‘Conventional’ landfill closure covers are made with compacted clay and/or plastic membrane layers to match the porosity of the landfill’s bottom liner, as required by the 1986 Resource Conservation and Recovery Act (RCRA). ‘Conventional’ landfill liners are designed to protect ground water by containing and collecting leachate. RCRA required that the cover porosity match the porosity of the liner, to prevent water filling the landfill like a bathtub. These ‘conventional’ tight covers were designed to shed precipitation like a ‘raincoat’. Alternative covers use different techniques to protect human health and the environment. Evapotranspirative (ET) caps develop a ‘sponge and pump’ mechanism to reduce through-cap water percolation. The ‘sponge’ consists of a water-holding top layer of soil and amendments. The ‘pump’ action is achieved by plant transpiration and surface evaporation. The rootable, less-compacted soil cover also creates a root zone reactor where adapted microbes oxidize methane that now escapes to the atmosphere. ET cover percolation models are becoming more accurate, so designs can be compared before construction, then monitored following closure. ET covers normally percolate much less precipitation than clay covers. In dry climates, ET covers achieve percolation performance equivalent to plastic membrane covers, but tend to leak more in wet climates. The US EPA has research facilities testing grass- and tree-based test cells placed across the country under the Alternative Cover Assessment Program (ACAP). This paper summarizes results from the Iowa and Georgia ACAP sites that compare clay, membrane, poplar, prairie grass and tree covers. The first ET landfill cover design based on poplar tree physiology was planted in 1990 at Lakeside Landfill, Beaverton, OR. Monitored performance data and documented trends resulted in an Oregon Department of Environmental Quality Permit # 214 for final closure. That original ET cover was planted to compare soil amendments, poplar clones, and post-installation agronomic care practices. The cover grew well. Lessons learned from this first alternative cover design were applied to future ET cover designs. ET cover data challenges the existing conventions for final closure of Subtitle D landfills. This paper discusses some possible future directions in the context of the publication of the EPA Research and Development Document (RD&D) focused on protecting human health and environment. Landfill chemistry requires water as a reactant to convert solid waste to methane and carbon dioxide. Landfills can be operated as bioreactor digesters. By managing through-cover water addition via natural percolation or artificial irrigation, waste stabilizing reactions will be stimulated by water addition and bioreactor operation. Evapotranspirative covers and bioreactors can be part of the future landfill design to achieve stability while maximizing landfill volume, reducing leachate volume,
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