Dual-phase Extraction (DPE)
Dual-phase extraction (DPE), also known as multi-phase extraction, is a technology that uses a high-vacuum system to remove both contaminated groundwater and soil vapor. In DPE systems a high-vacuum extraction well is installed with its screened section in the zone of contaminated soils and groundwater. Fluid/vapor extraction systems depress the water table and water flows faster to the extraction well. DPE removes contaminants from above and below the water table. As the water table around the well is lowered from pumping, unsaturated soil is exposed. This area, called the capillary fringe, is often highly contaminated, as it holds undissolved chemicals, chemicals that are lighter than water, and vapors that have escaped from the dissolved groundwater below. Contaminants in the newly exposed zone can be removed by vapor extraction. Once above ground, the extracted vapors and liquid-phase organics and groundwater are separated and treated. Use of dual-phase extraction with these technologies can shorten the cleanup time at a site, because the capillary fringe is often the most contaminated area.
In DPE, gas and liquids are conveyed from the extraction well to the surface in separate conduits by separate pumps or blowers. With a similar technology called two-phase extraction (in 1997, EPA distinguished dual-phase and two-phase extraction technologies), soil gas and liquid are conveyed from the extraction well to the surface in the same conduit. A single vacuum source (vacuum pump or blower) is used to extract both liquid and gaseous phases. This latter version is used more often when fuel contamination is present.
Limitations and Concerns
Site geology and contaminant characteristics influence the effectiveness of this technology.
Dual phase extraction requires both water treatment and vapor treatment.
Two-phase extraction requires an oil/water separator.
Technology Development Status
DPE is commercially available.
Other Resources and Demonstrations
A large number of private and public sites have used this technology. These include Air Force Plant 44 in Tucson (AZ), Fort Drum (NY), Lockheed-Burbank (CA), and a number of small gasoline stations.