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Title: Modeling colloid and microorganism transport and release with transients in solution ionic strength

Author
item Bradford, Scott
item TORKZABAN, SAEED - University Of California
item KIM, HYUNJUNG - Chonbuk National University
item SIMUNEK, JIRI - University Of California

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/20/2012
Publication Date: 9/7/2012
Citation: Bradford, S.A., Torkzaban, S., Kim, H., Simunek, J. 2012. Modeling colloid and microorganism transport and release with transients in solution ionic strength. Water Resources Research. doi:10.1029/2012WR012468.

Interpretive Summary: Mathematical models to simulate the movement of pathogens through aquifers and soils frequently do not consider the significant influence of transients in solution ionic strength (IS) and velocity on pathogen transport and release. The objective of this work is to modify a sophisticated transport model and to develop theory to mechanistically account for the transport, retention, and release of pathogens with transients in IS and velocity. The calibrated model provided a satisfactory description of the observed release behavior for a range of microbe types and sizes. Furthermore, analysis of fitted model parameters indicates that microscopic heterogeneities on the soil play an important role in pathogen interactions, especially for smaller sized organisms. This information will be of interest to scientists and engineers concerned with predicting the fate of pathogens in soils and aquifers.

Technical Abstract: The transport and fate of colloids, microorganisms, and nanoparticles in subsurface environments is strongly influenced by transients in solution ionic strength (IS). A sophisticated dual-permeability transport model that is capable of simulating exponential, hyperexponential, uniform, and nonmonotonic retention profiles is modified and theory is developed to mechanistically account for the transport, retention, and release of colloids with transients in IS. In particular, colloid release in the model is directly related to the balance of applied hydrodynamic and resisting adhesive torques that determines the fraction of the solid surface area that contributes to colloid immobilization (Sf). The colloid sticking efficiency (a) and Sf are explicit functions of IS that determine the rates of colloid interaction with the solid, immobilization on the solid, colloid release from the solid and back into the bulk aqueous phase, and the maximum amount of colloid retention. The developed model is used to analyze experimental transport and release data with transients in IS for 1.1 and 0.11 µm latex microspheres, E. coli D21g, and coliphage fX174. Comparison of experimental values of a(IS) and Sf(IS) with predictions based on mean interaction energies indicates that predictions need to account for the influence of physical and/or chemical heterogeneity on colloid interaction and immobilization. Furthermore, experimental values of Sf(IS) exhibited hysteresis with IS, especially for smaller colloids, due to microscopic heterogeneity. A sensitivity analysis indicates that colloid release with IS transients is not diffusion controlled, but rather occurs rapidly and with low levels of dispersion. The calibrated model provided a satisfactory description of the observed release behavior for a range of colloid types and sizes, and a solid theoretical foundation to develop predictions for the influence of solution chemistry on the transport, retention, and release of colloids.