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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. [ABSTRACT FROM AUTHOR]

A mathematical model is presented to describe the vertical transport of Cryptosporidium parvum oocysts through sediments. C. parvum oocyst has a particle diameter of 4.5-5.5 μm and density of 1.025-1.070 g cm-3, and so the gravitational sedimentation may play a role in the vertical transport of the oocysts. The settling velocity of the oocysts is calculated and on saturated porous media, was found to be between 2.2×10-5 and 8.910-5 cm sec-1. The permeability of porous media may be altered due to the deposition of the oocysts on the solid matrix. In the simulation, the permeability change is determined with an equation from other researches. The model equations are used to simulate the experimental data of an ealier work by other authors and the numerical results show a good fit with them. Simulation results show that when the oocyst suspension is injected continually, the relative permeability of sediments decreases since the volume of the oocysts deposited on the solid matrix increases. Sensitivity analysis demonstrates that the profile of the relative permeability is very sensitive to the change of the irreversible deposition rate coefficient of the oocysts, kci. As kci increases, the volume of the oocysts immobilized around the inlet boundary increases, resulting in the decrease of the relative permeability. [ABSTRACT FROM AUTHOR]