Frequently Asked Questions about Soil Remediation:
What does a typical soil remediation system look like?
What is soil vapor extraction?
Why consider SVE or Air Sparging?
Will SVE or Air Sparging work at every site?
What is Dual Phase extraction?
What does a typical soil remediation system look like?
Soil vapor extraction, known as SVE, is the most frequently selected method for treatment of contaminated soil at superfund sites, brown-fields, refineries, gas stations, dry cleaners, and grain elevators. It is a relatively simple process that physically separates contaminants from soil. As the name suggests, SVE extracts contaminants from the soil in vapor form. Therefore, SVE systems are designed to remove contaminants that have a tendency to volatilize or evaporate easily. SVE removes volatile organic compounds (VOCs) and some semi-volatile organic compounds (SVOCs) from soil beneath the ground surface in the unsaturated zone-that part of the subsurface located above the water table. By applying a vacuum through a system of underground wells, contaminants are pulled to the surface as vapor or gas. Often, in addition to vacuum extraction wells, air injection wells are installed to increase the air flow and improve the removal rate of the contaminant. An added benefit of introducing air into the soil is that it can stimulate bioremediation of some contaminants.
SVE is sometimes called in situ volatilization, enhanced volatilization, in situ soil venting, forced soil venting, in situ air stripping, or soil vacuum extraction.
Used alone, soil vapor extraction cannot remove contaminants in the saturated zone of the subsurface, the water-soaked soil that lies below the water table. At sites where contamination is in the saturated zone, a process called air sparging may be used along with the SVE system. Air Sparging means pumping air into the saturated zone to help flush (bubble) the contaminants up into the unsaturated zone where the SVE extraction wells can remove them.
For air sparging to be successful, the soil in the saturated zone must be loose enough to allow the injected air to readily escape up into the unsaturated zone. Air Sparging, therefore, will work fastest at sites with coarse-grained soil, like sand and gravel.
As with SVE, an added benefit of air sparging is that it provides an oxygen source that helps stimulate the bioremediation of some contaminants. Bioremediation is an innovative treatment technology that uses microorganisms, such as bacteria, that live in the soil or groundwater to break down contaminants into harmless substances. Air Sparging also can be a quick and effective treatment for VOCs in groundwater.
The first step to constructing an SVE system is to install vapor extraction wells and injection Wells (or air vents) in the contaminated area. Air injection wells use air compressors to force air into the ground. Air vents serve the same function as air injection wells, but are passive-instead of pumping air they just provide a passage for air to be drawn into the ground. When incoming air passes through the soil on its way to the extraction wells, contaminants evaporate out of the spaces between the soil particles and are pulled by the air to the wells and removed.
Vapor extraction wells can be placed either vertically or horizontally. Typically, they are placed vertically and are designed to penetrate the lower portion of the unsaturated zone.
Vapors extracted by the SVE process are typically treated using carbon adsorption, incineration, catalytic oxidation, or condensation. Other methods, such as biological treatment and ultraviolet oxidation, also have been used with SVE systems. The type of treatment chosen depends on which contaminants are present and their concentrations.
Carbon adsorption is the most commonly used treatment for contaminated vapors and is adaptable to a wide range of volatile organic compounds. When properly designed and operated, SVE is a safe, low maintenance process. Explosion-proof equipment is available to handle the potentially explosive mixtures of extracted gas that may be encountered on some landfill or gasoline spill sites.
Why consider SVE or Air Sparging?
SVE is very effective at removing VOCs from the unsaturated zone. With the addition of an air sparging system, contaminants can be removed from the saturated zone as well. Neither technique requires excavation of the contaminated soil. (Excavation is undesirable because it is expensive, creates dust, and allows volatile contaminants to escape untreated into the atmosphere.) The extracted vapors usually require treatment, but costs for treating extracted vapors and liquids are low compared to the costs of technologies requiring excavation. The technologies are relatively simple to install, can be used effectively in combination with other treatment technologies, and are effective under a variety of site conditions.
Will SVE or Air Sparging work at every site?
SVE and Air Sparging may be good choices at sites contaminated with solvents and other volatile organic compounds (such as trichloroethane, trichloroethylene, benzene, toluene, ethylbenzene, and xylene) and fuels. Because properties of the soil have such an important effect on the movement of soil vapors, the performance and design of SVE and Air Sparging systems depend greatly on the properties of the soil. SVE is best used at sites with loose unsaturated soil, such as sand, gravel, and coarse silt or fractured bedrock. However, it has been applied to sites with denser soils, although treatment may take longer. Also, the higher the moisture content of the soil, the slower SVE works.
What is Dual Phase extraction?
Dual phase extraction is a treatment system similar to SVE, but the extraction wells are sunk more deeply into the ground-below the water table into the saturated zone. Strong vacuum is applied through the extraction wells to simultaneously remove groundwater and vapors from the subsurface. Once above ground, the extracted vapors and groundwater are separated and treated. Dual phase extraction is more effective than SVE at sites with dense, clayey soil. When dual-phase extraction is combined with bioremediation, Air Sparging or bioventing, it can shorten cleanup times.