Permeable Reactive Barrier (PRB) Design for Groundwater Remediation
A permeable reactive barrier (PRB) is a passive, in-situ groundwater remediation technology installed across the flow path of a contaminant plume. Groundwater passes through the barrier naturally while reactive materials — most commonly zero valent iron (ZVI) — chemically degrade or immobilize contaminants in place. Since the installation of the first full-scale PRB in Sunnyvale, California in 1995, more than 200 ZVI-based PRBs have been installed across North America, Europe, and internationally. Hepure has supplied over 1.5 million pounds of Ferox PRB reactive iron powder to hundreds of project sites and provides full PRB design and implementation support.
How a Permeable Reactive Barrier Works
A PRB is constructed by excavating a trench across the flow path of a contaminated groundwater plume and filling it with reactive media. The barrier is designed to remain permeable so that groundwater flows freely through it under natural hydraulic gradient — no pumps or active intervention required.
When contaminated groundwater contacts the ZVI within the barrier, chemical reactions degrade or immobilize target contaminants:
- Reductive dechlorination — ZVI reduces chlorinated organic compounds such as TCE and PCE into non-toxic end products like ethane and ethene
- Metal precipitation — ZVI converts soluble metals into insoluble forms. For example, toxic soluble Cr(VI) is reduced to insoluble Cr(III) hydroxide, which precipitates out of the water column
- Anion and oxyanion reduction — ZVI reductively precipitates contaminants such as arsenic, nitrate, and sulfate
- PFAS degradation — ZVI provides a reductive defluorination pathway for PFOS and PFOA treatment, an increasingly important application as PFAS regulations tighten
Once installed, a properly designed ZVI PRB operates passively for 10 years or more with minimal maintenance, making it one of the most cost-effective long-term groundwater remediation technologies available.
Figure 1: Contaminated groundwater plume flowing through a permeable reactive barrier. Clean water exits the downgradient side of the barrier.
What Contaminants Can a PRB Treat?
ZVI-based permeable reactive barriers are effective across a wide range of groundwater contaminants. Both laboratory testing and field-scale results have demonstrated treatment of:
- Chlorinated volatile organic compounds (CVOCs) — TCE, PCE, DCE, vinyl chloride, TCA, carbon tetrachloride, and other chlorinated solvents (full contaminants list)
- Heavy metals — Chromium (Cr VI), arsenic, lead, cadmium, mercury, nickel, selenium, copper, zinc, and uranium
- PFAS — PFOS and PFOA (learn more about PFAS remediation)
- Inorganics — Nitrates, sulfates, and perchlorates
- Petroleum hydrocarbons and PAHs
- Pesticides and energetics — Including chloropicrin, DDT, lindane, TNT, and RDX
Some contaminant combinations can be addressed with ZVI alone, while others may require a combined approach. For example, a single barrier may treat both CVOCs and dissolved metals simultaneously, while sites with mixed organic and inorganic contaminants may benefit from a barrier design that incorporates both iron and biological treatment zones.
Advantages of PRBs Over Active Treatment Systems
| Factor | Permeable Reactive Barrier | Pump-and-Treat |
|---|---|---|
| Operation | Passive — no pumps, energy, or continuous operator involvement | Active — requires pumps, treatment systems, and ongoing operation |
| Operating Cost | Minimal annual cost after installation | Significant ongoing energy, labor, and maintenance costs |
| Design Life | 10+ years with properly designed ZVI media | Operates only as long as actively maintained |
| Surface Disruption | Below grade — no permanent surface infrastructure | Requires wellheads, piping, treatment building, discharge infrastructure |
| Waste Generation | No ongoing waste stream | Continuous generation of treatment residuals and discharge water |
PRB Design Considerations
A successful PRB installation depends on understanding the hydrogeology, geochemistry, and contaminant characteristics of the site. Hepure uses modeling tools to determine the optimal barrier dimensions, ZVI loading, and particle size distribution for each project.
Figure 2: Key PRB design variables — barrier width, depth, ZVI loading, and flow path length must be balanced to achieve target contaminant reductions while maintaining permeability and longevity.
