Open Cut » Environment
This project aimed to greatly reduce the water trapped below the crust of tailings dams by developing a 'Vegetative Water Pump'. The aim was to select plant species mostly natural to the area of the dam to penetrate the crust and pump the water out increasing the solids density in the dam. In doing so it would prevent liquefaction of the crust (caused by vibration or earth tremor) and prevent the potential for slurry formation and flow should a dam wall fail. The species selected were known to have some waterlogging or flooding tolerance. This proved useful because the tailings dam used was often flooded, sometimes for weeks. The flooding limited access to the dam for reasons of personnel safety. However, it also taught us much about the dam and the species used, which assists with a longer-term aim of rehabilitating tailings dams rather than capping them.
Based on performance, including flood tolerance, we selected four primary species: Casuarina glauca Sieber ex Spreng, Eucalyptus camaldulensis Dehnh., Eucalyptus robusta Smith., and Melaleuca quinquenervia (Cav.). M. quinquenervia became our Model Species.
An unexpected finding was that leaves of three of the primary species used were 'leaky'. That is, they lose water in the light and dark contrary to most terrestrial plants that only lose water during the day when stomata are open. Many wetland plants have hydathodes in their leaves and we did not know this to be true for the species used. Hydathodes are openings in the epidermis seen in three of the primary species used, that are positioned above the ends of minor veins. We believe we have preliminary estimates of their contribution to transpiration at night at about 37% of the total water lost in each 24 hours. They are likely to be a significant contributor in the daytime too.
The project found that plants transpired considerable amounts of water even in dryer times suggesting they had penetrated the crust. The crust was also found to be sinking by about 27cm per year which the transpiration stream may have contributed to.
Model species was calculated to be pumping a total of up to 8GL of water per year if in a group of 1000 plants of the size used in this experiment spread over 10 ha at summer temperatures. Eucalyptus species were found to be even leakier. Thus, dependent on the plant density, the lower levels of a dam below the crust could be made safe in a few years depending on the impacts of rainfall and flooding, as well as successful species selection and survival from surface flooding.