Citation:

Blowes, D. W., R W. Puls*, AND R. W. Gillham. AN IN-SITU PERMEABLE REACTIVE BARRIER FOR THE TREATMENT OF HEXAVALENT CHROMIUM AND TRICHLOROETHYLENE IN GROUND WATER: VOLUME 1 DESIGN AND INSTALLATION. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-99/095a (NTIS PB2000-105970), 2000.

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Description:

A 46 m long, 7.3 m deep, and 0.6 m wide permeable subsurface reactive wall was installed at the U.S. Coast Guard (USCG) Support Center, near Elizabeth City, North Carolina, in June 1996. The reactive wall was designed to remediate hexavalent chromium [Cr(VI)] contaminated ground water at the site, in addition to treating portions of a larger overlapping trichloroethylene (TCE) ground-water plume which has not yet been fully characterized. The wall was installed in approximately 6 hours using a continuous trenching technique, which simultaneously removed aquifer sediments and installed the porous reactive medium. The reactive medium was composed entirely of granular iron, with an average grain size (d50) of 0.4 mm. The reactive medium was selected from various mixtures on the basis of reaction rates with Cr(VI), TCE and degradation products, hydraulic conductivity, porosity, and cost. The continuous wall configuration was chosen over a Funnel-and-Gate configuration, based on three-dimensional computer simulations of ground-water flow and contaminant transport, and cost. The simulations indicated that both configurations could be designed to achieve the same capture areas and residence times with the same volume of reactive material. However, initial cost comparisons suggested that a reactive wall would have a lower material and installation cost than a Funnel-and-Gate. For this site, the installation and material cost was approximately $7550 U.S./linear meter for a 46 m long, 7.3 m deep and 0.6 m wide continuous reactive wall. The minimum required width of the granular iron wall was determined from simulations of TCE decay within the barrier, rather than Cr(VI) reduction because Cr(VI) reaction rates are significantly faster. Simulations of contaminant transport within the granular iron wall indicate that 10,000 g/L TCE, 900 g/L cis-dichloroethylene (cDCE) and 101 g/L vinyl chloride (VC) are reduced to less than maximum contaminant level (MCL) values of 5, 70, and 2 g/L respectively, within 0.3 m of travel through the wall under the maximum flow velocities expected at the site. The total project cost, including site assessment, reactive barrier design, installation, soil treatment and follow-up, was approximately $985,000 U.S. The U.S. Coast Guard anticipates that using this reactive barrier will result in a saving of $4 million U.S. in operation and maintenance costs over a 20 year period, compared to a pump-and-treat system.

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