PRB Design

Permeable Reactive Barrier (PRB) Design for Groundwater Remediation

Over 1.5 million pounds of Ferox PRB iron have gone into barriers we helped design, and the calls that precede those orders follow a pattern: a well-defined plume, a receptor downgradient, and a consultant who needs the barrier to still be working in year ten. This page is built from those conversations.

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 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 plume and filling it with reactive media — or, where trenching isn’t feasible, by injecting media to form the reactive zone. The barrier is designed to remain permeable so groundwater flows freely through it under natural hydraulic gradient — no pumps, no energy input, no continuous operator involvement.

When contaminated groundwater contacts the ZVI, chemical reactions degrade or immobilize target contaminants:

• Reductive dechlorination — ZVI reduces chlorinated organic compounds such as TCE and PCE toward non-toxic end products like ethene and ethane. Any intermediates that form are typically further degraded within a properly sized barrier.
• Metal precipitation — ZVI converts soluble metals into insoluble forms; 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 — a reductive defluorination pathway for PFOS and PFOA is under active research; see our FAQ on where ZVI stands on PFAS

Once installed, a properly designed ZVI PRB operates passively for 10 years or more with minimal maintenance — one of the most cost-effective long-term groundwater remediation technologies available.

What Contaminants Can a PRB Treat?

ZVI-based barriers are effective across a wide range of groundwater contaminants. 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, nickel, selenium, copper, zinc, and uranium
• Inorganics — nitrates, sulfates, and perchlorates
• Petroleum hydrocarbons and PAHs
• Pesticides and energetics — including chloropicrin, DDT, lindane, TNT, and RDX
• PFAS — research-stage for direct treatment; PRBs are field-proven for the chlorinated solvents and metals that co-occur at PFAS sites (PFAS remediation techniques)

Some contaminant combinations can be addressed with ZVI alone; a single barrier can treat CVOCs and dissolved metals simultaneously. Sites with mixed organic and inorganic contaminants may benefit from a 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	Wellheads, piping, treatment building, discharge infrastructure
Waste Generation	No ongoing waste stream	Continuous treatment residuals and discharge water

PRB Design Considerations

A successful 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.

Design Factor	Key Considerations
Barrier Configuration	Continuous trenches suit shallow plumes reachable with excavation equipment. Funnel-and-gate systems help direct groundwater in lower-permeability settings. Injection-based PRBs can be installed deeper than trenching allows.
Reactivity of the ZVI	Evaluated for the specific iron media and contaminants involved, typically with batch or column testing and a Kmass framework.
Thickness of Reactive Zone	Commonly 3 to 5+ feet, depending on ZVI reactivity, groundwater velocity, contaminant concentration, and required residence time.
Hydraulic Conductivity	The barrier should be equal to or more permeable than the surrounding aquifer to prevent bypass.
Residence Time	Controlled by media reactivity, barrier thickness, and groundwater velocity — critical for complete treatment.
Longevity	Depends on iron reactivity, groundwater geochemistry, and potential mineral precipitation or fouling.

Groundwater Flow

Flow rate is one of the most critical design inputs: it determines residence time — how long contaminated water stays in contact with the reactive media. Higher velocities require wider barriers or more reactive ZVI formulations for the same performance. Characterize flow rate, direction, and seasonal variability before design begins.

Site Geology and Matrix Characteristics

Soil type (sand, silt, clay, or fractured rock), heterogeneity, and depth to the treatment zone influence construction method, barrier geometry, and cost. Uniform sandy aquifers are generally the most straightforward; fractured rock or highly heterogeneous formations may require modified approaches.

Contaminant Type and Concentration

The specific contaminant or mixture determines the reaction chemistry and the amount of ZVI needed. Different classes — CVOCs, metals, inorganics — may call for different configurations; co-mingled plumes can often be addressed by a single ZVI barrier.

Geochemistry and Chemical Demand

The number that gets under-designed most often is the aquifer’s demand, not the plume’s. Dissolved oxygen, sulfate, hardness, and other naturally occurring constituents consume ZVI over time, and this chemical demand — not just the target contaminant load — determines the barrier’s effective lifespan. High demand combined with high velocity shortens barrier life significantly if it isn’t in the design. Characterize major cations and anions, mineral content, pH, buffering capacity, ion exchange capacity, salinity, and ORP.

