Landfill Liners and Covers Put a Lock on Leachate

There are two driving factors: the liner and the cover. How they are designed and installed, what materials are used, and how it is maintained play a significant role in achieving a high degree of assurance of environmental integrity throughout the period in which post-closure care is required. Experts agree that no liner is totally impermeable, but the right approach offers the lowest possible permeability against leachate and gas escaping into the environment. Geosynthetics serve the purpose of keeping a plastic liner from being punctured, depending on their strength and resistance to tears, impact, punctures, and environmental stress. The cap used for the landfill’s closure also is subject to the same factors that will lay the groundwork for success. [text_ad] Don Hullings, Cornerstone Environmental Group client manager, points out that geosynthetics are used “first and foremost” as a prescriptive standard to keep water out of leachate. As of late, a more pressing concern is to contain the gas as well, he says. “We don’t want to have odor or greenhouse gas emissions coming out. We want to have a beneficial gas use project and contain all of that gas within the landfill and collect it in a gas collection system, but not have it be emitted through the cover,” he adds. Geosynthetics offer a “very, very low permeability,” which is lower than what could be derived from a traditional compacted clay layer, which was used before the advent of geosynthetic systems, Hullings notes. He points out a geosynthetic membrane is not totally impermeable, given that water can seep through under “very long-term conditions” and through defects in seams and pinholes. Design and Materials Site-specific conditions and budgets will dictate landfill liner and cap designs and materials used. Todd Farmen, principal environmental engineer with Arcadis USA, says in areas with a lot of clay soils, “geosynthetics aren’t the right way to go. But in a lot of sites in some parts of the country that don’t have a readily available clay source, geosynthetics is certainly a more cost-effective way to construct either a cell or a cap.” There are many elements that go into a design, depending on the facility, says Farmen. “Is it in a seismic area? What kind of safety factors are they trying to achieve? Those are all typically governed by state or federal regulations that you’re designing under to meet those criteria,” he says. There are many types of geosynthetics from which to choose for a baseline or cover system, Farmen points out, adding that the chosen materials are site- and design-specific. “There are geosynthetic clay liners for containment, geocomposite systems for drainage, fabrics for separation and for cushioning effects on liner systems, and geogrids for soil reinforcement and strengthening,” notes Farmen. “If you’re dealing with a liner or a cover system that’s flat or doesn’t have as much of a gradient on it, then you’re typically going to use geomembranes or liner systems that are smooth,” he adds. “If you’re dealing with more slopes and need to have that higher degree of interface friction between the liner and cap materials, you’re going to go with a textured liner.” Arcadis USA has a project in Catskill, NY, recapping a landfill with ClosureTurf by Watershed Geo, which features a textured geomembrane component manufactured by AGRU America. ClosureTurf combines a drainage system and geomembrane barrier with a durable synthetic turf. [text_ad use_post='27767'] It is designed to install more than 50% faster than traditional closure systems. It eliminates the need for large quantities of soil and related equipment activities. Average maintenance costs are reduced by $1,300 per acre per year. ClosureTurf is designed to improve stormwater runoff quality and reduce carbon footprint by 80%. The benefit to the client for that site is that it significantly reduces the amount of long-term O&M on the 40-plus-acre landfill, notes Farmen. The primary overriding goal in designing a final cover system to effectively control the leachate is to ensure it is less permeable than the bottom liner system “because if it was more permeable, moisture would infiltrate in there somehow and it would start building in the landfill and have a bathtub effect, which Subtitle D federal regulations were set up to avoid,” says Ron DiFrancesco, principal and senior consultant with Golder Associates.

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