Submitted to: Water Resources Research
Publication Type: Other
Publication Acceptance Date: 8/7/2009
Publication Date: 9/25/2009
Citation: Torkzaban, S., Walker, S.L., Bradford, S.A. 2009. Reply to comment by William P. Johnson et al. on ‘‘Transport and fate of bacteria in porous media: Coupled effects of chemical conditions and pore space geometry’’. Water Resources Research. 45:W09604 Interpretive Summary: This article addresses the nature of colloid and microorganism association with solid surfaces in the presence of weak chemical interactions. There are currently different opinions in the literature on this topic, and we discuss the two competing theories and provide additional evidence to support our view of the associated forces and torques that act on colloids/microorganisms under these conditions. This information will be of use to scientists and engineers concerned with predicting the fate of microorganisms in the environment.
Technical Abstract: We are grateful for this opportunity to address the comment of Johnson et al.  on our manuscript. Their comment concerns the nature of colloid association with solid surfaces via the secondary energy minimum. There are currently different opinions in the literature on this topic. Johnson et al.  have assumed that colloids associated with collector surfaces via the secondary minimum are ‘‘freely mobile’’ in the presence of fluid drag and can therefore only be immobilized in stagnation regions. In contrast, we have applied a torque balance approach to determine locations where colloid retention is theoretically possible when the driving (hydrodynamic) torque is less than the resisting (adhesive) torque [Bradford et al., 2007; Torkzaban et al., 2007, 2008]. Each of these points of view will be discussed below.