Groundwater Flow
The groundwater flow rate is one of the most critical design inputs. It determines the residence time — how long contaminated water remains in contact with the reactive media. Higher flow velocities require wider barriers or more reactive ZVI formulations to achieve the same treatment performance. Accurate characterization of flow rate, direction, and seasonal variability is essential before design begins.
Site Geology and Matrix Characteristics
Soil type (sand, silt, clay, or fractured rock), heterogeneity, and depth to the target treatment zone all influence construction method, barrier geometry, and cost. Sites with uniform sandy aquifers are generally the most straightforward for PRB installation, while fractured rock or highly heterogeneous formations may require modified approaches.
Contaminant Type and Concentration
The specific contaminant or mixture of contaminants determines the reaction chemistry and the amount of ZVI needed. Different contaminant classes — CVOCs, metals, PFAS, inorganics — may require different barrier configurations. Some sites have co-mingled plumes where a single ZVI barrier can address multiple contaminant types simultaneously.
Geochemistry and Chemical Demand
One of the most overlooked design parameters is the site’s geochemistry. Dissolved oxygen, sulfate, hardness, and other naturally occurring constituents in the groundwater also consume ZVI over time. This chemical demand — not just the target contaminant load — determines the barrier’s effective lifespan. High chemical demand combined with high flow velocity can significantly reduce barrier longevity if not accounted for in design.
Key geochemical parameters to characterize include major cations and anions, mineral content, pH, buffering capacity, ion exchange capacity, salinity, and oxidation-reduction potential (ORP).
Reactivity and Longevity Modeling
Hepure uses two modeling approaches to optimize the amount of ZVI needed for each barrier:
Reactivity modeling determines the barrier width needed to achieve target contaminant reductions based on the ZVI reaction rate and groundwater residence time. The relationship is governed by first-order kinetics, where the required mass of ZVI is a function of the target reduction ratio (inlet vs. outlet concentration), the reaction rate constant for the specific contaminant, and the groundwater flow velocity through the barrier’s pore space.
Chemical demand modeling estimates the barrier’s effective lifespan by accounting for how much ZVI will be consumed over time by both target contaminants and background geochemistry. High chemical demand environments — those with elevated dissolved oxygen, sulfate, or hardness — require proportionally more ZVI mass to maintain reactive capacity over the design life.
Balancing these two factors — sufficient reactivity for treatment performance and sufficient mass for long-term durability — is the core engineering challenge of PRB design.
Ferox PRB: Hepure’s Reactive Iron for Permeable Barriers
Ferox PRB ZVI reactive iron powder has been specifically engineered to optimize both reactivity and longevity for permeable reactive barrier applications. High-purity iron with minimal surface oxidation (rust) ensures maximum reactive surface area, while a controlled particle size distribution balances treatment performance with long design life.
Ferox PRB is available in two particle size distributions:
| Product | Best For | Design Characteristics |
|---|---|---|
| Ferox PRB Standard | Most PRB applications | Balanced reactivity and longevity; reduces barrier construction cost while providing 10+ years of barrier life |
| Ferox PRB Coarse | Sites prioritizing extended barrier life | Coarser particle distribution extends barrier life beyond Standard; may require a wider barrier to maintain reactivity |
Hepure has supplied Ferox PRB to hundreds of project sites across the U.S. and internationally, totaling over 1.5 million pounds of reactive iron deployed in the field.
Ferox PRB Case Study
A full-scale Ferox PRB installation demonstrated effective long-term treatment of a chlorinated solvent plume in groundwater. Download the Ferox PRB case study (PDF) for details on barrier design, installation, and field performance results.
PRB Design Challenges and How to Address Them
Mineral Clogging
Over time, secondary minerals — iron oxides, carbonates, and sulfides — can precipitate within the barrier and reduce its permeability. Proper geochemical characterization during design helps predict clogging potential. Barrier width and ZVI loading can be adjusted to accommodate expected mineral buildup while maintaining adequate flow and reactivity throughout the design life.
