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For estimating ground water concentrations, Equation 1 can be <br /> rearranged as: <br /> CW = Csv/H (Equation 2) <br /> To make the units compatible, the following conversions are used: <br /> 1 ppm soil vapor � 1x10-6 atmosphere <br /> 1 mole = mole weight expressed in milligrams. <br /> The molecular weights of benzene and toluene are 78x103 mg/mole <br /> �.. .._..- _ .and_92xlO mg/mole,. respectively.. (EPA_L986b)- ThrAr Henry's-Law <br /> constants are 5.6 atm'L/mole and 6.4 atm'L/mole, respectively <br /> (EPA 1986b) . Using these data and Equation 2, the expected <br /> ground-water concentration of benzene can be estimated as: <br /> CW-Benzene � (Csv) (1x1O_6 atm/ppm) (78x103 mg/mole). <br /> (5.6 atm'L/mole) <br /> 0.014 Csv. <br /> For toluene, the ground--water concentration can be estimated by: <br /> Cw-toluene (Csv) (1x10-6 atm/ppm) (92x103 mg/mole) <br /> (6.4 atm'L/mole) _ <br /> 0.014 Csv. <br /> The highest measured benzene concentration at the Stockton site <br /> was 280 ppm at MW2. This corresponds to a ground--water <br /> concentration of 3.9 mg/L. The highest toluene concentration, <br /> 280 ppm found at MW1, corresponds to a ground-water concentration <br /> of 3.9 mg/L. The San Francisco Regional water Quality Control <br /> Board (.1985) found that water in equilibrium with gasoline will <br /> contain as much as 40 mg/L of benzene and from 9 to 76 mg/L of <br /> toluene. <br /> CHV18:81J 9 <br />