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Wendy Cohen - 2 - 15 November 2007 Former Mobil Oil Company Bulk FL. 'lant, Tracy From 1990 to 1995, Mobil completed about 30 onsite and offsite soil borings to characterize the extent of the contamination following excavation activities. The maximum post excavation soil concentrations of TPHg, TPHd, and benzene were 1,200 mg/kg, 6,700 mg/kg, and 8.4 mg/kg, respectively, in soil samples collected in 1994 from borings SB-7 and SB-8 at 11.5 feet bgs. Concentrations decreased rapidly with depth, and soil samples collected below 16.5 feet bgs were below laboratory detection limits for all constituents. Mobil estimated that following the 1990 excavation activities about 650 gallons of TPHd remained in soil between 10.5 to 12.5 feet bgs. This mass has likely reduced significantly in the 13 years since these soil concentrations were measured. Soil sample results are presented in Table 1. GROUNDWATER INVESTIGATIONS Mobil installed 12 groundwater monitoring wells (eight onsite and four offsite) to monitor groundwater beneath the Site (Figure 1). These wells were sampled quarterly from 1990 to 1997. Groundwater flow is north-northeast toward the Leprino facility. The maximum concentrations of TPHg, TPHd, and benzene were 4,200 pg/L (AW-3 in 1990), 22,000 pg/L (AW-3 in 1991), and 15 pg/L (AW-9 in 1992). During the last Mobil sampling event, first quarter 1997, only monitoring well AW-3 contained detections of TPHg (20 pg/L) and TPHd (490 pg/L) above the reporting limit. Benzene was not detected in any monitoring well sampled in 1997. Leprino continues to monitor wells AW-3, AW-5, AW-6, and AW-7, and now calls them MW-18, MW-16, MW-17, and MW--7, respectively. TPHd has decreased in MW-7, MW-16, MW-17, and MW-18 from 300 pg/L, 370 pg/L, 170 pg/L, and 22,000 pg/L in 1991 to <1 pg/L (MW-7 last sampled in 1997), < 48 pg/L, <48 pg/L, and 67 pg/L in March 2007, respectively. The Leprino groundwater extraction system is downgradient of the former Mobil site and any TPH migrating onto the Leprino property will be captured by that system. SENSITIVE RECEPTORS In 1997, Alton Geoscience conducted a sensitive receptor survey. The nearest downgradient surface water body is the Sugar Cut Slough more than 3,000 feet north of the Site. One municipal well was identified about 300 feet east of the Site. This well is listed as a municipal supply well for the City of Tracy and is about 920 feet deep and screened from 340 to 920 feet with a sanitary seal down to 200 feet. HUMAN HEALTH RISK ASSESSMENT Vapor Intrusion Hazard The site is currently a vacant lot with no site structures. Confirmation soil sampling and groundwater monitoring show that the gasoline contamination was removed and most of the remaining mass is TPHd. TPHd is not likely to present a vapor intrusion hazard. Based on these data, there is no evidence of a vapor intrusion hazard at the Site.