Colloid‐facilitated groundwater contaminant transport

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Abstract
Colloidal particles or dissolved organic matter (DOM) can act as carriers to enhance the transport of contaminants in groundwater by reducing retardation effects. When either of these materials is present, the system can be treated as consisting of three phases: an aqueous phase, a carrier phase, and the stationary solid matrix phase. The contaminant may be present in either or all of these phases. In the work reported, a mathematical model was developed to describe the transport and fate of the contaminant and carrier material in a porous medium. The model is based on mass balance equations describing the transport and fate of the contaminant and carrier in a three‐phase medium. Colloid/contaminant and colloid/matrix mass transfer mechanisms are represented by first‐order kinetics. Equilibrium partitioning of DOM acting as a carrier of the contaminant introduces a significant simplification in the model formulation. For a constant DOM concentration a much smaller retardation coefficient can be obtained in the three‐phase system than the coefficient obtained in a conventional advective/dispersive transport equation for a two‐phase system. The modified retardation coefficient reflects the presence of the mobile carrier by incorporating both the sorption of the contaminant and capture of the carrier on the solid matrix. Numerical solutions for the model were obtained by using a finite difference scheme to provide estimates of contaminant and carrier concentrations. Significant sensitivities to model parameters, particularly the rate constants of carrier capture and sorption were discovered. The numerical results of the DOM carrier effect matched favorably with experimental data reported in the literature.