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f <br /> Human Health Risk Assessment <br /> Former Mobil Oil Bulk Plant 04-343 <br /> 500 East Grant Line Road <br /> . - Tracy, California <br /> The second step in the model involves the calculation of the effective ,1fusion coefficient. The <br /> effective diffusion coefficient is the diffusion coefficient of a chemical in air, modified by a <br /> tortuosity factor that accounts for decreased cross-sectional flow area and increased flow path. <br /> The formula is referred to as the Millington-Quirk tortuosity formula and is utilized in <br /> volatilization models referenced in the California Site Mitigation Decision Tree <br /> ' Manual (DHS, 1986). <br /> The calculation of the diffusion coefficient of a chemical in soil gas is provided in the following <br /> equation: <br /> ' Da x Pss <br /> a <br /> De = <br /> P2 <br /> i <br /> Where: D = (cm' <br /> /sec) <br /> Diffusion coefficient of chemical in air (cm /sec) <br /> P, = Soil air porosity (dimensionless) <br /> P, = Total soil porosity (dimensionless) <br /> De = Effective diffusion coefficient of chemical (cmZlso c) <br /> The Bulk Density (sb) of soil and air-filled soil porosity (Pj are calculated according to the <br /> Wowing equations: <br /> sb = sp - (sp x Pt) <br /> Where: P, = Total soil porosity (dimensionless) <br />' Mdry = Dry weight moisture content of soil (dimensionless) <br /> sb = Dry soil bulk density (g/cm3) <br /> Pp = Air-filled soil porosity (dimensionless) <br /> 1: <br /> 30-0136-11 <br /> 5-7 <br /> 1 <br />