Air sparging is the process of injecting air directly into groundwater. Air sparging remediates groundwater by volatilizing contaminants and enhancing biodegradation. It is akin to blowing bubbles from a straw into a bowl of water. As the bubbles rise, the contaminants are removed from the groundwater by physical contact with the air (i.e., stripping) and are carried up into the unsaturated zone (i.e., soil). As the contaminants move into the soil, a soil vapor extraction system is usually used to remove vapors. The addition of oxygen to contaminated groundwater and soils also enhances biodegradation of contaminants in and above the water table, as it acts as a nutrient for bacteria. There are several enhancements to air sparging.
Air sparging is sometimes referred to as in situ air stripping. When used in combination with soil vapor extraction (SVE), air bubbles carry vapor phase contaminants to a SVE system which removes them. The SVE system controls vapor plume migration by creating a negative pressure in the unsaturated zone through a series of extraction wells. Using air sparging as an SVE enhancement technology increases contaminant movement and enhances oxygenation in the subsurface which increases the rate of contaminant extraction.
The first enhancement is called cometabolic air sparging. Propane, which is injected along with air, acts as another nutrient for microorganisms.
Another method is a newly patented technology, Bio-SpargeTM. It addresses a major problem of traditional pump-and-treat systems. When a pump-and-treat system is employed, it pumps contaminated water to the surface and pulls the groundwater down into a cone of depression. At the former upper extent of the groundwater (called the capillary fringe), most of the sorbed contaminant remains. In many instances, this area contains the most heavily contaminated soil. When groundwater pumping ceases, the groundwater returns to its former level and becomes re-contaminated by the sorbed material. This is called a rebound.
Bio-SpargeTM induces desorption within the capillary fringe by injecting a heated mixture of air, water, nutrients and bacteria. A downgradient well recovers the air for reinjection. As the air mixture moves across the contaminated zone, bacteria, with the aid of the nutrients, destroy many of the contaminants. Since the system is a closed loop, no release to the environment occurs. That is, there is no need for off-gas treatment and no need for an air emission permit.
Another method is called the C-Sparge. C-Sparging periodically injects an ozone/air mixture in conjunction with a pulsing pump. This is a two-phase process. First, fine bubbles are injected into the groundwater to extract dissolved volatile organic compounds (VOCs) out of contaminated groundwater. Second, the ozone contained within the bubbles reacts rapidly with the volatile compounds to decompose them into end products consisting of carbon dioxide, dilute hydrochloric acid, and water.
Limitations and Concerns
Air flow through the saturated zone may not be uniform.
There can be uncontrolled movement of potentially dangerous vapors.
Air sparging depends on two processes. One helps to biodegrade the contaminants by adding nutrients or other amendments, so transformation products should be monitored. The other process removes remaining contaminants from the groundwater, possibly emitting them into the air.
Air injection wells must be designed for site-specific conditions. Depth of contaminants and specific site geology must be considered. Soil heterogeneity may cause some zones to be relatively unaffected.
The target contaminant groups for air sparging are VOCs and fuels in groundwater. Air sparging is generally more applicable to the lighter gasoline constituents such as benzene, ethylbenzene, toluene, and xylene. It is less applicable to heavier constituents such as diesel fuel and kerosene.
Technology Development Status
Air sparging has been demonstrated at numerous sites. It is a commercial technology.
Other Resources and Demonstrations