Modeling colloid transport and retention in saturated porous media under unfavorable attachment conditions

Authors: Bradford, Scott; TORKZABAN, SAEED - University Of California; SIMUNEK, JIRI - University Of California

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2011
Publication Date: 10/6/2011
Citation: Bradford, S.A., Torkzaban, S., Simunek, J. 2011. Modeling colloid transport and retention in saturated porous media under unfavorable attachment conditions. Water Resources Research. 47:W10503. DOI:10.1029/2011WR010812.

Interpretive Summary: Existing mathematical models to simulate the movement of pathogens through agricultural soils and groundwater do not provide reliable predictions even under relatively simple, well defined conditions. The objective of this work is to present a mathematical model for pathogen transport and retention that accounts for observed trends in pathogen and soil size, velocity, chemical interactions, and concentration. Our approach considers pathogen transport in the bulk water and adjacent to the soil surface, and pathogen retention on only a fraction of the solid surface. The model provides a clear conceptual explanation for many incompletely understood observations of pathogen transport and retention in soils, and helps to identify areas where additional research and theory development are still needed. This information will be of interest to scientists and engineers concerned with predicting the fate of pathogens, colloids, nanoparticles, and colloid associated contaminants in soils and aquifers.

Technical Abstract: A mathematical model is presented for colloid transport and retention in saturated porous media under unfavorable attachment conditions. The model accounts for colloid transport in the bulk aqueous phase and adjacent to the solid surface, and rates of colloid collision, interaction, release and immobilization on the solid phase. Model parameters were estimated using: (i) filtration theory; (ii) calculated interaction energies in conjunction with the Maxwellian kinetic energy model of diffusion; (iii) information about the velocity magnitude and distribution adjacent to the solid phase that was obtained from pore-scale water flow simulations; (iv) colloid and collector sizes; (v) the balance of applied hydrodynamic and resisting adhesive torques; and (vi) time dependent filling of retention locations using a Langmuirian approach. The presented theory constrains the model parameters and output to physically realistic values in many instances, and minimizes the need for parameter optimization. Example simulations demonstrate that our modeling formulation is qualitatively consistent with observed trends for retention with colloid size and concentration, grain size, and velocity for many systems. The model provides a clear conceptual explanation for the causes of hyperexponential, exponential, uniform, and nonmonotonic retention profiles without invoking hypotheses with regard to colloid heterogeneity, aggregation, or multiple deposition rates. Furthermore, the model formulation and research presented herein helps to identify areas where additional research and theory development are still needed.