Water and Solute Transport Processes

Raoult-Wack, A.L., Guilbert, S., Le Maguer, M., and Rios, G. 1991. Simultaneous water and solute transport in shrinking media. 1. Application to dewatering and impregnation soaking process analysis (osmotic dehydration). Dry. Technol 9, 589-612. 

Nielsen, D. R., M. Th. van Genuehten, and J. W. Biggar. 1986. Water flow and solute transport processes in the unsaturated zone. Water Rec. Res. 22 89S—108S. 

It is also possible (at least, in a formal way) to apply a fractional version of Richards equation to simulate one-dimensional water transport in horizontal columns. We were able to fit the FADE to data on horizontal water infiltration (data not shown). However, the parameter a had to be set to values greater than two to fit the experimental data. This range of a is theoretically unjustified (Benson et al., 1999 Meerschaert et al., 1999). This example serves as a reminder about the danger of drawing analogies between water and solute transport in soils, since the underlying physical processes are different, Particles of soil water moving faster than others are affected by the structure of pore surfaces and move in films rather than in bulk volume by convection. One possible way to model the water transport is to use the diffusivity model proposed by Jumarie (1992) ... 

Pachepsky, Y., D.A. Benson, and DJ. Timlin. 2001. Transport of water and solutes in soils as in fractal porous media, p. 51-75. In H.M. Selim and D.L. Sparks (ed.) Physical and chemical processes of water and solute transport /retention in soil. SSSA Special Publ. 56, Madison, WI. 

Physical and Chemical Processes of Water and Solute Transport/Retention in Soil... 

Apple cubes subjected to osmotic process and treated by ultrasound, dewatered faster than the nontreated ones [20]. Water and solute transport rates were significantly higher in sonicated samples in comparison with those not sonicated during osmosis. 

Horne, R.A., Courant, R.A., and Johnson, D.S. (1966) The dependence of ion-, proton-, water and electron transport processes on solvent stmc-ture in aqueous electrolyte solutions. Ekctrochim. Acta, 11 (8), 987-996. 

Eor pesticides to leach to groundwater, it may be necessary for preferential flow through macropores to dominate the sorption processes that control pesticide leaching to groundwater. Several studies have demonstrated that large continuous macropores exist in soil and provide pathways for rapid movement of water solutes. Increased permeabiUty, percolation, and solute transport can result from increased porosity, especially in no-tiUage systems where pore stmcture is stiU intact at the soil surface (70). Plant roots are important in creation and stabilization of soil macropores (71). 

Rouen D, Scher H, Blunt M (1997) On the structure and flow processes in the capillary fringe of phreatic aquifers. Transp Porous Media 28 159-180 Rose CW (1993) The transport of adsorbed chemicals in eroded sediments. In Russo D, Dagan G (eds) Water flow and solute transport in soils. Springer, Heidelberg, pp 180-199 Rosenberry DO, Winter TC (1997) Dynamics of water-table fluctuations in an upland between two prairie-pothole wetlands in North Dakota. J Hydrol 191 266-289 Russo D (1997) On the estimation of parameters of log-unsaturated conductivity covariance from solute transport data. Adv Water Resour 20 191-205 Russo D, Toiber-Yasur 1, Laufer A, Yaron B (1998) Numerical analysis of field scale transport of bromacU. Adv Water Resour 21 637-647... 

We may conclude that our findings support independent water and salt permeation processes, and suggest that the salt permeation is governed by a solution-diffusion transport mechanism. 

In porous media the flow of water and the transport of solutes is complex and three-dimensional on all scales (Fig. 25.1). A one-dimensional description needs an empirical correction that takes account of the three-dimensional structure of the flow. Due to the different length and irregular shape of the individual pore channels, the flow time between two (macroscopically separated) locations varies from one channel to another. As discussed for rivers (Section 24.2), this causes dispersion, the so-called interpore dispersion. In addition, the nonuniform velocity distribution within individual channels is responsible for intrapore dispersion. Finally, molecular diffusion along the direction of the main flow also contributes to the longitudinal dispersion/ diffusion process. For simplicity, transversal diffusion (as discussed for rivers) is not considered here. The discussion is limited to the one-dimensional linear case for which simple calculations without sophisticated computer programs are possible. 

Closely linked to its extraordinary solvent capacities is water s role in transporting dissolved materials throughout the organism. With the exception of air-filled channels like the tracheal systems of insects, most of the transport processes of organisms involve movement of dissolved solutes. Diffusion of solutes within water is rapid, as is the translational and rotational movement of water itself. The extensive networks of hydrogen bonds that form among water molecules and between water and solutes do not impede this dynamic move-... 

Considering the membrane process as a binary system, the transport of solvent (e.g., water), and solute are involved. Designating solute and solvent by subscripts A and B, Eq. (5) can be written for solvent B as... 

The interactions of the chemical kinetic and macroscopic transport processes depend to a variable extent on each of the nine couples given in Table 1. For example, transport of solutes by water runoff is a result of atmospheric precipitation, land exposure to water, chemical reactivity of solids in an aqueous solution, and flow of water over the continental surface. The net result of this process is controlled by coupling among the physical, hydrological and chemical entities in Table 1, as shown below ... 

A number of experimental studies have established that both microbial and chemical degradation can be approximately described by first-order kinetics (24). Most pesticide models employ such an approach. As with linear sorption, this relatively naive representation of a fundamentally more complicated process is a simplifying assumption to make mathematical solutions possible and data requirements reasonable. Implicit in the assumption is the belief that the accuracy of simulation of pesticide fate is more dependent upon other factors than a very precise representation of the degradation process. These factors include spatial and temporal variability of the degradation process itself as affected by water, temperature, substrate, and pH, and variability in the transport of pesticide through the soil profile. There is little information to substantiate this assumption, although some field experiments on water and solute movement (discussed below) indicate it to be reasonable at this point in model development. 

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