Title: Permeable Reactive Barriers (PRBs) Text: Permeable Reactive Barriers are passive underground structures, which are placed across the flow path and used to treat contaminated groundwater at hazardous waste sites. The reactive cell can be filled with various treatment media, which are selected for their ability to clean up specific types of contaminants. Use your mouse to roll over areas of the graphic for more information.

Title: Permeable Reactive Barriers (PRBs) Text: PRBs can be used to treat a wide variety of contaminants including chlorinated solvents, organics, inorganics, metals, and radionuclides.

Title: Permeable Reactive Barriers (PRBs) Text: The range of contaminants that can be treated through PRBs is increasing due to the use of new reactive media. PRB media generally are classified as reactive, adsorptive, or biodegradation-enhancing.

Title: Permeable Reactive Barriers (PRBs) Text: As groundwater flows through the barrier, the contaminants come into contact with the reactive media and are degraded into nontoxic compounds. For example, zero-valent iron will react with chlorinated solvent compounds to form ethene and chloride as the end products. Although the groundwater exiting the barrier has been treated to target cleanup levels, it is not necessarily potable (drinking) water.

Title: Continuous Reactive Barrier Text: PRB configuration refers to the manner in which groundwater is directed through the treatment unit. One of the most common configurations is a continuous reactive barrier, where the treatment wall extends across the entire width and depth of the contaminant plume.

Title: Potential Flow Problems Text: If groundwater flow is irregular or changes seasonally, a continuous reactive barrier will not always be effective for plume containment.

Title: Funnel and Gate System Text: Another common configuration is the funnel-and-gate, where impermeable walls guide the influent groundwater through one or more treatment gates.

Title: Monitoring Considerations Text: Thorough site characterization, including characterization of the plume, hydrogeology, and geochemistry, is required on the more local scale of a prospective PRB location. Groundwater flow and solute transport modeling help address the variability and uncertainty inherent in most aquifers and PRB systems.

Title: Monitoring Well Placement Text: Thorough compliance and performance monitoring is critical to successful implementation of PRBs. Monitoring wells are typically located on all sides of the PRB, within the treatment medium, and at numerous upgradient and downgradient locations. Normal compliance monitoring parameters include the target contaminants, degradation products, and general water quality parameters. Typical performance monitoring parameters include hydrologic parameters, Eh, dissolved oxygen, and geochemical parameters (e.g., calcium, magnesium, and alkalinity). These parameters are used to monitor for potential loss of reactivity, decrease in permeability, decrease in residence time in the treatment zone, short-circuiting, and leakage.

Title: Definition of Eh Text: The redox potential (Eh) is a measurement of the “concentration” of electrons in groundwater. It indicates whether the groundwater is under oxidizing conditions (Eh > +200 mV) or whether the groundwater is under reducing conditions (Eh < +200 mV).

Title: Monitoring Hydraulic Performance of PRBs Text: Water level measurements are used to evaluate bulk flow and a wide variety of flow sensors can be used to measure point flow. However, even with water level measurements, determining the hydraulic capture zone in the upgradient aquifer and residence time in the PRB can be challenging. Groundwater flow and solute transport modeling can be used to model various flow scenarios and obtain a more robust design. Appropriate safety factors should be incorporated into the design in order to address inevitable uncertainties in hydraulic flow and long-term plume influent concentrations. Assessing the longevity and the hydraulic performance of the barrier media is essential for performance monitoring of PRBs.

Title: Media Types Text: Zero-valent iron currently is the most common reactive material used in a PRB, but a variety of other adsorptive, reactive, and biodegradation-enhancing materials also are being used. Assessing the longevity and the hydraulic performance of the barrier media is essential for performance monitoring of PRBs.

Title: PRB Construction Text: As the number of PRB field applications has grown, so too has the sophistication, reliability, and number of commercially available construction techniques adapted to PRB installation. The use of special geo­technical methods, such as jetting and hydraulic fracturing, has improved the ability to access deeper aquifers. One important advance in the use of backhoe trenching is the use of a biodegradable slurry (or bio­slurry) to shore up the excavation. Bioslurry is a highly viscous solution of guar gum that keeps the excavation open and eliminates the need for sheet piling or trench boxes. The PRB reactive material, usually iron, is tremied into the excavation and allowed to settle through the slurry.

