Computational Methods for Acid Mine Drainage Management: Simulation of Hydrogeochemical Processes in Abandoned Underground Mines1

Predicting post-mining water quality is a key task in managing mine drainage. The most scientifically-defensible approach would be to undertake thermodynamics-based modeling of coupled geochemical and hydrodynamic processes occurring in flooded and partly flooded underground workings. Most models currently in use make the fatal assumption of local equilibrium, or else do not include all important geochemical and geometrical components of an abandoned mine system. Hydrologic and geochemical data have been incorporated into a model (POllutant Sources & Sinks in Underground Mines) that models one-dimensional reactive transport along a mine roadway. POSSUM is an object-oriented program that models the evolution of mine water quality, accounting for contaminant sources and sinks, water level changes (due to rebound and seasonal fluctuations), reaction kinetics, variable flow regimes, infiltration through porous media, and inflow from secondary roadways. The main metals included in POSSUM’s analysis are Fe, Al, Mn, Ni, and Zn. Additionally, carbonates, including limestone, ankerite and siderite, and silicates, including K-feldspar, anorthite, albite and muscovite are represented in POSSUM. Reversible sorption is also accounted for. Contaminant transport is solved using the NOAH 1-D program developed at University of Newcastle. POSSUM solves for concentrations in both the aqueous and solid phases using sequential iterations of finite difference numerical methods. Data sets from the National Coal Mining Museum in Wakefield and Coal Authority boreholes have been collected for comparison with model results. In contrast to pre-existing models, POSSUM couples kinetically controlled multi-component reactions with variable transport and will further the ‘state of the art’ by improving the defensibility of predictions of water quality changes over time. Additional