Landfills that have reached the end of their useful lives must be capped and taken out of service. The final cover must adhere to the specifications provided in Subtitle D of the Resource Conservation and Recovery Act (RCRA). This section of the federal code deals with the management and disposal of nonhazardous municipal solid waste (MSW).
In essence, the final cover must be designed to minimize infiltration and erosion to prevent the “bathtub effect,” which occurs when the landfill’s cover system is too permeable and liquid seeps to the bottom liner system. The potential for generation of leachate greatly increases with the “bathtub effect.”
Specifically, RCRA requires the final cover to be a barrier layer that consists of at least 18 inches of earthen material and is capped by an erosion layer of at least six inches of earthen material. The cap must be able to sustain native plant growth. RCRA does allow for an alternative design for a landfill final cover if it demonstrates the equivalent performance as a conventional barrier.
An alternative design that is becoming increasingly more popular is the Evapotranspiration (ET) Cover System. ET cover systems can also be called water balance covers, alternative earthen final covers, vegetative landfill covers, soil-plant covers and store-and-release covers. An ET cover depends on a thick soil layer with a vegetative cover that is capable of storing water until it is lost to the environment through transpiration or evaporation.
This system considers all the hydrological processes at a specific site, including the water storage capacity of the soil, precipitation, surface runoff, evapotranspiration and infiltration. Percolation through the cover system can be minimized by using materials with high water storage capacity and evapotranspirative properties. Three main principles guide the design of an ET cover system––using fine-grained soils with relatively high water storage capacity, selecting native vegetation to increase evapotranspiration and streamlining construction and providing cost savings through the use of locally available soils.
Two general types of ET cover systems are monolithic barriers and capillary barriers. Monolithic barriers rely on a vegetated, relatively homogeneous, fine-grained soil layer to store water and limit deep drainage that results in the “bathtub effect.” A capillary barrier design consists of a fine-grained soil layer on top of a coarser material, such as sand or gravel. The fine-grained layer can retain more water than a monolithic cover system of equal thickness because the coarse layer forms a capillary break at the layer interface. The images below illustrate the differences between monolithic barriers and capillary barriers and are from the US EPA’s Evapotranspiration Landfill Cover Systems Fact Sheet:
ET cover systems are not a one-size-fits-all solution for MSW alternative final cover design issues. For instance, ET cover systems are considered potentially applicable only in areas that have arid or semi-arid climates, like the Great Plains or the Southwest U.S. Also site-specific conditions and local climatic conditions may make ET cover systems unfeasible. ET covers may not adequately control gas emissions. Gas may need to be captured and vented, which will require the ET cover design to be modified. Landfill gases can also be toxic to vegetation, and this can greatly restrict the use of ET cover systems.