Technical Abstract: Recent experimental and theoretical work has demonstrated that pore space geometry and hydrodynamics can play an important role in colloid retention under unfavorable attachment conditions. Computer 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, which frequently produce non-exponential profiles with distance. In this work, we highlight a dual permeability model formulation that can be used to account for enhanced colloid retention in low velocity regions of the pore space. The model accounts for different rates of advective and dispersive transport and first-order colloid retention and release in fast and slow velocity regions of the pore space. The model also includes provisions for the exchange of colloids from fast to slow regions in the aqueous phase and/or on the solid phase due to either rolling or sliding. A sensitivity analysis was performed with the dual permeability model parameters that indicated that low amounts of advective transport to low velocity regions had a pronounced influence on the colloid retention profiles, especially near the inlet. The developed model provided a good description of measured colloid breakthrough curves and retention profiles that were collected for a variety of conditions (colloid and porous media size, water velocity, and solution ionic strength), suggesting that it provided a reasonable approximation of the pore-scale physics controlling colloid retention under unfavorable attachment conditions.