A Citizen's   Guide   \o>  ^
Solidification  and  Sta
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
Binding agents can be injected into soil and mixed using augers.
                                                      Extreme cold or rainfall delays treatment.

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.
                                       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
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
Office of Solid Waste and
Emergency Response
                                                                            EPA 542-F-12-019
                                                                            September 2012