Reactive transport and multiphase gas model of coal tailings to predict potential long-term seepage quality

January 7, 2026

A geochemical reactive transport model was constructed to predict the long-term seepage quality from coal mining tailings backfilled into an opencast pit. The results of this model are intended to serve as a ‘source term’ for the tailings in a contaminant transport groundwater model. Initially, a 1D gas model was developed to simulate atmospheric oxygen ingress, considering the moisture content and hydraulic head in the backfilled tailings. Critical aspects required for modelling gas migration into the tailings included the Oxygen Consumption Rate (OCR), Oxygen Diffusion Coefficient (ODC), Soil Water Retention Characteristics, and the hydraulic properties of the tailings material. The mineral kinetic rate parameters were determined by simulating kinetic tests performed on tailings samples. These results, along with those from the gas transport model, were used as inputs for the reactive transport geochemical model, which was scaled to simulate site-specific conditions. The modelling domain was designed to allow for simulation of multiple output localities, representing seepage to the adjacent mining spoil backfill materials and the underlying aquifer. The results of the reactive transport model indicated that sulfide mineral oxidation and leaching were restricted to the unsaturated zone exposed to atmospheric conditions. The modelling therefore also suggested that the neutralisation mineral content, especially in the unsaturated zone was crucial in providing buffering capacity against acidification, as assessed through sensitivity analysis. An additional modelling scenario was also undertaken where oxygen availability was limited by submerging the tailings underwater. The results of which indicated it as an effective acid-generating prevention strategy, regardless of the availability of carbonate minerals, to ensure no severe deterioration in the seepage quality from the tailings.

TDu Toit, JH., Fourie, PJ., and Usher, BH. 2025 " Reactive transport and multiphase gas model of coal tailings to predict potential long-term seepage quality,” in Proceedings of the LOM-MWT Conference 2025, July 29 – 30, 2025. Brisbane, Australia: Australian Institute of Mining and Metallurgy (AusIMM)