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1 <br /> Vadose Zone Model <br /> 1 <br /> 1 where P is a loss term accounting for both dissolution and volatile losses The loss <br /> term is calculated differently depending on whether or not residual phase is present <br /> 1 <br /> A-4 5 Calculating Whether or Not Residual Phase Hydrocarbon is <br /> Present <br /> To determine if residual phase hydrocarbon is present, the following condition for <br /> ' multiple chemicals must be met <br /> N CT,Pb <br /> 1 > (A-15) <br /> S, hK�Fm +0. +O.Kw, <br /> ' where <br /> N = total number of chemicals in mixture <br /> C7, = total concentration of chemical i in soil [mg/kg] <br /> S, = liquid phase solubility for chemical i [mg/l or kg/m}] <br /> ' ph = soil bulk density of the source area [g/cm3] <br /> F,,, fraction organic carbon in soil [g oc/g soil] <br /> ' Ko4 r = organic carbon partition coefficient for chemical i [ml/g] <br /> KH , Henry's Law Constant for chemical i [(mg/l vapor)/(mg/l <br /> water)] <br /> 1 <br /> This partitioning equation assumes that the total soil concentration for each chemical, <br /> C1„ can be accounted for by summing the mass in the solid, liquid, and vapor phases <br /> (i a no residual is present) However, equation A-15 requires that all of the <br /> chemicals in the mixture are accounted for in the sum In risk assessments, more <br /> ' often the number of chemicals of concern (and characterized with respect to their <br /> concentration) is a small subset of the total number of chemicals present in the <br /> mixture The vadose zone model checks the following equation for each chemical <br /> used in the model <br /> ' __ Cr Pb <br /> C'. POC Fx +@x +6QKH (A-1 6) <br /> 1 <br /> I <br /> i <br /> A-11 <br /> I <br />