Reactive transport modelling of acid mine drainage within discretely fractured porous media: Plume evolution from a surface source zone

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.