Using airborne electromagnetics to assess groundwater connectivity and ecological risk in a dewatered mine setting

Authors: Kumar, D; Hodges, D; Smallacombe, J Download Paper Open access courtesy of:

DOI https://doi.org/10.36487/ACG_repo/2615_151

Cite As:
Kumar, D, Hodges, D & Smallacombe, J 2026, 'Using airborne electromagnetics to assess groundwater connectivity and ecological risk in a dewatered mine setting', in AB Fourie, G Boggs, J Heyes & M Tibbett (eds), Mine Closure 2026: Proceedings of the 19th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 1-14, https://doi.org/10.36487/ACG_repo/2615_151



Abstract:
Closure planning for an iron ore mine in the Pilbara incorporates protection of a woodland, classed as a protected ecological community (PEC). The PEC relies on a shallow aquifer, with mine operations dewatering a deeper aquifer. A thick clay aquitard separates the shallow and deeper aquifers; however, recent declining tree health has questioned the aquitard’s continuity. Assessments in this area are complex and integrate within local cultural sensitivities requiring non-destructive methods. Hence, to assess the aquitard, a multiple lines of evidence study was initiated that integrated airborne electromagnetic (AEM) surveying, electrical resistivity imaging, borehole data, hydrogeological monitoring and ecological observations. The AEM survey provided high-resolution mapping of shallow and deep subsurface conductivity, enabling improved delineation of key hydrostratigraphic units, particularly the clay aquitard. Results confirmed that the aquitard is laterally continuous beneath the PEC but thins or is absent to the north, indicating a potential zone of vertical hydraulic connection. This interpretation was strengthened through comparison with electrical resistivity imaging and available lithological logs. Furthermore, groundwater levels show a declining trend in the deep and shallow aquifers. Drawdown in the deep aquifer is associated with mine dewatering, while shallow groundwater levels fluctuate with long-term seasonal cycles primarily driven by large rainfall events associated with cyclonic activity and climate. Although no direct hydraulic connection was detected, increased vertical gradients following dewatering create the potential for downward leakage in areas where the aquitard thins or is absent. Complementary evidence, including potential mine pit wall seepage, isotope datasets and ecological assessments, was included to holistically evaluate the factors influencing tree health. The findings support development of mitigation options for closure to allow for shallow aquifer recharge. Overall, the multiple lines of evidence approach provided a robust foundation for closure planning by clarifying groundwater behaviour, risks and necessary future work.

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