Soil water transport diffusion

It is probable that capillary flow of water contributes to transport in the soil. For example, a rate of 7 cm/year would yield an equivalent water velocity of 8 x 10-6 m/h, which exceeds the water diffusion rate by a factor of four. For illustrative purposes we thus select a water transport velocity or coefficient U6 in the soil of 10 x 10 6 m/h, recognizing that this will vary with rainfall characteristics and soil type. These soil processes are in parallel with boundary layer diffusion in series, so the final equations are... 

Contaminants in the soil compartment are associated with the soil, water, air, and biota phases present. Transport of the contaminant, therefore, can occur within the water and air phases by advection, diffusion, or dispersion, as previously described. In addition to these processes, chemicals dissolved in soil water are transported by wicking and percolation in the unsaturated zone.26 Chemicals can be transported in soil air by a process known as barometric pumping that is caused by sporadic changes in atmospheric pressure and soil-water displacement. Relevant physical properties of the soil matrix that are useful in modeling transport of a chemical include its hydraulic conductivity and tortuosity. The dif-fusivities of the chemicals in air and water are also used for this purpose. 

The Rooting-Zone Soil Root-zone soil includes the A horizon below the surface layer. The roots of most plants are confined within the first meter of soil depth. In agricultural lands, the depth of plowing is 15-25 cm. In addition, the diffusion depth, which is the depth below which a contaminant is unlikely to escape by diffusion, is on the order of a meter or less for all but the most volatile contaminants. Soil-water content in the root zone is somewhat higher than that in surface soils. The presence of clay in this layer serves to retain water. Contaminants in root-zone soil are transported upward by diffusion, volatilization, root uptake, and capillary motion of water transported downward by diffusion and leaching and transformed chemically primarily by biodegradation or hydrolysis. 

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) ... 

This conclusion is confirmed by the fact that the calculated flux would be 356 ng cm day based on water movement through the soil at 50% RH of 0.274 mL cm day and a Ce of 1300 ng mL in equilibrium with a soil concentration of 10 JLg Reducing the relative humidity to 15% increased the water movement to 0.501 mL cm day and the mass transport of Lindane to 651 ng cm day Note that with dry nitrogen the flux increased to a point, D, and then decreased, which was doubtless due to the soil surface drying out increasing the sorption. Thus evaporation of a compound incorporated in soil will depend first on diffusion through the soil and mass transport to the surface in the soil water when the water is evaporating. Both processes are influenced by properties of the soil and the chemical. 

Methane and carbon dioxide produced in soils are transported into the atmosphere by diffusion and mass flow via two pathways (1) the aerenchyma tissues of plant roots and stems and (2) flux from soil to the overlying water column (Figure 5.61). Gas exchange in plants is discussed in detail in Chapter 7. Carbon dioxide is highly soluble and undergoes various chemical reactions, and it may be difficult to estimate flux accurately without considering aU associated reactions. Because of the potency (on molecule-to-molecule basis, methane absorbs 25 times as much infrared radiation as carbon dioxide) of methane as greenhouse gas, we will focus our discussion on methane emissions from wetlands. 

Advective flux of solutes in wetlands can result from pressure gradients that force pore water from soil pores to overlying water, carrying solutes and line particulate matter with it across the soil-water interfaces. The flux is influenced by hydraulic gradients, associated water, and adjacent upland area (Figure 14.3). Pore water movement and its transport could be significant in sandy, permeable soils and sediments. In low-permeability soils such as those with high silt and clay contents, molecular diffusion (see Section 14.3) and bioturbation (see Section 14.4) can be the major transport processes. 

Besides electrokinetic transport, chemical reactions also occur at the electrode surfaces (i.e., water electrolysis reactions with production of at the anode and OH at the cathode). Common mass-transport mechanisms like diffusion or convection and physical and chemical interactions of the species with the medium also occur. In a low-permeable porous medium under an electrical field, the major transport mechanism through the soil matrix during treatment for nonionic chemical species consists mainly of electro-osmosis, electrophoresis, molecular diffusion, hydrodynamic dispersion (molecular diffusion and dispersion varying with the heterogeneity of soils and fluid velocity [8]), sorption/ desorption, and chemical or biochemical reactions. Since related experiments are conducted in a relatively short period of time, the chemical and biochemical reactions that occur in the soil water are neglected [9]. 

F.J. Molz and G.M. Homberger, Water transport through plant tissues in the presence of a diffusible solute. Soil Sci. Soc. Am. Proc., 37 SS3 (1973). 

Soils have been the repository of much of the world s refuse. Toxic substances from this refuse may be transported by convection or diffusion to underground reservoirs, lakes, and streams. In this way, our supplies of drinking water are jeopardized. Neither convection nor diffusion will occur appreciably in the soil in the absence of water. Soil water is in close contact with the surfaces of soil particles, especially the surfaces of clay minerals. If these surfaces modify the water, its role as a medium of transport for toxic substances will be affected. It is important, therefore, that we know how such surfaces interact with the water adjacent to them. The present paper reports some of the author s observations on clay/water interaction and indicates how this interaction affects the convective and diffusive transport of ions and neutral molecules. 

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