A Citizen’s Guide to

In Situ Chemical Oxidation

What Is In Situ Chemical Oxidation?

Chemical oxidation uses chemicals called “oxidants” 

to help change harmful contaminants into less toxic 

ones. It is commonly described as “in situ” because it 

is conducted in place, without having to excavate soil 

or pump out groundwater for aboveground cleanup. 

In situ chemical oxidation, or “ISCO,” can be used to 

treat many types of contaminants like fuels, solvents, 

and pesticides. ISCO is usually used to treat soil and 

groundwater contamination in the source area where 

contaminants were originally released. The source area 

may contain contaminants that have not yet dissolved 

into groundwater. Following ISCO, other cleanup 

methods, such as pump and treat or monitored natural 

attenuation, are often used to clean up the smaller 

amounts of contaminants left behind. (See A Citizen’s 

Guide to Pump and Treat [EPA 542-F-12-017] and A 

Citizen’s Guide to Monitored Natural Attenuation [EPA 

542-F-12-014].)

How Does It Work?

When oxidants are added to contaminated soil and 

groundwater, a chemical reaction occurs that destroys 

contaminants and produces harmless byproducts. 

To treat soil and groundwater in situ, the oxidants 

are typically injected underground by pumping them 

into wells. The wells are installed at different depths 

in the source area to reach as much dissolved and 

undissolved contamination as possible. Once the 

oxidant is pumped down the wells, it spreads into the 

surrounding soil and groundwater where it mixes and 

reacts with contaminants.

To improve mixing, the groundwater and oxidants 

may be recirculated between wells. This involves 

pumping oxidants down one well and then pumping 

the groundwater mixed with oxidants out another 

well. After the mixture is pumped out, more oxidant is 

added, and it is pumped back (recirculated) down the 

first well. Recirculation helps treat a larger area faster. 

Another option is to inject and mix oxidants using 

mechanical augers or excavation equipment. This may 

be particularly helpful for clay soil. 

The four major oxidants used for ISCO are 

permanganate, persulfate, hydrogen peroxide and 

ozone. The first three oxidants are typically injected 

as liquids. Although ozone is a strong oxidant, it is a 

gas, which can be more difficult to use. As a result, it 

is used less often.

Catalysts are sometimes used with certain oxidants. A 

catalyst is a substance that increases the speed of a 

chemical reaction. For instance, if hydrogen peroxide 

is added with an iron catalyst, the mixture becomes 

more reactive and destroys more contaminants than 

hydrogen peroxide alone.

Following treatment, if contaminant concentrations 

begin to climb back up or “rebound,” a second or 

third injection may be needed. Concentrations will 

rebound if the injected oxidants did not reach all of the 

contamination, or if the oxidant is used up before all the 

contamination is treated. It may take several weeks to 

months for the contamination to reach monitoring wells 

and to determine if rebound is occurring. 

ISCO may produce enough heat underground to cause 

the contaminants in soil and groundwater to evaporate 

and rise to the ground surface. Controlling the amount 

of oxidant helps avoid excessive heat, and if significant 

gases are produced, they can be captured and treated.
United States Office of Solid Waste and EPA 542-F-12-011

Environmental Protection Emergency Response September 2012

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

www.cluin.org

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.

Example

Groundwater near a former 

wastewater treatment plant at 

the Naval Air Station Pensacola 

in Florida was contaminated 

with solvents and acids from 

painting and electroplating. 

A groundwater pump and 

treat system had operated for 

more than 10 years to control 

migration of contaminated 

groundwater. However, it did 

not do much to lower the 

concentrations of contaminants. 

ISCO using hydrogen peroxide 

with an iron catalyst was 

chosen to reduce contaminant 

concentrations in the source 

area enough to allow monitored 

natural attenuation to complete 

the cleanup. 

The natural chemistry of the 

site’s groundwater was found 

to limit the effectiveness of the 

first phase of injections. In the 

second phase, a chemical was 

added to the reagent mix to 

stabilize the oxidant mixture. 

Contaminant levels fell 

substantially. The successful 

use of ISCO at this site was 

estimated to save several 

million dollars compared with 

continued pump and treat.

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/chemox

How Long Will It Take?

ISCO works relatively quickly to clean up a source area. Cleanup may take a few months 

or years, rather than several years or decades. The actual cleanup time depends on 

several factors that vary site to site. For example, ISCO will take longer where: 

• The source area is large.

• Contaminants are trapped in hard-to-reach areas like fractures or clay.

• The soil or rock does not allow the oxidant to spread quickly and evenly.

• Groundwater flow is slow.

• The oxidant does not last long underground.

Is ISCO Safe?

The use of ISCO poses little risk to the surrounding community. Workers wear 

protective clothing when handling oxidants, and when handled properly, these 

chemicals are not harmful to the environment or people. Because contaminated 

soil and groundwater are cleaned up underground, ISCO does not expose 

workers or others at the site to contamination. Workers test soil and groundwater 

regularly to make sure ISCO is working.

How Might It Affect Me?

Nearby residents and businesses may see drilling rigs and tanker trucks with 

oxidants and supplies as they are driven to the site. Residents may also hear the 

operation of drilling rigs, pumps, and other equipment leading up to and during 

the injection period. Following an injection, however, the cleanup process occurs 

underground with little aboveground disruption. Workers may visit the site to 

collect soil and groundwater samples to monitor cleanup progress.

Why Use ISCO?

ISCO is usually selected to 

clean up a source area, where it 

destroys the bulk of contaminants 

in situ without having to dig up 

soil or pump out groundwater for 

aboveground treatment. This can 

save time and money. ISCO has 

successfully cleaned up many 

contaminated sites and has been 

selected or is being used at around 

40 Superfund sites and many 

other sites across the country.

ISCO system installed behind a small drycleaning facility.