Saturation, zone

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. 

Steam-foaming agents that efficiently mobilize heavy cmde oil by heat transfer can reduce the residual oil saturation. This can increase foam stabihty and improve the diversion of subsequently injected steam into oil saturated zones thereby increasing oil recovery (204). 

U.S. EPA. Assessing UST Corrective Action Technologies Early Screening of Cleanup Technologies for the Saturated Zone, EPA/600/2-90/027, prepared by P. J. Reidy and co-workers, U.S. Environmental Protection Agency, Washington, D.C., 1990. 

The majority of trichloroethylene present on soil surfaces will volatilize to the atmosphere or leach into the subsurface. Once trichloroethylene leaches into the soil, it appears not to become chemically transformed or undergo covalent bonding with soil components. When trichloroethylene was absorbed onto kaolinite and bentonite, the nuclear magnetic resonance (NMR) spectra showed no evidence of chemical reactions (Jurkiewicz and Maciel 1995). Because trichloroethylene is a dense nonaqueous phase liquid, it can move through the imsaturated zone into the saturated zone where it can displace soil pore water (Wershaw et al. 1994). 

Trichloroethylene in soil and groundwater were found to be correlated (r 0.9994) in samples taken during well instillation at the U.S. Army Cold Regions Research and Engineering Laboratory in Hanover, NH (Hewitt and Shoop 1994). Concentrations of trichloroethylene in soil from the saturated zone were 0.008-25 mg/kg, while concentrations in the groundwater were 0.044-180 ppm. 

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. 

Figure 32. The saturated zone defines the groundwater table...

The combination of SVE with air sparging technology. Air sparging involves the injection of air into the saturated zone of contaminated groundwater. The air bubbles enhance the... 

Introduction of the flushing solution may occur within the vadose zone, the saturated zone, or both. Flushing solutions may consist of plain water, or surfactants, co-solvents, acids, bases, oxidants, chelants, and solvents. The infiltrating flushing solution percolates through the soil and soluble compounds present in the soil are dissolved. The elutriate is pumped from the bottom of the contaminated zone into a water treatment system to remove pollutants. The process is carried out until the residual concentrations of contaminants in the soil satisfy given limits. 

Water Movement in Saturated 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. 

Atmospheric pressure is not obvious, because it is balanced in opposite directions. The combination of atmospheric pressure and the weight of the overlying water create the total pressure in the saturation zone.24... 

The most important factor for movement in the saturated zone is the hydraulic gradient. The velocity head, which is generally more than ten orders of magnitude smaller than the pressure and gravitational head, may be neglected because of the slow water movement. Equation 18.4 can therefore be simplified to... 

Solubility causes gasoline compounds to be more mobile in association with the movement of groundwater. Dissolved gasoline compounds reach the saturated zone in several ways ... 

In the saturated zone, the most important phase of gasoline is its free product above the groundwater, then the gasoline as adsorbate in the soil the gasoline as solute in the ground-water is less important. 

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. 

In some configurations, the vacuum used in MPE increases the effective drawdown of ground-water (i.e., the increase or lowering of the depth of the groundwater table) locally near the pumped well. This has the effect of increasing exposed soil in the saturated zone and the removal of volatile contaminants located above and below the original water table. 

In all of the workshops, but especially in the FAT and Exposure Assessment workshops, the need for better understanding and model representation of soil systems, including both unsaturated and saturated zones, was evident. This included the entire range of processes shown in Table II, i.e., transport, chemical and biological transformations, and intermedia transfers by sorption/desorption and volatilization. In fact, the Exposure Assessment workshop (Level II) listed biological degradation processes as a major research priority for both soil and water systems, since current understanding in both systems must be improved for site-specific assessments. 

Even in the absence of fractures and thief zones, the volumetric sweep efficiency of injected fluids can be quite low. The poor volumetric sweep efficiency exhibited in waterfloods is related to the mobility ratio, M. This is defined as the mobility of the injected water in the highly flooded (watered-out) low oil saturation zone, m, divided by the mobility of the oil in oil-bearing portions of the reservoir, m, (253,254). The mobility ratio is related to the rock permeability to oil and injected water and to the viscosity of these fluids by the following formula ... 

Even where it is not occluded, the mineral surface may not be reactive. In the va-dose zone, the surface may not be fully in contact with water or may contact water only intermittently. In the saturated zone, a mineral may touch virtually immobile water within isolated portions of the sediment s pore structure. Fluid chemistry in such microenvironments may bear little relationship to the bulk chemistry of the pore water. Since groundwater flow tends to be channeled through the most permeable portions of the subsurface, furthermore, fluids may bypass many or most of the mineral grains in a sediment or rock. The latter phenomenon is especially pronounced in fractured rocks, where only the mineral surfaces lining the fracture may be reactive. 

Fig. 3. Sample collection deeper into the saturated zone results in more reducing conditions (a) and increase in redox sensitive elements such as Fe (b).

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