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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...

Wheat grown after Brassica crops normally yields more than wheat grown after wheat. Previously we reviewed 33 experiments and concluded that wheat after canola yielded about 19 % more than wheat after wheat and that the gross margin of a canola-wheat sequence was 27 % greater than a wheat-wheat sequence. Further analysis of other published, replicated experiments revealed that the mean increase in wheat yield after brassicas was better represented as a fixed amount rather than a percentage. The mean yield benefit of canola (B napus) to subsequent wheat yield was similar (0.8 t ha-1) over 180 experiments where wheat yield varied from 1.1 to 9.5 t ha -1. Over 36 experiments where canola and juncea canola (B juncea) were compared the break-crop effects were identical at 0.6 t ha -1 . Part of the reason for the additional wheat yield is increased uptake of soil water and nutrients, which may explain the fixed, rather than percentage, increase in mean yield as a response to the limited supply of these resources. The earlier conclusion about canola-wheat providing a 27 % increase in gross margin over wheat-wheat must be revised since the financial benefit is relatively greater at low levels of wheat yield. A canola-wheat sequence provided an 85% increase in gross margin over wheat-wheat at low (2 t ha -1 ) yield levels but only 3% at high (6 t ha -1 ) yield levels.

Groundwater contamination by nonaqueous liquids such as organic solvents and petroleum hydrocarbons frequently occurs as a result of surface spills, tank leaks, and improper disposal practices. This first of two papers examines the physics governing the emplacement and movement of a separate phase in porous media, the role of sorption, and the conditions necessary to mobilize a separate phase. The movement of the separate phase is controlled by capillary forces, and ganglia displacement by groundwater is not possible under reasonable hydraulic gradients. In addition, because of mass transfer limitations in liquid phase dissolution, groundwater extraction at contaminated sites is shown to be ineffective in removing the nonaqueous contaminant within a reasonable time frame. Therefore other means of mobilizing the trapped second phase are needed, steam displacement is proposed and steam front propagation through contaminated porous media is evaluated. The results of laboratory experiments supporting some of these analytical results are presented in the second paper (Hunt et al., this issue).