ZVI Depletion
As ZVI corrodes through reactions with both contaminants and background groundwater chemistry, its reactive capacity decreases. This is expected behavior and is accounted for in the chemical demand modeling. Selecting the right particle size distribution (Standard vs. Coarse) and ZVI loading rate ensures the barrier maintains performance over its target design life.
Site-Specific Hydrogeology
Every site presents unique challenges — heterogeneous soils, seasonal flow variations, depth limitations, or the presence of utilities and infrastructure. Successful PRB design requires thorough site characterization and experienced engineering judgment. Hepure’s team has designed barriers across a wide range of geologic settings and can help identify the optimal configuration for your site conditions.
Frequently Asked Questions About Permeable Reactive Barriers
How long does a permeable reactive barrier last?
A properly designed ZVI PRB can function effectively for 10 years or more. Barrier life depends on ZVI loading, particle size distribution, groundwater flow rate, and geochemical demand. Hepure’s Ferox PRB Coarse formulation is designed for applications where extended barrier life is a priority. Published field studies have documented effective PRB performance at 15+ years at some sites.
How much does a permeable reactive barrier cost?
PRB costs vary significantly depending on barrier dimensions (width, depth, length), ZVI loading, site geology, and construction method. While upfront installation costs can be higher than starting a pump-and-treat system, PRBs typically have much lower total lifecycle cost because they eliminate ongoing energy, labor, and waste disposal expenses. Hepure can provide cost estimates based on your site-specific design parameters.
Can a PRB treat PFAS?
Yes. ZVI provides a reductive defluorination pathway for PFOS and PFOA. PFAS is an increasingly common target contaminant for PRB applications as regulatory standards tighten. Hepure’s Ferox ZVI products list PFOS and PFOA among their documented treatable contaminants.
What is the difference between a PRB and pump-and-treat?
A PRB is a passive system — groundwater flows through it naturally and contaminants are treated in place with no ongoing energy input or active operation. Pump-and-treat is an active system that extracts groundwater, treats it above ground, and discharges or reinjects the treated water. PRBs generally have lower lifecycle costs and require less long-term management, but they require suitable hydrogeologic conditions and are best suited for plume containment rather than source area mass removal.
What reactive materials are used in PRBs besides ZVI?
While ZVI is the most widely used PRB reactive media, organic materials (such as mulch or compost) can be used to create biologically active zones for treating contaminants like nitrate and sulfate through anaerobic biodegradation. Some barrier designs incorporate both ZVI and biological treatment zones in a single installation to address complex contaminant mixtures. Hepure provides both chemical (ZVI) and biological PRB solutions.
References
- ITRC. (2011). Permeable Reactive Barrier: Technology Update PRB-5. Interstate Technology & Regulatory Council. www.itrcweb.org
- ITRC. (2005). Permeable Reactive Barriers: Lessons Learned / New Directions. Interstate Technology & Regulatory Council. www.itrcweb.org
- EPA. (1998). Permeable Reactive Barrier Technologies for Contaminant Remediation. EPA/600/R-98/125.
- Gillham, R.W., Vogan, J., Gui, L., Duchene, M., & Son, J. (2010). Iron Barrier Walls for Chlorinated Solvent Remediation. In H.F. Stroo & C.H. Ward (Eds.), In Situ Remediation of Chlorinated Solvent Plumes. Springer.
- Wilkin, R.T., Acree, S.D., Ross, R.R., Puls, R.W., Lee, T.R., & Woods, L.L. (2014). Fifteen-year assessment of a permeable reactive barrier for treatment of chromate and trichloroethylene in groundwater. Science of the Total Environment, 468–469, 186–194.
- Henderson, A.D. & Demond, A.H. (2007). Long-term performance of zero-valent iron permeable reactive barriers: A critical review. Environmental Engineering Science, 24(4), 401–423.
Get Help with PRB Design for Your Site
Hepure has been designing and supporting permeable reactive barrier installations since entering the ZVI remediation market in 2000, with over 1.5 million pounds of Ferox PRB deployed at hundreds of sites. Our team can assist with site assessment review, barrier design modeling, ZVI product selection, and field implementation support.
- Call to discuss your project: 866.727.4776
- Email site assessment documents for review: Contact Hepure
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