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M Human Health Risk Assessment <br /> Former Mobil Oil Bulk Plant 04-343 <br /> 500 East Grant Line Road <br /> Tracy, California <br /> Henry's Law <br /> For a dilute solution, Henry's Law can be written in a ger+gyral form as follows <br /> ., <br /> (Adamson, 1979): <br /> limxj-0 Pi = k x, <br /> Where: P; = Vapor pressure of solute component i in the solution <br /> j k, = Henry's Law constant (dimensionless) <br /> xl = Mole fraction of solute component i in the solution <br /> 1 <br /> Henry's Law can be used to predict the vapor pressure of solute compor,ent i in a solution where <br /> component i does not exist as a free phase and is a miscible component in the dilute solution. <br /> That is, the solute and solvent do not have similar properties (matlic,natically written x;-0). <br /> ' Henry's Law is more appropriately used when the chemical concentr,3 �;-)n is less than 1 ppm. <br /> 5.3.2.2 Calculation of Chemical Concentrations in the Soil Gas <br /> The chemical (e.g., TPH and BTEX) exist in a dissolved-phase in tai. thin, immobile film of <br /> ' water surrounding the soil particles. When contaminants such as pet •_.leum hydrocarbons are <br /> present in a system containing both water and air, they will partition themselves between the two <br /> ' phases in proportion to their values of Henry's Law constant (Lyman, Roider, and Levey, 1992). <br /> The equation used to quantify the soil gas concentration is shown below: <br /> C s = C * H <br /> s8( ) sorl � ) Dw <br /> M.t <br /> Where: Cs,,;, = Concentration of chemical in soil (mg/kg) <br /> ' H = Henry's Law constant (dimensionless) <br /> MW, = Wet moisture content of soil (dimensionless) <br />' D,,, = Density of Water (9/cm3) <br /> Cg(s) = Concentration of chemical in soil gas from soil (g/cm) <br /> 30-0136-11 <br />�� 5-b <br />