United States
Environmental Protection
Agency
Office of Solid Waste and
Emergency Response
(5102G)
EPA 542-F-01-005
April 2001
www.epa.gov/superfund/sites
www.cluin.org
vvEPA A Citizen's Guide to
Permeable Reactive Barriers
EPA uses many methods to clean up pollution at Superfund and other sites. Some, like permeable reactive
barriers, are considered new or innovative. Such methods can be quicker and cheaper than more common
methods. If you live, work, or go to school near a Superfund site, you may want to learn more about cleanup
methods. Perhaps they are being used or are proposed for use at your site. How do they work? Are they safe?
HiisaGifizen's Guide is one in a series to help answer your questions.
What are permeable reactive barriers?
A permeable reactive barrier or PRB is a wall built below ground to clean up polluted
groundwater. The wall is permeable, which means it has tiny holes that allow groundwater to
flow through it. Reactive materials in the wall trap harmful chemicals or change the chemicals
into harmless ones. Clean groundwater flows out the other side of the wall.
"polluted .
fgiundwater
How do they work?
A PRB is built by digging a long, narrow trench in the path of the polluted groundwater. The
trench is filled with a reactive material that can clean up the harmful chemicals. Iron, lime-
stone, and carbon are common types of reactive materials that can be used. The reactive
materials may be mixed with sand to make it easier for water to flow through the wall, rather
than around it. At some sites, the wall is part of a funnel that directs the polluted groundwater
to the reactive part of the wall. The filled trench or funnel is covered with soil, so it usually
cannot be seen above ground.
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For more
information
write the Technology
Innovation Office at:
U.S.EPA(5102G)
1200 Pennsylvania Ave.,
NW
Washington, DC 20460
or call them at
(703) 603-9910.
Further information also
can be obtained at
www.cluin.org or
www.epa.gov/
superfund/sites.
The material used to fill the trench depends on the types of harmful chemicals in the groundwa-
ter. Different materials clean up pollution through different methods by:
• Trapping or sorbing chemicals on their surface. For example, carbon has a surface that
chemicals sorb to as groundwater passes through.
• Precipitating chemicals that are dissolved in water. This means the chemicals settle out of the
groundwater as solid materials, which get trapped in the wall. For example, limestone can
cause dissolved metals to precipitate.
• Changing the chemicals into harmless ones. For example, iron can change some types of
solvents into harmless chemicals.
• Encouraging tiny bugs or microbes in the soil to eat the chemicals. For example, nutrients and
oxygen in a PRB help the microbes grow and eat more chemicals. When microbes com-
pletely digest the chemicals, they can change them into water and harmless gases such as
carbon dioxide. (A Citizen's Guide to Bioremediation [EPA 542-F-01-001]
describes how microbes work.)
\ How long will it take 7
Cleaning groundwater with a PRB may take many years. The time it
takes depends on two major factors that vary from site to site:
• type and amount of pollution present in the groundwater
• how fast the groundwater moves through the PRB
Groundwater may move a few inches to hundreds of feet per year. Its speed variej
from site to site.
Are PRBs safe?
PRBs have a good safety record. Once built, they have no moving parts, equipment, or noise.
The reactive materials placed in the PRB trench are not harmful to the groundwater or to
people. The polluted groundwater is cleaned underground so cleanup workers can avoid
contact with it. Some soil, which may be polluted, must be removed when digging the trench.
EPA makes sure that the polluted soils are handled safely. For example, they cover loose soil to
keep dust and harmful gases out of the air.
EPA tests the air to make sure that dust and gases are not released. If the soil is polluted, it may
be cleaned using another cleanup method. Or the soil is disposed of properly in a landfill. The
groundwater is tested regularly to make sure the PRB is working.
Why use PRBs?
PRBs work best at sites with loose, sandy soil and a steady flow of groundwater. The pollution
should be no deeper than 50 feet. PRBs clean up many types of pollution underground. Since
there is no need to pump polluted groundwater above ground, PRBs can be cheaper and faster
than other methods. Very little waste needs to be disposed of in a landfill, which also saves
money. There are no parts to break, and there is no equipment above ground so the property
can be used while it is being cleaned up. There are no energy costs to operate a PRB because it
works with the natural flow of groundwater. PRBs have been installed at more than 40 sites in
the United States and Canada.
NOTE: Tills fact sheet is intended solely as general guidance and 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.
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