Installation Date:
06/01/1996

Contaminants:
Chromium, Trichloroethene

Reactive Media:
Fe⁰

Cost:
$675,000

Construction:
Continuous trench

Point of Contact:
Robert W. Puls
USEPA
Tel: 580-436-8543
Fax: 580-436-8706
Email: puls.robert@
epa.gov
National Risk Management Research Laboratory
P.O. Box 1198
Ada , OK 74820

A full-scale demonstration of a permeable reactive barrier (PRB) to remediate ground water contaminated with chromium and chlorinated organic compounds was initiated at the U.S. Coast Guard Support Center site in Elizabeth City, NC, in 1995. The primary contaminants of concern are hexavalent chromium (Cr⁺⁶) and trichloroethylene (TCE). Initial maximum concentrations were more than 4,320 µg/L for TCE and more than 3,430 µg/L for Cr⁺⁶. The contaminant plume was estimated to cover a 34,000-ft² area. The plume is adjacent to a former electroplating shop that operated for more than 30 years prior to 1984 when operations ceased.

Ground water begins approximately 4-6 ft below ground surface, and a highly conductive zone is located 15-21 ft below the surface. This layer coincides with the highest aqueous concentrations of chromium and chlorinated organic compounds found on the site. A low-conductivity layer—clayey, fine sand to silty clay—is located at a depth of about 22 ft. This layer acts as an aquitard to the contaminants located immediately above.

A continuous wall composed of 100% zero-valent iron (Fe⁰) was installed in June 1996 using a trencher that was capable of installing the granular iron to a depth of 24 ft. The continuous trenching equipment used for the installation has a large cutting chain excavator system to remove native soil combined with a trench box and loading hopper to emplace the iron.

The trenched wall was designed to meet cleanup goal concentrations of 0.05 mg/L of Cr⁺⁶ and 5 µg/L of TCE. It is approximately 2 ft thick and about 150 ft long. The wall begins about 3 ft below ground surface and consists of about 450 tons of granular iron. The total installation cost was $500,000. This includes the cost of design, construction, materials, and the iron, which cost about $175,000.

Performance monitoring has been conducted on a quarterly basis since November 1996. In addition to compliance wells (2-in PVC), the wall is monitored using a series of multilevel sampling (MLS) ports to monitor the geochemical mechanisms occurring in the barrier and in the downgradient aquifer. As of June, 2001 sampling results for chromium indicate that all chromium continues to be removed from the ground water within the first 6 in of the wall as expected. No chromium has been detected downgradient of the wall either in the MLS ports or in the compliance wells located immediately behind the wall. Results indicated that the barrier was successfully reducing TCE, c-DCE, and vinyl chloride concentrations to less than MCL levels for the vast majority of the monitored portions of the wall. Of 29 downgradient MLS ports, MCLs for TCE and vinyl chloride were exceeded in 1 and 3 ports, respectively. TCE concentrations were generally below 5 µg/L within the wall, but exceeded 50 µg/L at the lowest depth. There were some indications that the TCE plume may have dipped lower in this part of the aquifer following wall installation. The slight elevation beyond target levels for vinyl chloride seen in the MLS ports were not reflected in adjoining compliance wells. Downgradient vinyl chloride concentrations in the MLS ports had declined with time. Nowhere did c-DCE concentrations exceed regulatory limits.

There has now been 5 years of post installation performance monitoring of the system and continued long-term performance is being assessed as part of a new project entitled “Long-term” performance monitoring for permeable reactive barriers.

Lessons Learned

Researchers have investigated the possibility that the TCE plume dipped lower in the aquifer after the wall was installed and a small portion was moving under the wall. A significant amount of recharge occurred into the reaction zone following installation due to removal of the concrete parking lot covering the site. This recharge may have driven the plume deeper than had previously been observed allowing some of the plume to move under the wall. Interestingly, there was still significant treatment below the wall where no iron resides.

Based on limited preliminary electrical conductivity profiles, the wall is approximately 19-21 inches thick, compared to the design thickness of 23 inches. Some minor vertical discontinuities were observed in the conductivity data and have been confirmed with coring. These small gaps are probably due to bridging within the trencher hopper during iron emplacement.

Analysis of iron cores have indicated a slow rate of corrosion resulting in a loss of porosity of only about 1-2% of total porosity per year. Loss of porosity is not expected to affect wall permeability for at least 10 years. No changes in hydraulics at the site have been observed after 5+ years of quarterly monitoring.

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