itizen's Guide to
" ">able ReactK
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
Water Table
Groundwater Flow
Permeable Reactive Barrier
PRB treats a plume of groundwater contaminants.
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.
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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.
Construction of a PRB in Sunnyvale, CA
PRBs have been selected or are being used at more than 30 Superfund sites
across the country.
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-feetdeep. 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 on this
and other technologies in the
Citizen's Guide Series, contact:
U.S. EPA
Technology Innovation &
Field Services Division
Technology Assessment Branch
(703)603-9910
Or visit:
http://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.
United States
Environmental Protection
Agency
Office of Solid Waste and
Emergency Response
(5102G)
EPA 542-F-12-017
September 2012
www.epa.gov/superfund/sites
www.cluin.org
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