Air sparging is most often used together with soil vapor extraction (SVE), but it can also be used with other remedial technologies or even alone. When air sparging (AS) is combined with Soil Vapor Extraction (SVE), the SVE system creates a negative pressure in the unsaturated zone through a series of extraction wells to capture vapors and control the vapor plume migration. This combined system is called AS/SVE. An Air Sparging pilot test is recommended for adequate design.

Application
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:

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.

Operation Principles (Back to Top)
The effectiveness of air sparging depends primarily on two factors:

1. 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.
2. 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).

System Design (Back to Top)
The essential goals in designing and air sparging system are to configure the wells and monitoring points in such a way to:

1. optimize the influence on the plume, thereby maximizing the removal efficiency of the system, and
2. 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.
3. optimize injected air flow to allow for air stripping affect.

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:

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 Soil Vapor Extraction (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.

An air sparging pilot test can provide a 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 best indication of actual ROI is to measure subsurface air flow. Other methods used to estimate the air sparging ROI such as mounding, oxygen concentration, and subsurface pressure are inaccurate and can result in ROI that is greater than the actual air flow ROI. These methods are more effective at measuring ROI for bio sparging.

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.

Advantages and Disadvantages (Back to Top)