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.