Phyto-cover of landfill sites: a sustainable alternative to conventional clay cover

Abstract

A large scale field research was carried out in five active landfill locations across Australia to investigate the viability and sustainability of a new phyto-capping system of landfill final cover as a replacement for the conventional clay cover which is expensive to build and maintain. Phytocovers consists of a deep layer of lightly compacted soil on top of the waste over which native plant species of grass, shrub and tree are grown to uptake and transpire the infiltrating water thus preventing such water getting into the underlying waste. Trees on all sites with adequate rain showed rapid growth, providing near 100% coverage of the plots in the last round of field studies. Although the soil layer was initially compacted to 85% of maximum proctor test, plant root penetration on most sites remained unaffected, extending to the bottom of the soil profile within 3 years of planting, thus removing and transpiring water from the entire soil profile. Within the study period there were a few notable changes in the soil characteristics, namely a decrease in pH and soil phosphorus. It was hypothesised that the declining soil pH could be attributable to reactions between landfill gas and soil moisture. However, methane gas emission from the underlying wastes did not appear have any adverse effect on the plant growth and survival on any of the study sites. Overall results suggest that phytocover is capable of reducing or eliminating seepage into the waste by transpiring back into atmosphere most of the infiltrating water into soil profile. The project is continuing but the results so far suggest that phytocover can successfully replace conventional clay cover under most Australian soil and climatic conditions, from Mediterranean climatic regions to tropics.