Environmental performance of a phytocapped landfill

Landfilling is still the most economical way of disposing urban wastes, and will continue to occur as long as humans live in communities. The operational and post-closure management costs of landfills have increased recently due to the introduction of stringent environmental laws and regulations. Research is therefore needed to develop alternative methods to the current capping technique that will minimise the construction and management costs and also demonstrate increased environmental performance and ecologically sustainable development. An alternative technique known as ‘Phytocapping’ was trialed in Rockhampton, Australia. Twenty one tree species were established in each of two phytocaps, viz thick phytocap (1400 mm soil) and thin phytocap (700 mm soil). Plant growth and water relation parameters (e.g. transpiration, soil moisture, ccanopy interception) were monitored over a three year period. This data, together with 15 years of meteorological data were used to simulate site water balance using HYDRUS 1D, under two scenarios, viz, with or without vegetation. The results from monitoring and modelling showed percolation rates of 16.7 mm yr-1 in the thick phytocap and 23.8 mm yr-1 in thin phytocap. These values were orders of magnitude lower than those expected from a clay cap (78 mm yr-1 @10% of the total rainfall) or from non-vegetated cap (153 mm yr-1). Results of this study suggest that the phytocaps are effective in minimising percolation rates thereby reducing leachate generation. The results also demonstrate the role played by the trees in site water balance, both as biopumps and as canopy rainfall interceptors. Furthermore, none of the established tree species accumulated heavy metals suggesting that growing trees on landfills is not environmentally hazardous. Root distribution was mainly concentrated in top 700 mm of the soil cover.