Evapotranspirative Tree Caps: Research prototype results, full-scale case histories, and possible future designs

摘要

‘Conventional’ landfill closure covers are made withrncompacted clay and/or plastic membrane layers tornmatch the porosity of the landfill’s bottom liner, asrnrequired by the 1986 Resource Conservation andrnRecovery Act (RCRA). ‘Conventional’ landfill linersrnare designed to protect ground water by containing andrncollecting leachate. RCRA required that the coverrnporosity match the porosity of the liner, to prevent waterrnfilling the landfill like a bathtub. These ‘conventional’rntight covers were designed to shed precipitation like arn‘raincoat’.rnAlternative covers use different techniques to protectrnhuman health and the environment. Evapotranspirativern(ET) caps develop a ‘sponge and pump’ mechanism tornreduce through-cap water percolation. The ‘sponge’rnconsists of a water-holding top layer of soil andrnamendments. The ‘pump’ action is achieved by plantrntranspiration and surface evaporation. The rootable,rnless-compacted soil cover also creates a root zonernreactor where adapted microbes oxidize methane thatrnnow escapes to the atmosphere. ET cover percolationrnmodels are becoming more accurate, so designs can berncompared before construction, then monitored followingrnclosure. ET covers normally percolate much lessrnprecipitation than clay covers. In dry climates, ETrncovers achieve percolation performance equivalent tornplastic membrane covers, but tend to leak more in wetrnclimates.rnThe US EPA has research facilities testing grass- andrntree-based test cells placed across the country under thernAlternative Cover Assessment Program (ACAP). This paper summarizes results from the Iowa and GeorgiarnACAP sites that compare clay, membrane, poplar,rnprairie grass and tree covers.rnThe first ET landfill cover design based on poplar treernphysiology was planted in 1990 at Lakeside Landfill,rnBeaverton, OR. Monitored performance data andrndocumented trends resulted in an Oregon Department ofrnEnvironmental Quality Permit # 214 for final closure.rnThat original ET cover was planted to compare soilrnamendments, poplar clones, and post-installationrnagronomic care practices. The cover grew well.rnLessons learned from this first alternative cover designrnwere applied to future ET cover designs.rnET cover data challenges the existing conventions forrnfinal closure of Subtitle D landfills. This paperrndiscusses some possible future directions in the contextrnof the publication of the EPA Research andrnDevelopment Document (RD&D) focused on protectingrnhuman health and environment.rnLandfill chemistry requires water as a reactant tornconvert solid waste to methane and carbon dioxide.rnLandfills can be operated as bioreactor digesters. Byrnmanaging through-cover water addition via naturalrnpercolation or artificial irrigation, waste stabilizingrnreactions will be stimulated by water addition andrnbioreactor operation.rnEvapotranspirative covers and bioreactors can be part ofrnthe future landfill design to achieve stability whilernmaximizing landfill volume, reducing leachate volume,