THE PRACTICE OF USING engineered tree caps to cover landfills is beginning to grow roots in the waste industry. Although low-permeability clay or geomembrane covers generally are used to cap waste and prevent water from penetrating a site, several landfills now are using the “sponge and pump” process provided by trees to manage precipitation.
The sponge and pump method relies on rootable soil, amendments and vegetation that are placed over waste. The sponge can consist of cover soils, borrow soils, composts and waste materials such as biosolids, wood chips, street sweepings and power plant ash. Trees and grasses provide the “pump” by taking water up through their root systems and releasing it into the atmosphere via evapotranspiration. Using a hydrologic model, engineers can determine which sponges are best-suited to store water during the trees' growing and dormant seasons.
Generally, a landfill permit requires the sponge and pump to become functional quickly once it's implemented. Consequently, fast-growing trees — hybrid poplars, cottonwoods and aspens — are preferred. Poplar trees are most frequently used because they grow quickly, have high water-uptake rates, root aggressively, and are tolerant of low-oxygen conditions often present at landfills.
Since 1990, more than 15 tree caps have been installed at U.S. landfills that are pre-Subtitle D, Subtitle D on a demonstration basis, construction and debris (C&D), and Superfund sites. The U.S. Environmental Protection Agency's National Risk Management Research Laboratory (NRMRL), Cincinnati, also is testing the process and seeing positive results.
For example, the Oliver Landfill in Waterford Township, Pa., is a pre-Subtitle D site that is using hybrid poplar trees to cap its waste. When the landfill closed in 1971, it then was covered with 2 feet of soil and vegetated with grass. Then in the 1999, a 13-acre 11,000 hybrid-poplar tree cap was designed and installed by Ecolotree Inc., North Liberty, Iowa, and Cranbury, N.J.-based Blasland, Bouck & Lee Inc. for Waste Management of Pennsylvania. Today, the site is owned by the Waterford Township and is a recreational area, which includes baseball and soccer fields located next to the landfill.
The site's design has helped to limit access to the inactive waste disposal cell, according to Waste Management. Indigenous trees and shrubs were planted as a “living fence” around the inactive waste cells. Also, hybrid poplar trees were planted on top of the inactive cells to manage groundwater. A wetland was constructed to mitigate environmental displacement and to manage surface water runoff from the recreation facilities. The project has improved groundwater quality because of the trees high rate of water uptake, and allowed the community to create an aesthetically pleasing area without having to purchase additional land for its Little League fields.
“Environmental protection was enhanced at the site, and the community gained a valuable asset,” says Trevan Houser, the Waste Management project manager who proposed using the tree cap design.
Additionally, the EPA's Alternative Cover Assessment Program (ACAP) is currently evaluating a licensed tree cap called ECap, at a Subtitle D landfill and a Resource Conservation and Recovery Act (RCRA) waste disposal site. At the Subtitle D location in Iowa, the tree cap leaked 3 inches of the total 39 inches of precipitation during a nine-month period. This means the cap had a 92 percent effectiveness rate compared with a geomembrane cap, which had a 99 percent effectiveness rate, and a compacted clay cap, which had a 96 percent rate.
At the RCRA ACAP site in southern Georgia, a tree cap outperformed a prescribed compacted clay cap. The tree cap was 90 percent effective between 2000 and 2002, leaking 9.5 inches out of 101 inches of total applied water. The clay cap was an estimated 70 percent to 75 percent effective in 2002. However, it did not have sufficient grass cover for comparison purposes before 2002. Costs to cap the site's 17 acres and perform 30 years of operation and maintenance were estimated to be $10.5 million for the compacted clay cap and $5.4 million for a tree cap.
Tree caps are estimated to cost 30 percent to 50 percent less than geomembrane covers. However, tree covers have limitations. For example, they are not suitable for arid climates because sufficient water is needed to support tree growth. Also, trees must grow for approximately four to six years to reach their maximum pumping capacity. During this time, the evapotranspiration pump is not at full capacity and the potential for leakage is greater than after full development.
Besides preventing water from infiltrating waste, trees have other uses at landfills. For example, they can intercept and treat leachate plumes, capture odors and wind-blown waste, and mitigate irrigated leachate. Tree caps also can help to control erosion, reduce subsurface landfill gas movement off-site and sequester greenhouse gases.
Data still is being gathered to evaluate tree caps, but their popularity is growing.
Visit the Alternative Cover Assessment Program Web site www.acap.dri.edu for more information.