Equilibrium approaches to organic contaminant transport modeling in a multi-phase groundwater system.

Mathematical models were developed in this study based on equilibrium sorption approaches to simulate transport and retardation of organic contaminants such as polycyclic aromatic hydrocarbons in a multi-phase groundwater system. In a model development, the Langmuir or Freundlich sorption isotherms were incorporated into a retardation factor to account for nonlinear sorption of contaminants and colloids on solid matrix. Numerical solutions of model equations were obtained using a fully implicit finite difference method. Simulation results showed that the contaminant retardation factor varied temporally and spatially in a multi-phase system (dissolved phase, solid phase, and two mobile colloidal phases). Simulation also indicated that colloidal concentration and mobility would be key factors in controlling the contaminant transport in a multi-phase system. Sensitivity analysis demonstrated that the models were sensitive to changes of nonlinear isotherm coefficients and equilibrium distribution coefficients.