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Layered-over landfills
13th December 2008
Covering waste dumps with trees could smother greenhouse gas emissions
Decomposing waste in landfill sites is a major source of carbon dioxide and methane emissions. But covering dumps with a layer of soil and then putting trees, shrubs and other plants on top – called phytocapping – could be a good solution. It would not only trap the greenhouse gases inside, but also create new habitats and corridors for local wildlife, according to a study in the January, 2009, issue of the International Journal of Environmental Technology and Management.
Previous attempts in the U.S. to put caps over landfill sites with clay – to prevent the rubbish from being rained on (and therefore rotting) – were less effective because in hot and dry areas the clay would crack, letting rain in and hence greenhouse gases out into the atmosphere.
But scientists from Central Queensland University found that covering an Australian landfill site with a layer of soil more than a metre thick and planting it with acacias, figs and eucalyptus trees reduced methane emissions by four to five times compared with an uncapped site. They tried out different thicknesses of soil and 19 different tree species to see which would best prevent water from reaching the waste.
Phytocaps could work in Canada as well, says the study’s lead author, Kartik Venkatraman of Central Queensland University.
“There have been 11 successful trials in the U.S. and Certainly there is a possibility for this capping system to be promoted in Canada,” he said via e-mail.
But could heavy metals or toxic chemicals from the waste be absorbed by the plants and passed on through the food chain? Not in this study, at least.
Mr. Venkatraman found all but one tree species had heavy-metal concentrations “well below the threshold limit” in their leaves. Only one – the small-leaved cheese tree – had high cobalt levels, so it will not be used in the future.
Published in The Green Report in The Globe and Mail
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