Reactivity and Longevity Modeling

Hepure uses two modeling approaches to size the ZVI for each barrier. Reactivity modeling determines the barrier width needed to hit target reductions from the ZVI reaction rate and groundwater residence time — first-order kinetics, where required ZVI mass is a function of the target reduction ratio, the contaminant’s rate constant, and pore velocity through the barrier. Chemical demand modeling estimates effective lifespan by accounting for ZVI consumed by both target contaminants and background geochemistry. Balancing the two — enough reactivity for performance, enough mass for durability — is the core engineering challenge of PRB design.

When a PRB Is — and Isn’t — the Right Tool

Good fit: groundwater flow direction is well defined, the plume can be intercepted hydraulically, long-term containment or treatment is required, and site access allows trenching or injection. Poor fit: highly heterogeneous flow, poorly defined plume geometry, or extremely low-permeability formations — unless an injected PRB approach is feasible. PRBs are best suited to plume interception rather than source-area mass removal; see in situ groundwater remediation for how they fit in a treatment train.

Ferox PRB: Hepure’s Reactive Iron for Permeable Barriers

Ferox PRB reactive iron has been engineered to optimize both reactivity and longevity for barrier applications. High-purity iron with minimal surface oxidation ensures maximum reactive surface area, while a controlled particle size distribution balances treatment performance with long design life (see how ZVI particle size is measured).

Media	Best For	Design Characteristics
Ferox PRB Standard	Most trenched 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 distribution extends life beyond Standard; may require a wider barrier to maintain reactivity
Ferox Flow	Injection-based PRBs	Fine ZVI media for injected barrier concepts where trenching is not feasible

Hepure has supplied Ferox PRB to hundreds of project sites across the U.S. and internationally — over 1.5 million pounds of reactive iron deployed in the field. See a full-scale installation in the Ferox PRB case study (PDF), and read how a direct-push mZVI barrier stopped a cDCE/vinyl chloride plume in our ZVI PRB case study article.

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 permeability. Geochemical characterization during design predicts clogging potential; barrier width and ZVI loading can be adjusted to accommodate expected buildup while maintaining flow and reactivity through the design life.

ZVI Depletion

As ZVI corrodes through reactions with contaminants and background chemistry, reactive capacity decreases. This is expected and accounted for in chemical demand modeling; the right particle size distribution (Standard vs. Coarse) and loading rate keep the barrier performing over its target life.

Site-Specific Hydrogeology

Heterogeneous soils, seasonal flow variation, depth limits, utilities and infrastructure — every site has something. Hepure’s team has designed barriers across a wide range of geologic settings and can help identify the right configuration for yours.

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, depending on ZVI loading, particle size distribution, flow rate, and geochemical demand. Ferox PRB Coarse is designed for applications where extended life is the priority. Published field studies have documented effective performance at 15+ years at some sites.

How much does a permeable reactive barrier cost?

Costs vary with barrier dimensions, ZVI loading, geology, and construction method. Upfront installation can exceed the cost of starting a pump-and-treat system, but PRBs typically have much lower total lifecycle cost because they eliminate ongoing energy, labor, and waste disposal. We provide estimates from your site-specific design parameters.

Can a PRB treat PFAS?

Research has demonstrated a reductive defluorination pathway for PFOS and PFOA on ZVI, and it remains an active study area rather than standard full-scale practice — today’s field-practical PFAS toolset centers on adsorption. Where a ZVI barrier earns its place at a PFAS site is the co-occurring chlorinated solvents and metals, which it treats reliably while the PFAS fraction is managed separately.

What is the difference between a PRB and pump-and-treat?

A PRB is passive — groundwater flows through it naturally and contaminants are treated in place with no energy input. Pump-and-treat is active — groundwater is extracted, treated above ground, and discharged or reinjected. PRBs generally have lower lifecycle costs and less long-term management, but need suitable hydrogeologic conditions and suit plume containment better than source mass removal.

What reactive materials are used in PRBs besides ZVI?

Organic materials such as mulch or compost can create biologically active zones for treating nitrate and sulfate through anaerobic biodegradation, and some designs combine ZVI and biological zones in one installation. 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.
• 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 designed and supported permeable reactive barrier installations since entering the ZVI remediation market in 2000. Our team assists with site assessment review, barrier design modeling, ZVI product selection, and field implementation support. Call 866-727-4776 — Pat Randall (x1) in the West, Bob Kelley, Ph.D. (x2) in the East — or send site assessment documents for review.