Landfill-covering system

Source U.S. EPA, Evapotranspiration Landfill Cover Systems Fact Sheet, EPA 542-F-03-015, U.S. Environmental Protection Agency, Washington, DC, September 2003. 

Typical applications of geotextiles using the filter function are drainage trenches, earth dams, bank revetments, coastal structures, retaining walls, landfill cover system, etc. 

In landfill-covering system, geotextiles are often used as a drainage layer for gas collection. Koemer et al. (1984) conducted tests on transmissivity to air of nonwoven geotextiles by a radial flow device similar to that illustrated in Fig. 13.1(b). Transmissivity to air was more than two orders higher than that to water imder comparable conditions. Furthermore, their test results showed that normal pressure did not have a significant influence on the air permeability of the geotextiles. 

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. 

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... 

Because of the water-holding properties of soils and the fact that most precipitation returns to the atmosphere via ET, it is possible to devise a landfill cover to meet remediation requirements, and yet contain no barrier layer. The ET cover consists of a layer of soil covered by native grasses it contains no barrier or impermeable layers. The ET cover uses two natural processes to control infiltration (1) soil provides a water reservoir and (2) natural evaporation from the soil plus plant transpiration (ET) empties the soil water reservoir.32-38 The ET cover is an inexpensive, practical, and easily maintained biological system that will remain effective during extended periods of time—perhaps centuries—at low cost. 

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. 

Further, landfill characteristics, such as production of landfill gases, may limit the use of ET covers. The cover system may not adequately control gas emissions since typical ET cover designs do not have impermeable layers to restrict gas movement. If gas collection is required at the site, it may be necessary to modify the design of the cover to capture and vent the gas generated in the landfill. In addition, landfill gas may limit the effectiveness of an ET cover, because the gases may be toxic to the vegetation.13 45... 

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... 

Because borrow soils will be mixed and modified during placement, the cover soil for an ET landfill cover, as constructed, will be unique to the site. However, the soil properties may be easily described. The design process requires an evaluation of whether or not the proposed soil and plant system can achieve the goals for the cover. Numerous factors interact to influence ET cover performance. A mathematical model is needed for design that is capable of (1) evaluating the site water balance that is based on the interaction of soil, plant, and climate factors and (2) estimating the performance of an ET landfill cover during extended future time periods. 

A major requirement of a landfill cover is to control the amount of precipitation that enters the waste. The amount of water that percolates through the cover and may enter the waste is called PRK. PRK is a part of a much bigger hydrologic system and must be assessed in parallel with the other parts. Therefore, it is necessary to estimate the entire hydrologic water balance for the cover in order to assess its behavior.49... 

The U.S. Department of Agriculture (USDA) soil classification system was developed for use in describing soils in which plants grow.63-66 The USDA system is now universally accepted within the United States and it should be used to describe soils used in ET landfill covers. 

Control layers, such as those used to minimize animal intrusion, promote drainage, and control and collect landfill gas, are often included for conventional cover systems and may also be incorporated into ET cover system designs. For example, a proposed monolithic ET cover at Sandia National Laboratories in New Mexico will have a biointrusion fence with 1/4-in. squares between the topsoil layer and the native soil layer to prevent animals from creating preferential pathways, potentially resulting in percolation. The biointrusion layer, however, will not inhibit root growth to allow for transpiration. At another site, Monticello Uranium Mill Tailings Site in Utah, a capillary barrier ET design has a 12-in. soil/rock admixture as an animal intrusion layer located 44 in. below the surface, directly above the capillary barrier layer. 

In more recent applications, several types of ET cover designs also have incorporated synthetic materials, such as geomembranes, which are used to enhance the function of minimizing water into the waste. For example, the Operating Industries Inc. Landfill in California has incorporated a soil layer with a geosynthetic clay liner in the design. The cover system for this site will reduce surface gas emissions, prevent oxygen intrusion and percolation, and provide for erosion control.68... 

Percolation monitoring can also be evaluated indirectly by using leachate collection and removal systems. For landfills underlain with these systems, the amount and composition of leachate generated can be used as an indicator of the performance of a cover system (the higher the percolation, the more leachate that will be generated).22... 

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... 

Stormont, J.C., Incorporating capillary barriers in surface cover systems, Environmental Science and Research Foundation, Proceedings, Landfill Capping in the Semi-Arid West Problems, Perspectives, and Solution, Grand Teton National Park, WY, ESRF-019, May 21-22, 1997, pp. 39-51. 

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 1 illustrates the components of municipal solid waste (MSW) landfill waste containment systems. MSW landfills are by far the most common type of engineered waste containment system, followed (with respect to frequency of occurrence) by cover systems (caps) for uncontrolled dumps and hazardous waste sites and then by hazardous waste landfills. Caps and containment systems for hazardous waste landfills employ similar elements to MSW landfills. The performance of ancillary facilities at a waste disposal site subject to seismic loading, including leachate and gas treatment facilities, surface water control systems, access roadways, and landfill monitoring systems, is also an important consideration but will not be addressed herein. 

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