Once fill capacity is reached, landfills are closed for rehabilitation. With the exception of older landfills that are left abandoned with minimal human interference, most postclosure landfills are rehabilitated using engineering and ecological approaches. Landfilled wastes are isolated physically from the biosphere by bottom barrier layers and surface cap technologies. Barrier systems can be sophisticated with multiple layers of geotextiles and impermeable synthetic membranes. The surface is usually covered with soil of 1-2 m thickness. Subsequent site development entails the establishment of a vegetation cover on the landfill soil, with the primary aims of minimizing environmental impact and making good value of its designated afteruse. Technically, closed landfills can be rehabilitated by either spontaneous ecological development in the absence of human intervention, manipulated succession followed by natural development, or habitat creation which involves intensive and prolonged management. Sole natural development is unreliable and slow, and lacks control of the ecological outcome. The aftercare period for a landfill can be as long as 30 years, but public safety and engineering concerns are usually of higher priority than the ecological function of the reclaimed site.
The criteria for selecting afteruses for former landfills include landuse planning policies, site characteristics, soil resource availability, social needs, and cost consideration. As construction on postclosure landfills is generally prohibited due to severe subsidence as a result of organic matter decomposition, and fire hazards associated with landfill gas, it is a usual practice to reclaim urban sites for soft end uses in order to provide amenity facilities such as parks, botanical gardens, golf courses, and playing fields that are safe for use by the public. Alternatives end uses for agriculture, nature conservation, and forestry are also common. Grassland has been one of the most popular end uses for rural sites, but agricultural conversion is not always appropriate because of the lack of quality topsoil. Nature conservation is sometimes a more suitable afteruse as it requires less intensive aftercare and is more flexible on the postclosure ecological design, though the transformation to wildlife habitat is not imperative. End use after closure can be mixed landscapes as in the Fresh Kills Landfill in New York, USA, which is converted to an amenity parkland with a range of landuses which include forests, dry lowlands, tidal wetlands, freshwater wetlands, waterways, and wildlife habitats.
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