A Citizen’s Guide to

In Situ Thermal Treatment

What is In Situ Thermal Treatment?

In situ thermal treatment methods move or “mobilize” 

harmful chemicals in soil and groundwater using heat. 

The chemicals move through soil and groundwater 

toward wells where they are collected and piped 

to the ground surface to be treated using other 

cleanup methods. Some chemicals are destroyed 

underground during the heating process. Thermal 

treatment is described as “in situ” because the heat 

is applied underground directly to the contaminated 

area. It can be particularly useful for chemicals called 

“non-aqueous phase liquids” or “NAPLs,” which do 

not dissolve readily in groundwater and can be a 

source of groundwater contamination for a long time 

if not treated. Examples of NAPLs include solvents, 

petroleum, and creosote (a wood preservative).

How Does It Work?

In situ thermal treatment methods heat contaminated 

soil, and sometimes nearby groundwater, to very 

high temperatures. The heat vaporizes (evaporates) 

the chemicals and water changing them into gases. 

These gases, also referred to as “vapors,” can move 

more easily through soil. The heating process can 

make it easier to remove NAPLs from both soil and 

groundwater. High temperatures also can destroy 

some chemicals in the area being heated.

In situ thermal methods generate heat in different ways: 

•	 Electrical resistance heating (ERH) delivers 

an electrical current between metal rods called 

“electrodes” installed underground. The heat 

generated as movement of the current meets 

resistance from soil converts groundwater and 

water in soil into steam, vaporizing contaminants. 

•	 Steam enhanced extraction (SEE) injects steam 

underground by pumping it through wells drilled in 

the contaminated area. The steam heats the area 

and mobilizes and evaporates contaminants.

•	 Thermal conduction heating (TCH) uses 

heaters placed in underground steel pipes. TCH 

can heat the contaminated area hot enough to 

destroy some chemicals.

The chemical and water vapors are pulled to collection 

wells and brought to the ground surface by applying a 

vacuum. (See A Citizen’s Guide to Soil Vapor Extraction 

and Air Sparging [EPA 542-12-018] for information on 

how this is done.) The vapors are then treated above 

ground using one of several cleanup methods available. 

Or, if concentrations are high, the vapors can be 

condensed back to liquid chemicals and reused. 

How Long Will It Take?

In situ thermal treatment might take a few months to a 

few years to clean up a site. The actual cleanup time 

will depend on several factors. For example, it might 

take longer where:

• Contaminant concentrations are high. 

• The contaminated area is large or deep. 

• A variety of soil types are present, causing the 

ground to heat unevenly. 

• The soil has a lot of organic matter, which causes 

chemicals to stick to the soil and not evaporate easily.

These factors vary from site to site. 







 


















United States Office of Solid Waste and EPA 542-F-12-013

Environmental Protection Emergency Response September 2012

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

www.cluin.org

Example

SEE was used to speed clean 

up of the Southern California 

Edison Co., Visalia Pole Yard 

Superfund site in California. Use 

of chemicals to treat wooden 

utility poles contaminated soil 

and groundwater at the facility. 

Conventional “pump and treat,” 

begun in 1984, did not show 

much progress in meeting 

cleanup objectives. In 1997, 

14 steam injection wells were 

installed around the contaminated area. Steam was injected 

into the ground at depths of 

80-100 feet, vaporizing the 

chemicals and forcing them 

toward the collection wells. 

Initially, about 13,000 pounds 

of contaminants were pumped 

from the collection wells every 

day. SEE was stopped after 

three years when the wells 

began collecting less than 4 

pounds per day, indicating that 

most of the chemicals had been 

removed. The pump and treat 

system was turned off in 2004. 

Overall, about 1.3 million pounds 

of contaminants were removed, 

and groundwater contaminant 

concentrations were reduced to 

below drinking water standards. 

By using SEE as part of the 

cleanup effort, the overall site 

cleanup was reduced from an 

estimated 120 years to 20 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/products/

Thermal In

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.

Are In Situ Thermal Treatment Methods Safe?

In situ thermal treatment methods do not pose a threat to site workers or 

the community when properly operated. For instance, when using ERH, the 

electrical current is prevented from traveling outside of the treatment area or to 

aboveground structures by using common electrical grounding techniques. A 

thermal treatment area is usually covered with an impermeable surface cover 

(such as concrete, asphalt, or a heavy-duty tarp) to keep the heat and steam 

underground. Such seals also help prevent the release of chemical vapors 

to the air. In addition, workers test air samples to make sure that vapors are 

being captured. 

How Might It Affect Me?

In situ thermal treatment requires the use of drilling equipment and other 

heavy machinery to install wells or electrodes and to collect and treat vapors. 

Neighborhoods near the site may experience some increased truck traffic as 

the equipment is delivered and later removed. Nearby residents and businesses 

also may hear operating equipment.

Why Use In Situ Thermal Treatment?

In situ thermal treatment methods speed the cleanup of many types of 

chemicals, and are among the few in situ methods that can clean up NAPLs. 

Thermal treatment can be used in silty or clayey soil where other cleanup 

methods do not perform well. They also can reach contamination deep 

underground or beneath buildings, which would otherwise be difficult or costly 

to dig up to treat above ground. In situ thermal treatment has been selected 

or is being used in cleanups of at least 12 Superfund sites as well as dozens of 

other sites across the country.

ERH system cleans up contaminated soil and groundwater.

Ground

Cover

Vapor 

Treatment 

Building

Vapor 

Collection 

Well

Pipes to 

Vapor 

Treatment