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l <br /> Human Health Risk Assessment <br /> Former Mobil Oil Bulk Plant 04-343 <br /> 500 East Grant Line Road <br /> Tracy, California <br /> Indoor air concentration of a chemical are influenced by its physical a•nd chemical properties. <br /> For example,amp e, high vapor pressure and low aqueous solubilities increase the likelihood that <br /> organic compounds found in water will also be found in air (Roberts and Dandliker, 1983). <br /> 5.3.2.1 Description of Model Used to Predict Airborne Concer,,;ations of <br /> ' Indicator Chemicals <br /> ' The transport of chemicals from soil to the atmosphere is a complex process which is influenced <br /> by a number of factors. These factors include the physical properties of the chemical <br /> (i.e., vapor pressure and solubility) and physical properties of the soi? matrix (i.e., soil bulls <br /> density, porosity, moisture content, etc.). The model used in this anaysis is based on one used <br /> by the California DHS (DHS, 1986). The model consists of five separate <br /> ' components: 1) calculation of soil gas concentration, 2) calculation of the effective diffusion <br /> coefficient of the indicator chemicals into the soil gas, 3) calculation= •-A chemical flux to soil <br /> ' surface, 4) adjustment for attenuation through a foundation, and 5) d germination of chemical <br /> concentration in the indoor air. <br /> The first component of the model predicts the chemical concentration inore <br /> p gas. Depending <br /> on site-specific conditions, one of two equations can be used to estin_ate the concentration of <br /> chemical in soil gas. The two models are based on Raoult's Law :1A Henry's Law. Brief <br /> descriptions of the two laws are presented next. <br /> Raoult's Law <br /> For a mixture of compounds that are essentially undistinguishable (e.g., hydrocarbon compounds <br /> that comprise gasoline), Raoult's Law can be written as follows (Adarlison, 1979): <br /> limxj_1 P�- o= P� xr <br /> 1 <br /> Where: P; = Vapor pressure of compound i in mixture <br />' P;° = Vapor pressure of component i at the sante temperature in a pure <br /> phase <br /> X_ = Mn1P frartinn of r`mmnnnant ;r% th, 1;,, —i <br /> 30-0136-11 <br /> �^ 5-5 <br />