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McDowell&Davis Towing <br /> Risk Assessment and Conceptual Site Model <br /> The dispenser island release was confirmed by soil samples collected from depths of <br /> �. 15 and 25 feet in boring B2. The release is characterized by extremely high <br /> concentrations of benzene and MTBE suggesting a more recent spill. Very low <br /> contaminant concentrations were detected at a depth of 25 feet and contamination was <br /> not detected at depths of 35 and 45 feet. Contaminant cross sections are presented in <br /> Figures 5 and 6. An estimate of the remaining contaminant mass in the two releases is <br /> shown below: <br /> Estimated Soil Contaminant Mass <br /> Tank TK2 Dispenser Island <br /> cu feet Lbs. Cu ft Ibs. <br /> Gas 0.00147 0.064189 Gas 0.00574 0.250737 <br /> Benzene 0 0 Benzene 5.59E-07 2.44E-05 <br /> Toluene 0 0 Toluene 2.58E-05 0.001127 <br /> E. Benzene 0.00275 0.12012 E. Benzene 0.00153 0.06683 <br /> Xylenes 0.015681 0.684948 Xylenes 0.051 2.22768 <br /> MTBE 0.022809 0.996288 MTBE 0.087136 3.8061 <br /> TBA 0.000054 0.002359 TBA 5.45E-05 0.002379 <br /> Groundwater contamination has been recognized at low concentrations in all three <br /> monitoring wells in the quarterly monitoring events. Water quality levels for benzene, <br /> �. toluene, and gasoline petroleum hydrocarbons were exceeded in the November 2000, <br /> June 2001, and February 2002 monitoring events. In the February 2002 event only <br /> benzene at 1.1 ug/1 exceeded the maximum contaminant level of 1.0 ug/1 in one well. <br /> As referenced above, the May 2002 monitoring event (report in preparation) did not <br /> detect contaminants in excess of the established water quality goals. The limited <br /> groundwater impact appears to be degrading and attenuating. Groundwater laboratory <br /> analyses are presented in table I. <br /> 3.3 Receptors and Potential Pathways <br /> Potential receptors were identified as ingestion, dermal exposure, contaminated <br /> groundwater ingestion, and vapor inhalation. Potential pathways for exposure <br /> i- including onsite and offsite ingestion and dermal exposure to contaminated surface <br /> soils; onsite and offsite exposure to contaminate vapors sourced from surface <br /> contaminated soil, subsurface contaminated soil, and contaminated groundwater, and <br /> '-' onsite and offsite ingestion of contaminated groundwater. <br /> The excavation and off site disposal of all surface and shallow contaminated soil has <br /> eliminated the potential exposures through dermal exposure and ingestion of surface <br /> contaminated soil and inhalation of vapors from contaminated surface soil. Drinking <br /> water for the McDowell and Davis Towing facility is supplied by the City of Escalon <br /> from offsite production wells, which removes the possibility of onsite ingestion of <br /> contaminated groundwater. <br /> Potential receptors and pathways identified in the preparation of the risk assessment <br /> are listed below: <br /> 4 <br />