Vadose Zone Soil Contamination

At the DOD s Letterkenny Army Base in Letterkenny, Pennsylvania, the Geo-Cleanse process was used to treat vadose zone soils contaminated with chlorinated solvents and benzene, toluene, ethylbenzene, and xylenes (BTEX). The acmal remediation costs were approximately 700,000 (D200964, pp. 9, 16, 17). 

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. 

Laney, D. F., 1988, Hydrocarbon Recovery as Remediation of Vadose Zone Soil/Gas Contamination In Proceedings of the National Water Well Association Second National Outdoor Conference on Aquifer Restoration, Groundwater Monitoring and Geophysical Methods, Vol. Ill, Las Vegas, NV, May, pp. 1147-1171. 

Vaportech enhanced volatilization is designed to remove volatile contaminants from vadose zone soils in situ. The technology operates by injecting clean air around the perimeter of the contaminated area and withdrawing contaminated air from the middle. 

The Harding ESE, Inc., in situ vadose zone soil treatment uses indigenous bacteria and formulations of mineral nutrients to treat biodegradable chemicals in soil. The technology only treats contaminants in the soil s vadose zone (the zone below the surface but above the water table also known as zone of aeration). 

Oleophilic suction lysimetry (OSL) uses a membrane-covered oil recovery lysimeter to recover non-aqueous-phase liquids (NAPLs) from contaminated vadose zone soil. The lysimeter is placed into the soil to intercept a region of soil contaminated by NAPL. Vacuum pressures applied to the lysimeter draw NAPL through the membrane. Recovered NAPL is then conveyed to the ground surface for storage prior to disposal. 

During the passage through dry soil, there is considerable potential for leaching with rain water and it is common to find PAH-contaminated vadose-zone soils depleted of the naphthalenes because of their relatively high water solubility (Bossert Bartha, 1986). The leached material will ultimately appear in the... 

There are a number of engineering variations of in situ bioremediation strategies potentially applicable for soil and/or groundwater contaminated by PAHs. Recent reviews of in situ bioremediation technologies by Norris et al. (1993) provide excellent sources of references and offer case studies for myriad in situ bioremediation applications. These include bioventing vadose zone soil, biosparging saturated zone soil, vacuum-vaporized well (UVB) technology, and in situ bioreactors. 

The vadose zone soils at the New Mexico State Highway and Transportation Department (NMSHTD) District 2 Maintenance Patrol Yard in Artesia, New Mexico (Artesia Yard) consist primarily of massive to poorly stratified silty clay, clay, or clayey silt. Soils of such low permeability (<10 10 square centimeters) are generally not amenable to the use of SVE (U.S. EPA 1995). However, as this case history shows, operation of a high-vacuum SVE system, with periodic monitoring and adjustment, can be very successful in removing sizeable secondary sources of petroleum hydrocarbons (both PSH and residual soil contamination) from the subsurface. 

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... 

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. 

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)... 

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 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. 

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. 

When contaminant air rises above the water table into the vadose zone, the VOCs are captured by soil-venting extraction, escape to the atmosphere, or are treated as they encounter indigenous bacteria present in that zone. 

Despite its humorous name, this technology is a fairly efficient procedure to combine the benefits of vacuum-enhanced recovery and bioventing to promote vapor recovery and in situ biodegradation. Integration of these technologies into a single step results in LNAPL recovery and remediation of residual soil contamination in the vadose zone. 

The primary focus of this chapter is to introduce approaches that limit (or prevent) migration of hydrocarbon and organic contaminants from the vadose zone into underlying water-bearing zones. The more conventional strategies, including a brief discussion of the process, relative cost, practical constraints, and limitations, are presented. Also presented are two typical soil vapor extraction case histories. 

In the vadose zone, liquid or dissolved contaminants exist in a complex environment that involves interaction among the chemicals, soil grains, water attached to soil grains (or between the soil pores), the atmosphere in void spaces, and numerous... 

Depth to Contamination In general, the shallower the release in the vadose zone, the more rapid the diffusion of soil gas, and the greater the indigenous microbial density. 

The zone between land surface and the water table, which forms the upper boundary of the groundwater region, is known as the vadose zone. This zone is mostly unsaturated— or more precisely, partially saturated— but it may contain a saturated fraction in the vicinity of the water table due to flucmations in water levels or capillary rise above the water table. The near-surface layer of this zone—the soil—is generally partially saturated, although it can exhibit periods of full saturation. Soil acts as a buffer that controls the flow of water among atmosphere, land, and sea and functions as a sink for anthropogenic contaminants. 

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