Flow in the Unsaturated Zone

The saturated zone is the region within which chemical pollution is generally of most concern because the saturated zone is the source of drinking water. Before pollutant chemicals spilled or disposed of on the land surface can reach the saturated zone, however, they must first move through the unsaturated zone (vadose zone). While vadose zone contamination is of concern per se, the fate and transport of chemicals in the vadose zone also are of interest because they affect the transmission of chemicals to the saturated zone. 

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Zhang K., Wu Y.S., et al. Parallel commuting simulation of fluid flow in the unsaturated zone of Yucca Mountain, Nevada. 2003 Journal of Contaminant Hydrology.62-... 

It is more complex to model water flow in the unsaturated zone than in the saturated zone because water flow in the vadose zone occurs only via water-filled pores, and the fraction of the pore spaces filled with water (the percent saturation) is highly variable. The water content 0 in a porous medium refers to the volumetric fraction that contains water it can range between zero and n, the porosity. The relationship between hydraulic conductivity and water content is complex and difficult to predict therefore, it is usually measured experimentally and expressed in the form of a K-6 curve, as shown in Fig. 3.21. One major complication in the K-6 relationship is hysteresis hydraulic conductivity differs depending on whether the porous media most recently have been dried or wetted to obtain a given moisture content. For dry material, regardless of its texture, hydraulic conductivity is low. 

For vapor to move in the unsaturated zone, the soil formations must be sufficiently dry to permit the interconnection of air passages among the soil pores. Vapor concentration and vapor flow govern its movement. Vapor can move by diffusion from areas of higher concentration to areas of lower concentration and ultimately to the atmosphere. Therefore, the transportation of the vapor phase of gasoline components in the unsaturated zone can pose a significant health and safety threat because of inhalation and explosion potential. 

There are some important differences between the behavior and flow of water in the unsaturated zone (vadose zone) above the water table and the saturated zone below the water table. The surface tension of water or other fluids becomes important when there is a gas phase in contact with the fluid phase and the solid phase. If the total volume of a porous medium (VT) is divided into the volume of the solid portion (Vs,),... 

The viscosity of separate LNAPL products varies significantly, ranging from far less to many times that of water. Flow of LNAPL in the unsaturated zone is largely dependent upon viscosity and soil grain size. Finer-grained materials have a higher residual saturation of water, which restricts the number of pores available for LNAPL entry in this region. 

The transfer of chemical molecules from oil to water is most often a surface area phenomenon caused by kinetic activity of the molecules. At the interface between the liquids (either static or moving), oil molecules (i.e., benzene, hexane, etc.) have a tendency to disperse from a high concentration (100% oil) to a low concentration (100% water) according to the functions of solubihty, molecular size, molecular shape, ionic properties, and several other related factors. The rate of dispersion across this interface boundary is controlled largely by temperature and contact surface area. If the two fluids are static (i.e., no flow), an equilibrium concentration will develop between them and further dispersion across the interface will not occur. This situation is fairly common in the unsaturated zone. 

In the unsaturated zone, soil venting is an effective and inexpensive procedure to provide air. When the bulk of the product is held above the water table, supplies of air can be provided by the use of vacuum wells located in this zone. The flow of air is drawn either from the subsurface to the well (Figure 13.8a) or through vent wells (Figure 13.8b). The alternative benefit of this approach is that volatile portions of the product are removed by the lower-pressure operation. Soil vapor venting is particularly well suited to less permeable silt and clay soils. 

Calculation of the flow in the saturated portion of the subsurface is generally much easier than that in the unsaturated zone. However, calculation of flow in either requires a fundamental understanding of groundwater pressure and energy. 

Finally, Chapters 14 and 15 address the remediation of the unsaturated zone. In Chapter 14, researchers from the University of Virginia study the effects of natural atmospheric pressure variations on the flow of air into and out of the unsaturated zone at the Picatinny Arsenal in northern New Jersey. This barometric pumping contributes to the natural remediation of the shallow, trichloroethylene-contaminated groundwater. In the last chapter of the book, Richard Meixner and co-workers present a detailed field study documenting the effectiveness of soil-vapor extraction to remediate gasoline hydrocarbons in the unsaturated zone. 

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. 

Dreier, R.B., Solomon, D.K., and Beaudoin, C.M., Fracture characterization in the unsaturated zone of a shallow land burial facility, in Flow and Transport Through Unsaturated Rock, Evans, D.D. and Nicholson, T.J., Eds., Geophys. Monogr., 42, 51, 1987. 

In a classic paper, Pillsbury (1981) described how recycling of salts via irrigation and agricultural return flow controls the salinity of downstream rivers in arid zones. Once the natural salt balance is disturbed and salts begin to accumulate, either in the unsaturated zone or in drainage waters, the salinity increases. The salinity of the Colorado River is derived from a century of activity that includes upstream diversion of freshwater, massive irrigation, evapotranspiration and salt accumulation in the soil, and return of saline drainage back to the river (Pillsbury, 1981). Similarly, the rise of the salinity in the Nile Delta has been attributed to a disturbance of the natural salts balance after the construction of Assuan dam and the reduction of the natural outflow of salts from the Nile River to... 

In the unsaturated zone, water movement is caused by both gravity and by pore water pressure differences arising from variations in the water content from one location to another water may even move vertically upward through the soil profile if evaporation or plant roots remove it from the near-surface soil. Water flow is impeded, however, by the fact that water can only move via the relatively thin film of water coating the particles. Such flow contrasts with water flow in the saturated zone, where water can move through the entire pore volume and occupy the full cross-sectional area of the pore spaces. 

Flow is more difficult to model in the unsaturated zone because hydraulic... 

Water flow in the vadose zone also is much more difficult to measure than is water flow in the saturated zone. Piezometers placed in unsaturated porous media do not fill with water, so head is less readily measured than it is in saturated media, and thus the direction and magnitude of the hydraulic gradient are more difficult to determine. For values of ifj between 0 and — 1 atm, tensiometers may be employed to directly measure pore water pressure. For very low values of i/j, other techniques may be used, such as the indirect assessment of if from measurements of the humidity of the air in the pore spaces. 

Bottrell, S.H. Atkinson, T.C. (1992) Tracer study of flow and storage in the unsaturated zone of a karstic limestone aquifer. In Hotzl, H. Werner, A. (Eds) Tracer Hydrology. Rotterdam Balkema, pp. 207-211. 

Flow in the unsaturated vadose zone is best described using values of hydraulic conductivity obtained through experimental observation. Not surprisingly, prediction of unsaturated flow rates is considerably less satisfactory than prediction of rates in the saturated zone. 

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