Fact Sheet #2000-02 Soil Vapor Extraction and Biosparging A fact sheet providing information about two remedial cleanup technologies for contaminated soil at MMR. The purpose of this fact sheet is to describe technologies that will be used to clean up some of the soil contamination associated with the Massachusetts Military Reservation (MMR). Some portions of contaminated soils or source areas can contribute to groundwater contamination, so it is important to remediate these sites to complete the overall cleanup of MMR. Currently, 29 source areas are scheduled to begin cleanup in 2000 using one or more of the following soil cleanup technologies:
This fact sheet describes the first two technologies, soil vapor extraction and biosparging. The third, cold-mix asphalt batching, is described in detail in the Air Force Center for Environmental Excellence’s (AFCEE) fact sheet, Cold Mix Asphalt Batching (#99-05, July 1999). Remedial project managers from the AFCEE, the U.S. Environmental Protection Agency (EPA), and the Massachusetts Department of Environmental Protection (DEP) have evaluated and selected these technologies as the most appropriate cleanup solutions for several source areas at MMR.
Words that appear in italics are defined in the glossary at the end of this fact sheet. Background Each of the 29 source areas that are scheduled to begin cleanup activities in 2000 has undergone various public involvement processes. These include, but are not limited to: preliminary assessments & site investigations, proposed plans, public comment periods, public meetings and record of decisions. Soil vapor extraction (SVE), is one of the most frequently selected soil cleanup technologies at Superfund sites. 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. By applying an air vacuum through a system of underground wells, contaminants are pulled to the surface in vapor form. Often, in addition to vacuum extraction wells, air injection wells are installed to increase the airflow and improve the removal rate of the contaminant vapor. How does SVE work? The first step to constructing an SVE system is to install vapor extraction wells and air injection wells in the contaminated area. Air injection wells use air compressors to force air 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 carried by the air to the extraction wells and removed. Vapor extraction wells are typically 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, similar to carbon filtration of contaminated groundwater. Once the carbon filters are spent, they are taken off-site and recycled. Why consider SVE at MMR? When properly designed and operated, SVE is a safe, low maintenance process. It 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. 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. SVE and air sparging are good choices at sites contaminated with solvents (such as perchloroethylene (PCE) and trichloroethylene (TCE)) and fuels. SVE is best used at sites with loose unsaturated soil, such as sand and gravel. MMR has the ideal geological environment for this technology to work. It has already been demonstrated to work effectively at the FS-12 source area where, from October 1995 to May 1998, AFCEE removed over 44,580 pounds of contaminants using SVE. Air Sparging Used alone, soil vapor extraction cannot remove contaminants in the saturated zone of the subsurface. 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 both the saturated and unsaturated zones to help flush or bubble the contaminants up into the unsaturated zone where the SVE extraction wells can remove them (Figure 1).
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 more effectively 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. Biosparging is essentially air sparging that uses injected air (oxygen) to speed up bioremediation-a treatment process that uses naturally occurring microorganisms (yeast, fungi, or bacteria) to break down hazardous substances into less toxic or nontoxic substances. Microorganisms, just like humans, eat and digest organic substances for nutrients and energy. Certain microorganisms can digest organic substances such as fuels or solvents that are hazardous to humans. The microorganisms break down the organic contaminants into harmless products -- mainly carbon dioxide and water. Once the contaminants are degraded, the microorganism population is reduced because they have used all of their food source. How does biosparging work? Microorganisms must be active and healthy in order for bioremediation to take place. Bioremediation technologies such as biosparging assist growth of microorganisms and increase microbial populations. Increased bacterial growth, combined with creating optimal living conditions, provides the bacteria an environment to detoxify the maximum amount of contaminants. Bioremediation can take place under aerobic and anaerobic conditions. In aerobic conditions, microorganisms use available atmospheric oxygen in order to function. With sufficient oxygen, microorganisms will convert many organic contaminants to carbon dioxide and water. Anaerobic conditions support biological activity when no oxygen is present. The microorganisms break down chemical compounds in the soil to obtain the energy they need. At MMR, biosparging will treat the contaminated soils in the location in which they were found and like SVE, it requires no excavation. The loose, unsaturated sandy soils of MMR provide an ideal environment for this technology to work. Air Monitoring Periodic air monitoring will be performed to check air emissions to the atmosphere after treatment. Although unlikely, contaminant vapors that may escape to the surface as a result of the airsparging will also be monitored, and risk reductions put in place to protect public health and the environment. Conclusions SVE and biosparging technologies have been success-fully used at many Superfund sites, including MMR. They are cost-effective alternatives to excavating hazardous materials and transporting them off-site. Further, SVE and biosparging have been recognized by EPA and MassDEP as effective in removing certain hazardous chemicals, such as PCE and TCE in soils. Bibliography This fact sheet summarizes information found in the following EPA documents:
aerobic—activity occurring only in the presence of oxygen. air sparging— pumping air into the saturated zone to help flush the contaminants upward where the SVE extraction wells can remove them anaerobic—lacking oxygen. adsorption—to attach by physical or chemical attraction. bioremediation—an innovative treatment technology that uses microorganisms, such as bacteria, that live in the soil or groundwater to break down contaminants. perchloroethylene (PCE)—also referred to as tetrachloroethylene; a man-made solvent commonly used for degreasing metal and dry-cleaning clothes. Preliminary Assessment/Site Investigation—The Preliminary Assessment and Site Inspection are used by EPA to evaluate the potential for a release of hazardous substances from a site. Proposed Plan—a document that summarizes the preferred remedial action for a site and presents the rationale for the preference. Record of Decision (ROD)—a document presenting the remedial action selected under agreement with the regulatory agencies and also includes a record of public comments and the responsiveness summary saturated—unable to hold or contain more liquids. semivolatile organic compounds (SVOCs)—organic chemical compounds that do not evaporate readily to the atmosphere. trichloroethylene (TCE)—a solvent used to dissolve or disperse another substance such as oil; often used in degreasing metal. unsaturated zone—soils that lie within the area underground from the surface to the water table. volatile organic compounds (VOCs)—organic chemical compounds that evaporate readily to the atmosphere. For More Information: Doug Karson, Community Involvement Specialist Jim Murphy, Community Involvement Coordinator Ellie Grillo, Community Involvement Coordinator
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