A Citizen's Guide \o> ^
Solidification and Sta
ation
What Are Solidification
And Stabilization?
Solidification and stabilization refer to a group of
cleanup methods that prevent or slow the release of
harmful chemicals from wastes, such as contaminated
soil, sediment, and sludge. These methods usually do
not destroy the contaminants. Instead, they keep them
from "leaching" above safe levels into the surrounding
environment. Leaching occurs when water from rain
or other sources dissolves contaminants and carries
them downward into groundwater or over land into
lakes and streams.
Solidification binds the waste in a solid block of
material and traps it in place. This block is also less
permeable to water than the waste. Stabilization
causes a chemical reaction that makes contaminants
less likely to be leached into the environment. They are
often used together to prevent people and wildlife from
being exposed to contaminants, particularly metals
and radioactive contaminants. However, certain types
of organic contaminants, such as PCBs and pesticides,
can also be solidified.
How Does It Work?
Solidification involves mixing a waste with a binding
agent, which is a substance that makes loose materials
stick together. Common binding agents include cement,
asphalt, fly ash, and clay. Water must be added to most
Binding Agent -""
Injected Into Soil
Solidified Soil
Augers Spin
"and Mix Soil
mixtures for binding to occur; then the mixture is allowed
to dry and harden to form a solid block.
Similar to solidification, stabilization also involves
mixing wastes with binding agents. However, the
binding agents also cause a chemical reaction with
contaminants to make them less likely to be released into
the environment. For example, when soil contaminated
with metals is mixed with water and lime — a white
powder produced from limestone — a reaction changes
the metals into a form that will not dissolve in water.
Additives can be mixed into the waste while still in
the ground (often referred to as "in situ"). This usually
involves drilling holes using cranes with large mixers or
augers, which both inject the additives underground and
mix them with the waste. The number of holes needed
depends on the size of the augers and the contaminated
area. Dozens of holes may need to be drilled. When
the waste is shallow enough, the contaminated soil
or waste is excavated and additives are mixed with it
above ground (often referred to as "ex situ"). The waste
is either mixed using backhoes and front end loaders
or placed in machines called "pug mills." Pug mills can
grind and mix materials at the same time.
Solidified or stabilized waste mixed above ground is
either used to fill in the excavation or transported to a
landfill for disposal. Waste mixed in situ is usually
covered with a "cap" to prevent water from contacting
treated waste (See A Citizen's Guide to Capping
[EPA 542-12-004].)
How Long Will It Take?
Solidification and stabilization may take weeks or
months to complete. The actual time it takes will
depend on several factors. For example, they may
take longer where:
• The contaminated area is large or deep.
• The soil is dense or rocky, making it harder to mix
with the binding agent.
• Mixing occurs above ground, which requires
excavation.
Binding agents can be injected into soil and mixed using augers.
Extreme cold or rainfall delays treatment.
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Are Solidification And Stabilization Safe?
The additives used in solidification and stabilization often are materials used in
construction and other activities. When properly handled, these materials do
not pose a threat to workers
or the community. Water or
foam can be sprayed on the
ground to make sure that
dust and contaminants are
not released to the air during
mixing. If necessary, the
waste can be mixed inside
tanks, or the mixing area can
be covered to minimize dust
and vapors. The final solidified
or stabilized product is tested
to ensure that contaminants
do not leach. The strength
and durability of the solidified
materials are also tested.
Large augers inject and mix binding agent with
contaminated soil.
How Might It Affect Me?
Nearby residents or businesses may notice increased truck traffic as equipment
and additives are brought to the site or as treated waste is transported to a
landfill. They also may hear earth-moving equipment as waste is excavated or
mixed. When cleanup is complete, the land often can be redeveloped.
Why Use Solidification Or Stabilization?
Solidification and stabilization provide a relatively quick and lower-cost way
to prevent exposure to contaminants, particularly metals and radioactive
contaminants. Solidification and stabilization have been selected or are being
used in cleanups at over 250 Superfund sites across the country.
Contaminated soil mixed with cement in a pug mill is
spread on the ground as pavement.
Example
Solidification and stabiliza-
tion were used to clean up
contaminated sludge and soil
at the South 8th Street Landfill
Superfund site in Arkansas.
From the 1960s to 1970s,
municipal and industrial
wastes were disposed at the
site, including a 2.5-acre pit
of waste-oil sludge. In the
1980s, that area was found
to be contaminated with oily
wastes, PCBs, pesticides,
and lead.
In 1999, cranes with augers
were used to inject and
mix limestone, fly ash,
and Portland cement with
40,000 cubic yards of sludge
and soil in the pit. These
additives helped solidify the
mixture as well as stabilize
the lead and other metals.
The hardened material was
left in place and covered
with a soil cap. Evaluations
in 2004 and 2009 indicated
that the cleanup approach is
still protecting human health
and the environment. The site
has been deleted from the
National Priorities List, the list
of the nations most serious
hazardous waste sites.
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
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-12-019
September 2012
www.epa.gov/superfund/sites
www.cluin.org
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