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V �., McDowell & Davis Towing 1360 Escalon Avenue, Escalon, California Risk Assessment and Conceptual Site Model 1.0 PROJECT IDENTIFICATION A site evaluation and risk assessment was conducted of the McDowell and Davis, Escalon, CA leaking underground storage tank site to characterize the risks posed by the petroleum contamination discovered during the removal of the underground tanks. The San Joaquin County Environmental Health Department (SJCEHD), in a January 22, 2002 review letter of the ongoing groundwater monitoring; required a well survey, the calculation of the contaminant mass in the shallow soils, an evaluation of the potential risk posed by the contamination, and determination of the need to remediate the contamination. 1.1 Site Description and History Four underground storage tanks, three fuel and one waste oil, were removed on March 15, 1999 by SEMCO under permit from the SJCEHD. Hydrocarbon odors were noted in the soil beneath the west end of the dispenser island area and in soils at the west end of the center fuel tank. Soil samples for analysis were collected as required from beneath the tanks, the dispenser islands, the piping lines, and from the soil stockpiles. Analytical results disclosed up to 210 ppb of MTBE, 2,700 ppb TBA and 570 ppb xylenes from the former tank pit, and 5,000 ppb TBA and 110 ppb xylenes in a grab sample form the former dispenser island area. Laboratory analyses �- of soil samples from the tank removal and exploratory borings and wells are presented in Table II. �- M. J. Kloberdanz &Associates constructed four soil borings to depths of 20 to 45 feet in the vicinity of the former underground tanks and fuel islands. Boring B-1 was drilled next to the former tank excavation to a depth of 40 feet. Laboratory samples collected from various depths including the bottom of this boring were reported as "non-detect for the compounds analyzed"by the analytical laboratory. Boring B-2 was drilled to a depth of 45 feet in the fuel island area, near grab sample PL# 7, where soil contamination was detected during the tank removal. Petroleum contamination was apparent to a depth of 20 feet in the boring. Boring B-3 was drilled to 20 feet southeast of boring B-2 between the two fuel islands. Laboratory soil samples, collected at 15 and 20 feet in depth, were reported as "non-detect for the compounds analyzed"by the analytical laboratory.