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

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 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 times to allow the models to be effectively used as a tool. The challenge was therefore to ensure that DHI could increase the resolution and accuracy of the model while keeping simulation run times reasonable and proportional to the increased complexity. The model simulation time is a critical element for project delivery that is often overlooked and can cause significant frustration to project managers later in the project delivery cycle. In this case—and as expected—the HSRSM model resulted in increased run times but provided greater precision and accuracy in its predictions.   

Most importantly, DHI’s detailed modeling enabled FMG to quantify the amount of available fresh or brackish groundwater and to predict groundwater salinity reliably. The three major processes considered were density-driven flow, mine dewatering, and groundwater injection of saline and brackish water. With DHI’s HRSM model in hand, FMG was able to continue planning mine dewatering and re-injection operations while ensuring the environmental safety of the region. The HSRSM modeling confirmed that FMG’s approach was sound, and allowed the mining company to minimize any uncertainty. Ultimately, the model could be used to:

Design practical dewatering and re-injection solutions to meet specific water management goals;

Optimize designs for economic efficiency;

Evaluate potential effects of mining activities on environmental features such as the marsh;

Understand and mitigate saline water ingress; and

Predict impacts of alternative hydrological or development scenarios to assist FMG in decision-making.

DHI is an independent, international consulting and research organization that offers technological development, governance, and competence in the fields of water, environment and health. The company has offices in almost 30 countries worldwide and employs more than 1,000 people. For additional information, contact Andrew Druzynski (adr@dhigroup.com).