Contaminants:
Trichloroethene, 1,1-Dichloroethene, Vinyl chloride

Reactive Media:
Fe⁰

Construction:
Continuous Walls with Overlapping Panels

Point of Contact:
Jerry Hansen
Technology Center Division
Tel: 210-536-4353
Fax: 210-536-4330
Email: jerry.hansen@
hqafcee.brooks.af.mil
Environmental Restoration Directorate
U.S. Air Force Center for Environmental Excellence
3207 North Road
Brooks AFB , TX 78235-5363

Side-by-side, pilot-scale demonstrations of two emplacement techniques for permeable reactive barriers (PRBs) are being conducted at the industrial area of Cape Canaveral Air Station, FL. The site is contaminated with 90 mg/L of trichloroethylene (TCE), 7 mg/L of vinyl chloride (VC), and 170 mg/L of dichloroethylene (DCE).

The water table at the site is about 5 ft below ground surface. Ground-water flow is in the range of 0.1-0.5 ft/day and changes with depth.

A major objective of the demonstration was to compare the two emplacement methods. Both wall systems included a 50-ft main wall followed by 10-ft wall placed 4 ft downgradient from it and a third 10-ft wall placed 4 ft downgradient of the second. This provided a total target length of 70 linear feet for each technique. In the first installation, a hollow mandrel, or vibrated beam, created a void that is 4 in thick, about 45 ft deep, and 32 in long for each panel of the wall. A vibratory hammer drove the beam to the required depth. The void was filled with the reactive material through a chute at the top of the mandrel. About 98 tons of 100% zero-valent iron was used to construct the wall, and adjacent panels were overlapped to provide continuity in the wall. In the second installation, high-pressure water jets, guided by a 36-in I-beam, were used in addition to the water to create the void for each wall panel. A vibratory hammer was used to drive the beam to depth. The void was filled with a slurry made by mixing zero-valent iron with guar gum and a binder. About 107 tons of zero-valent iron was used for this emplacement. As in the first installation, adjacent wall panels were overlapped to provide continuity.

Total cost for the two barriers at this site was $809,000. This includes design, construction, materials, and the reactive media. The design cost for both walls totaled $292,000. Mobilization and demobilization, construction, materials, and the reactive material for the mandrel system was $279,000. Mobilization and demobilization, construction, materials, and the reactive material for the jet-assisted grout system was $238,000.

Dedicated in situ flow sensors and ground-water monitoring wells were installed after construction of the walls to track performance.

Quarterly monitoring is scheduled to continue until November 1998, and a report of demonstration results is expected to be issued in 1999.