Closure Postclosure and End

Both the design and operation must consider the closure and postclosure periods, as well as the end use. Typical end uses include green areas, parks, and golf courses. As phases are closed, the final or intermediate cover may be applied depending on whether the top elevation of the landfill has been reached. Vertical gas vents or recovery wells can be installed as the final elevations are reached. Horizontal gas recovery wells are installed at specified height intervals as the phases are filled. As the side slopes of the landfill are completed, many aspects of final closure can also be completed, including final cover installation and revegetation.

Figure 10.13.6 shows a typical final cover cross section. The surface layer consists of top soil and is used to support vegetation. The vegetation reduces erosion and aesthetically improves the landfill. Grasses are the most common vegetation used, but other plants are used, including trees. Just below the surface layer is the optional drainage layer, used to minimize the hydraulic head on the barrier layer. The drainage layer can be sand or a geonet and is protected from clogging by a geotextile. The next layer is the hydraulic barrier. Current regulations require it to have a hydraulic conductivity at least as low as the bottom liner. Therefore, the barrier layer usually includes a geomem-brane. A subbase layer may be necessary to protect the barrier layer.

Final closure involves installing the remaining final cover, planting the remaining vegetation, and adding any fencing required to maintain site security. Revegetation depends on a number of factors (O'Leary and Walsh 1992b). First, the cover soil must be deep enough to sustain the planted species. Grasses require at least 60 cm, while trees require at least 90 cm. The final cover topsoil should be

Vegetation (Helps control erosion)

Surface Layer (Supports vegetation)

Ground Surface

Geotextile

Drainage Layer (Drains water off barrier layer)

Barrier Layer — Clay and Geomembrane (Keeps water out, directs gases toward venting and collection system)

Subbase (In some cases a subbase is required to protect the geomembrane)

Waste

FIG. 10.13.6 Typical final cover.

stabilized with vegetation as soon as possible to avoid erosion. Operators should determine the soil characteristics before planting and add lime, fertilizer, or organic matter as required. The bulk density should be measured, and, if too high, amended. Species should be chosen that are landfill tolerant (Gilman, Leone, and Flower 1981; Gilman, Flower, and Leone 1983). Grasses and ground covers should be planted first. If possible, seeds should be embedded in the soil. Trees or shrubs, if used, should be planted only one or two years after grasses are planted. If grasses cannot survive on the landfill, the same will be true of trees and shrubs. The most common problems encountered with revegetation of landfill surfaces are poor soil, root toxicity, low oxygen concentration in the soil pore space, low nutrient value, low moisture content, and high soil temperature. Operators should develop a landfill closure plan which addresses control of leachate and gases, drainage and cover design, and environmental monitoring systems. The postclosure period is currently specified by regulation to be at least thirty years after closure. During this time, surface water drainage control, gas control, leachate control, and monitoring continue. The general problems that must be addressed during this period are the maintenance of required equipment and facilities, the control and repair of erosion, and the repair of problems associated with differential settlement of the landfill surface.

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