Water preferential flow paths

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. 

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. 

Figure 16 shows the results when 20 pore volumes of an emulsion having a 3.1- xm mean droplet size is injected into an 1170-mD sand pack and is followed by several pore volumes of water (ii). After emulsion injection, a permeability reduction of about 50% is observed. With water injection, the effluent concentration drops to 0 after one pore volume, whereas the permeability is unaltered. For this dilute emulsion, the droplets are captured in the porous medium, and this capture leads to blocking of the flow paths. Figure 16 shows that once the droplets are captured, they do not re-enter the flow stream, velocity being constant. Soo and Radke (ii) proposed the following physical interpretation for the results of Figure 15. Initially oil droplets are preferentially captured in the small-size pores, and as injection proceeds, more and more of the small pores become blocked. This blockage leads to a flow diversion toward larger size pores, and the rate...

At some point an area will develop where oil is preferentially removed perhaps because the porosity of the rock is greater, the oil less viscous, etc. In any case, the water flowing through this area will increase in volume and velocity further increasing the removal rate. Eventually, this area consumes much of the total water flow. Some recovery wells see much decreased water flow and therefore much less oil production. The area has become the path of least resistance to the water flow and because it steals most of the water flow, it is known in the industry as a thief zone. The development of such a thief zone is illustrated in Figure 12.74. Colloidal silica can be used to seal off such a zone and help restore higher production... 

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