The authors are to be commended for their thorough and timely review [McDowell-Boyer et al., 1986] on the subject of particle transport through porous media. It was felt, however, that they have not followed far enough their own streams of thoughts. Despite the detailed descriptions of current perceptions how particles get separated from the carrying liquid and how they interact with the porous media, little attention was paid to the fact why they can be transported over hundreds of meters. The authors mentioned the studies by Smith et el.[1985], who reported microbial breakthrough at the 0.3-m depth of undisturbed soil columns within 17-50 min. Gerba and Goyal's [1985] study is also cited according to which bacteria had moved over distances of more than 800 m. However, no discussion is offered about the discrepancy between these and many other reported observations on fast and farreaching particle transport in porous media and the lack of models dealing with this kind of transport. Corapcioglu and Haridas [1985], for instance, presented an approach to the transport of microorganisms through porous media. No matter how these authors stressed the flow parameters (they went to the permissible limits), their model strongly indicated that bacteria cannot be transported over distances exceeding 0.2 m under the commonly accepted constrains of the model.The reasons for these deficiencies in the perception of particle transport through porous media, as they are well represented in this review article, can be traced back to the early beginnings of groundwater hydrology. It was Darcy's [1856] cracks. Voids which support particle transport have diameters at least an order of magnitude greater than the particles that move. This movement results in readily recognizable accumulations of silt (siltants) or clay (argilans) coatings on the surfaces of the voids, eventually plugging them. Filter theory conceptualizes the porous media as homogenous, whereas microscopists conceptualize the same media as inhomogeneous [Jeans, 1986; Bullock et al., 1985]. Thus surface accumulation is, indeed, observed but at much smaller scales. Pores only 50/•m wide are often distinctly lined with clay particles. Apparently, water carrying them through those pores lost its dragging power due to loss of momentum. This 10ss can be caused by decreasing flow velocity, by the reduction of mass flow due to water sorption into the finer pores surrounding the flow paths, or by any combination of the two. Gerba and Biton [1984], for instance, reported from groundwater flow studies that the bulk of larger Escherichia coil appeared about 1 hour earlier in an observation well 160 m downstream from an injection well than the bulk of the much smaller coliphage f2. Likewise, Harvey et al. [1986] reported that the peak abundance during fractional breakthrough of carboxylated microspheres at 1.5 m from the point of injection was highest for the 1.2-/•m diameter size class, followed by the 0.7-and 0.2-#m microspheres. This indicates that some of the larger partic...

Particles in Saturated Porous MediaMany waterborne contaminants are particulate in nature or are associated with small particles or colloids. Thus, consideration of the mobility of colloids in porous media is necessary for an understanding of contaminant transport in groundwater. Immobilization of colloids in porous media has been studied extensively for water filtration applications, but the reverse process of mobilization is seldom considered. This study addresses the latter process. Theoretical treatment of the mobilization process in steady flow situations generally involves consideration of London-van der Waals attractive forces and electrical double-layer attractive or repulsive forces at the point of contact between media and particle surface. Laboratory sand columns were used to test the suitability of this approach with respect to observed mobilization under changing chemical conditions. Colloids deposited on the porous media surface were mobilized when exposed to lower ionic strength solution without disturbing the flow rate. Although results did not contradict theory qualitatively, further theoretical development is necessary to consider a broader range of surface forces likely to affect mobilization behavior. Flow rate dependence of mobilization is indicated and must be investigated further in order to understand its basis.

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