The following description of air sparging is an excerpt from Chapter VII of OUST's publication: How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites: A Guide for Corrective Action Plan Reviewers. (EPA 510-B-95-007). This publication also describes 9 additional alternative technologies for remediation of petroleum releases. You can download PDF files of every chapter of the document at: http://www.epa.gov/swerust1/pubs/tums.htm.
Air sparging is an in situ remedial technology that reduces concentrations of volatile constituents in petroleum products that are adsorbed to soils and dissolved in groundwater. This technology, which is also known as "in situ air stripping" and "in situ volatilization," involves the injection of contaminant-free air into the subsurface saturated zone, enabling a phase transfer of hydrocarbons from a dissolved state to a vapor phase. The air is then vented through the unsaturated zone.
Air sparging is most often used together with soil vapor extraction (SVE), but it can also be used with other remedial technologies. When air sparging (AS) is combined with SVE, the SVE system creates a negative pressure in the unsaturated zone through a series of extraction wells to control the vapor plume migration. This combined system is called AS/SVE.
When used appropriately, air sparging has been found to be effective in reducing concentrations of volatile organic compounds (VOCs) found in petroleum products at underground storage tank (UST) sites. Air sparging is generally more applicable to the lighter gasoline constituents (i.e., benzene, ethylbenzene, toluene, and xylene [BTEX]), because they readily transfer from the dissolved to the gaseous phase. Air sparging is less applicable to diesel fuel and kerosene. Appropriate use of air sparging may require that it be combined with other remedial methods (e.g., SVE or pump-and-treat). An air sparging system can use either vertical or horizontal sparge wells. Well orientation should be based on site-specific needs and conditions.
Air sparging should NOT be used if the following site conditions exist:
- Free product is present. Air sparging can create groundwater mounding which could potentially cause free product to migrate and contamination to spread.
- Nearby basements, sewers, or other subsurface confined spaces are present at the site. Potentially dangerous constituent concentrations could accumulate in basements unless a vapor extraction system is used to control vapor migration.
- Contaminated groundwater is located in a confined aquifer system. Air sparging cannot be used to treat groundwater in a confined aquifer because the injected air would be trapped by the saturated confining layer and could not escape to the unsaturated zone.
The existing literature contains case histories describing both the success and failure of air sparging; however, since the technology is relatively new, there are few cases with substantial documentation of performance.
The effectiveness of air sparging depends primarily on two factors:
- Vapor/dissolved phase partitioning of the constituents determines the equilibrium distribution of a constituent between the dissolved phase and the vapor phase. Vapor/dissolved phase partitioning is, therefore, a significant factor in determining the rate at which dissolved constituents can be transferred to the vapor phase.
- Permeability of the soil determines the rate at which air can be injected into the saturated zone. It is the other significant factor in determining the mass transfer rate of the constituents from the dissolved phase to the vapor phase.
In general, air sparging is more effective for constituents with greater volatility and lower solubility and for soils with higher permeability. The rate at which the constituent mass will be removed decreases as air sparging operations proceed and concentrations of dissolved constituents are reduced.
Soil characteristics will also determine the preferred zones of vapor flow in the vadose zone, thereby indicating the ease with which vapors can be controlled and extracted using SVE (if used).
Stratified or highly variable heterogeneous soils typically create the greatest barriers to air sparging. Both the injected air and the stripped vapors will travel along the paths of least resistance (coarse-grained zones) and could travel a great lateral distance from the injection point. This phenomenon could result in the contaminant-laden sparge vapors migrating outside the vapor extraction control area.
The essential goals in designing and air sparging system are to configure the wells and monitoring points in such a way to:
- optimize the influence on the plume, thereby maximizing the removal efficiency of the system, and
- provide optimum monitoring and vapor extraction points to ensure minimal migration of the vapor plume and no undetected migration of either the dissolved phase or vapor phase plumes. In shallow applications, in large plume areas, or in locations under buildings or pavements, horizontal vapor extraction wells are very cost effective and efficient for controlling vapor migration.
