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RISC Manual Version 4 0 <br /> the source volume and assuming that this concentration leaves the source with the <br /> bulk groundwater flow <br /> In order to estimate the dissolved phase concentration, it must be determined whether <br /> or not residual phase hydrocarbon (NAPL) is present If residual phase hydrocarbons <br /> are present, the dissolved phase source concentration for each chemical is assumed to <br /> be equal to its effective solubility This is the same approach used in the Vadose Zone <br /> model (discussed in Appendix A) <br /> To determine if residual phase hydrocarbon is present, the following condition for <br /> multiple chemicals must be met <br /> Cr Pb C-2 <br /> 1>all dwMteals S,(Pb Ka.F.,: +0. +9a KH/ ( } <br /> where <br /> Cr = total concentration of chemical in soil [mg/kg] <br /> S, = dissolved phase solubility for chemical i [mg/1] <br /> pb = soil bulk density of the source area [g/cmI <br /> Foy = fraction organic carbon in soil [g oc/g soil] <br /> K,. = chemical-specific organic carbon partition coefficient <br /> [ml/g or m P. <br /> KH = Henry's Law constant [(mg/1)/(mg/1)] <br /> 8Q = air-filled porosity of vadose zone [cm3 of air/cm3 total <br /> soil volume] <br /> 8,,, = water-filled porosity of vadose zone [cm3 of water/cm3 <br /> total soil volume] <br /> This partitioning equation assumes that the total soil concentration, Cr, can be <br /> accounted for by summing the mass in the solid, liquid, and vapor phases (i a no <br /> residual is present) However, Equation C-2 requires that all of the chemicals in the <br /> mixture are accounted for in the sum In risk assessments, the number of chemicals of <br /> concern is often a small subset of the total number of chemicals present in the <br /> C-4 <br />