Harck, T & Faulkner, D 2026, 'From detonation to discharge: nitrate risks at mine closure', 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. 1-8, https://doi.org/10.36487/ACG_repo/2615_125
(https://papers.acg.uwa.edu.au/p/2615_125_Harck/)
Abstract: The closure of mining operations in arid regions presents significant environmental challenges, particularly concerning the long-term impacts of waste rock landforms on groundwater quality. This paper reviews the process of waste rock seepage generation under arid climate conditions and its role in nitrate contamination of groundwater, focusing on the delayed discharge of nitrate from waste rock landforms after mine closure.
Waste rock landforms, constructed during mining operations, undergo a transitional wetting phase before reaching hydraulic breakthrough, a process that can span decades. During this phase, limited seepage occurs, but once breakthrough is achieved, basal seepage increases significantly, mobilising stored solutes, including nitrate. Nitrate contamination in mining environments is primarily attributed to residues of ammonium nitrate based explosives, which are highly soluble and persist in waste rock for extended periods. Under the arid conditions typical of regions like the Pilbara, nitrate behaves conservatively, with limited attenuation due to the assumed absence of denitrification processes in the predominantly oxic waste rock.
This paper presents a conceptual model of nitrate release from waste rock based on hydrogeochemical modelling and field data from iron ore mining operations in the Pilbara region of Western Australia. It highlights the role of waste rock landforms as key sources of nitrate, with waste rock seepage dynamics controlling the timing and magnitude of nitrate release. The review suggests that nitrate concentrations in groundwater could remain elevated for decades after mine closure, potentially posing risks to groundwater-dependent ecosystems, human users and surface water systems.
The conceptual model is consistent with the spatial and temporally variable concentrations of nitrate observed in monitoring data. However, there remains considerable uncertainty regarding the understanding of seepage dynamics and nitrate transport mechanisms. This is a challenge for mine closure planning. Enhanced monitoring frameworks and targeted studies are needed to quantify nitrate mass loading, and identify and mitigate long-term environmental risks.