Citizen's Guide
itu Thermal"
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.
1
11
I
1 1
Electrical
_ Current
( K
1^
J Elecl
1
Wv*
*
**~^
Wv*
-^
rode
JJL
|
|
^
1
«w*
wwy
^
1
*
**~^
WV*
-^
Jb..
1
1
=
Ground
.Surface
Vapor
— Collection
Well
Vapors
^^^^^3
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.
-------
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.
Vapor
Treatment
Building
Vapor
Collection
Well
Ground
Cover
Pipes to
Vapor
Treatment
ERH system cleans up contaminated soil and groundwater.
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 contami-
nated 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 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.duin.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.
United States
Environmental Protection
Agency
Office of Solid Waste and
Emergency Response
(5102G)
EPA 542-F-12-013
September 2012
www.epa.gov/superfund/sites
www.cluin.org
------- |