| Abstract: | The presence of colloids or of dissolved organic matter (DOM) acting as carriers can 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 this study, a kinetic model, a hybrid-equilibrium model, a full-equilibrium model and a fly ash dissolution model were developed to describe the transport and fate of the contaminant and carrier material in a porous medium. The models are based on mass balance equations describing the transport and fate of the contaminant and carrier in a three-phase medium. In the first three models, colloid/contaminant mass partition mechanisms are represented by first order kinetics or local equilibrium assumption (LEA). It was shown that equilibrium partitioning of colloids or DOM acting as a carrier of the contaminant in the hybrid-equilibrium model, introduces a significant simplification in the model formulation. For a constant DOM concentration in the full-equilibrium model, a much smaller retardation coefficient is obtained in the three-phase system than the coefficient obtained in a conventional advection/dispersion 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 models were obtained by using a finite difference scheme to provide estimates of contaminant and carrier concentrations. A significant sensitivity to model parameters, particularly the rate constants of carrier capture and sorption was discovered. The computed results of the DO M carrier effect matched favorably with experimental data reported in the literature. The relationship between the kinetic and LEA models developed in this study was discussed and a comparison of the two types of models was perform ed. Dissolution of colloidal fly ash in groundwater was simulated by a similar kinetic model. Results of this study show that the three-phase models can be a useful tool for predicting colloid facilitated groundwater transport. |
|---|