Harck, T, Landers, M, Nalecki, P & Faulkner, D 2026, 'No pit left behind: closure pit water modelling at scale', 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_147
(https://papers.acg.uwa.edu.au/p/2615_147_Harck/)
Abstract: Predicting long-term water quality in residual mine voids is a central challenge for closure planning, particularly where multiple pits exhibit contrasting hydrological and geochemical behaviours. This paper presents a scalable hydrogeochemical modelling framework applied to an Australian hard rock operation containing 11 post-closure voids, including both open pits and backfilled systems. The objective was to generate structured, defensible forecasts of pit water evolution over a 100-year closure horizon. The purpose was to support preliminary risk assessment and future regulatory engagement while acknowledging uncertainty through grouped scenarios based on median and 95th percentile source term representations and bounded water balance inputs. 
The modelling integrates stochastic water balance simulations with equilibrium geochemical calculations. Daily rainfall–evaporation sequences were generated to simulate long-term hydrological behaviour, with separate conceptual models representing recharge-dominated open pits and through-flow backfilled voids. Annualised flow proportions were coupled to geochemical source terms derived from monitoring data, laboratory leach testing and field run-off characterisation. These inputs were processed using thermodynamic speciation modelling to account for evapoconcentration, mineral precipitation and adsorption reactions, enabling prediction of pH, salinity and trace element trends through time.
Scenario testing explored variability in infiltration rates and source term concentrations to bracket credible ranges of outcomes. Results highlight first-order contrasts between voids driven by material composition, groundwater interaction and hydrologic connectivity. While several voids are predicted to remain circumneutral and dilute, others show persistent acidity or elevated salinity under conservative assumptions. The modelling framework supports risk ranking across the pit portfolio and identifies priority targets for monitoring and adaptive management. At this stage, the model has not been calibrated against observed void behaviour or reviewed by regulators and therefore should be interpreted as providing first-order, riskbased predictions.
The study demonstrates that closure forecasting can be extended from single-pit assessments to site-wide systems without sacrificing transparency. By combining probabilistic hydrology, geochemical equilibrium modelling and structured uncertainty treatment, the approach provides a practical foundation for iterative closure planning and future regulatory engagement as additional data become available.