A Citizen’s Guide 

to Capping

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.

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.
United States Office of Solid Waste and EPA 542-F-12-004

Environmental Protection Emergency Response September 2012

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

www.cluin.org

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.

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.

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

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.