EVAPOTRANSPIRATION (ET) COVERS

Evapotranspiration (ET) cover systems are increasingly being used at municipal solid waste (MSW) landfills, hazardous waste landfills, at industrial monofills, and at mine sites. Conventional cover systems use materials with low hydraulic permeability (barrier layers) to minimize the downward migration of water from the surface to the waste (percolation), ET cover systems use water balance components to minimize percolation. These cover systems rely on soil to capture and store precipitation until it is either transpired through vegetation or evaporated from the soil surface. Compared to conventional membrane or compacted clay cover systems, ET cover systems are expected to cost less to construct.

They are often aesthetic because they employ naturalized vegetation, require less maintenance once the vegetative system is established, including eliminating mowing, and may require fewer repairs than a barrier system. All cover systems should consider the goals of the cover in terms of protectiveness, including the pathways of risk from contained material, the lifecycle of the containment system.

The containment system needs to be protective of direct contact of people and animals with the waste, prevent surface and groundwater water pollution, and minimize release of airborne contaminants. While most containment strategies have been based on the dry tomb strategy of keeping waste dry, there are some sites where adding or allowing moisture to help decompose organic waste is the current plan. ET covers may work well in places where complete exclusion of precipitation is not needed.

The U.S. EPA Alternative Cover Assessment Program (ACAP), USDOE, the Nuclear Regulatory Commission, and others have researched ET cover design and efficacy, including the history of their use, general considerations in their design, performance, monitoring, cost, current status, limitations on their use, and project specific examples. An on-line database has been developed with information about specific projects using ET covers.

There are three general approaches for non-conventional cover systems to achieve approval for installation; the first is when equivalent performance to conventional final cover systems can be demonstrated directly on site. This is the approach used by the Sandia study, by most ACAP sites, and the Rocky Mountain Arsenal. A second approach is used when there are data from a site specific study such as an ACAP installation at a site that has analogous soil and climate conditions. Several sites in Colorado and Southern California have achieved approval based on data from similar sites. The third most common approach for regulatory approval is by installation of data collection systems with the agreement that the permanence of the ET cover installation is contingent on success of the cover in meeting certain performance goals. This article is intended as an introduction to the topic and is not intended to serve as guidance for design or construction, nor indicate the appropriateness of using an ET cover systems at a particular site.