Pyrite oxidation and its implications for flooding of heterogeneous lignite mine dumps: a reactive transport modelling study

Pyrite oxidation in internal lignite mine dumps is the primary source of acidity, sulfate, and iron in flooded open-pit lignite mines. While the associated geochemical reactions have been extensively studied through modeling, field observations, and laboratory experiments, uncertainties remain due to site-specific factors such as the heterogeneous distribution of sediments and pyrite within the dumps. In particular, these heterogeneities complicate predictions of the temporal contaminant release into surrounding aquifers.

This study investigates the development of reaction fronts in an internal mine dump, focusing on how sediment and pyrite distribution, defined by correlation lengths and mineral content, affects reactive transport processes. Geostatistical methods are combined with geochemical modelling to conduct 2D reactive transport simulations that incorporate pyrite oxidation kinetics.

Results show that heterogeneous scenarios reduce tracer breakthrough time by up to 15 % compared to a homogeneous setup. The total tracer outflux varies between 89 % and 139 % of that observed in the homogeneous reference case. Reaction fronts in heterogeneous configurations cover a larger area and extend deeper into the modelling domain than those in the homogeneous scenario. Simulations that exclude reaction kinetics require more computational time, but result in smoother reaction front edges and more detailed redox gradients compared to equilibrium-based approaches.

The findings indicate that preferential flow paths, that arise from structural heterogeneity, can accelerate flow-through times and enhance solute outflux quantities. The effect scales with oxygen availability for pyrite oxidation. However, while higher oxygen concentrations increase peak and average solute concentrations in the dump, the spatial and temporal patterns of outflux are primarily governed by heterogeneity. Accurate prediction of contaminant release from specific mine dumps remains challenging due to the difficulty of characterising internal structures in the field. However, simulating multiple plausible scenarios allows for estimating ranges of outflux timing and magnitude, supporting risk assessment and groundwater management. The impact of 3D heterogeneities on preferential flow path development in similar geochemical systems remains unexplored and should be addressed in future research.

Literature

Schnepper, T., Kühn, M. and Kempka, T. (2025a): Reaction Path Modeling of Water Pollution Implications of Pumped Hydropower Storage in Closed Open-pit Lignite Mines. Mine Water and the Environment, 44, 107-121. DOI: 10.1007/s10230-025-01039-y

Schnepper, T., Kapusta, K., Strugała-Wilczek, A., Roumpos, C., Louloudis, G., Mertiri, E., Pyrgaki, K., Darmosz, J., Orkisz, D., Najgebauer, D., Kowalczyk, D. and Kempka, T. (2025b): Potential hydrochemical impacts of pumped hydropower storage operation in two European coal regions in transition: the Szczerców-Bełchatów mining complex, Poland, and the Kardia Mine, Greece. Environmental Earth Sciences, 84, 9, 247. DOI: 10.1007/s12665-025-12198-0

Schnepper, T., Kühn, M. and Kempka, T. (2025c): Effects of Permeability and Pyrite Distribution Heterogeneity on Pyrite Oxidation in Flooded Lignite Mine Dumps. Water, 17, 21, 3157. DOI: 10.3390/w17213157