Coupled Geochemical-Transport Modelling of Acid Mine Drainage Formation and Attenuation in Nigerian Abandoned Mine Catchments

This dissertation develops a coupled geochemical and transport modelling framework for predicting acid mine drainage formation, propagation, and natural attenuation in catchments containing abandoned sulphide mineral mine workings in Nigeria, making original theoretical and methodological contributions to mine drainage environmental engineering. Abandoned mine sites containing sulphidic tailings and waste rock in Plateau, Nasarawa, and Zamfara States generate acid mine drainage that degrades surface water and groundwater quality in receiving catchments, but the predictive tools available for quantifying long-term drainage loads and evaluating remediation effectiveness in these specific geological and climatic settings are insufficiently developed. The theoretical contribution is a novel coupled reaction-transport model that integrates pyrite oxidation kinetics under tropical temperature and oxygen conditions, secondary mineral precipitation and dissolution reactions specific to the mineralogy of Nigerian crystalline basement terrains, unsaturated zone transport in seasonally wetted tailings profiles, and catchment hydrological routing under the strongly seasonal Nigerian rainfall regime. The model integrates PHREEQC geochemical calculations with a catchment hydrology model in a dynamic coupling scheme. Model parameters are constrained by laboratory column leaching experiments using mine material from three study sites, field pore water chemistry from multi-level samplers installed in tailings profiles, and stream chemistry data from paired catchments. The validated model is used to evaluate the long-term performance of passive alkalinity-generating treatment systems as remediation alternatives. Original contributions include the tropical mine drainage kinetics parameterisation and the seasonal coupling scheme for arid-to-subhumid climate conditions. Keywords: acid mine drainage, geochemical modelling, abandoned mines, Nigeria, coupled transport.