An investigation was performed on the importance of the rates of adsorption and desorption of neutral organic pollutants onto sediments and onto the natural dissolved organic materials which reversibly partition onto sediments. Dichlorobenzene exchange rates and equilibrium partitioning onto the fulvic acid fraction of natural dissolved organic carbon (DOC) were experimentally determined to be significantly different than that onto the humic acid and humin fraction. From the experimental results, a numerical model of the sorption kinetics was incorporated into a finite difference pollutant transport model consisting of the system of partial differential equations for transport and transfer rates between forms for the free dissolved, bound to sediments, and bound to naturally occurring dissolved organic carbon pollutant concentrations with respect to time and space. The model was applied to the case of hydrophobic species in the transverse mixing zone of fast streams and compared with other formulations for partitioning. The comparison showed that the phase distribution of a pollutant cannot be correctly predicted from simple equilibrium models.

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