Final cover system

Alternative final cover systems, such as the innovative evapotranspiration (ET) cover systems, are increasingly being considered for use at waste disposal sites, including municipal solid waste (MSW) and hazardous waste landfills when equivalent performance to conventional final cover systems can be demonstrated. Unlike conventional cover system designs that use materials with low hydraulic permeability (barrier layers) to minimize the downward migration of water from the cover to the waste (percolation), ET cover systems use water balance components to minimize percolation. These cover systems rely on the properties of soil to store water until it is either transpired through vegetation or evaporated from the soil surface. 

Final cover systems are intended to remain in place and maintain their functions for an extended period of time. 

For hazardous waste landfills, RCRA Subtitle C provides certain performance criteria for final cover systems. While RCRA does not specify minimum design requirements, U.S. EPA has issued guidance for the minimum design of these final cover systems. Figure 25.1b shows an example of an RCRA Subtitle C cover at a hazardous waste landfill.30... 

FIGURE 25.1 Examples of final cover systems, (a) MSW landfill and (b) hazardous waste landfill.15... 

Limited data are available to describe the performance of ET cover systems in terms of minimizing percolation, as well as the covers ability to minimize erosion, resist biointrusion, and remain effective for an extended period of time. While the principles of ET covers and their corresponding soil properties have been understood for many years, their application as final cover systems for landfills has emerged only within the past 10 years. Limited performance data are available on which to base applicability or equivalency decisions.39,43 46... 

Numerical models are used to predict the performance and assist in the design of final cover systems. The availability of models used to conduct water balance analyses of ET cover systems is currently limited, and the results can be inconsistent. For example, models such as Hydrologic Evaluation of Landfill Performance (HELP) and Unsaturated Soil Water and Heat Flow (UNSAT-H) do not address all of the factors related to ET cover system performance. These models, for instance, do not consider percolation through preferential pathways may underestimate or overestimate percolation and have different levels of detail regarding weather, soil, and vegetation. In addition, HELP does not account for physical processes, such as matric potential, that generally govern unsaturated flow in ET covers.39 42 47... 

Although the ability to minimize percolation is a performance criterion for final cover systems, limited data are available about percolation performance for final cover systems for both conventional and alternative designs. Most of the recent data on flux rates have been generated by two federal research programs, the Alternative Landfill Cover Demonstration (ALCD)84 and the Alternative Cover Assessment Program (ACAP). From these programs, flux rate performance data are available for 14 sites with demonstration-scale ET cover systems.5,39,85... 

Final cover systems are another important component of waste containment systems used at landfills. While liner systems are installed beneath the waste, final cover (or closure) systems are installed over the completed solid waste mass. For hazardous waste landfills, 40 CFR 264 requires that the landfill be closed with a final cover system that meets certain performance criteria, most notably, that they have a permeability less than or equal to the permeability of any bottom liner system or natural subsoils present. U.S. EPA guidance documents517 recommend that final cover systems for hazardous waste landfills consist of at least the following, from top to bottom ... 

FIGURE 26.28 Final cover system. (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)... 

Modern landfill facilities, waste lagoons, and leachate ponds use geotextiles in their key designed systems to contain wastes within the facility. The main designed systems are groundwater and gas collection systems underneath the base liner system, typically consisting of a leak detection system and a leachate collection and removal system, and within the final cover system to collect gas and water below and above the barrier system, respectively. 

Figure 19.2 Final cover system of a modem waste containment facility typically containing a geomembrane barrier layer.

Gas transmission beneath liners and surface water drainage in and above lined final cover systems... 

The site conditions for an on-site landfill, such as location, geology, hydrogeology, physiography, climate, and so on, should also be suitable. Landfill should meet the minimum technology requirements and regulations for hazardous waste landfills such as double liners and leachate collection and removal systems, leak detection systems, closure procedures and final cover, and construction quality assurance.59... 

The design of cover systems is site-specific and depends on the intended function of the final cover—components can range from a single-layer system to a complex multilayer system. To minimize percolation, conventional cover systems use low-permeability barrier layers. These barrier layers are often constructed of compacted clay, geomembranes, geosynthetic clay liners, or combinations of these materials. 

In addition to being called ET cover systems, these types of covers have also been referred to in the literature as water balance covers, alternative earthen final covers, vegetative landfill covers, soil-plant covers, and store-and-release covers. 

Basically, U.S. EPA regulations and guidance16 require that the final cover be no more permeable than the liner system. In addition, the cover must be designed to function with minimum maintenance, and to accommodate settlement and subsidence of the underlying waste. The regulations do not specify any design criteria for liner materials to meet the performance standard for permeability. 

The guidance16 recommends a three-layer cap design consisting of a vegetative top cover, a middle drainage layer, and a composite liner system composed of an FML over compacted low-permeability soil. The final cover is to be placed over each cell as it is completed. 

Another important implication is that highly permeable soil liners generally have defects, such as cracks, macropores, voids, and zones, that have not been compacted properly. One opportunity to eliminate those defects is at the time of construction. Another opportunity arises after the landfill is in operation, and the weight of overlying solid waste or of a cover over the whole system further compresses the soil. This compression, however, occurs only on the bottom liners, as there is not much overburden stress on a final cover placed over a solid waste disposal unit. This is one reason why it is more difficult to design and implement a final cover with low hydraulic conductivity than it is for a bottom liner. Not only is there lower stress acting on a cover than on a liner, but also the cover is subjected to many environmental forces, whereas the liner is not. 

CES This closure evaluation system consists of three systems to aid in review of closure plans regarding vegetation cover, final cover, and leachate collection system (prototype). PC-Plus Knowledge-Pro Micro D. Greathouse/EPA/RREL... 

The data for surface phonon dispersion determined either experimentally or theoretically for adsorbed covered systems is reported and compared with the surface phonon dispersion of the corresponding bare system. The data is organised according to the electrical properties of the material firstly metals, secondly elemental semiconductors and insulators, and finally compound semiconductors, oxides and salts. The reported systems are collected in Table I. 

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