Groundwater modelling for mine closure planning

Authors: Foley, G; Shokri, A; Gresswell, R; Calabrese, N Download Paper Open access courtesy of:

DOI https://doi.org/10.36487/ACG_repo/2615_124

Cite As:
Foley, G, Shokri, A, Gresswell, R & Calabrese, N 2026, 'Groundwater modelling for mine closure planning', 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. 1-9, https://doi.org/10.36487/ACG_repo/2615_124



Abstract:
Loy Yang mine is a coal mine operated by AGL and located in the Latrobe Valley, approximately 150 km east of Melbourne, Australia. Mining commenced in 1982, supplying coal for power generation for both Loy Yang A (LYA) Power Station and Loy Yang B (LYB) Power Station. LYA power station is currently scheduled to cease operations in 2035. Depressurisation of the confined aquifers commenced in 1986 and is required to maintain safe mining conditions, having reached a maximum volume of 16 GL/y in 2009. Pumping from the shallower M1B aquifer commenced in 2025 and will increase as mining expands eastwards, exposing the aquifer in the batters. Placement of overburden in the western mine floor commenced in 2017. The proposed mine rehabilitation plan includes development of a pit lake by utilising surface water and groundwater sources to achieve a safe and sustainable landform subject to an extensive legislated approvals process. Groundwater modelling is a critical component of closure planning to assess the stability of the mine floor as confined aquifer pressures recover and the connection between the groundwater system and pit lake. Groundwater modelling provides inputs for other technical studies to inform pit water quality and surface water studies. A groundwater model has been developed using MODFLOW-USG across the Gippsland Basin, with local refinement around Hazelwood, Loy Yang and Yallourn mines to enable mine-scale effects as well as the cumulative mining and rehabilitation impacts of all 3 mines to be considered within a single model. This paper describes the application of groundwater modelling to support mine closure planning at Loy Yang mine and includes rigorous calibration and benchmarking of the model against long-term groundwater levels and subsidence measurements to increase model confidence while also modelling the potential effects of climate change on post-closure recovery of aquifer pressures. This paper demonstrates how groundwater modelling can be used as a decision-support tool for mine closure planning, linking regional aquifer recovery with mine-scale closure design. By representing the 3 Latrobe Valley mines within a single MODFLOW-USG model, the approach captures both cumulative regional groundwater impacts and local closure risks at Loy Yang mine. The model outputs are used to inform pump bore decommissioning, floor heave risk management, pit lake development and overburden placement requirements. Groundwater modelling has been used to assess and manage the risk of floor heave during the transition to closure. In the longer term, an assessment of overburden spoil thickness to achieve a sufficient counterweight against predicted recovered aquifer pressures was completed to manage the risk of floor heave and provide a stable landform. Modelling is also in progress to assess the impact of overburden placement providing hydraulic resistance between the unconfined M1B aquifer and pit lake, on long- and short-term M1B aquifer seepage rates.

Keywords: groundwater, modelling, closure, heave

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