Review of models used to simulate hydrological and geochemical issues related to closure of open pit mines

Executive Summary

Open pit mining operations that extend below the natural water table require substantial groundwater dewatering to sustain dry and safe working conditions. Upon cessation of mining and dewatering, these excavations often fill with water to form pit lakes. The management of these post-mining pit lakes presents a complex challenge due to the long-term evolution of both hydrological and geochemical conditions. This report, developed as part of CRC TiME Project 4.9 – Mine Pit Lake Assessment and Management: A National Initiative to Support Mine Closure and Regional Opportunities, synthesises current knowledge and modelling practices to support improved prediction, management, and sustainable closure of pit lakes.

Hydrological and Geochemical Evolution of Pit Lakes

Following mine closure, pit lakes evolve into new hydrological systems and may act as terminal sinks, throughflow systems, or groundwater recharge lakes depending on climate, pit geometry, aquifer characteristics, and surface inflows. In arid zones, pit lakes often become evaporative sinks, while in wetter environments, they may exchange water with surrounding aquifers or surface streams. These regimes may change over time due to climate variability. Geochemically, pit lakes undergo slow evolution driven by wall-rock interactions, water balance dynamics, and solute transport mechanisms. Risks include Acid Rock Drainage (ARD), salinity build-up, and mobilisation of heavy metals such as arsenic, iron, and manganese. Stratification – either thermal or chemical – can further complicate water quality, creating anoxic layers and vertical gradients in contaminant concentrations. Predicting and managing pit lake water quality requires accounting for mineral dissolution/precipitation, redox reactions, and microbial activity, especially in systems prone to long-term salinisation or ARD. The integration of physical, chemical, and biological processes into predictive models is therefore critical for assessing risks and supporting beneficial post-mining land uses.

The Role of Modelling

The report reviews several modelling tools used in pit lake hydrology, geochemistry, and closure planning. Groundwater models such as MODFLOW (including NWT, SURFACT, and SEAWAT) and FEFLOW, are widely used to simulate dewatering, groundwater recovery, and post-closure groundwater fluxes. MINEDW has also been used at quite a few sites. Several surface water models, including AWBM, AWRA-L and SWAT, are used to evaluate estimate runoff, lake inflows, groundwater percolation and flood risks, with the potential for coupling with hydraulic models like HEC-RAS and TUFLOW. Hydrodynamic models such as CE-QUALW2 and DYRESM simulate lake stratification, mixing, and evaporation. Geochemical models like PHREEQC, MINTEQ, and PHT3D are applied to assess the chemical evolution of groundwater and pit lake waters. This report presents examples of the use of these and other models, and briefly discusses their complexity and the different processes that are simulated in these models. Integrated or coupled models are increasingly employed to address the interlinked processes affecting pit lake environments. For example, MODFLOW can be coupled with PHREEQC to simulate reactive transport, or with surface water models to capture climate-driven runoff and recharge. Software platforms like GoldSim facilitate probabilistic simulation and scenario testing, supporting uncertainty analysis and decision-making under variable future conditions.

Climate Change, Risk, and Sustainability

Climate variability and change are projected to significantly impact pit lake hydrology through altered precipitation, evaporation, and flood frequency. Modelling pit lake responses under future climate scenarios is essential for robust closure planning. Increased rainfall intensity may raise flood risks and pit overtopping, while prolonged droughts could exacerbate salinity and lower groundwater recovery rates.

Uncertainty and sensitivity analyses are vital components of mine closure modelling. They inform data collection priorities, support adaptive management strategies, and ensure transparency in regulatory and stakeholder communication. Risk-based modelling frameworks are particularly important where post-mining water quality may affect environmental receptors, groundwater-dependent ecosystems, or downstream users.

Recommendations for Practice

Effective mine closure planning requires careful model selection based on the site’s hydrological complexity and associated risk profile. This may mean use of simpler models where applicable. However, for high risk environments, integrated models that couple groundwater, surface water, and geochemical processes are recommended to capture the full range of interactions. Incorporating uncertainty and scenario testing is essential. Long-term monitoring of key parameters, including water levels, groundwater heads, and water quality, is critical post closure to validate model predictions and support adaptive management. Where necessary, model coupling should be employed to simulate pit lake dynamics with higher accuracy. Coupled hydrological, hydrodynamic, and geochemical tools can enhance predictive reliability. Importantly, all modelling efforts should align with regulatory expectations by addressing potential impacts on environmental values, groundwater users, and cultural assets, while incorporating the latest climate change guidance.