HGS RESEARCH HIGHLIGHT – Reactive transport modelling of acid mine drainage within discretely fractured porous media: Plume evolution from a surface source zone
Molson, J., Aubertin, M., & Bussière, B. (2012). Reactive transport modelling of acid mine drainage within discretely fractured porous media: Plume evolution from a surface source zone. In Environmental Modelling & Software (Vol. 38, pp. 259–270). Elsevier BV. https://doi.org/10.1016/j.envsoft.2012.06.010
“One of the more advanced models for simulating flow and transport through porous and discretely fractured porous media is HydroGeoSphere (and its predecessor FRAC3DVS; Therrien and Sudicky, 1996). Adaptations of this code have recently been applied to simulate reactive systems (Ghogomu and Therrien, 2000; Graf and Therrien, 2007) and for simulating groundwater flow around tailings-filled mining pits within fractured rock (ben Abdelghani et al., 2009).”
Figure. 1. Validation of the discrete fracture approach in the AMD transport model for a conservative solute: (a) 400-day concentration contours in the plane perpendicular to the fracture, (b) A comparison of longitudinal profiles from the numerical and CRAFLUSH analytical model (Sudicky and Frind, 1982), and (c) Comparison of transverse profiles at 400 days, at depths of 3, 6 and 9 m. For clarity, not all numerical data points are shown.
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Summary:
We’re pleased to highlight this publication by J. Molson, M. Aubertin, and B. Bussière, which focuses on simulating the fate and transport of acid mine drainage (AMD) through fractured porous media using a discrete fracture network (DFN) modelling approach. This research addresses a critical environmental challenge in mining regions— predicting how acidic contaminants generated by sulphide mineral oxidation migrate through complex geological formations and interact with host rocks over time.
The study introduces POLYMIN/DFN, a new numerical modelling tool designed to simulate reactive transport of AMD within a saturated, fractured porous medium. The model accounts for multi-component advective-dispersive transport, geochemical speciation, and water-rock buffering reactions. It specifically examines how AMD infiltrates from a surface waste source, such as tailings, into a silicate-rich fractured host rock. The model was validated using analytical solutions and then applied to conceptual field-scale simulations incorporating various fracture densities, matrix porosities, and mineral compositions.
Key findings show that fracture networks strongly influence AMD plume evolution. Compared to traditional equivalent porous media (EPM) models, DFN simulations revealed longer, more dispersed plumes and significantly earlier contaminant arrival times— highlighting the importance of explicitly representing fractures in predictive models. The research also demonstrated how diffusion-limited buffering reactions within the porous matrix can moderate pH drops and control the extent of mineral dissolution and precipitation, particularly for calcite, siderite, gypsum, and ferrihydrite.
Although the model assumes geochemical equilibrium for most reactions, which may not capture slower silicate weathering processes, it offers valuable insight into the long-term behavior of AMD plumes and the limitations of simplified modelling approaches. The results highlight how fracture connectivity, aperture variability, and matrix diffusion all play pivotal roles in contaminant migration.
By advancing modelling capabilities in fractured media and bridging geochemical and hydrological processes, this study contributes to better environmental risk assessments and mine waste management strategies. It also sets the stage for future development of reactive transport models that incorporate kinetic reactions and 3D fracture representations, enhancing our ability to protect groundwater resources in complex mining environments.
Abstract:
A numerical model is developed for investigating the transport behaviour and geochemical evolution of acidic mine drainage (AMD) in discretely fractured porous media. The simulation approach is tested using a conceptual model of a reactive mine waste system in which an active source of AMD overlies a fractured silicate-rich porous host rock with a low but non-zero matrix permeability. Source composition is based on measured data from an existing mine tailings site. The numerical model includes groundwater flow, AMD infiltration, multi-component advective-dispersive transport, equilibrium geochemical speciation and water-rock pH-buffering reactions within a discrete fracture network (DFN). An analytical solution for parallel-fractures is used to verify the model, which is then applied to simulate the evolution of pH, the major aqueous species from the AMD source, as well as selected mineral buffers. As the acidic drainage water infiltrates into the initially uncontaminated fracture networks, high concentration gradients develop within the matrix along fracture interfaces inducing diffusion-limited pH buffering and precipitation of secondary minerals within the rock matrix. A comparison of AMD evolution in three fracture networks shows that even within a densely fractured network, AMD plume evolution can be significantly different from that obtained from assuming an equivalent porous medium (EPM). The paper also addresses issues of time scales and matrix diffusion. The results have implications for predicting environmental impacts of acid mine drainage in complex mining environments and for coupling of hydro-geochemical and geotechnical models. The model can also be applied to other hydrogeological systems including fractured clays and tills, to other contaminants including hydrocarbons or organic solvents, and to simulate geochemical evolution in natural flow systems.