A Citizen’s Guide to

Solidification and Stabilization

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 

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. 

• Extreme cold or rainfall delays treatment. Binding agents can be injected into soil and mixed using augers.
United States Office of Solid Waste and EPA 542-F-12-019

Environmental Protection Emergency Response September 2012

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

www.cluin.org

Example

Solidification and stabilization 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 about 

this and other technologies in 

the Citizen’s Guide Series, 

visit:

www.cluin.org/remediation

www.cluin.org/products/

citguide

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 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. 

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. 

Large augers inject and mix binding agent with 

contaminated soil.

Contaminated soil mixed with cement in a pug mill is 

spread on the ground as pavement.