Field pilot studies are necessary to adequately design and evaluate any air sparging system. However, pilot tests should not be conducted if either of the following conditions exist:
- free product is known to exist at the groundwater table,
- uncontrolled migration of vapors into confined spaces, sewers, or buildings,
- the contaminant source is in a confined aquifer.
The air sparge well(s) used for pilot testing should be located in an area of no more than moderate constituent concentrations. Testing the system in areas of extremely low constituent concentrations may not provide sufficient data, and because sparging can induce migration of constituents, pilot tests are generally not conducted in areas of extremely high constituent concentrations. The air sparging pilot study should include an SVE pilot study if SVE is to be included in the design of the air sparging system.
The placement and number of air sparge points required to address the dissolved phase plume is determined primarily by the permeability and structure of the soil as these affect the sparging pressure and distribution of air in the saturated zone. Coarse-grained soils (e.g., sand, gravel) have greater intrinsic permeability than fine-grained soils (e.g., clay, silt) and it is easier to move air (and water) through more permeable soil. Greater lateral dispersion of the air is likely in fine-grained soils and can result in lateral displacement of the groundwater and contaminants if groundwater control is not maintained.
Design radius of influence (ROI) for air sparging wells. The ROI is the most important parameter to be considered in the design of the air sparging system. The ROI is defined as the greatest distance from a sparging well at which sufficient sparge pressure and airflow can be induced to enhance the mass transfer of contaminants from the dissolved phase to the vapor phase. The ROI will help determine the number and spacing of the sparging wells. Air sparging wells should be placed so that the overlap in their radii of influence completely cover the area of contamination.
The sparging air flow rate required to provide sufficient air flow to enhance mass transfer is site-specific and will be determined via the pilot test.
- Readily available equipment; easy installation.
- Implemented with minimal disturbance to site operations.
- Short treatment times (usually less than 1 to 3 years under optimal conditions).
- At about $20-50/ton of saturated soil, air sparging is less costly than aboveground treatment systems.
- Requires no removal, treatment, storage, or discharge considerations for groundwater.
- Can enhance removal by SVE.
- Cannot be used if free product exists (i.e., free product must be removed prior to air sparging).
- Cannot be used for treatment of confined aquifers
- Stratified soils may cause air sparging to be ineffective.
- Some interactions among complex chemical, physical, and biological processes are not well understood.
- Lack of field and laboratory data to support design considerations.
- Potential for inducing migration of constituents.
- Requires detailed pilot testing and monitoring to ensure vapor control and limit migration.
Brown, L.A. and R. Fraxedas. 1992. Air sparging extending volatilization to contaminated aquifers, in Proceedings of the Symposium on Soil Venting, April 29-May 1, 1991, Houston, Texas, pp. 249-269. U.S. EPA, Office of Research and Development. EPA/600/R-92/174.
Johnson, R.L., P.C. Johnson, D.B. McWhorter, R.E. Hinchee, and I. Goodman. 1993. An overview of in situ air sparging. Ground Water Monitoring Review. Vol. 13, No. 4, pp. 127-135.
Hinchee, R.E. 1994. Air Sparging for Site Remediation. Boca Raton, FL: Lewis Publishers.
Marley, M., D.J. Hazenbronck, and M.T. Walsh. 1992. The application of in situ air sparging as an innovative soils and groundwater remediation technology. Ground Water Monitoring Review. Vol. 12, No. 2, pp. 137-145.
Martin, L.M., R.J. Sarnelli, and M.T. Walsh. 1992. Pilot-scale evaluation of groundwater air sparging: site-specific advantages and limitations, in Proceedings of R&D 92-National Research and Development Conference on the Control of Hazardous Materials. Greenbelt, MD: Hazardous Materials Control Research Institute.
U.S. Environmental Protection Agency (EPA). 1992. A Technology Assessment of Soil Vapor Extraction and Air Sparging. Washington, D.C.: Office of Research and Development. EPA/600/R-92/173.