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a <br /> 3.2.1 Site Aerial Photographs <br /> ' Available aerial photographs were reviewed between the years 1957 to 1996 and are summarized <br /> on Table 1 The aerial photographs are provided in Appendix A The photographs document the <br /> ' presence of the residential homes, the commercial tire shop, and parked vehicles No other <br /> structures or activities were found to be present <br /> 3.2.2 Site Previous Investigation <br /> ' Groundwater contamination on the 541 Site was initially suspected (via petroleum odors) during <br /> the geotechnical investigation performed by the San Joaquin Company for a proposed building in <br /> ' 1999 A preliminary subsurface investigation was performed at the 541 Site in August 2000 by <br /> Advanced GeoEnvironmental, Inc (AGE) Six direct-push borings, designated B-1 to B-6, were <br /> ' drilled on the 541 Site as shown on Figure 3 <br /> Soil and groundwater grab samples were collected during the AGE investigation and chemically <br /> analyzed for total petroleum hydrocarbons (TPH) as gasoline, diesel and motor oil, benzene, <br /> ' toluene, ethylbenzene and xylenes (BTEX), and methyl tertiary butyl ether(MTBE) The results <br /> are summarized on Tables 3 and 4 The soil samples were collected below the top of the <br /> ' groundwater at 10 and 15-foot depths The highest contaminant levels detected in groundwater <br /> were at B-4 with TPH-gasoline at 50,000 parts per billion (ppb), TPH-diesel at 15,000 ppb, <br /> ' TPH-motor oil at 1,500 ppb, and benzene at 420 ppb B-4 is located near the west central <br /> portion of the 541 Site adjacent to the Tracy Express Wash & Lube (Figure 3) The AGE Report <br /> is appended to the Site Assessment Report(Treadwell &Rollo, 2003a) <br /> ' 3.3 Known and Potential Offsite Contaminant Sources <br /> ' Because there is no evidence of onsite source areas including a fueling station or underground <br /> storage tank at the 541 Site, the contamination in the groundwater is believed to be from an <br /> offsite source(Treadwell & Rollo, 2003a) There are numerous documented LUST sites located <br /> upgradient and cross-gradient to the 541 Site (Figure 2) These LUST sites have contaminated <br /> 1 <br /> Field Investigation Report 4 8 December 2003 <br /> ' TREADWELL&ROLLO,INC <br />