Title: Advances in PRB Construction Techniques Text: Construction methods have improved to allow PRBs to be placed at a depth of up to 120 feet or more, and the development of bioslurries have allowed for more precise placement of the barriers. Click on a technique to learn more about it.

Title: Construction Methods Text: Conventional backhoes excavate trenches up to 5.6 feet wide and up to 30 feet bgs. Trench boxes or sheet piling are used to stabilize the excavation prior to back­filling with treatment medium. Modified backhoes used recently at PRB sites have reached depths of 80 feet, although slurry is required to keep the trench open. Crane-operated clamshells have been used to excavate trenches to depths of 120 feet bgs with the help of trench support slurry. This technique has been implemented at PRB installations in Sunnyvale, CA, and former NAS Moffett Field, CA.

Title: Construction Methods Text: The use of biodegradable slurry (bioslurry) is a recent advance in PRB trenching. The jelly-like guar gum/water mixture is added to the trench to stabilize the excavation walls, thereby eliminating the need for trench boxes or sheet piling. Preservatives and pH adjustments prevent bioslurry breakdown during construction. After the reactive medium (usually iron) has been installed, bioslurry degradation is initiated by adding a liquid enzyme breaker. This technique has been implemented at several sites, including Pease AFB, NH and Somersworth Landfill Site, NH.

Title: Construction Methods Text: Caissons are large-diameter load-bearing enclosures that are driven into the ground. Once installed, the native material is excavated and replaced with the treatment medium, and the caisson then is removed. Caissons were used to install the Savannah River Site Geosiphon (25 feet deep), and the Dover AFB treatment gates (45 feet deep).

Title: Construction Methods Text: The continuous trencher is equipped with a boom apparatus similar to a chainsaw for excavation, a trench box for stabilizing the trench walls, and a hopper to backfill the excavation with reactive medium. It can excavate and immediately backfill trenches 1 to 2 feet wide. This technique has been implemented at several sites, including U.S. Coast Guard site, NC, and Naval Weapons Industrial Reserve Plant, TX.

Title: Construction Methods Text: Two or three special augers equipped with mixing paddles are lined up in series. As the augers penetrate the ground, they mix fine iron and soil together. The iron also can be introduced in biodegradable slurry. Alternatively, iron-filled casings can be driven into the ground with a vibratory hammer, and the iron later mixed with the soil using the mixing paddles. A variation of this method was used at Launch Complex 34, Cape Canaveral Air Station, FL.

Title: Construction Methods Text: A series of wells are installed along the length of the PRB. A vertical fracture is propagated within each well, and the fractures are filled with granular iron/guar gum slurry. The reactive iron slurry in one fracture coalesces with the adjacent fracture, creating a continuous vertical wall. This technique has been implemented at Massachusetts Military Reservation and at the Caldwell Trucking Co. Site, NJ.

Title: Construction Methods Text: Jet grouting involves the injection of grout or slurry at high pressure into the ground. A triple-rod injection system delivers a high-pressure mixture of granular iron, guar gum, air, and water to the subsurface. Injection starts at the bottom of the PRB wall and continues as the rod is lifted, creating a column or panel of reactive medium. Multiple rows of overlapping columns or panels create the continuous passive treatment wall. This technique has been implemented at Travis AFB, CA.

Title: Construction Methods Text: This technique involves driving an H-beam or mandrel with a sacrificial shoe at the bottom to create a void space. As the beam is raised, a slurry or grout containing the reactive medium is injected into the void space through a special nozzle at the bottom of the beam. By driving beams in an overlapping panel, a continuous treatment wall is created. This barrier can be installed at an angle up to 45 degrees to avoid utilities or structures. The vibrated beam technique was used at an industrial site in Tifton, GA, and a mandrel was used at Hangar K, Cape Canaveral Air Station, FL.

Title: Advantages/Limitations Text: The following are the advantages/limitations for the use of PRBs: Advantages: In situ remediation Passive operation No required above ground structures Potentially less expensive than pump-and-treat systems. Limitations: Mass flux of certain native dissolved solids can cause a decrease in PRB longevity When barrier reactivity decreases by a factor of 2, the barrier should either be replaced or regenerated, or another remedial action should be implemented. Determining hydraulic capture zone in upgradient aquifer and residence time in PRB can be challenging without use of groundwater flow and solute transport modeling.

Title: Feedback Text:

Title: Contact Information Text: For more information about PRBs, please contact: NFESC POC (805) 982-1656 PRTH_NFESCT2@navy.mil