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Workplan for Additional Assessment 49 South Wilson Way May 4, 1998 Page 2 • The government records research indicated that a Texaco gasoline service station currently located adjacent,on the southwest corner of the intersection, is identified as a leaking underground storage tank(LUST) site. Phase II Subsurface Investigation A Phase II Subsurface Investigation was performed by Building Analytics in February 1996,to further investigate the potential of an-environmental impact by the former on-site gasoline service station. Soil samples were collected from five Geoprobe borings at depths of up to 25 feet below ground surface (bgs). All soil samples revealed non-detectable concentrations for total petroleum hydrocarbons as gasoline and diesel (TPH-g and TPH-d) and benzene, toluene, ethylbenzene and xylenes (BTEX). Total recoverable petroleum hydrocarbons (TRPHs) were found at concentrations ranging from ten parts per million(ppm) to 31 ppm, which is well below the Maximum Contaminant Level (MCL) of 100 ppm as set forth by the Regional Water Quality Control Board(RWQCB). Based on the above-mentioned analytical results, it was concluded that the on-site soil contamination,although below regulatory action levels,was likely caused by the spilling of waste oil and motor oil at the former on-site gasoline service station. There was no indication of diesel or gasoline contamination. Groundwater was expected at a depth of approximately 60 feet bgs, and was not sampled during this subsurface investigation. February 1997, Follow-up Phase II Subsurface Investigation, Building Analytics completed a Follow-up Phase II Subsurface Investigation in February 1997, which included the installation of three monitoring wells. The following is..a summary of the findings from the expanded investigation: • The groundwater flow direction appears 'to be to the northeast, with a hydraulic gradient of 0.0025; • TPH-g at levels up to 1,300 ppm, and benzene at levels up to 0.39 ppm, were detected in the soil samples collected from Boring B2, at depths between 35 and 55 feet bgs; _ • TPH-g and BTEX were not reported in the groundwater samples collected from MW-1 or MW-3; • Benzene was not reported in the groundwater sample collected from MW-2; • TPH-g (1.6 ppm), ethylbenzene (120 ppm) and xylenes (370 ppm) were reported in the groundwater sample collected from MW-2; and • Methyl-tert-butyl ether (MTBE) was not reported in any of the soil or groundwater samples collected. 2 198075-WFB Stockton.workplan