Vadose zone contamination

Figure 3. Common bioventing system for treatment of vadose-zone contaminants using oxygen as a terminal electron acceptor. Reprinted from In Situ Bioremediation When Does It Work Copyright 1993 by the National Academy of Sciences. Courtesy of the National Academy Press, Washington, DC.

Vacuum-vaporized well (German Unterdruck-Verdampfer-Brunnen abbreviation UVB) technology for in situ treatment of the capillary fringe, phreatic zone, and vadose zone contaminated with volatile organic compounds, including NAPLs, represents to us one of the most promising in situ bioremediation technologies (U.S. 

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. 

Environmental Problems—This section describes N Reactor operational activities that affected the environment. The affected environs are groundwater contamination (radionuclides, volatile organics, polychlorinated biphenyls (PCB), metals, and other Inorganics) soil contamination (surface and vadose zone contamination from radionuclides and organic compounds) biota contamination (flora [vegetation] and fauna [animals] from radionuclide uptake by plants or Ingestion by animals) and evaluated radiation at the Columbia River (unshielded sediments In the 1301-N Liquid Waste Disposal Facility). 

Of course the presence of a Hquid phase of hydrocarbon in a soil gives rise to vapor contamination in the vadose zone above the water table. This can be treated by vacuum extraction, and the passage of the exhaust gases through a biofilter (see above) can be a cheap and effective way of destroying the contaminant permanently. 

In Situ Air Stripping. An innovation to conventional pump and treat air stripping is in situ air stripping. Two horizontal wells are installed, one below the water table and one in the vadose zone. Air is injected in the lower well while contaminated soil vapor is extracted by vacuum through the upper well. 

A U.S. EPA study (41) showed that soil vapor extraction (SVE) is an effective treatment for removing volatile contaminants from the vadose zone. Sandy soils are more effectively treated than clay or soils with higher organic content because higher air flows are possible in sand and clays—organic soils tend to adsorb or retain more contaminants. Removal of volatiles is rapid in the initial phase of treatment and thereafter decreases rapidly thereafter-an important consideration in the design of air emissions control over the life of the project. 

Lappala, E. and G. Thompson. Detection of Groundwater Contamination by Shallow Soil Gas Sampling in the Vadose Zone and Applications. In Management of Uncontrolled Hazardous Waste Sites Proceedings, Hazardous Materials Control Research Institute,Washington, D.C., 1984. 

As more sensitive analytical methods for pesticides are developed, greater care must be taken to avoid sample contamination and misidentification of residues. For example, in pesticide leaching or field dissipation studies, small amounts of surface soil coming in contact with soil core or soil pore water samples taken from further below the ground surface can sometimes lead to wildly inaccurate analytical results. This is probably the cause of isolated, high-level detections of pesticides in the lower part of the vadose zone or in groundwater in samples taken soon after application when other data (weather, soil permeability determinations and other pesticide or tracer analytical results) imply that such results are highly improbable. 

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... 

Pumping of the groundwater to lower the water table and enlarge the vadose zone, with simultaneous treatment of contaminated groundwater.10... 

Phase Distribution of Organic Contaminants in the Vadose Zone... 

Organic contaminants can be present in the vadose zone in four distinct phases (Figure 14.2) ... 

FIGURE 14.2 Phase distribution of organic contaminants in the vadose zone. The solid arrows in the three-and four-phase models represent the equilibria taken into consideration in the equations of Table 14.3. 

The airflow equations presented above are based on the assumption that the soil is a spatially homogeneous porous medium with constant intrinsic permeability. However, in most sites, the vadose zone is heterogeneous. For this reason, design calculations are rarely based on previous hydraulic conductivity measurements. One of the objectives of preliminary field testing is to collect data for the reliable estimation of permeability in the contaminated zone. The field tests include measurements of air flow rates at the extraction well, which are combined with the vacuum monitoring data at several distances to obtain a more accurate estimation of air permeability at the particular site. 

The concentrations and the mass distribution of toluene in the four phases, as calculated from this set of equations, are presented in Table 14.4. As seen in the table, the major part of the toluene, i.e., 68.9%, remains in the vadose zone as free NAPL, 27.6% is adsorbed on the surfaces of solid particles, and only 3.5% is distributed between the aqueous and gas phases. Free NAPL occupies only a small part of the available pore volume, and it is not expected to disturb the movement of air through the contaminated zone. 

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. 

Soils and vadose zone information, including soil characteristics (type, holding capacity, temperature, biological activity, and engineering properties), soil chemical characteristics (solubility, ion specification, adsorption, leachability, cation exchange capacity, mineral partition coefficient, and chemical and sorptive properties), and vadose zone characteristics (permeability, variability, porosity, moisture content, chemical characteristics, and extent of contamination)... 

Multiphase extraction uses a vacuum system to remove various combinations of contaminated groundwater, separate-phase petroleum product, and vapors from the subsurface. The system lowers the water table around the well, exposing more of the formation. Contaminants in the newly exposed vadose zone are then accessible to vapor extraction. Once above ground, the extracted vapors or liquid-phase organics and groundwater are separated and treated. 

SVE has been an effective technique for removing VOCs such as TCE and some petroleum compounds from the vadose zone of contaminated soil.72 The following presents some of the newly developed technologies. 

May cause a lateral spread of dissolved or separate phase contaminant plume Contamination may be transferred from groundwater to die vadose zone Has limited applicability at sites with confined aquifers Low soil permeability or other heterogeneous conditions may reduce effectiveness... 

In some cases, such as areas of lower contaminant concentrations or in remote locations, contaminants stripped from the groundwater may be allowed to attenuate naturally in the vadose zone. 

In an SVE system, the primary mechanism for contaminant removal from the soil to the vadose zone is the volatilization of contaminants present in the pure or adsorbed phase onto soil into the vapor phase, as the vapor phase is continually extracted. The property that shows the extent to which this transfer can take place during SVE is vapor pressure, which provides an indication of the extent to which each contaminant will partition between the liquid phase and the vapor state at equilibrium conditions. Generally, a contaminant with a greater vapor pressure more readily volatilizes than one with a lesser vapor pressure. 

Robert M., Terce M. Effect of gel and coatings on clay mineral chemical properties. In Inorganic Contaminants in the Vadose Zone, B. Bar-yosef, N.J. Barrow, J. Goldsmith, eds. Berlin, 1989. 

Groundwater can be found in the traditional sense at the water table below which the soil pore spaces are essentially saturated and the water is free to move, and in the unsaturated zone (or vadose zone) above the water table. It is possible for water to migrate through both of these zones, transporting dissolved components (or contaminants). The interaction of the various forces involved will determine the direction and rate of migration. 

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