Remediation of TNT and RDX in Groundwater Using Zero-Valent Iron Permeable Reactive Barriers

Site Name:

Cornhusker Army Ammunition Plant (CAAP)


Grand Island, Nebraska

Period of

November 2003 to July 2005 (20 months)


Pilot Scale

Zero-Valent Iron Permeable Reactive Barriers (ZVI PRB)

  • The in situ remediation system involved emplacement of iron filings/shavings in a trench or similar structure oriented perpendicular to groundwater flow to form a PRB or "wall."
  • A mixed iron/sand PRB (30% by weight iron) was used. The PRB was approximately 50 feet (ft) long by 15 ft deep by 3 ft thick.
  • The PRB was located within a large groundwater plume from a diffuse source resulting from the production of munitions.
  • A network of groundwater sampling points was installed around the PRB and sampled periodically over a 20-month period.

Cleanup Authority:
Environmental Security Technology Certification Program (ESTCP)


Rick Johnson
Oregon Health and Science University
20000 N.W. Walker Road
Beaverton, OR 97006
Telephone: 503-748-1193
Fax: 503-748-1273

Paul Tratnyek
Oregon Health and Science University
20000 N.W. Walker Road
Beaverton, OR 97006
Telephone: 503-748-1023
Fax: 503-748-1273

Tom Krug
GeoSyntec Consultants
130 Research Lane, Suite 2
Guelph, ON N1GSG3
(519) 822-2230, Ext. 242
(519) 822-3151

2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-trinitro-1,3,5-triazine & cyclonite (RDX)

Waste Source:
Munitions loading, assembly, and packing operations

Type/Quantity of Media Treated:

Purpose/Significance of Application:
The purpose of the demonstration was to evaluate the performance and cost effectiveness of ZVI PRB for treating explosives-contaminated groundwater.

Regulatory Requirements/Cleanup Goals:
Two main performance objectives were established for the site: 1) Minimal effort to operate (i.e. passive operation after installation) 2) Greater than a 90% reduction (or less than 1 micrograms/liter (µg/L)) in concentration of TNT and RDX downgradient of the treatment zone

Data from the demonstration site indicated the following:

  • All the primary performance criteria for this project were met. The ZVI PRB reduced concentrations of TNT and RDX to below detection limits for the duration of the project.
    Significant changes in groundwater chemistry occurred up to at least 8 meter (m) downgradient due to the PRB. Dissolved sulfate concentrations decreased substantially as groundwater flowed through the PRB.

  • In situ reactivity testing suggests that the PRB has the capacity to degrade explosives for an extended period of operation. The hydraulics of the PRB are more likely to limit long-term performance at this site.

  • Guar, used in installation of the PRB, may have been interfered with the flow of water through the PRB and caused a portion of the water upgradient of the PRB to be diverted beneath it. This was likely the result of a combination of factors including the presence of guar remaining in the subsurface following PRB installation.

Cost Factors:
The installation costs for the pilot scale barrier were $138,000. The barrier was 50 ft long by 15 ft deep (i.e., ~750 ft2), so the cost per sq ft is approximately $180. Total demonstration costs (including site characterization, design, installation, and operation and maintenance) were $603,600.

The demonstration was conducted at the Cornhusker Army Ammunition Plant (CAAP) near Grand Island, Nebraska. CAAP was used for the production of artillery shells, mines, bombs and rockets. Explosive wastes and residues associated with munitions loading, assembly, and packing operations have resulted in a groundwater contamination plume that originates at former waste leach pits and cesspools associated with CAAP load lines and extends east-northeastward into the city. The main contaminants at the site are TNT and RDX. Groundwater at the site is at 15-20 feet (ft) below ground surface (bgs) and the groundwater velocity at the site is approximately 1 to 2 ft/day.

Demonstration activities included a field study that installed a ZVI PRB. The ZVI PRB creates strongly reducing conditions in water as it passes through the barrier. Those conditions lead to the abiotic reduction of a broad range of contaminants in the water. In this case, TNT and RDX in water are reduced to corresponding amino compounds within the PRB. The PRB was constructed at CAAP in November 2003. Preliminary monitoring of groundwater conditions in and around the PRB was conducted in December 2003, about 4 weeks after installation. A network of groundwater sampling points was installed around the PRB and sampled periodically over a 20-month period. The following parameters were monitored: changes in groundwater chemistry, the long-term performance of the PRB, groundwater flow through the PRB, and hydraulic conductivity. Samples were also collected and analyzed to determine the contaminants treated. The results of this demonstration showed that groundwater was flowing through the PRB and that the explosives were being reduced to below detection limits in groundwater. The results also showed that due to hydraulic conditions some groundwater passed under, rather than through, the PRB.