United States Office of Solid Waste and EPA 542-F-01-022

Environmental Protection Emergency Response December 2001

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

www.cluin.org

A Citizen’s Guide

to Capping

? ? EPA uses many methods to clean up pollution at Superfund and other sites. If you live, work, or go to school near

a Superfund site, you may want to learn more about these methods. Perhaps they are being used or are proposed

for use at your site. How do they work? Are they safe? This Citizen’s Guide is one in a series to help answer your

questions.

What is capping?

Capping involves placing a cover over contaminated material such as the waste buried at a

landfill. Such covers are called “caps.” Caps do not clean up the contaminated material. They

just keep it in place so it will not come into contact with people or the environment.

How does it work?

Sometimes digging up and removing contaminated material can be difficult or expensive.

Instead, a cap will be placed over it to keep it in place. A cap works in three main ways:

1) It stops rainwater from seeping through the hazardous material and carrying the

pollution into the groundwater, lakes or rivers.

2) It stops wind from blowing away the hazardous material.

3) It keeps people and animals from coming into contact with the contaminated material

and tracking it off the site.

The Citizen’s Guide Series

water table

ground water

polluted

soil

grass

top soil

ground

surface

monitoring well

monitoring well

geomembrane

gravel

clay
For more

information

write the Technology

Innovation Office at:

U.S. EPA (5102G)

1200 Pennsylvania Ave.,

NW

Washington, DC 20460

or call them at

(703) 603-9910.

Further information also

can be obtained at

www.cluin.org or

www.epa.gov/

superfund/sites.

2

Constructing a cap can be as simple as placing a single layer of asphalt on top of the contaminated material. More often, however, caps are made of several layers. The top layer at the

ground surface is usually soil with grass or other plants. Plants take up rainwater with their roots

and help prevent it from soaking down into the next layer. They also keep the topsoil from

eroding. The second layer down drains any water that comes through the first layer. It is usually

constructed of gravel and pipes. A third layer may be added to control gasses that come from

the hazardous material. The bottom layer lies directly on the contaminated material. It is usually

made of clay. The clay is covered by a sheet of strong synthetic material called a geomembrane. Together the clay and the geomembrane help stop further flow of water downward.

Is capping safe?

When properly built and maintained, a cap is a safe method for keeping contaminated material

in place. A cap will continue to work safely as long as it is not broken or eroded. Regular

inspections are made to make sure that the weather, plant roots or some human activity have not

damaged the cap. Also, groundwater monitoring wells are placed around the edges of the cap

so that any leakage from the site can be found and fixed.

Why use capping?

Caps have been used at hundreds of sites because they are an effective method for keeping

wastes contained. Caps are usually only part of a cleanup remedy. Often they are used with

pump and treat systems (See A Citizen’s Guide to Pump and Treat [EPA 542-01-025]). The

pumping and treating cleans up polluted groundwater, while the cap prevents contaminated

materials from reaching the groundwater.

Building a cap can take a few days up to several months.

The length of time depends on several factors that vary from site to site:

• size of the area

• thickness and design of the cap

• availability of clean topsoil and clay

Caps can be effective for many years as long as they are properly maintained.

NOTE: This fact sheet is intended solely as general guidance and 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.

How long will it take?