Premining and post-mining land suitability of the Newlands open-cut coal mine

Abstract

This study was the initial phase of a project undertaken by the University of Queensland to define the rehabilitation options and methods for the Newlands open-cut coal mine. The principles of the United Nations Food and Agriculture method of land evaluation were used to define pre-mining land suitability for a number of land uses, and the requirement and rehabilitation measures necessary to achieve those land uses after mining.Initially, the land suitability for seven land uses was evaluated for the pre-mining landscape to define feasible objectives for the rehabilitation program. This phase involved a detailed land resource survey, in which a total of 106 unique mapping areas (UMAs) were identified and grouped into 21 mapping units at a scale of 1:20,000. The chemical and physical properties of the soils found within each mapping unit in the study area were then assessed in more detail to define the properties of UMAs. The land suitability of each UMA was assessed for each land use from an interpretation of how adequately the land could satisfy the requirements placed upon it if it were used for that land use. Suitable land uses were mainly limited to extensive grazing on either native or improved pastures, but 10% of the area was also suitable for opportunity cropping. The total area suitable for opportunity cropping was probably insufficient to warrant development.The properties of the post-mining landscape were then estimated from a detailed study of the overburden properties and the expected changes that take place during an open-cut mining operation. Initially, the pH, salinity and acid- producing potential of a total of 850 overburden samples from 22 boreholes were assessed to identify strata with similar properties for further analyses. These strata were studied in more detail to define their chemical and physical properties. A small weathering trial was also used to estimate the likely changes in salinity with time in the field. It was predicted that, immediately after mining, the land resources would not support any of the feasible land uses satisfactorily, and rehabilitation measures would be required. The required rehabilitation measures were then estimated as those needed to change the properties of the land after mining to a form that would satisfy the requirements for the different land uses. These requirements were defined previously when assessing the pre-mining land suitability. The only agricultural land uses that were identified as being feasible after mining were cattle grazing on native vegetation and improved pastures. Additionally, the final void was predicted to be potentially suitable for a water reservoir which may be used for either recreation, a wildlife reserve or as water storage for agricultural land uses.The rehabilitation measures required to establish either native vegetation or improved pastures varied with the properties of the overbidden in the mine, but included selective placement of overburden in some areas, reduction of slopes, an adequate surface drainage network, soil replacement in many areas, surface preparation and planting of the desired species. The volume of suitable surface and subsoil material was also estimated to identify areas where there were insufficient or excess soil reserves for the required rehabilitation. Successful rehabilitation of the final void would depend on designing a drainage system that would divert water into the pit area after mining was finished, and reducing the amount of groundwater losses during the wet periods when the water levels in the final void were higher than the adjacent groundwater.The range of suitable land uses after mining will depend on the level of rehabilitation measures that are undertaken. The different landscape and socio-economic conditions that exist after mining will reduce the importance of many of the limitations present before mining, but other limitations are expected to become more severe. Land that was marginally suitable for opportunity cropping before mining will be lost, but there is potential to increase the area under sown pastures. Additionally the final void could become a valuable water reservoir for both recreation and agricultural land uses in the area. The relative advantages of adopting the different suitable land uses are discussed.