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P.O.Box 516981-69131 Modesto,California 95357 erdanz� Associates Phone: (209)577-8186 Ob Fax, (209) 577-8158 Environmental Assessment ---- 3 July 7, 2003 Lori Duncan San Joaquin County Environmental Health Division P.O. Box 2009 Stockton, California 95201 RE: Work Plan Vertical Soil Contamination Investigation McDowell & Davis Towing, 1360 Escalon Avenue, Escalon, California 1.0 PROJECT IDENTIFICATION The objective of this plan is to provide a scope of work that would further define the vertical and lateral extent of the hydrocarbon plume in the soil near former boring B-2 at the McDowell & Davis Towing facility. Soil contamination associated with the former underground storage tanks was recognized at two locations during the removal and closure of the underground storage tanks, at the western end of the center tank, TK2, and at the western dispenser island. The dispenser island release was confirmed by soil samples collected from depths of 15 and 25 feet in boring B2. The release is characterized by high concentrations of benzene and MTBE suggesting a more recent spill. Very low contaminant concentrations were detected at a depth of 25 feet and contamination and not detected at depths of 35 and 45 feet. A risk assessment based on the interpreted contaminant concentrations determined that a potential risk may be present to int6rior air exposure to benzene sourced from the release. The objective of this study is to refine the estimate of soil contamination and reevaluate the potential risk posed by the release. 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.