Description:

Figure 4-39b:
Typical NoVOCs^TM system

Figure 4-39c:
Typical DDC system  

In-well air stripping technology air is  injected into a vertical well that has been screened at two depths. The lower screen is set in the groundwater saturated zone, and the upper screen is in the unsaturated zone, often called as  vadose zone. 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 groundwater 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. The vapors are drawn off by a soil vapor extraction (SVE) system. The partially treated ground water is never brought to the surface; it is forced into the unsaturated zone, and the process is repeated as water follows a hydraulic circulation pattern or cell that allows continuous cycling of ground water. As ground water circulates through the treatment system in situ, contaminant concentrations are gradually reduced. In-well air stripping is a pilot-scale technology.

Modifications to the basic in-well stripping process may involve additives injected into the stripping well to enhance biodegradation (e.g., nutrients, electron acceptors, etc.). In addition, the area around the well affected by the circulation cell (radius of influence) can be modified through the addition of certain chemicals to allow in situ stabilization of metals originally dissolved in ground water.

The duration of in-well air stripping is short- to long-term, depending contaminant concentrations, Henry's law constants of the contaminants, the radius of influence, and site hydrogeology.

Circulating Wells

Circulating wells (CWs) provide a technique for subsurface remediation by creating a three-dimensional circulation pattern of the ground water. Ground Water is drawn into a well through one screened section and is pumped through the well to a second screened section where it is reintroduced to the aquifer. The flow direction through the well can be specified as either upward or downward to accommodate site-specific conditions. Because ground water is not pumped above ground, pumping costs and permitting issues are reduced and eliminated, respectively. Also, the problems associated with storage and discharge are removed. In addition to ground water treatment, CW systems can provide simultaneous vadose zone treatment in the form of bioventing or soil vapor extraction.

CW systems can provide treatment inside the well, in the aquifer, or a combination of both. For effective in-well treatment, the contaminants must be adequately soluble and mobile so they can be transported by the circulating ground water. Because CW systems provide a wide range of treatment options, they provide some degree of flexibility to a remediation effort.

Synonyms:

Applicability:

CW systems are most effective at treating sites with volatile contaminants with relatively high aqueous solubility and strong biodegradation potential, e.g., halogenated and non-halogenated VOCs. CWs operate more efficiently with horizontal conductivities greater that 10^-3 cm/sec and a ratio of horizontal to vertical conductivities between 3 and 10. A ratio of less than 3 indicates short circulation times and a small radius of influence. If the ratio is greater that 10, the circulation time may be unacceptably long.

Limitations:

• UVB-type systems only treat the water in the stripping well.
• In general, in-well air strippers are more effective at sites containing high concentrations of dissolved contaminants with high Henry's law constants.
• Fouling of the system may occur by infiltrating precipatation containing oxidized constituents.
• Shallow aquifers may limit process effectiveness.
• Effective CW installations require a well-defined contaminant plume to prevent the spreading or smearing of the contamination. They should not be applied to sites containing NAPLs to prevent the possibility of smearing the contaminants.
• CWs are limited to sites with horizontal hydraulic conductivities greater that 10^-5 cm/sec and should not be utilized at sites that have lenses of low-conductivity deposits.
• In well air stripping may not be efficient in sites with strong natural flow patterns.

Data Needs:

Performance Data:

Cost:

References:

Treatment Technologies for Site Cleanup: Annual Status Report (ASR), Tenth Edition, EPA 542-R-01-004

Innovative Remediation Technologies:  Field Scale Demonstration Project in North America, 2nd Edition

Abstracts of Remediation Case Studies, Volume 4,  June, 2000, EPA 542-R-00-006

Guide to Documenting and Managing Cost and Performance Information for Remediation Projects - Revised Version, October, 1998, EPA 542-B-98-007

Melinda A. Trizinsky, 1999. Groundwater Circulating Wells with In-Well Air-Stripping: Despite drawbacks, in situ technology holds promise.

• Fuel Dispensing Area 1595, Watertwon, NY
• Lawrence Livermore National Laboratory
• Site Demo March AFB, CA
• Savannah River Site, GA 1994
• NEL Demo: NAS North Island Installation Restoration (IR) Site 9, CA
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A list of vendors offering In Situ Physical/Chemical Water Treatment is available from EPA REACH IT which combines information from three established EPA databases, the Vendor Information System for Innovative Treatment Technologies (VISITT), the Vendor Field Analytical and Characterization Technologies System (Vendor FACTS), and the Innovative Treatment Technologies (ITT), to give users access to comprehensive information about treatment and characterization technologies and their applications.

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