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Deep Wall Emplacement
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The Technology
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Permeable reactive barriers (PRBs) are subsurface structures that are placed across the flow path of a contaminant plume. As the plume passes through the PRB, the reactive material precipitates, sorbs, or degrades the contaminants. These barriers are commonly constructed with zero-valent metals, such as iron, to treat groundwater contaminated with chlorinated solvents. Through a process known as reductive dechlorination, the chlorinated solvents react with surface-bound Fe(II) at the iron-water interface. The result is contaminant degradation.
The iron used in PRBs may be installed as pure iron filings, or mixed with a substrate (e.g., sand, gravel) to improve permeability. Although treatment barriers are a recent innovation, they have proven to be an effective remediation technology and are becoming more widely used. PRBs have been used at more than 120 sites worldwide.
| How It Works |  |
NASA's Deep Wall Emplacement Method for installing permeable reactive barriers (PRBs) involves the following steps
- Several feet (typically 4 feet) of surface soil are removed.
- A deep soil mixing rig is used to loosen the area where the barrier will be emplaced.
(Video of steps 1-2)
- Using vibration techniques, a hollow casing is inserted into the subsurface to the desired depth (usually to the containment layer).
- The casing is filled with zero-valent reactive material, along with bulking material to increase permeability.
(Video of steps 3-4)
- When the casing is removed, the column of reactive material is left in place.
- The previous steps are repeated until an adequate number of columns have been emplaced. (Extensive modeling determines the optimum number of columns and spacing.)
- The deep soil mixing rig blends the reactive material with the soil surrounding the columns.
(Video of steps 5-7)
NASA’s Deep Wall Emplacement integrates the strengths of both caisson-based emplacement and deep soil mixing. Caisson-based emplacement allows for the installation of PRBs at significant depths without dewatering. However, this technique requires existing soils to be augered out of the center of the caissons. The result is large quantities of contaminated soils that require treatment or disposal.
By first using deep soil mixing to loosen the subsurface, caissons can be emplaced without removing significant amounts of soil. In addition, using deep soil mixing after caisson emplacement helps increase soil permeability. This process ultimately enhances the hydraulic gradient around the wall, improving PRB performance.
A field demonstration of NASA's Deep Wall Emplacement Method was completed at Kennedy Space Center Launch Complex 34 in 1998. The technology was used at the site to install a PRB designed to treat groundwater contaminated with chlorinated solvents. Based on analytical results, the majority of the contaminants were degraded below target levels after passing though only a portion of the treatment barrier.
| Benefits | |
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Allows emplacement of reactive barriers at depths of 100 feet or greater
(Video illustrating depth capability) |
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Lowers waste disposal costs by reducing the amount of spoils generated |
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Promotes increased permeability in soil around the barrier |
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Eliminates risks associated with open excavations |
| Commercial Uses | |
As a result of the U.S. Environmental Protection Agency's regulations enforcing the cleanup of contaminated sites, this technology has great potential for use in the groundwater remediation industry. Between 1992 and 2000, the number of active groundwater treatment projects in the United States jumped from 2,692 to 7,633. According to a report from the Groundwater Remediation Technologies Analysis Center, over half of the known treatment barrier projects target organic contaminants, such as chlorinated solvents. Trichloroethene (TCE), the most common chlorinated contaminant, has been found at more than 790 of 1,300 sites on the National Priority List.
NASA's Deep Wall Emplacement Method can be used to install iron barriers at a variety of sites including:
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Electronic Manufacturers |
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Plating Facilities |
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Chemical Manufacturers |
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Dry Cleaners |
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Sanitary and Industrial Landfills |
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Automobile and Aircraft Maintenance Facilities |
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Semiconductor Manufacturers |
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Solvent Recycling Facilities |
Click here for licensing information on Deep Wall Emplacement.
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