Mobile organic sorbents (MOS) such as dissolved and colloidal phase organic matter control flow of water and transport of solutes in soils. We studied the effect of MOS on contaminant fate by systematic numerical case studies. The scenarios considered were (i) enhanced mobility, (ii) reduced mobility due to cosorption, and (iii) reduced mobility due to cumulative sorption. The enhanced mobility and cosorption scenario require contaminant sorption to the MOS. The cosorption and cumulative sorption scenario require sorption of MOS to the immobile sorbent. Simulations were run for physicochemically different fractions of dissolved organic matter and two model contaminants. Mobile organic sorbents mobility is characterized by a wide range of retardation parameters. Continued import of MOS to subsoil material high in MOS-specific sorption sites will increase the solid phase organic matter content. Enhanced mobility was observed for soils without MOS-specific sorption sites or for situations where MOS are in sorptive equilibrium with the immobile sorbent. Cosorption resulted in reduced contaminant mobility. The extent to which reduced mobility was observed depended on the ratio of the affinities of the free contaminant and the MOS-bound contaminant. As the sorption of the MOS-bound contaminants is controlled by the properties of the MOS, the characterization of these properties is a crucial step for the estimation of the effect of MOS on contaminant mobility. Cumulative sorption resulted in reduced contaminant mobility as well. However, this result is the consequence of the increase of the sorption capacity due to the sorption of MOS, a long-term process that may last for years to decades.

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