Research Highlights

Field-Testing Alternative Landfill Covers

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Modern landfills are complicated and costly structures. Closely regulated by state and federal statutes, they are designed to protect buried trash from contact with air, light, and water. This “dry tomb” technology relies on various systems of liners and surface capping, using clay, plastic membranes, or both.

The high costs associated with traditional impermeable landfill covers and a growing interest in alternative designs prompted EPA to create the Alternative Cover Assessment Program (ACAP) in 1997. This national program, supervised by NRMRL researchers, was the first field-scale, side-by-side comparison of traditional and alternative covers.

In collaboration with a dozen private and public partners, the ACAP researchers in 14 communities over a 6-year period examined when and how much water passed through the test cover systems. In so doing, the program generated the world’s largest body of data on landfill cover performance.

The data provide a body of historical information on how systems succeed or fail across a wide range of climates and soil conditions. They also provide the technical foundation for a computer-based methodology that can be used by engineers, regulators, and other decision makers to design and evaluate alternative covers.

The Back Story

Covers of compacted clay or a composite of geomembrane over clay have been the standard practice since the Resource Conservation and Recovery Act of 1976. Typically, these covers require 18 inches of earthen material with an erosion overlay of at least 6 inches of soil capable of supporting native plant growth. Questions about the long-term performance of these impermeable covers had gone unanswered for lack of substantial data. Meanwhile, by the late 1990s, alternative evapo-transpiration (ET) covers were being developed.

Using site-specific variables of soil, plants, and climate, ET covers rely on a “sponge and pump” action in which soil and plants absorb precipitation, store it, and then later release much of the moisture back into the atmosphere through evaporation (from the soil) and transpiration (from the plants).

Also known as water balance or vegetative caps, ET covers can be based on either a continuous layer of soil or on layers that create capillary breaks, a feature that improves the water-holding capacity of the soil. Plantings include grasses, bushes, or fast-growing trees appropriate to the site. The ACAP research was designed as an on-site, data-driven program to answer performance questions about both ET and traditional covers.

The test sites in California, Georgia, Iowa, Montana, Nebraska, Ohio, and Oregon were chosen for a variety of climates, soils, and climax (stable) vegetation. Each test site contained sensors and devices for measuring runoff, soil moisture, meteorological data, and percolation through the cover.

Key Findings

ACAP research yielded the following key findings:

As a measurement program for drainage performance and for process and design variables, ACAP was a complete success. The program has led to a better understanding of testing cover designs.
In all locations, alternative covers performed as well or better than compacted clay covers.
To date, installation cost savings total $205 million at the 30 sites using alternative covers.
ACAP data and design methodologies have been used in support of regulatory decisions to approve permits for and installation of alternative cover systems at many conventional cover sites, including four Superfund sites.

While decisions about landfill cover design still require site-specific evaluation, the information gleaned from the ACAP data makes this technology a very useful tool.

For more information about the ACAP research, see Alternative Landfill Cover Project Profiles .

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