Unsaturated zones

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

Models for transport distinguish between the unsaturated zone and the saturated zone, that below the water table. There the underground water moves slowly through the sod or rock according to porosity and gradient, or the extent of fractures. A retardation effect slows the motion of contaminant by large factors in the case of heavy metals. For low level waste, a variety of dose calculations are made for direct and indirect human body uptake of water. Performance assessment methodology is described in Reference 22. 

On the downside, various studies (23—25) have shown that hydrogen peroxide decomposes rapidly after soil contact, it is cytotoxic at a 3% solution and unless stabilized, oxygen bubbles can escape prematurely through the unsaturated zone before they have a chance to disperse well in the ground water. 

Bioventing. Bioventing is soil venting that enhances biodegradation while extracting volatile compounds from the unsaturated zone. 

The most commonly used remediation technique for the recovery of organic contaminants from ground water has been pump- and-treat, which recovers contaminants dissolved in the aqueous phase. In this regard, the application of carbon adsorption has found extensive, but not exclusive use. Vacuum extraction (also called soil venting) has also become popular for removal of volatile organic contaminants from the unsaturated zone in the gaseous phase. Both of these techniques can, in the initial remediation phase, rapidly recover contaminants at concentrations approximately equal to the solubility limit (pump-and-treat), or the maximum gas phase concentration of the contaminant (vacuum extraction). The... 

Vadose zone Unsaturated zone of soil above the groundwater, extending from the bottom of the eapillary fringe all the way to the soil surfaee. 

Suction lysimeters are required for some field-scale groundwater monitoring studies to monitor the transport of compounds of interest through the unsaturated zone. Unlike monitoring wells or water supply wells that sample water from the saturated zone, suction lysimeters sample water from the unsaturated zone. This section provides a summary of the installation and sampling procedures for pressure-vacuum suction lysimeters. A detailed discussion of unsaturated zone sampling devices is available elsewhere. 

Soil vapor extraction (SVE) is a relatively new yet widely applied technology for the remediation of soils contaminated with volatile organic compounds (VOC) in the unsaturated zone above the water table (vadose zone). The process consists of generating an airstream through the contaminated soil subsurface in order to enhance the volatilization of organic contaminants and thus remove them from the soil matrix.913... 

Figure 14.1 presents the main components of a typical in situ SVE system.910 Vertical extraction wells are installed inside the contaminated zone at appropriate distances from one another. The SVE wells are typically constructed of PVC pipe, with a screened interval, which is placed within the contaminated zone. The wells are connected to blowers or vacuum pumps, which induce a continuous airflow through the pores of the unsaturated zone. The soil surface is sometimes covered with an impermeable seal, made from high density polyethylene (HDPE) or bentonite clay for example, to prevent the vertical influx of air from the surface, which might cause short-circuiting problems, and promote horizontal gas flow through the contaminated area. The airstream, which contains the contaminant vapors, passes initially through an air-water separation unit to remove the entrained moisture and is then directed to the gas treatment unit, where the contaminants are thermally destroyed or removed by adsorption. 

The pump-and-treat method is comparable to soil flushing. In fact, the pump-and-treat method can treat both groundwater and aquifer soil at the same time, and can also be directly applied to unsaturated soil zones. The soil flushing method is mainly considered as a treatment in unsaturated zones. 

Water Movement in Unsaturated Zone of Soil Formation. 701... 

Subsurface formations can be divided into the overburden (unconsolidated) and bedrock according to its solidarity. The upper subsurface can be further divided into the unsaturated zone and the saturated zone depending on pore structure and moisture saturation. The saturated zone is the zone in which the voids in the rock or soil are filled with water at a pressure greater than atmospheric. The water table is at the top of a saturated zone in an unconfined aquifer. The unsaturated zone is the zone between the land surface and the water table, and is also called the zone of aeration or the vadose zone. The pore spaces contain water at less than atmospheric pressure, air, and other gases. This zone is unsaturated except during periods of heavy infiltration. 

In the lower region of the unsaturated zone, immediately above the water table, is the capillary fringe, where water is drawn upward by capillary attraction. Above the capillary fringe, moisture coats the solid surfaces of the soil or rock particles. If the liquid coating becomes too thick to be held by surface tension, a droplet will pull away and be drawn downward by gravity. The fluid can also evaporate and move through the air space in the pores as water vapor. 

