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Biological and Environmental Engineering, Riley Robb Hall, Cornell University, Ithaca, NY 14853
* tss1{at}cornell.edu
Abbreviations: AW, air–water • AWS, air–water–solid • AWmS, air–water meniscus–solid interface
Wan and Tokunaga's (2005) comments give us an opportunity to relate our recent findings to our initial paper (Crist et al., 2004). The main purpose of our first paper (Crist et al., 2004) was "to describe a novel pore-scale visualization technique" and "help reconcile some inconsistencies among earlier studies," in particular by demonstrating the need to refine the widely held assumption used to explain observed breakthrough curves, namely that colloids are retained at air–water (AW) interfaces in unsaturated media. Contrary to the statement of Wan and Tokunaga (2005), attempting "to provide pore-scale insights into mechanisms responsible for controlling colloid transport in unsaturated porous media" was not the aim of the initial Crist et al. (2004) paper. We discussed insights into retention mechanisms in a subsequent paper (Crist et al., 2005) under conditions in which evaporation effects on colloid retention were greatly reduced by using steady-state flow. Furthermore, the length of the unsaturated zone in this subsequent paper was increased by using larger diameter sand particles and employing a steeper chamber slope. We are therefore disappointed that Wan and Tokunaga restricted their comments "for brevity" mainly to our 2004 paper.
It is of interest that we did not observe the colloid straining conceptualizations that Wan and Tokunaga (2005) depicted in their Fig. 2b ("with evaporation," where colloids form a wide band at air–water–solid (AWS) interfaces in pendular rings) unless colloid diameter and water film thickness were approximately the same. With some slight adjustments, we observed phenomena similar to their Fig. 2a, with one difference: while Wan and Tokunaga (1997)(p. 2418) envisioned that "colloids are constrained to rolling along the grain surfaces..." below critical saturation, in our case colloids were stagnant after they attached at the AWS interface, indicating a slightly different mechanism. Note that by "AWS interface" we refer to the location where the meniscus either meets the dry grain surface or diminishes in thickness to a thin water film covering the grain surface. In our recent work (Zevi et al., 2005) we have proposed the term air–water meniscus–solid interface (abbreviated AWmS) for this concept, denoting the region where the water meniscus ends by thinning to a thin film, which more correctly represents the interfaces observed in unsaturated media. Our equipment at the time of the Crist et al. (2004) experiments could not observe thin films. Wan and Tokunaga's (2005) reasons for distinguishing whether a thin film exists at the AWS interface are not clear to us at this time, although it could prove to be important in the future.
We are also puzzled by Wan and Tokunaga's (2005) comment that the "magnitude of [our] observations [of colloid retention at the AWS] are not likely to occur anywhere except within the most superficial few centimeters of soils." In this initial paper we did not claim (nor understand) that it was an important mechanism, noting that we "fully expected" not to find "the rings" of colloids in vertical columns. Only at the end of the paper did we suggest that retention at the AWS interface could reconcile the contrasting results of their earlier work, in which they found negatively charged colloids at the AW interface (Wan et al., 1994a, 1994b), but did not find them in air bubbles in their Wan and Tokunaga (2002) paper. In our subsequent work with both better experimental controls and fewer evaporation effects, we proved that the tentative findings of retention at the AWS in Crist et al. (2004) were both real and significant.
The comment of Wan and Tokunaga (2005) also claims that evaporation increases the advection of colloids to AWS contact lines where they are subsequently deposited. In this reply we show that although evaporation has an effect (as was noted in the original paper [Crist et al., 2004]; e.g., the legend of Fig. 6) in terms of increasing the color intensity of the visible bands at the AWS surfaces, it is clearly not the only process that influences colloid retention at the AWS interface. In the flowing water experiments described in Crist et al. (2005), the same pattern of immobilization of colloids near the AWS interface was clearly seen under conditions where any effect of evaporation would be negligible. The videos accompanying that paper visually document the high flux of water across grain surfaces and within pendular rings, and demonstrate that retention did not occur at the AW interface, but only at or near what we termed the AWS interface.
Since the initial set of experiments reported in Crist et al. (2004), we have continued experimentation with better visualization equipment, including a more powerful bright field microscope and a longer chamber to avoid problems that arise from the presence of capillary fringes in the field of visualization (Zevi et al., 2005). In this new set of experiments, we similarly found colloid attachment to the AWS (termed AWmS in that paper) interface. More importantly, colloid attachment occurred as soon as the colloids were added and before there was any opportunity for evaporation to exert an effect.
To further determine the relative significance of evaporation, we repeated the Crist et al. (2004) experiment in a larger chamber using a bright field microscope, but this time with a cover plate on the chamber to prevent evaporation. The microscope halogen light source produced less heat in the chamber than was the case with the light source used in Crist et al. (2004). The image in Fig. 1 (which is of only moderate quality due to the optical effects of the cover) clearly shows that colloids are attached at the AWS (AWmS) interface. Wan and Tokunaga's (2005) argument, if we understand it correctly, is that the movement of colloids to the AWS interface is driven only by evaporation. In contrast, Fig. 1 shows that colloid retention rapidly occurs at the AWS interface in the absence of evaporation. Bands in this case are less numerous than in Crist et al. (2004).
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Wan and Wilson's (1994a)(1994b) ground-breaking experiments should be credited for stimulating the discussion of colloid retention mechanisms in unsaturated media. That these early findings are being refined with better visualization equipment does not reduce the significance of their contributions. The aim of our 2004 manuscript was to facilitate this process by introducing a novel three-dimensional visualization system by employing translucent silica sand. We suggest, however, that readers who want to follow the advice of Wan and Tokunaga (2005) to "perform independent tests of their own" follow the procedures detailed in our recently published paper (Crist et al., 2005), or even better those in our most recent work (Zevi et al., 2005; contact us directly for additional experimental details when needed). These visualization tests can then also reveal under what conditions the conceptual model of film straining of Wan and Tokunaga (1997)(depicted in Fig. 2) would be valid. We are not aware of any direct visual evidence of these interesting model conceptualizations.
ACKNOWLEDGMENTS
Our work has been carried out by combining relatively small amounts of money from many sources: the U.S. Air Force allowed J.T. Crist to study at Cornell for two years; the Cornell Biocomplexity and Biogeochemistry Initiative (BBI) is providing support for Annette Dathe; a portion of John McCarthy's effort was supported by the U.S. Department of Energy's Environmental Management Science Program (DE-FG07-02ER63496); Rosemarie Fehrman was a National Science Foundation Research Experiences for Undergraduates (NSF-REU) fellow in the summer of 2004; and the Department of Biological and Environmental Engineering at Cornell University provided partial support for Yuniati Zevi.
REFERENCES
This article has been cited by other articles:
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  P. Sharma, H. M. Abdou, and M. Flury Effect of the Lower Boundary Condition and Flotation on Colloid Mobilization in Unsaturated Sandy Sediments Vadose Zone J., August 1, 2008; 7(3): 930 - 940. [Abstract] [Full Text] [PDF]
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S. A. Bradford and S. Torkzaban Colloid Transport and Retention in Unsaturated Porous Media: A Review of Interface-, Collector-, and Pore-Scale Processes and Models Vadose Zone J., May 27, 2008; 7(2): 667 - 681. [Abstract] [Full Text] [PDF]
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