Evapotranspiration Covers
Overview
Modern landfills are designed to protect both public health and the environment by providing physical containment of trash, isolation from air and water, and control of landfill gas, leachate, odors, disease vectors, and other nuisances. Modern landfill technology provides hydrologic isolation with various systems of liners and surface capping using compacted clay, plastic membranes, or both; as well as an overlay of about 18 inches of earthen material and at least six inches of soil to support native plant growth and control erosion. Final cover systems are also expected to meet aesthetic and other post-closure site end use criteria for waste management sites. These systems are intended to achieve their functional requirements for time periods of many decades to hundreds of years.
Questions about the long-term performance of the low-permeable layers in conventional cover designs have prompted significant interest and research into alternative designs. Alternative landfill cover systems emerged in the late 1990s as a possible cost-effective option to conventional covers and have been applied at some landfills and other types of waste management units (e.g., waste piles and surface impoundments). The implementation of an alternative cover system is dependent on specific site characteristics. The site characteristics that have a dominant influence on the choice of an appropriate final cover include climate, soils, landfill waste characteristics, hydrogeology, gas production, seismic environment, and reuse of landfill areas.
There are various alternatives to conventional covers. This site focuses on covers that utilize natural processes to manage water precipitating on waste containment sites, commonly known as evapotranspiration (ET) covers. These covers have proven an effective means of containing waste at municipal landfills, hazardous, and industrial waste landfills. There are other types of alternatives to conventional cover systems, such as asphalt barriers. However, these are not addressed on this site because they are not evapotranspiration covers.
ET covers are also known as store and release covers, vegetative covers, sponge and pump covers, alternative final covers (AFC), alternative final earthen covers (AFEC), and other names. They include various combinations of earthen materials and plants, and generally can be categorized into the following cover types:
- Monolithic: Any precipitation water is stored in a layer of soil and later removed through evapotranspiration.
- Capillary break: This cover uses a two layer system to increase the water storage capacity of the cover. A capillary break is formed by two layers – a layer of fine soil over a layer of coarser material (e.g. sand or gravel). Capillary force causes the layer of fine soil overlying the coarser material to hold more water than if there were no change in particle size between the layers.
- Dry barrier: The dry barrier cover uses wind-driven airflow through the layer of coarse material to remove water from a storage layer.
For further reading:
The Community Guide series (formerly Citizen's Guides) is a set of two-page fact sheets describing cleanup methods used at Superfund and other hazardous waste cleanup sites. Each guide answers six questions about the method: 1) What is it? 2) How does it work? 3) How long will it take? 4) Is it safe? 5) How might it affect me? 6) Why use it?
An effective cap design requires that the cap limit percolation into the underlying soils. For earthen caps, design consists of selecting an arrangement of earthen materials that can be used to divert water away from the cap or store the water until it can be removed later by evaporation and/or plant transpiration.
International Journal of Phytoremediation 14(1):1-25(2012)
ET cover systems are being used increasingly at municipal solid waste landfills, hazardous waste landfills, industrial monofills, and mine sites. This paper is intended only to provide an introduction to the topic of ET covers; it does not offer technical guidance.
This Fact Sheet updates the 2003 Fact Sheet on Evapotranspiration Covers and provides information on the regulatory setting for ET covers, general considerations in their design, performance, and monitoring, and status at the time of writing (2011). Also several examples with supporting performance data are provided, as well as a list of sites that have proposed, approved, and installed ET covers.
Links to several documents focusing on alternative landfill technologies prepared by the Interstate Regulatory Technology Council.
