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Soil Mass Balance Methodolozy and Assumptions <br /> • Benzene is a human carcinogen and the chemical of concern(COC) in the vadose zone <br /> • Approximate extent of soil impacted by benzene was based on soil results from previous <br /> investigations at the site <br /> • Soil impacted by petroleum hydrocarbons is homogenous <br /> • Impacted volumes were determined from only the soil samples collected within the <br /> impacted area All other soil samples outside this area of extent were not used in <br /> calculating the mass <br /> • Each soil sample was considered to be within the impacted area, if the COC exceeded the <br /> analytical reporting limits (ARLs) Only one sample (T14-11 5A at 0 087 mg/kg <br /> benzene) exceeded the ARL for benzene Samples collected from beneath the product <br /> dispensers were not used in the calculation of mass balance because this soil was <br /> removed by excavation <br /> • The average vadose zone thickness was estimated by averaging the depths to recorded water <br /> levels The average vadose zone thickness is(8 89 feet+ 12 37 feet)/2 = 10 63 feet <br /> • The volume of impacted soil was estimated based on the areas between isoconcentration <br /> lines shown on the COC isoconcentration map (Figure 2), assuming an average thickness <br /> for the vadose zone Using sample T14-11 5A (0 087 mg/kg benzene), the volume of <br /> iimpacted soil was estimated to be a cylinder with a radius of 5 feet and an average <br /> thickness of 10 63 feet To be conservative, the benzene concentration of 0 087 mg/kg was <br /> assumed as the "representative" concentration for the cylinder, even though this sample was <br /> collected at a deeper depth (11 5 feet)than the average thickness of the vadose zone <br /> • The product of the volume and "representative" concentration was multiplied by appropriate <br /> conversion factors to provide the estimate of pounds of benzene remaining in the soil <br /> Mass of benzene in soil (pounds) = V (ft3) * g (Ib/ft) * C (°/amass) * 0 01 <br /> where <br /> V=volume of benzene-impacted soil <br /> g=unit weight of soil <br /> C =soil concentration in%mass <br /> Mass of benzene in soil (pounds) _ [p (5ft)- (10 63 ft)] * 116 lb/ft' <br /> [0 087(0 0001)%mass] * 0 01 <br /> = 0 008 pounds <br /> • Soil concentration (mJkg) =parts per million, where 1 ppm= 0 0001% mass <br /> • 0 01 is used as a mathematical conversion of percent <br /> • A typical dry unit weight of 116 lb/ft' was used based on a mixture of sands and silts <br /> I <br />