A method for the geochemical modelling of transient contaminant release from rehabilitated opencast coal mines, including calibration against existing data, is proposed. The need for such a methodology is illustrated by a directive received by a decommissioned opencast coal mine in Mpumalanga, South Africa. Groundwater monitoring data, geochemical analyses, numerical flow modelling and geochemical modelling are used to model the hydrogeochemical evolution of mine water over time. Models presented in this study are based on a conceptual model detailing groundwater levels and flow directions, hydraulic conductivities, groundwater chemistry, precipitation, evaporation, surface water bodies and potential sources. Additional to this, mineralogical analyses, leaching tests and acid-base accounting were performed to obtain a better understanding of the site geochemistry. A geochemical model was constructed which was used to obtain a statistically representative mineral assemblage based on laboratory data which was calibrated against leaching test data. This assemblage was simulated in field conditions as input to a numerical flow and transport model. The transport of sulfate was modelled accordingly and sulfate concentrations from monitoring data were used for chemical calibration. Following this, long term contaminant release was simulated. Calibration graphs from the transport model indicated concentrations within a 20 mg/L error margin, showing that the proposed methodology can be used to calculate contaminant concentrations in an aquifer over time within an acceptable range. This approach could provide an improved estimate of the duration of the first flush which, upon completion, will transform decommissioned collieries into large scale reservoirs of utilisable groundwater.