Preferential flow paths

System failure for in situ bioremediation efforts is often the result of ineffective transport of nutrients and electron acceptors due to channeling into preferential flow paths, heterogeneities, adsorption, biological utilization, and/or chemical reactions in the soil. Many of these problems can be overcome using electric fields for transport and injection instead of conventional groundwater injection by hydraulic techniques. 

Cracks and root zones may lead to preferential flow paths resulting in channeling (Borkovec et al., 1991). 

To decompact, and relax the resin bed thereby reducing pressure drop and eliminating any tendency towards encouraging preferential flow paths, or channelling, which would otherwise lead to a reduced utilization of resin exchange capacity and premature column breakthrough. 

The longitudinal fluxes that may arise from the preferential flow paths of water and solutes along large root channels are even less well documented than the radial fluxes of trace elements in the rhizosphere. It is well known that roots tend to colonize former biopores, as indicated in Section 4.2. As many of those macropores, especially vertical/subvertical biopores created by earthworms or root activity, are prone to preferential flow during re-saturation events, they may complicate our understanding of (1) the directions of trace elements and water fluxes, and ultimately, (2) the origin of the solutes circulating in the root environment. This process has received virtually no attention so far, however, possibly because of the practical difficulties associated with its quantitative assessment. 

In-line banks of high-finned tubes are not usually used in air-cooled heat-transfer equipment because the preferential flow path between the fin tips of adjacent tubes allows flow bypassing and reduces the apparent heat-transfer coefficient. This effect tends to disappear in the deeper tube banks used in convection sections of fired heaters. 

FIGURE 9. Observed preferential flow in an undisturbed sandy lysimeter. Results of a brilliant blue tracer experiment is shown. A discontinuous clay layer is situated at a depth of 20-30 cm causing the brilliant blue to funnel in a preferential flow path. (Source Javaux et al., 2005). 

In some cases, the metal-organic is a solid which sublimates. In this case mass transport can be much more difficult to calculate. Several works have examined sublimation and diffusion sources to calculate mass transport. Specific problems are that gas passing over a solid bed does not necessarily yield maximum transport efficiency or gases passing through a solid tend to channel (develop preferential flow paths through the material), and flux varies in time. To minimize this problem, first the internal bubbler configuration may be modified in order to maximize the surface area. Secondly, bub-... 

Shear zones are of considerable thickness at the area (5 to 20 m). At the intersection with tunnels, they display major outflows indicating their relevance as preferential flow paths. Lamprophyre dykes have also considerable dimensions (thickness up to several meters), although their relevance as preferential flow paths is not as important as shear zones. A preferential flow path within these dykes is the contact surface between the lamprophyre and the host rock. Figure 2 provides a geological cross section of the Grimsel area and shows the position of the FEBEX drift, bounded by two main shear zones... 

To density a bulk material by expressing air from the voids, it is essential that the bulk material is not in a fluidized condition, otherwise the material will leak backwards through the screw form. When the supply material is in a highly dilated state, the feed hopper design should be of mass flow form with sufficient residence time for the material to de-aerate to a stable flow condition. Note that a mass flow hopper will not prevent the development of a preferential flow path for a bulk material in a fluidized condition. If necessary, techniques to accelerate de-aeration should be employed to secure a bulk condition that is amenable to screw compaction, e.g. vibrating rod frames, as described by the author in Bulk Solids Handling, 1986, Vol. 6, No. 1, p. 77. 

The amount, distribution and placement of the instruments installed in the centrifuge models and the flexibility of the coimecting cables are carefully chosen to minimize the disturbance on soil behavior. In general, large concentration of instruments was avoided and cables were positioned so that soil reinforcement and creation of preferential flow paths was hampered. The loading of the model was carefully performed to minimize the 1-g vibrations induced in the model, which could affect the instruments position. 

Recent tritium data from Wells 199-N-67 and 199-N-69 (Color Illustration 2-2), however, show a different pattern. These data indicate significant contamination at depth, and in both Wells 199-N-67 and 199-N-69 higher levels of contamination at depth than at water table. For example, tritium activity observed in Wells 199-N-67 and 199-N-69 in November/December 1989 was 42,600 pCi/L and 78,400 pCi/L, respectively. The higher tritium levels at depth in the vicinity of 116-N-l may indicate a deeper, preferential flow path for discharge from 116-N-3, particularly in the most direct pathway towards the river. The Sr activity associated with discharges to 116-N-3 is presumably not observed at this point in the migration pathway because it was previously removed through adsorption in the immediate vicinity of the crib. 

Oostindie, K., Dekker, L.W., Wesseling, J.G., Ritsema, C.J., 2008. Soil surfactant stops water repellency and preferential flow paths. Soil Use Manage. 24,409. 

In some other successful examples, zeolite nanoparticles have been incorporated into a polymer matrix to form a thin-film nanocomposite RO membrane and to create a preferential flow path for water molecules, leading to enhanced water transport through the membrane [64,65]. Use of zeolite in the development of TFN for RO was first reported by Hoek and co-workers [66]. Similarly, Jeong et al. [64] prepared a thin-film RO nanocomposite membrane by interfacial in situ polymerization on porous polysulfone support, in which NaA zeolite nanoparticles were incorporated into a thin PA film. Introduction of zeolite nanoparticles into a conventional PA RO thin film has enhanced flux to more than double of the conventional membrane with a salt rejection of 99.7%, which is attributed to the smoother and more hydrophilic negatively charged surface. Silica nanoparticles of various sizes have also been incorporated into a PA polymer matrix for RO desalination [67]. Presence of silica nanoparticles was found to remarkably modify the PA network structure, and subsequently the pore structure and transport properties with only 1-2 wt% of silica, a membrane was fabricated with significantly enhanced flux and salt rejection. 

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