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GEOMATAIX Mr. Robert Mihalovich Chevron Environmental Management Company April 3, 2002 Page 3 SB-5, SB-6, and SB-7 did not contain detectable concentrations of benzene, ethylbenzene, MtBE, or PAHs; only low concentrations of toluene (one sample at 0.0037 mg/kg) and xylenes (two samples; 0.0087 and 0.009 mg/kg)were detected. Samples from these borings contained low concentrations (less than 200 mg/kg) of TPHd,TPHmo, and TPHg; these TPH ranges are subsets of the overall TPH quantified as crude oil (TPHc)range. Samples collected from borings SB-8 and MW-1 did not contain benzene and concentrations of TPHd ranged from 480 mg/kg to 9800 mg/kg. Concentrations of ethylbenzene, toluene, and xylenes were less than 9 mg/kg and con- centrations of various PAHs were as high as 340 mg/kg(chrysene) to 5800 mg/kg(fluorene). Boring MW-1 is the deepest boring within the area of affected soil and extended to a depth of 52 feet bgs. Petroleum hydrocarbon affected soil was detected in the deepest soil sample from MW-1 at 46.5 feet bgs at 6900 mg/kg, and an"odor" was observed in the soil boring at 51 feet bgs. In summary, the lateral extent of residual petroleum at the site has been adequately assessed and appears to be constrained within the area of SB-2, SB-8, and MWA. The vertical extent of resid- ual petroleum has been identified above and below the water table between depths of approxi- mately 15 and 50 feet bgs using field observations; based on experience at other OVP sites, visual observation has been effective to delineate the extent of residual petroleum, particularly within the saturated zone. As shown on Figure 3, the area of affected soil is located around SB-2, SB-8, and MW-1 and bounded in four directions by borings SB-3 through SB-6. Groundwater Conditions Based on data from the site monitoring wells, groundwater occurs between 31 to 33 feet bgs (104 to 106 feet mean sea level) and generally has a horizontal hydraulic gradient to the northeast at approximately 0.005 feet per linear foot. Groundwater monitoring and sampling was performed in July and November of 1998. The analytical results and potentiometric surface for the 1998 inves- tigation are shown on Figure 3. As shown in Table 4, TPHmo, TPHg, and BTEX were not detected in groundwater samples from either of the July or November 1998 sampling events. Additionally, PAHs were not detected in either event. TPHd was detected at concentrations slightly above water quality objectives'` (WQO) in the July 1998 samples at concentrations of 230, 53, and 150 micrograms per liter(4g/1) in monitoring wells MW-2 through MW-4 respectively. TPHd was not detected above the laboratory reporting limit in groundwater samples from the November 1998 groundwater sampling event; for this event, TPHd analysis was performed following a silica gel preparation procedure. This indi- cates that the TPHd detections in July 1998 may be due to the presence soluble biodegradation byproducts resulting from intrinsic bioremediation of the residual crude oil present in the subsur- face. Monitoring well MWA was not sampled during either event due to the presence of approxi- mately 6 inches of separate-phase petroleum. ` The water quality objective utilized by the Central Valley Regional Water Quality Control Board for diesel fuel in 100µg/l and based on secondary criteria(i.e.,taste and odor threshold).