Air Sparging: Is it the Right Choice for Your Next Remediation Project?
Defining a Typical Air Sparge System
Air sparging (AS) is a treatment technology that uses injected air to remove volatile contaminants from the saturated region of the soil. It can remove volatile contaminants such as gasoline, some solvents, and even jet fuel. Air sparging provides oxygen for aerobic microbial degradation of contaminants, which can be utilized in the later stages of remediation.
The main components of a standard air sparging system consist of air injection or sparge wells, air compressors, or blowers to supply the injection air, and monitoring sensors and equipment. The injection wells are normally vertical and screened below the contamination level. Using these wells, air is injected into the saturated zone, which forces the contaminants to separate from the liquid phase into the vapor phase, a process commonly known as “stripping”. Occasionally, the air injection wells can be replaced or supplemented by air vents, which allow for the passive introduction of surface air into the contaminated ground water and soil. From there they are transported to the vadose zone for removal. The resultant air is either treated through a Soil Vapor Extraction (SVE) system, or released directly into the atmosphere with the proper regulatory approval.
Air sparging is an in-situ remedial technology which reduces the concentrations of volatile portions in petroleum products that are usually adsorbed to soil particles and dissolved in ground water. This technology is most often used in conjunction with soil vapor extraction systems (SVE), but it can also be used with other remedial systems and technologies, such as the introduction of chemical or biological agents such as hydrogen peroxide to accelerate the degradation process.
Air Sparging Vs. Vapor Extraction
As described above, air sparging uses uncontaminated air to help separate and remove harmful contaminants from below the saturated zone. When air is introduced into the ground water, the chemicals in question will evaporate faster, and therefore volatilize into the vadose zone. At this point, the increased vapors can be either be removed through a companion SVE system, or forced out to awaiting treatment systems by the air sparge blowers. This is accomplished by the influx of air creating a positive pressure gradient in the soil. The positive pressure then creates a pressure gradient between the area around the air injection well and the removal apparatus, usually a SVE extraction well. Since pressure equilibrium is preferred, the volatilized contaminates travel to the lower pressure area at the extraction well, where they are subsequently removed.
Applications of Air Sparging
Air sparging is usually utilized in the corrective action plans of areas dealing with petrouleum or solvent contamination. Because of their relative volatility, these contaminants are easily affected by the introduction of air into the ground water, and the smear zone. Many of the chemical compounds contained within fuels, such as benzene and toluene have a vapor pressure between 50 and 250 mm Hg at standard temperature and pressure. These relatively high vapor pressure values indicate that these chemical constituents have a higher chance to evaporate under normal conditions. For air sparging to be effective, a vapor pressure higher than 5 mm Hg at standard conditions is desired. In addition, air sparging is also used to remediate source contamination, especially if the source is close to or at the water surface. It should be noted, however, that this method shouldn’t be used in confined aquifers, since there is little chance for the vapors to volatize to the surface. A prime example of this would be a gasoline underground storage tank (UST). On occaision, a leaking UST is located within the seasonal ground-water level fluctuation zone. If this is the case, an air sparge system might be a viable option. Unlike an SVE system, where the introduction of ground water into the extraction wells can lead to potential problems, an air sparge (AS) system is designed to be working within the saturated zone. If the system is designed correctly, an AS system can quickly make short work of a contaminated source area, making sure that the source is volatilized and diluted enough to make it a less hazardous situation. Once that occurs, ongoing bioremediation or removal through an SVE system can occur.
While an air sparge remediation system can be utilized to help remove residual soil contamination and the dissolved ground-water phase, it can also be used to introduce chemical agents that can be used to accelerate the degradation process. For example, an air sparge well system can be used to introduce higher concentrations of hydrogen peroxide as well as an oxidizing agent such as iron. This combination allows hydroxyls (radicals) to form, which in turn help to break down complex chlorinated solvents into smaller portions, which in turn can be either volatilized more easily, or degraded further through the use of microbes already contained within the soil.
In order for an AS system to be an effective remediation system, there are a number of site characteristics that much be considered. The first is the distribution of contaminants as related to the potential air distribution in the target treatment zone. The physical characteristics of the soil, such as composition, texture, porosity and permeability can easily affect the air flow into the saturated and vadose zones, so detailed information about the geological and hydrological characteristics of the soil is paramount for a good system design.
In addition to knowing about the site, it is extremely important to understand the nature of the contaminants of concern while using an AS system. By understanding the chemical and physical properties of the contaminants such as its density, solubility, and chemical bonds can allow an AS system to be designed to introduce the most effective means of degrading the contaminants.
With all characteristics being considered, AS systems usually operate best in situations where there is a contaminant of higher volatility, and in an environment with an increased amount of potential air channels in the soil. Soils that contain an abundance of sands or gravels will usually find that AS systems to be more effective than ones with higher clay concentrations, for example. In addition, areas with large macro scale heterogeneities, such as clay or silt lenses in areas of coarse sand can potentially inhibit the effectiveness of this remediation technique. The differing soil textures and compositions can potentially affect the uniform air distribution that is integral in an effective AS remediation.
When to Use it and When to Look to Other Alternatives
While air sparging is often a cost effective method of remediation, there are certain situations where other options might be more effective. Of course, as any remediation designer or engineer knows, situations are always changing, so re-evaluation is necessary.
If free product (example: undiluted gasoline) is present at the site, air sparging is not only an ineffective choice, but it can potentially create ground-water mounding, which could cause free product to migrate and the contamination to spread. So, before implementing an AS system, make sure all free product has been removed from the remediation site. Another area characteristic that should be taken into account is the presence of nearby basement, sewers, or other subsurface confined spaces. These areas can potentially become sites of contaminant or vapor accumulation, especially if an effective SVE system is not utilized. Since many of the contaminants remediated in this manner have adverse effects on the human body in higher concentrations, it is extremely important that precautions are taken, and routine monitoring is undertaken during the remediation process. Finally, if the contaminated ground water is located within a confined aquifer system, air sparging is not the best remediation option. Once the air is injected into the ground water of the confined aquifer it would be trapped there, and not escape to the unsaturated zone, making the whole AS system essentially ineffective.
Overall Cost Effectiveness of Air Sparging
Air sparging, along with SVE sytems are some of the most cost effective in-situ remediation techniques available on the market today. The systems are usually easy to install and maintain, and can also be set up as temporary systems. In addition, they can effectively reach greater depths and lateral extents than a typical excavation can. AS systems are also effective at removing a number of contaminants, including many of the light non-aqueous phase liquids (LNAPLs) such as gasoline and solvents that are involved in many cotaminated sites.