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1 � � reported to be at approximately 10 feet below ground surface (bgs) in August 2000 (AGE, 2000) However, during the construction of the Sherwin Williams building and storm water basin in i2001-2002, the depth to groundwater was noted to be at approximately 6 to 6 5 feet bgs (Bird, 2003) Groundwater contamination at the Site was initially suspected (via petroleum odors) during the geotechnical investigation performed by the San Joaquin Company for a proposed building in 1999 (SJC, 1999) A preliminary subsurface investigation was performed at the Site in August ' 2000 by Advanced GeoEnvironmental,Inc (AGE) Six direct-push borings, designated B-1 to ' B-6,were drilled at the Site as shown on Figure 3 Soil and groundwater grab samples were collected during the AGE investigation and chemically ' analyzed for total petroleum hydrocarbons (TPH) as gasoline, diesel and motor oil, benzene, toluene, ethylbenzene and xylenes (BTEX), and methyl tertiary butyl ether (MTBE) The soil ' samples were collected below the top of the groundwater at 10 and 15-foot depths Most ' compounds, except MTBE, were detected in the majority of the soil and groundwater samples collected The highest levels detected in groundwater were at B-4 with TPH gas at 50,000 ppb, ' TPH-diesel at 15,000 ppb, TPH-motor oil at 1,500 ppb, and benzene at 420 ppb B-4 is located near the west central portion of the Site adjacent to the Tracy Express Wash& Lube (Figure 3) ' The results are summarized in the Site Assessment Report(Treadwell &Rollo, 2003) The maximum contaminant levels detected in soil and groundwater are provided in the Site Health ' and Safety Plan provided in Appendix A ' 3.3 Known and Potential Offsite Contaminant Sources Because there is no evidence that a fueling station, underground storage tank, or other activities of concern were ever present at the Site, the contamination in the groundwater is believed to be from an offsite source There are numerous documented leaking underground storage tanks ' (LUSTs) sites located upgradient and cross-gradient to the Site (Figure 2) These LUST sites ihave contaminated the groundwater with gasoline, diesel, and/or motor oil, which are the same Field Investigation Work Plan 4 2 September 2003 ' TREADWELL&ROLLO,INC.