In-Well Air Stripping
In-well air stripping technology injects air into a vertical well that has been screened at two depths. The lower screen is set in the saturated zone (i.e., groundwater), and the upper screen is in the unsaturated zone (i.e., called the vadose zone, otherwise referred to as soil). Pressurized air is injected into the well below the water table, aerating the water. The aerated water rises in the well and flows out of the system at the upper screen. Contaminated water is drawn into the system at the lower screen. The volatile organic compounds (VOCs) vaporize within the well at the top of the water table, as the air bubbles out of the water, very similar to an above-ground air stripper. The vapors are drawn off by a soil vapor extraction (SVE) system and treated.
The partially treated groundwater is never brought to the surface. After it is released to the unsaturated zone, the water percolates back down to the groundwater. Contaminant concentrations are gradually reduced as the process is repeated.
Limitations and Concerns
These systems only treat the water that passes through the stripping well. Thus the radius of influence is limited by the pumping capacity of each well and the hydrogeologic characteristics of the site. Much remains to be learned about specific applications of in-well stripping, particularly regarding the questions of radius of influence and groundwater flow regime around the well.
Effective installations require a well-defined contaminant plume and well-placed screens to prevent the spreading of the contamination.
Some contaminants are transferred from the groundwater to the soil. Depending on site geology, these contaminants may be sorbed to soil particles and slowly desorbed after treatment is complete.
In general, in-well air strippers are most effective at sites containing high concentrations of dissolved contaminants.
Effectiveness may be limited in shallow aquifers.
To prevent smearing the contaminants in the area immediately above the groundwater level, the process should not be used at sites containing non-aqueous phase liquids (NAPLs).
In-well stripping generally takes a very long time.
This technology remediates VOCs, some of the lighter semi-volatile organic compounds (SVOCs), and fuels in groundwater. When combined with a vacuum extraction system, it also remediates contaminated soil. Typically, in-well air stripping systems are more cost-effective for remediating VOC-contaminated groundwater at sites with deep water tables because the water does not need to be brought to the surface.
Technology Development Status
In-well air stripping is a commercial technology. There are several variations of this technology. The most common is called Unterdruck-Verdampfer Brunner (UVB) and has been used at numerous sites in Germany.
Stanford University has developed another variation of this process called NoVOCs that has been evaluated as part of the U.S. Department of EnergyÍs Integrated Technology Demonstration Program and by the U.S. Navy. Wasatch Environmental, Inc. has developed an in-well vapor stripping system that enhances bioremediation and discharges extracted vapors into the unsaturated zone for degradation by naturally-occurring microorganisms.
Other Resources and Demonstrations