A Citizen’s Guide to 

Evapotranspiration Covers

What Are Evapotranspiration Covers?

Evapotranspiration (ET) covers are a type of cap placed 

over contaminated material, such as soil, landfill waste, 

or mining tailings, to prevent water from reaching it. 

They differ from other types of caps (See A Citizen’s 

Guide to Capping [EPA 542-F-12-004].) in the way they 

prevent water from seeping into the waste. ET covers 

store water from rainfall and snowmelt until drier or 

warmer weather evaporates the water, or until the water 

is taken in by plant roots and released to the air as water 

vapor through the leaves and stems. This process is 

called “transpiration.” 

How Do They Work?

Like other caps, ET covers do not destroy or remove 

contaminants. Instead, they isolate them and keep 

them in place to prevent the spread of contamination 

and protect people and wildlife from the contaminated 

material. ET covers are constructed by placing a 2- to 

10-foot-thick layer of fine-grained soil containing silt 

and clay over the contaminated material. The type of 

soil is chosen for its ability to store water and promote 

plant growth. The thickness of the cover depends on 

how much rainfall and snowmelt is expected in the 

area. Grass, shrubs, or small trees that form extensive 

root systems and survive the local climate are usually 

planted in the soil. Plants native to the area often 

work best. 

The soil-plant layer of an ET cover slows the downward 

movement of rainwater and snowmelt and promotes 

storage of the water. The stored water will either 

evaporate or transpire. Together, evaporation and 

transpiration (“evapotranspiration”) keep water from 

seeping into contaminated material and carrying 

contaminants downward into groundwater.

Construction of an ET cover sometimes involves placing 

the layer of fine-grained soil over a 1-to 2-foot-thick layer 

of coarse-grained soil, such as sand or gravel. This 

extra layer allows the fine-grained soil layer to hold more 

water through a process known as “capillary action.” 

This type of cover usually needs a smaller amount 

of fine-grained soil to have the same water storage 

as a regular ET cover. Use of clean, locally-available 

soil for these layers will speed up construction and 

decrease costs.

How Long Does It Take?

Building an ET cover can take a few days to several 

months. Construction may take longer when: 

Example of ET cover used at Operating Industries, Inc. 

Landfill Superfund site.
United States Office of Solid Waste and EPA 542-F-12-006

Environmental Protection Emergency Response September 2012

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

www.cluin.org

Example

An ET cover was installed over 

wastes buried in the former 

Box Canyon Landfill, one of 

several contaminated areas at 

the Camp Pendleton Marine 

Corps Base Superfund site in 

California. The 28-acre landfill 

received municipal solid waste 

and commercial wastes from 

1974 to 1984. In the 1990s, low 

concentrations of contaminants 

were found in groundwater and 

soil around the landfill.

Rather than excavating the 

wastes, in 2002 a 6-foot thick 

ET cover was constructed 

over the entire landfill. 

Quick-growing non-native 

plants were in the original plant 

mix to provide erosion control. 

These later were replaced 

with native grasses and brush 

to return the site to a natural 

coastal sage scrub habitat. The 

cover is inspected every six 

months to make sure the cover 

is in good condition and the 

plants are healthy.

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

www.cluin.org/products/evap/

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.

• The contaminated area is large.

• A thick cover is needed.

• Supplies of clean soil, gravel, or other cap materials are not available locally.

• The growing time for the plants is long.

ET covers must be maintained for as long as the contaminated materials 

remain in place to ensure the plants and soil continue to keep water away 

from contamination.

Are They Safe?

When designed for local conditions, ET covers offer a very safe and effective 

way to isolate wastes. Regular inspections are made to ensure that the weather, 

plant roots, and animal activity have not damaged the soil cover and that any 

plants that are part of the cover are still growing. Also, groundwater wells around 

the covered area are sampled to ensure the cover is working and contaminants 

remain isolated.

How Might It Affect Me?

Residents and businesses close to a site may see increased truck traffic as 

materials are brought to the site. Construction of the cover may involve bulldozers, 

backhoes, and other noisy equipment, and some soil may need to be excavated 

for use in the cap. Any dust from excavation and construction can be controlled 

by spraying water or covering stockpiled materials with tarps. 

Why Use ET Covers?

ET covers can be a quick, relatively inexpensive way to isolate landfill wastes 

and other buried contaminated materials. Like conventional caps, installing 

an ET cover can avoid the 

excavation of large amounts 

of soil or waste having low 

levels of contamination. ET 

covers can be designed to 

provide equal performance 

to conventional caps, and the 

plants can make the site more 

attractive. They are also less 

likely than conventional caps 

to be damaged by repeated 

freezing and thawing as 

seasons change. ET covers 

are more commonly used in 

dry climates where there is 

little rainfall.

Wheatgrasses, sage bush, pinyon and juniper are part 

of an ET cover at the Monticello Mill Tailings Superfund 

site in Utah.