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r <br /> The relationship between the adsorbed concentration and the free <br /> concentration can be expressed as a linear partition equation: <br /> ` S = KdaC ( 2 ) <br /> where <br /> Kd = the distribution coefficient. <br /> This may be differentiated with respect to time: <br /> aS = Kd OC <br /> at at <br /> ( 3 ) <br /> and solved with substitution as <br /> ` aC = D/Rf a2C - v/Rf aC <br /> t ax2 ax ( 4 ) <br /> where <br /> Rf (the retardation factor ) = 1 + p/n Kd. <br /> ., In many ground water systems, the velocity term (as opposed to <br /> the dispersion term) dominates transport. It has been found for <br /> organic compounds in an aqueous system flowing through soil that <br /> Kd can generally be related to the soil organic content <br /> (expressed as total organic carbon ) and to the compound' s hydro- <br /> phobicity (Mackay et al. 1985 ) . Reinhard and Hopkins ( 1983 ) have <br /> demonstrated retardation even in relatively organic-free ( 0 . 2 <br /> percent organic carbon ) sand. The relative migratory velocity of <br /> a compound in ground water ( i.e. , its velocity relative to the <br /> ` ground water ) can be estimated from either the solubility or the <br /> octanol-water partition coefficient ( Kow) , both of which are <br /> indicators of hydrophobicity. Differential compound solubility <br /> and affinity for the soil or rock matrix in which a compound is <br /> 31 <br />