ARCHIVED REPORTS FINAL SITE INVESTIGATION

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FMS #24 Site Investigation Report California Army National Guard which is greater than 100 mg/kg (the recommended Tri -Regional Board guideline). The highest • ethylbenzene (up to 18 mg/kg) and xylenes (up to 50.4 mg/kg) concentrations were also reported at the same depth and boring. Other BTEX compounds were also detected at concentrations less than residential preliminary remediation goals (PRGs) (EPA, 2004). The maximum TPHG concentration present in soils at the location of the former USTs was in boring B30 at a concentration of 47 mg/kg at 40 feet bgs, which is less than 100 mg/kg. The highest concentrations of benzene (up to 1.2 mg/kg) and toluene (up to 0.88 mg/kg) were also reported at the same depth and boring. All detected concentrations of benzene and toluene in soil were less than residential PRGs. Only one analyte (TPHD, detected at 260 mg/kg) had concentration levels detected greater than 100 mg/kg. TPHD concentrations have not been defined laterally from boring B31 at 35 feet bgs in all directions, except to the north by borings B30 and B32. The concentrations detected indicate that soils beneath the area southeast and northeast of the previous diesel UST location have likely been impacted by leaks from the former diesel UST, and contaminant concentrations are present within soil greater than Tri - Regional Board guidelines (Regional Water Quality Control Board [RWQCB], 1990) but less than residential PRGs (RWQCB, 2003). Figures 3-2 through 3-4 show the detected concentrations for each analyte at 25, 35, and 40 feet bgs, respectively. TPHD concentrations have been defined vertically within B31 at 40 feet bgs, where TPHD was non -detect. At 40 feet bgs, all detected TPHG, BTEX, and lead concentrations were less then 100 mg/kg and residential PRGs. 3.2.2 Groundwater samples were collected from the three boring locations in the first groundwater encountered around the vicinity of the previous USTs. Groundwater samples were analyzed for the following: TPHG, TPHD, BTEX, and oxygenates, and lead by EPA Methods SW8015G and D, SW 8260B, and SW6010B, respectively. Groundwater sampling analysis results indicate that the maximum concentrations of TPHD (1,400 micrograms per liter [µg/I.]) and TPHG (74,000 µg/L) in boring B30 exceed taste and odor thresholds of 100 ltg/L and 5 lig/L, respectively. The highest • concentrations of benzene (up to 24,000 µg/L) and ethylbenzene (up to 2,300 µg/L) were reported in boring B32 and exceed the primary maximum contaminant levels (MCLS) of benzene (1 µg/L) and ethylbenzene (300 µg/L), respectively. The highest concentrations of toluene (up to 6,000 µg/L) and xylenes (up to 4,900 µg/L) were reported in boring B30, and exceed the primary MCLS of toluene (150 µg/L) and xylenes (1,750 µg2), respectively. All detected BTEX concentrations, except for xylenes in boring B31, in groundwater were greater than their associated primary MCL. The detected concentrations stated above indicate that groundwater beneath the previous diesel and gasoline UST locations has been impacted by leaks from the former diesel and gasoline USTs, and TPHG, TPHD, and BTEX contaminant concentrations are present within groundwater greater than taste and odor thresholds and primary MCLS, respectively. Figure 3-5 shows the detected concentrations for each analyte in groundwater beneath the former diesel and gasoline UST locations. Concentrations of TPHG, benzene, toluene, and ethylbenzene were not been defined laterally from borings B30 to B32 in all directions; in addition, all concentrations for these contaminants exceed their water quality goals for all boring locations. TPHD concentrations have been defined laterally to the south and southwest by borings B31 and B32 where TPHD was not detected; however, TPHD remains undefined laterally in all other directions from the former UST location because TPHD concentra- tions within boring B30 was detected greater than the taste and odor threshold (100 µg/L). Xylenes concentrations have been defined to the MCL laterally to the south by boring B31, where xylenes were detected less then primary MCL. However, xylenes remain undefined laterally in all other directions from the former UST location because concentrations of xylenes within borings B30 and B32 were detected greater than primary MCL (1,750 µg/L). 4 December 2007