Perspectives On Modeling Colloid Transport and Retention in Porous Media.

Colloid and colloid-facilitated transport in porous media has traditionally been assumed to be controlled by chemical interactions between the colloids and the solid-water and air-water interfaces.  Recent experimental and theoretical work has demonstrated that pore space geometry and hydrodynamics can also play an important role in colloid transport and retention under unfavorable attachment conditions.  Conceptual models that only consider the average pore-water velocity and a single attachment rate coefficient are therefore not always adequate to describe colloid retention processes.  In this work, various modeling approaches will be discussed to account for enhanced colloid retention in low velocity regions, time and concentration dependent colloid deposition, transients in solution chemistry, and non-exponential profiles of retained colloids with distance.  Specific model formulations that will be considered include: a two-site chemical nonequilibrium model, a physical and chemical nonequilibrium model, a multispecies model, a stochastic stream tubes model, and a dual permeability model.  The contribution of Dr. Martinus Th. van Genuchten to the development of these models will be outlined.