Engineering and Mining Journal - Whether the market is copper, gold, nickel, iron ore, lead/zinc, PGM, diamonds or other commodities, E&MJ takes the lead in projecting trends, following development and reporting on the most efficient operating pr
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O P E R AT I N G S T R AT E G I E S Minimizing Mine Dewatering Problems and Risk in Western Australia Faced with the task of predicting how present and future mining could affect three different types of groundwater reserves in its area of operations, a major iron ore producer needed a better hydrogeological model—and got it Environmental consulting group DHI produced a high-resolution groundwater model that allowed Fortescue Metals Group to confidently coordinate its mining and water management strategies to protect the arid Pilbara environment and avoid unexpected water-related problems in the pits. (Photo courtesy of Fortescue Metals Group) The dry, dusty Pilbara region of Western Australia doesn't make you think about water—unless you get thirsty looking at this arid landscape. However, the water that's hidden beneath the red plains poses a great challenge for mining companies operating in this region. For these companies, modeling provides the basis for developing reliable water management plans that ensure protection of environmental ecosystems which depend on the groundwater, and reduce the risks associated with mine development. Fortescue Metals Group (FMG) has two major iron ore sites in the Pilbara region, located on a narrow east-west strip bordered by the Chichester range in the north and Fortescue Marsh in the south. Groundwater is hyper-saline (three to four times that of seawater salinity) underneath the marsh, but is fresh to brackish in the mining area. The interface between saline and brackish waters is located about 2 km to the north of the marsh. Many activities related to mining—such as pit dewatering, water treatment and 96 E&MJ; • AUGUST 2013 re-injection of groundwater into different aquifers—can cause changes to the groundwater in surrounding areas. To allow for proper and prudent water management and mine dewatering plans, these impacts have to be investigated. In addition, mines require the use of groundwater for operational purposes. Minimizing the ingress of highly saline water into fresh or brackish aquifers or into the mine pits is a prime concern and requires understanding the effects of dewatering pumping. To optimize operations and understand the dynamics of the interface between the fresh, brackish and saline groundwater, FMG called on DHI to design a high spatial resolution salinity groundwater model (HSRSM) for the mining area. This would enable FMG to predict the movement of the saline/brackish groundwater interface, as well as the quantity and salinity of dewatered groundwater in each mining pit under different mining plans. Moreover, the model could be used to assess the effects of various water management plans involving dewatering and water re-injection. Previous hydrogeological analyses and regional-scale numerical models predicted that the saline/brackish interface would move toward the mining area under the proposed mine-dewatering conditions for a new mining area. However, the available information did not provide sufficient certainty to develop a reliable mine water management plan. FMG had previously developed a density-driven groundwater flow and transport model. That model was used to predict mine-dewatering volume and groundwater drawdown/mounding. However, the spatial resolution of the model was not sufficient to provide certainty with respect to predicted salinity changes associated with groundwater pumping from the mining pits. DHI performed a review and analysis of the FMG models and gained an understanding of the challenges to be resolved. For example, a realistic prediction of the groundwater salinity interface relies on high spatial resolution but, at the same time, must offer reasonable simulation www.e-mj.com