A 4-year field study was conducted to assess the ability of landfill covers to control percolation into the waste. Performance of one conventional cover was compared to that of two ET tree covers, using large (7 x 14 m) lined lysimeters at the Leon County Solid Waste management facility in Tallahassee, FL. ET tree covers vegetated with cottonwood or eucalyptus performed well in the North Florida climate as an alternative to a conventional cover with a geosynthetic clay liner.

As the field of phytoremediation has evolved over the last 10 years, there has been a growing interest in the use of vegetative covers as alternative landfill caps. Consequently, a number of vegetative caps (grass, shrubs, and trees) have been installed at landfills across the United States, with tree caps being the focus of this paper. The two primary objectives of vegetative caps are to minimize water percolation into landfill waste via a "sponge and pump" mechanism and to prevent surface soil erosion. The soil pores hold precipitation like a sponge until plant roots can access the water; plants take up this water for growth and release it into the atmosphere by transpiration. The erosion control objective is achieved by canopy interception of rainfall, minimizing the "splash effect," and the growing of a root matrix that interlaces through soil particles. Secondary benefits of vegetative caps include aesthetic appeal for the neighboring community, sequestration of greenhouse gases, reduced construction costs, reduced operation costs, and creation of wildlife habitat. When properly designed and managed, vegetative caps can minimize landfill owner liability while creating a future asset for the surrounding community. The Ecolotree Cap (ECap) (Ecolotree, Inc., North Liberty, Iowa) is a patented phytoremediation system that consists of carefully designed cover soils planted with perennial trees and understory grasses. This system has been installed at 13 landfills in the United States (in nine states) and at one landfill in Europe (Slovenia). The sites include pre-Subtitle D landfills, Subtitle D demonstrations, a construction debris landfill, and a landfill on the Superfund National Priorities List. The ECap cover is currently being evaluated for Subtitle D landfill closure at two sites by the U.S. Environmental Protection Agency Alternative Cover Assessment Program.

A remedial solution in the form of a vegetative cap was implemented at the Site 17/1400 Area Landfill. The cap will minimize migration of contaminated ground water through hydraulic control by groves of phreatophytic trees, limit rain water from reaching the water table, and stabilize the soil cover. This paper details the construction of the remedy, lists the plants selected, and describes the lessons learned during the course of installing a plant-based system. In 2000, a cap was installed comprised of a two-foot layer of clean fill planted with specially selected trees such as hybrid poplars, willows, and sycamores--over 900 hybrid poplar trees and 800 trees of other species. The limitations of the planting and growing season rendered scheduling and delivery of supplies of vital importance, which this paper emphasizes in its detailing of lessons learned. Resources research for nurseries and fertilizers should be undertaken early and extensively, and tree quantities and delivery coordinated well before they are needed. Coordinate soil sources up front because blending of soils from varied sources may be necessary to achieve the optimum mixture, which will contain at least 5% organic matter. Install an irrigation system to ensure adequate moisture control on the cap. Allow for unexpected environmental conditions involving both weather and wildlife that may affect the growth of plants and trees. The author credits the overall success of this project to keeping all key players well informed at every stage and seeking their input. The site was an operating gravel pit between 1953 and 1969. From 1970 through 1978, the depression created by the graveling operation was used as a landfill for municipal waste, as well as construction debris, aircraft parts, and ordnance casings. The Site 17 Landfill originally covered about 6.0 acres. As a result of the Remedial Action, the landfill now covers 3.5 acres and is properly controlled. The cost of the project was $1,800,000.

The establishment of a vegetative cover to enhance evapotranspiration and control runoff and drainage was examined as a method for stabilizing a landfill containing coal combustion waste. Suitable plant species and pretreatment techniques in the form of amendments, tilling, and chemical stabilization were evaluated. A randomized plot design consisting of three subsurface treatments (blocks) and five surface amendments (treatments) was implemented. The three blocks included (1) ripping and compost amended, (2) ripping only, and (3) control. Surface treatments included (1) topsoil, (2) fly ash, (3) compost, (4) apatite, and (5) control. Inoculated loblolly (Pinus taeda) and Virginia (Pinus virginiana) pine trees were planted on each plot. After three growing seasons, certain treatments were shown to be favorable for the establishment of vegetation on the basin. Seedlings located on block A developed a rooting system that penetrated into the basin media without significant adverse effects to the plant. However, seedlings on blocks B and C displayed poor rooting conditions and high mortality, regardless of surface treatment. Pore-water samples from lysimeters in block C were characterized by high acidity, Fe, Mn, Al, sulfate, and trace-element concentrations. Water-quality characteristics of the topsoil plots in block A, however, conformed to regulatory protocols. A decrease in soil-moisture content was observed in the rooting zone of plots that were successfully revegetated, which suggests that the trees, in combination with the surface treatments, influenced the water balance by facilitating water loss through transpiration and thereby reducing the likelihood of unwanted surface runoff and/or drainage effluent.