A Citizen’s Guide to

In Situ Chemical Reduction

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) 

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.

Illustration of the treatment of contaminated water with a PRB 

made of ZVI.
United States Office of Solid Waste and EPA 542-F-12-012

Environmental Protection Emergency Response September 2012

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

www.cluin.org

Example

ISCR was used to treat soil 

and groundwater contaminated with chrome 6 at 

the Macalloy Corporation 

Superfund site in South 

Carolina. Leaks and disposal 

of wastes at the former 

iron-chrome alloy manufacturing 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 contamination 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 about 

this and other technologies in 

the Citizen’s Guide Series, 

visit::

www.cluin.org/remediation

www.cluin.org/products/

citguide

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.

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.