Phytocapping : importance of tree selection and soil thickness

An alternative landfill capping technique known as ‘Phytocapping’ (establishment of perennial plants on a layer of soil placed over the waste) was trialed in Rockhampton, Australia. In this technique, trees were used as ‘bio-pumps’ and ‘rainfall interceptors’ and soil cover as ‘storage’ of water. The environmental performance of the phytocapping system was measured based on its ability to minimise water percolation into the waste. The percolation rate was modelled using HYDRUS 1D for two different scenarios (with and without vegetation) for the thick and thin caps respectively. Results from the modelling showed percolation rates of 16.7 mm yr-1 in thick cover and 23.8 mm yr-1 in thin cover, both of which are markedly lower than those expected from a clay cap. Results from monitoring and observations showed that 19 trees out of 21 tree species grew well in the harsh landfill environment. Top ten performing species have been identified and are recommended to be grown on phytocaps in the Central Queensland region.