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' concentration of 1 µglm3 in air. The cancer slope factor and unit risk factor for benzene were <br /> modified from the default values as appropriate for California evaluations as described below: <br /> Benzene: <br /> , Benzene is classified by the US EPA as a known carcinogen, weight of evidence <br /> Category A . The slope factor determined by the US EPA and used as the default value <br /> in the ASTM document is 2.9 E-2 mg/kg-day`. However, the California EPA Office <br /> of Environmental Health Hazard Assessment has derived a slope factor of 1.00 E-1 <br /> mg/kg-day'` and equivalent unit risk factor of 2.9 E-5/µg/m3 (Cal EPA, 11194). These <br /> more conservative figures were substituted for the default values in assessing the risk due <br /> ' to exposure to benzene. <br /> ' Contaminated Soil Area <br /> The default contaminated soil area is approximately 2,420 ft . However, based on the soil <br /> ' samples collected from borings in May 1995 and November 1999, this value was changed to <br /> approximately 700 ft2. <br /> ' Vadose Zone Thickness <br /> The default vadose zone thickness is approximately 9.7 feet. However, based on the current <br /> ' depth to groundwater the value was changed to approximately 30 feet. <br /> Depth to Groundwater <br /> I <br /> The default depth to groundwater is approximately 10 feet. This value was changed to <br /> approximately 30 feet based on current measurements. <br /> Depth to Top of Affected Soil <br /> ' The default depth to top of affected soil is approximately 3.5 feet. This value was changed to <br /> approximately 17.5 feet based on the soil samples collected from the borings completed in May <br /> 1995. <br /> ' Thickness of Affected Subsurface Soil <br /> ' The default thickness of affected subsurface soil is approximately 6.5 feet. This value was <br /> changed to approximately 15 feet based on the soil samples collected from the borings completed <br /> in May 1995. <br /> ,V&DeYMwqWrw .W <br /> � � 4 <br /> I <br />