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The Alternative Covers Assessment Program (ACAP)

Note: EPA no longer updates this information, but it may be useful as a reference or resource.

Alternative covers attempt to achieve equivalent performance to conventional impermeable covers through an action that has been described as 'sponge and pump.' In this type of cover system, the soil and plants absorb moisture from precipitation, store it in the plant and soil structure, and later release much of that moisture back to the atmosphere through evaporation directly from the soil or through transpiration from the plants. This is an entirely different method of excluding water from an underlying waste mass than the more established impermeable layer approach. It is difficult to directly compare the two systems, which use such different mechanisms.

One way to determine equivalent performance is to build the two cover systems side by side and compare the relative exclusion of infiltration. The Alternative Cover Assessment Program was established in part to provide exactly that comparison. A series of test facilities were constructed across the country to collect performance data on site-specific designs. The sites include a variety of climatic conditions, with varying rainfall, temperature, altitude and growing season. An ACAP test facility is designed to simulate a landfill cover system for a waste containment facility. Each test section contains:

  • an impermeable lysimeter pan with a bottom drain to collect percolation from the base of the cover;
  • a collection system for surface runoff; and sensors to monitor hydrologic variables within the cover soils, percolation and runoff volumes, and
  • meteorological data.

ACAP facilities resemble aboveground swimming pools filled with soil and other landfill cover components and monitoring instruments.


Steven Rock
U.S. EPA National Risk Management Research Laboratory
Land Remediation and Pollution Control Division
26 W. Martin Luther King Dr.
Mail Code: CHL
Cincinnati, OH 45268

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