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RISC User's Manual Version 4.0 R=1 f p�d for inorganic chemicals (B-2b) where Foy = fraction organic carbon in dry soil [g oc/g soil] Ko, = organic carbon normalized partition coefficient [ml/g or m3/kg] Kd = inorganic distribution coefficient[ml/g] pb = soil bulk density of the saturated zone [g/cm3] 0 = porosity(effective)of the saturated zone [cm3/cm3] If a non-zero value for Kd is entered in the RISC chemical database, the code uses equation B-2b For organic chemicals that do not have Kd values the code assumes that the only process causing retardation is the sorption of the chemical due to the presence of organic carbon in the aquifer In this situation, the quantity Ko,*Fo, is used to estimate the distribution coefficient (Kd) In reality there may be several processes (such as the presence of silts and clays) contributing to the retardation (or . sorption) of the chemical The equations in RISC (and most fate and transport models)assume that the retardation can be completely predicted using the relationship in Equation B-2a This will usually under-predict the amount of sorption (and hence retardation) that is actually occurring Usually this will be conservative for purposes of estimating risk If it appears that there may be much more retardation occurring than what the site-specific measured value of Fo,would indicate (usually the case for low F,soils), the actual partitioning may be measured in a lab and then the measured Kd value could be entered for the chemical being modeled This retardation equation assumes that sorption and desorption processes are instantaneous and fully reversible See Chapter 11 (the chemical database) for equations for estimating KoG from other chemical parameters B.2 3 Dispersion Coefficients;w`",� i The dispersion coefficients in equation B-1 are calculated using the following relationships B-4