A Citizen's Guide
1 tapping A
What Is Capping?
Capping involves placing a cover over contaminated
material such as landfill waste or contaminated soil.
Such covers are called "caps." Caps do not destroy or
remove contaminants. Instead, they isolate them and
keep them in place to avoid the spread of contamination.
Caps prevent people and wildlife from coming in contact
with contaminants.
How Does It Work?
A cap isolates and prevents the spread of contamination
in several ways. For example, it can:
• Stop rain and snowmelt from seeping through
the material and carrying contaminants to the
groundwater.
• Keep storm water runoff from carrying contaminated
material offsite or into lakes and streams.
• Prevent wind from blowing contaminated material
offsite.
• Control releases of gas from wastes containing or
producing "volatile" chemicals (those that evaporate).
• Keep people and wildlife from coming into
contact with the hazardous material and tracking
contaminants offsite.
Sand and Gravel
Drainage Layer
Geomembrane
Clay
Vegetative
Layer
The cap design selected for a site will depend on
several factors, including the types and concentrations
of contaminants present, the size of the site, the
amount of rainfall the area receives, and the future
use of the property. Construction of a cap can be
as simple as placing a single layer of a material over
lightly contaminated soil to placing several layers of
different materials to isolate more highly contaminated
wastes. For example, an asphalt cap might be selected
to cover low levels of soil contamination on a property
whose future reuse requires a parking lot. A cap for
a hazardous waste landfill, however, might require
several layers, including a vegetative layer, drainage
layer, geomembrane, and clay layer. The following are
some of the options for caps:
• Asphalt or concrete: A layer of these materials
can serve as a parking lot or building slab
foundation.
• Vegetative layer: A top layer of soil planted with
grass or other vegetation can help prevent soil
erosion and make the area look more natural and
attractive. An evapotranspiration or "ET" cover is a
vegetative cap in which the plants and underlying
soil keep rain and snowmelt from soaking down
into the contaminated area. (For more information,
please see A Citizen's Guide to Evapotranspiration
Covers [EPA 542-F-12-006.)
• Drainage layer: A layer of sand and gravel, often
containing rows of slotted pipes, is built to collect
and drain any water that makes it through the top
layers of a cap.
• Geomembrane: A sheet of strong plastic-like
material is used to prevent downward drainage of
water and upward escape of gases.
• Clay: A layer of compacted clay also can help
prevent the downward drainage of water.
Some landfill covers, such as those for municipal
landfills, may also include collection and venting
systems for methane and other gases that could build
up underground.
Example of a cover with several layers.
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How Long Will It Take?
Building a cap can take a few days up to several months. Construction may take
longer when:
• The contaminated area is large.
• The design of the cap is thick or complex.
• Supplies of clean topsoil, clay, or other cap materials are not available locally.
Caps can be effective for many years when they are properly maintained. They
are maintained for as long as the contaminated materials remain in place.
Is Capping Safe?
When properly built and maintained, a cap can safely keep contaminated
material in place. A cap will continue to isolate contamination as long as it does
not erode or develop cracks or holes that allow water to reach the contaminated
material. Regular inspections are made to make sure that the weather, plant
roots, and human activity have not damaged the cap and that plants on
vegetative caps are still growing. Also, groundwater monitoring wells are placed
around the capped area and sampled to help determine if leaks occur.
How Might It Affect Me?
Residents and businesses close to a site may see increased truck traffic as cap
materials are brought to the site. Construction of the cap may involve bulldozers,
backhoes, and other noisy equipment, and some soil may need to be excavated
for use in the cap. Dust from excavation and construction can be controlled by
spraying water or covering stockpiled materials with tarps.
Why Use Capping?
Capping is the traditional
method for isolating landfill
wastes and contaminants. It
sometimes is used to address
large volumes of soil or waste
with low-levels of contamination.
Caps made of asphalt or
concrete, or even a layer of soil
planted with grass, can allow
some sites to be reused. Caps
have been selected for use at
many Superfund sites across
the country.
Spring grasses grow on the cap of a hazardous waste
landfill.
Example
Capping is one of several
methods being used to protect
people and the environment from
contamination at the Roebling
Steel Superfund site in New
Jersey. Drums and other wastes
were removed from one 5-acre
area of the site. Two areas of soil
that remained contained metals
and other contaminants from
steel manufacturing. In 2005, this
soil was covered with two types
of caps: asphalt and clean soil
planted with grass. The purpose
of these caps was to avoid the
spread of contaminants and to
prevent people from coming into
contact with contaminated soil.
The caps also were designed
with the future use of the site in
mind. A station for New Jersey's
light rail system was constructed
on the property, and the asphalt
cap serves as its parking lot. The
grassy landscaping surrounds
the remainder of the property. A
plan is in place for the long-term
maintenance and monitoring
of the caps to ensure that
they remain protective. Future
excavation through the soil cap is
not permitted.
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-12-004
September 2012
www.epa.gov/superfund/sites
www.cluin.org
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