Present and future flow simulation in a tailings pile at a former mine in France

Water flow is an essential component of the long-term environmental management of former mine sites. Flow through tailings storage facilities (TSF) often generates chemical reactions and releases acidic water. In the case of static leaching, this acidification can last for multiple decades depending on the acid remaining in the tailings. This mining water is then collected and treated in treatment plants before it is released in the environment in compliance with environmental standards. The understanding of the current hydrogeological functioning of the TSF is essential to properly adapt water management today. Given the potential impact of climate change, simulation of future hydrogeological behaviour is also required to ensure sustainable water management over this century.

We developed a daily time step model with HYDRUS 2D to represent the unsaturated hydrogeological functioning of a tailings pile of the former mine of Le Cellier (France). A granulometric analysis over the pile height provided reliable hydraulic properties and showed that the pile heterogeneity can be distributed into three layers. The historical monthly monitoring and the new daily hydrogeological monitoring implemented in 2021 measured the rainfall and discharges from the various drains that collect the water from the pile. As cross-correlations confirmed the fast reaction of drains discharges to rainfall (1 day), we simulated the water flow with the dual porosity package of HYDRUS. We also implemented the vegetation transpiration due to the presence of bushes and coniferous trees over the pile.

The model performance was evaluated by comparing the observed (monthly and daily) discharges and the simulated one. The calibrated model reproduces correctly the annual discharges for the period 2014-2022 as well as the pile fast reaction to rainfall. To evaluate the climate change impacts on the hydrogeological functioning of the pile, we used as input of our calibrated model the daily precipitations and temperatures of the Coupled Model Intercomparison Project (CMIP5) for three climatic scenarios (RCP2.6, RCP4.5 and RCP8.5). The calculation of the Mann-Kendall trend test on the predicted water balance components leads to the conclusion that the effective rainfall should remain stable over the next 100 years. At the end of the century, the frequency of extreme events could increase by 50% and their intensity could rise by 9%. With the calibrated model, we simulated the discharges at the pile outlet and studied their annual changes as well as the pile response to extreme events under climate change. These simulations are essential to ensure an accurate water management for this century.