Phytocapping: an alternative technology for the sustainable management of landfill sites

Landfill remains the predominant means of waste disposal throughout the globe. Numerous landfills exist in developed and underdeveloped countries, engineered with contrasting degrees of effectiveness. Modern landfill closure in developed countries involves the conventional capping of waste with materials such as compacted clay or geosynthetic clay liners, typically overlain with other soil materials. Conventional capping technologies are now accepted to be increasingly ineffective in reducing percolation into waste. Cost-effective alternative systems are of increasing interest, including the use of plants to control and limit water entry into waste, otherwise known as “Phytocapping”. Phytocapping reduces percolation through three main mechanisms: (a) canopy interception of rainfall, (b) storage of moisture in the soil layers, and (c) evapotranspiration (i.e., hydraulic lift) of stored water. Phytocapping has been shown to be at least as effective as clay capping in reducing percolation through landfill cover materials, provided site specific conditions are factored into design, and providing many additional benefits, including increased cap stability, reduced erosion of capping materials, reduction of wind-blown dust, enhanced biological diversity, increased opportunity to establish commercial plants, carbon sequestration, and enhanced methane oxidation from microbial communities. Phytocapping has been suggested as having potential in phytoremediation of landfill leachate. The most common phytocapping approach to date is the construction of vegetation assemblages for the purposes of creating natural vegetation nodes. Phytocapping technology can be enhanced by appropriate selection of soil amendments such as biosolids, biochar, compost, or other materials. Appropriate selection of plant species and soil amendment products can enhance methane oxidation in capping soils. There is considerable potential for the use of high biomass energy plants but further work is needed in choosing appropriate plant species that will serve both purposes of site water balance as well as commercial (e.g., timber, bioenergy) and biodiversity needs of the community.