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Title: Modeling the Coupled Effects of Pore Space Geometry and Velocity on Colloid Transport and Retention

Author
item Bradford, Scott
item TORKZABAN, SAEED - UC RIVERSIDE
item LEIJ, FEIKE - UC RIVERSIDE
item SIMUNEK, JIRI - UC RIVERSIDE
item Van Genuchten, Martinus

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/2008
Publication Date: 2/12/2009
Citation: Bradford, S.A., Torkzaban, S., Leij, F., Simunek, J., Van Genuchten, M.T. 2009. Modeling the Coupled Effects of Pore Space Geometry and Velocity on Colloid Transport and Retention. Water Resources Research. 45:1-15.

Interpretive Summary: Computer models for clay and microorganism (colloid) transport in soil and groundwater environments typically only considers the average water velocity and retention behavior at a particular location. Recent research finding, however, indicates that colloid retention is greatly enhanced in low velocity regions of soils that occur in small pores and near soil grain contact points. A computer model was developed to simulate these observations by dividing the soil into high and low velocity regions and accounting for colloid exchange between these regions. Simulation results indicate that this model provided a good description of a wide variety of experimental observations, and a reasonable approximation of the physics that control colloid transport and retention in many natural environments. This model and information will be of use of scientists, regulators, and consultants who need to quantify the transport fate of pathogens, colloids, and colloid-associated contaminants in subsurface environments.

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.