Citizen's Guide to
~ • Chemical R
What Is In Situ Chemical Reduction?
In situ chemical reduction, or "ISCR," uses chemicals
called "reducing agents" to help change contaminants
into less toxic or less mobile forms. It is described
as "in situ" because it is conducted in place, without
having to excavate soil or pump groundwater above
ground for cleanup. ISCR can clean up several types
of contaminants dissolved in groundwater. It can also
be used to clean up contaminants known as "dense
non-aqueous phase liquids" or "DNAPLs," which do
not dissolve easily in groundwater and can be a source
of contamination for a long time. ISCR is most often
used to clean up the metal chromium and the industrial
solvent trichloroethene, or "TCE," which is a DNAPL.
How Does It Work?
When reducing agents are added to contaminated
soil and groundwater, a chemical reaction occurs that
changes contaminants into other forms. For example,
a very toxic form of chromium called "hexavalent
chromium," or "chrome 6," can be changed to chrome
3 when reducing agents are injected into contaminated
groundwater. Chrome 3 is a much less toxic form of
the metal. Chrome 3 is also less mobile because it
does not dissolve as easily in water.
Common reducing agents include zero valent metals,
which are metals in their pure form. The most common
metal used in ISCR is zero valent iron, or "ZVI." ZVI
must be ground up into small granules for use in ISCR.
In some cases, micro- or nano-scale (extremely small)
T Water
^=- Table
Treated
Water
and Chrome 3
Groundwater
Contaminated
.with Chrome 6
Permeable
Reactive Barrier
Illustration of the treatment of contaminated water with a PRB
made of ZVI.
particles are used. The smaller particle size increases
the amount of iron available to react with contaminants.
Other common reducing agents include polysulfides,
sodium dithionite, ferrous iron, and bimetallic materials,
which are made up of two different metals. The most
common bimetallic material used in ISCR is iron
coated with a thin layer of palladium or silver.
There are two ways of bringing reducing agents into
contact with contaminated soil and groundwater: direct
injection and construction of a permeable reactive
barrier, or "PRB."
Direct injection involves mixing the reducing agent
with water (or sometimes vegetable oil) to create a
slurry, which is pumped down holes drilled directly
into the contaminated soil and groundwater. This
method is often used to treat highly contaminated
source areas, including DNAPLs. Nano-scale ZVI
is usually used when injecting iron underground, but
micro-scale ZVI also is used.
A PRB is a wall built below ground, usually by digging
a trench and filling it with a reducing agent. Iron filings,
which are larger granules of ZVI, are commonly used.
Because the wall is permeable, groundwater flows
through the PRB allowing contaminants to react with
the reducing agent; treated water flows out the other
side. A PRB is used to treat contaminants dissolved
in groundwater. It will only treat the water that flows
through it. (See A Citizen's Guide to Permeable
Reactive Barriers [EPA 542-12-015].)
How Long Will It Take?
ISCR may take as little as a few months to clean up a
source area using direct injection, and PRBs may take
several years. The actual cleanup time will depend on
several factors that vary from site to site. For example,
ISCR will take longer where:
• The source area is large, or contaminants are
trapped in hard-to reach areas like fractures
or clay.
• The soil or rock does not allow the reducing
agent to spread quickly and evenly or reach
contaminants easily.
• Groundwater flow is slow.
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Is In Situ Chemical Reduction Safe?
The use of ISCR poses little risk to the surrounding community. Workers wear
protective clothing while handling reducing agents, and when handled properly,
these chemicals are not harmful to the environment or to people. Because
contaminated soil and groundwater are cleaned up underground, ISCR does
not expose workers or others at the site to contamination. If contaminated
soil is encountered when digging the PRB trench, workers will need to wear
protective clothing. They also cover any loose contaminated soil to keep dust
and contaminants out of the air before disposing of it. Groundwater and soil
are tested regularly to make sure ISCR is working.
How Might It Affect Me?
Residents and businesses near the site may see increased truck traffic when
drilling rigs, earth-moving equipment, and reducing agents are delivered to
the site. Residents also may hear the operation of equipment during injections
or installation of PRBs. However, when injections and PRB installations
are complete, ISCR requires no noisy equipment. Cleanup workers will
occasionally visit the site to collect soil and groundwater samples to make
sure ISCR is working.
Why Use In Situ Chemical Reduction?
ISCR can treat some types of contaminants including DNAPLs that are difficult
to clean up using other methods. It can destroy most of the contamination
in situ without having to pump groundwater for treatment or dig up soil for
transport to a landfill or treatment facility. This can save time and money. In
addition, no energy is needed to operate a PRB because it relies on the natural
flow of groundwater. ISCR is a relatively new method for cleaning up hazardous
waste sites, but is seeing increased use at Superfund sites across the country.
Injection of reducing agent into a hole drilled underground.
Example
ISCR was used to treat soil
and groundwater contami-
nated with chrome 6 at
the Macalloy Corporation
Superfund site in South
Carolina. Leaks and disposal
of wastes at the former
iron-chrome alloy manufac-
turing plant contaminated the
groundwater, which flows into
a nearby creek.
In December 2005, five
PRBs (and later another four)
were constructed to contain
and treat groundwater before
it could enter the creek. Soil
excavated from trenches was
mixed with gravel and a blend
of ferrous iron and sodium
dithionite. The mixture was
placed back in the trenches
to form the PRBs.
A 2010 review showed that
concentrations of chrome 6
and the extent of contami-
nation are decreasing at
the site. Cleanup goals are
being met in most of the
wells sampled. The PRBs
are expected to continue to
reduce chrome 6 over the
next five years.
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:
www.cluin.org/ISCR
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-12-012
September 2012
www.epa.gov/superfund/sites
www.cluin.org
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