From liability to legacy: understanding and managing post-closure pit lake water quality

Authors: Usher, B; Fitzpatrick, A; Zheng, J; Vink, S Download Paper Open access courtesy of:

DOI https://doi.org/10.36487/ACG_repo/2615_113

Cite As:
Usher, B, Fitzpatrick, A, Zheng, J & Vink, S 2026, 'From liability to legacy: understanding and managing post-closure pit lake water quality', 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. 1-15, https://doi.org/10.36487/ACG_repo/2615_113



Abstract:
Understanding long-term (multi-decadal to centennial, typically simulated over 100 years) water quality of pit lakes is critical for mine closure planning and relinquishment. Water quality influences post-mining land use options and determines the extent of ongoing management required to ensure stability and environmental protection. While acid mine drainage is usually of greater concern, most coal mine voids in Queensland and the Hunter Valley exhibit neutral to slightly alkaline conditions, shifting the primary risk toward increasing salinity driven by evaporation. Two case studies are presented in this paper. For case study 1, field monitoring was combined with predictive modelling to evaluate the evolution of water quality, with 2 coal mine pit lakes as focus. This included vertical profiling of salinity, pH, dissolved oxygen, and temperature to assess seasonal stratification. Results indicated that both pits function as groundwater sinks, with no overflow expected under typical climatic conditions. Monitoring showed that salinity increases over time, reflecting evapoconcentration, while pH remains neutral, largely reducing metal solubility. To predict long-term trends, coupled water balance and salinity models were developed using GoldSim as a water balance frame. Groundwater fluxes were informed by external groundwater models, while PHREEQC was used to assess hydrogeochemical controls on salinity evolution. The models incorporated site-specific data, spoil geochemistry and recent research on evaporation estimation. Calibration (~6 years of data) reproduced water levels within ±0.5–1.5 m (MAE <1 m) and salinity within ±10–20%, with deviations driven by unmodelled operational inputs. Monitoring indicates salinity increased to ~5,000 µS/cm over ~10 years (~500 µS/cm/yr), while pH remained neutral (8–9). Models predict salinity will reach ~20,000–30,000 mg/L total dissolved solids (TDS) within 100 years under evaporation-dominated conditions. Sensitivity analysis identified evaporation and spoil-related salt loads are the primary controls on salinity evolution for the voids. A second case study evaluates a contrasting approach at a metal mine. Tailings is backfilled into a deep pit, rapidly inundated with river water to establish a >150 m water cover. The pit is later reconnected to the regional river system via controlled high-flow exchanges. Modelling indicates that periodic flushing limits salinity accumulation, and with the planned water cover, suppresses sulphide oxidation. The findings support early integrated assessment of pit lake water quality to inform closure strategies. Proactive assessment allows environmental risks and post-mining options to be understood. Incorporating robust monitoring, predictive modelling and sensitivity analysis into closure planning provides improved basis for decision-making and supports sustainable outcomes. This study demonstrates how integrated monitoring and coupled hydrological–geochemical modelling can quantitatively predict salinity evolution and distinguish between closure paradigms, providing a robust basis for pit lake closure design. The primary scientific contribution of this study is the demonstration that contrasting closure designs can produce fundamentally different long-term water quality trajectories, and that coupled monitoring and modelling provides a quantitative basis for evaluating these outcomes during closure planning.

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