A permeable reactive barrier (PRB) is a zone of reactive material placed underground to intercept and react with a contaminant plume in ground water. Typically, PRBs are emplaced by replacing soils with reactive material in a trench cut through a contaminated ground water aquifer. PRBs can also be installed above ground to provide access to the treatment media. Above ground systems are also referred to as ground water treatment cells.

PRBs can be installed using several methods including trenching or through injection wells. Treatability studies are typically conducted to evaluate the performance of reactive materials to treat ground water under site-specific conditions. Selection of material for the barrier is based on results of these treatability studies. Computer modeling is used to simulate the complex chemical reactions that occur in a PRB.

The material in the barrier is permeable, which allows the ground water or contaminant plume to flow through the barrier. When the targeted contaminant encounters the reactive material in the barrier, a chemical reaction occurs with the barrier material that results in adsorption, mineral precipitation, or degradation to a harmless compound. Reactive barriers that do not incorporate motors or mechanical devices are considered passive treatment.

PRBs are widely used to control organic contamination in ground water; however, less effort has been made to apply the technologyto metal and radionuclide contaminants. This technology may be more cost effective than pump-and-treat methods.

Field Projects

The U.S. Department of Energy (DOE) installed PRBs at several Legacy Management (LM) sites. Installation of a PRB hydraulically downgradient of the Monticello, Utah, millsite was completed June 30, 1999, as an Interim Remedial Action. DOE also installed a PRB in October 1995 to treat ground water from a uranium mill tailings disposal site at Durango, Colorado. In addition to operating the PRBs, ESL personnel conduct tests and help evaluate performance at other PRB sites, such as Cotter Corporation’s Cañon City site in Colorado.

Monticello, Utah – A PRB of zero-valent iron is helping to clean up ground water at a former uranium and vanadium ore processing mill at Monticello, Utah. DOE Legacy Management managed remediation of tailings and tailings-contaminated material at this site. Cleanup of the millsite is regulated under the Comprehensive Environmental Response, Compensation, and Liability Act. 

Arsenic, molybdenum, nitrate, selenium, uranium, and vanadium are contaminants of concern in ground water at the site. An Interim Record of Decision designated emplacement of a PRB hydraulically downgradient of the millsite to remove these contaminants. Results of both laboratory and field treatability studies were used to help design the PRB and its reactive material components. The Accelerated Site Technology Deployment Program within DOE's Office of Science and Technology funded design, installation, and initial monitoring of the PRB.

An extensive monitoring network was installed during summer 1999 to evaluate the performance of the PRB. The PRB has been effective in reducing contaminant concentrations. Concentrations of arsenic, selenium, uranium, and vanadium are reduced to nondetectable levels on the downgradient side of the PRB. In addition, concentrations of molybdenum and nitrate are reduced to near nondetectable levels. As expected, concentrations of iron increase as ground water passes through the PRB. Concentrations of iron exiting the wall are lower than expected (based on the treatability studies) and are well within an acceptable risk range.

The Variation in Hydraulic Conductivity Over Time at the Monticello Permeable Reactive Barrier document describes the hydraulic behavior of the PRB since its installation based primarily on results of three separate rounds of slug tests and routine water-level monitoring.

The Alternatives for Mending a Permeable Reactive Barrier at a Former Uranium Milling Site: Monticello, Utah, report evaluates seven technologies that might be used to mend areas where ground water mounding upgradient from the PRB occurs. The report also includes laboratory results from one possible mending option, injection of ferrous iron. Results suggest that all seven alternatives are nearly cost competitive.

DOE and the U.S. Environmental Protection Agency are conducting additional investigations of rejuvenation and performance assessment. Results of portions of this investigation are provided in Final Report Phase II: Performance Evaluation of Permeable Reactive Barriers and Potential for Rejuvenation by Chemical Flushing, U. S. Environmental Protection Agency Region 8 Support and Final Report: Rejuvenating Permeable Reactive Barriers by Chemical Flushing, U.S. Environmental Protection Agency, Region 8 Support.

Steel wool installation in a PRB
using a drain-field design at the
Durango, Colorado, Disposal Site