A Citizen’s Guide to

Permeable Reactive Barriers

What Are Permeable Reactive Barriers?

A permeable reactive barrier, or “PRB,” is a wall created 

below ground to clean up contaminated groundwater. 

The wall is “permeable,” which means that groundwater 

can flow through it. Water must flow through the PRB 

to be treated. The “reactive” materials that make up the 

wall either trap harmful contaminants or make them 

less harmful. The treated groundwater flows out the 

other side of the wall.

How Do They Work?

A PRB is usually built by digging a long, narrow trench in 

the path of contaminated groundwater flow. The trench 

is filled with a reactive material, such as iron, limestone, 

carbon, or mulch, to clean up contamination. Due to 

limitations of excavation equipment, walls typically 

can be no deeper than 50 feet. However, a deeper 

but usually shorter PRB can be built by drilling a row 

of large-diameter holes or by using fracturing (See A 

Citizen’s Guide to Fracturing [EPA 542-12-008]) and 

other new techniques.

The reactive material selected for the PRB will 

depend on the types of contaminants present in the 

groundwater. The material may be mixed with sand to 

make the wall more permeable so that it is easier for 

groundwater to flow through it, rather than around it. 

Side walls filled with an impermeable material such as 

clay may be constructed at an angle to the PRB to help 

funnel the flow of contaminated groundwater toward the 

reactive materials. The filled trench is covered with soil, 

and is not usually visible at the ground surface.

Depending on the reactive material, contaminants are 

removed through different processes:

• Contaminants sorb (stick) to the surface of the

reactive material. For example, carbon particles

have a surface onto which contaminants, such as

petroleum products, sorb as groundwater passes

through.

• Metals dissolved in groundwater precipitate, which

means they settle out of the groundwater by forming

solid particles that get trapped in the wall. For

example, limestone and shell fragments can cause

dissolved lead and copper to precipitate in a PRB.

• Contaminants react with the reactive material to

form less harmful ones. For example, reactions

between iron particles and certain industrial

cleaning solvents can convert the solvents to less

toxic or even harmless chemicals.

• Contaminants are biodegraded by microbes in the

PRB. Microbes are very small organisms that live in

soil and groundwater and eat certain contaminants.

When microbes digest the contaminants, they

change them into water and gases, such as carbon

dioxide. (A Citizen’s Guide to Bioremediation [EPA

542-F-12-003] describes how microbes work.)

Organic mulch frequently is used as reactive

media in this type of PRB. Mulch barriers consist

of plant-based materials, such as compost or

wood chips, and naturally contain many different

microbes. Groundwater flow through the PRB

also releases organic carbon from the mulch wall,

creating another reactive zone for contaminants

just beyond the wall.

Over time, reactive materials will fill up with 

contaminants or treatment products and become less 

effective at cleaning groundwater. When this occurs 

the contaminated reactive material may be excavated 

for disposal and replaced with fresh material. PRB treats a plume of groundwater contaminants.
United States Office of Solid Waste and EPA 542-F-12-015

Environmental Protection Emergency Response September 2012

Agency (5102G) www.epa.gov/superfund/sites

www.cluin.org

Example

A PRB with iron as the reactive 

material was installed in 1995 

to clean up groundwater 

at a former semiconductor 

manufacturing site in Sunnyvale, 

California. Concentrations 

of industrial solvents in the 

groundwater plume were 

extremely high. 

Due to changing groundwater 

flow directions, low-permeability 

walls were installed below 

ground and perpendicular to 

the PRB to direct the flow of 

contaminated groundwater 

toward the PRB. The PRB itself 

is about 8-feet wide, 40-feet long 

and 20-feet deep. The objective 

of the PRB is to reduce solvent 

concentrations to below the 

cleanup standards set by the State 

of California. As of 2009, solvent 

concentrations in groundwater 

samples collected within the 

treatment zone remain below 

the cleanup standards. Use of 

a PRB has allowed the metals 

machining facility currently at 

the site to continue operating 

during cleanup.

For More Information

For more information about 

this and other technologies in 

the Citizen’s Guide Series, 

visit:

www.cluin.org/remediation

www.cluin.org/products/

citguide

www.cluin.org/prb

NOTE: This fact sheet is intended solely as general information to the public. It is not intended, nor can it be relied upon, to create any 

rights enforceable by any party in litigation with the United States, or to endorse the use of products or services provided by specific 

vendors. The Agency also reserves the right to change this fact sheet at any time without public notice.

How Long Will It Take?

PRBs may take many years to clean up contaminated groundwater. The cleanup 

time will depend on factors that vary from site to site. For example, cleanup may 

take longer where: 

• The source of dissolved contaminants (for instance, a leaking drum of

solvent) has not been removed.

• The contaminants remain in place because they are not easily dissolved by

groundwater.

• Groundwater flow is slow.

Are PRBs Safe?

The reactive materials placed in PRBs are not harmful to groundwater or people. 

Contaminated groundwater is cleaned up underground so treatment does not expose 

workers or others onsite to contamination. Because some contaminated soil may be 

encountered when digging the trench, workers wear protective clothing. Workers also 

cover loose contaminated soil to keep dust and vapors out of the air before disposing of 

it. Groundwater is tested regularly to make sure the PRB is working.

How Might It Affect Me?

During construction of the PRB, nearby residents may see increased truck 

traffic when materials are hauled to the site or hear earth-moving equipment. 

However, when complete, PRBs require no noisy equipment. Cleanup workers 

will occasionally visit the site to collect groundwater and soil samples to ensure 

that the PRB is working. When the reactive materials need to be replaced, the old 

materials will have to be excavated and hauled to a landfill.

Why Use PRBs?

PRBs are a relatively inexpensive 

way to clean up groundwater. No 

energy is needed because PRBs rely 

on the natural flow of groundwater. 

The use of some materials, such 

as limestone, shell fragments, and 

mulch, can be very inexpensive, 

if locally available. No equipment 

needs to be above ground, so the 

property may continue its normal 

use, once the PRB is installed. 

PRBs have been selected or are being used at more than 30 Superfund sites 

across the country.

Construction of a PRB in Sunnyvale, CA