally ND for benzene, toluene, ethylbenzene, and xylenes (BTEX) and oxygenates. Low levels of methyl tertiary butyl ether (MTBE) were encountered, and xylene concentrations ranged from 16 to 128 mg/kg in SB-21. Constituent concentrations generally decreased with depth. Soil vapor samples were collected from all soil vapor wells except V-7 in which groundwater was encountered at 1.5 feet bgs. Laboratory analytical results for a groundwater sample collected from V-7 were ND for all constituents except for ethyl tertiary butyl ether (ETBE). There is no WQO established for this compound. Soil vapor samples obtained adjacent to the Site offices and a garage from six feet bgs were one to three orders of magnitude higher than the California Human Health Screening Levels (CHHSLs) for benzene. The commercial and residential CHHSLs for benzene are 122 micrograms per cubic meter (Ng/m3) and 36.2 pg/m3, respectively, indicating that further investigation is needed. Shell conducted ambient air sampling to address this concern. The results showed that benzene concentrations were below method detection limits (MDLs). Therefore, Shell concluded that these results indicated that hydrocarbon vapors are not migrating above the ground surface at detectable concentrations.