The moisture in the upper unsaturated zone can be affected by plant transpiration and atmospheric conditions. Some scholars classify the unsaturated zone into subzones such as the soil water zone and the intermediate zone.23... 

A typical profile of the pressure potential of soil moisture tested by a tensometer across the free-water surface shows a negative pressure (lower than atmosphere pressure) in the capillary zone (Figure 18.2). The negative pressure in the capillary zone indicates that the capillary zone belongs to the unsaturated zone. 

In an unsaturated zone, the capillary force becomes predominant, and the pressure gradient becomes a suction gradient. Hydraulic conductivity is no longer constant, but is a function of the water content or suction, which is greatest in value when the soil is saturated and decreases in value steeply when the soil water suction increases and the soil loses moisture. 

There are many components of gasoline that readily dissolve in water and are transported as solutes in the groundwater. Most gasoline products are volatile and can release gas into the soil void in gaseous form, particularly in the unsaturated zone. Besides these three forms, gasoline components can be adsorbed by the soil matrix and exist in the soil as adsorbates. 

The movement of free product is dependent on soil permeability and moisture. The released gasoline first infiltrates downward vertically, mainly governed by the gravity force, into and through the unsaturated zone, then reaches the water table. If there is an impermeable layer above the water table, the free product will be purged and may not reach the water table directly. In the... 

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. 

The level of vapor movement in the unsaturated zone is much less important than transport in liquid form. However, this might not be true if the water content of the soil is very low or if there is a strong temperature gradient. The movement of vapor through the unsaturated zone is a function of temperature, humidity gradients, and molecular diffusion coefficients for water vapor in the soil. 

In the upper unsaturated zone (above the capillary fringe), multiphase movement and transformation are typical. Vapor-phase gasoline becomes more important gasoline adsorption by soil, dissolution in pore water, and free product in the pore space can also be significant. 

Most contaminations of aquifers are a result of material being released above the saturated zone. The contaminant pumping method is limited to the cleanup of the saturated zone. Contaminants in the unsaturated zone can still be a source of future contamination. In situ bioremediation techniques can also be designed to clean up the unsaturated zone simultaneously. 

Soil models tend to be based on first-order kinetics thus, they employ only first-order rate constants with no ability to correct these constants for environmental conditions in the simulated environment which differ from the experimental conditions. This limitation is both for reasons of expediency and due to a lack of the data required for alternative approaches. In evaluating and choosing appropriate unsaturated zone models, the type, flexibility, and suitability of methods used to specify needed parameters should be considered. 

Many models are available in the literature, and some of these models can be applied only to specific environmental situations and only for chemicals for which they were developed. Obviously, all models do not provide the same numerical results when employed to provide answers to a particular problem, so care must be taken in choosing an appropriate unsaturated zone model, or when specifying a volatilization rate. For modeling algorithms, and numerical examples the reader is referred to the work of Lyman et al. (6), Bonazountas Wagner (5) and others listed in these references. 

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-... 

NY Effluent standards Maximum allowable concentrations into saturated or unsaturated zones 0.05 mg/L CELDS 1990a... 

However, so far there is a lack of knowledge concerning the potential geochemical and ecological impact of the artificial recharge, in both unsaturated zone and the aquifer itself. Another measure can be the relocation of certain crops in higher altitude because of a temperature increase. 

Tritium extracted from soil moisture in the unsaturated zone at various depths below the surface has been used to infer the progress of recharge of underlying aquifers. Studies of recharge in arid and semi arid zones where water moves very slowly in a downward direction have been particularly instructive [20, 32-35]. 

Smith, D. B., Wearn, P. L., Richards, H. J., Rowe, P. C., Water movement in the unsaturated zone of high and low permeability strata by measuring natural lithium, In Isotope Hydrology 1970, Vienna, Interna. Atomic Energy Assoc., p. 73-87, 1970. 

Wood, W. 1973. A technique using porous cups for water sampling at any depth in the unsaturated zone. Water Resources Research, 9, 486-488. 

Kerfoot [6] examined the performance of a grab sampling technique for soil-gas measurement analyses, at a site with groundwater known to be contaminated with chloroform. The study assessed the correlation between soil-gas and groundwater analyses with chloroform as a model volatile organic compound. Chloroform concentration in soil gas increased linearly with depth in the unsaturated zone. 

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