PhD Thesis (2024-2027)

Title: Return to the geochemical equilibrium of the flooded reservoirs of underground coal mines. Application to the Gardanne mine.

Supervisors: Pauline Collon (RING Team), Laurent Vaute (BRGM)

In 2022, I earned  an engineering degree from the ENSG (Nancy School of Geology), specializing in hydrogeology. Following that, I worked for a year and a half at BRGM (French Geological Survey) as a hydrogeologist modeler, focusing on groundwater rise issues in post-mining management contexts. Since April 2024, I have been pursuing a PhD in collaboration with the BRGM and GeoRessources-RING, focusing on the return to equilibrium of the geochemical conditions in the Gardanne coal mine reservoir.

The abandoned underground coal mines in France are managed on behalf of the State by the Mining Prevention and Safety Department of the BRGM. At the outlets of the large mines of Gardanne and Lorraine, which are now flooded, the concentrations of iron, manganese, and sulfate observed in the pumped waters over the past 10 years do not show the significant decrease initially predicted by various initial mixing-dilution models.

This doctoral thesis aims to understand the geochemical mechanisms driving the evolution of contaminant concentrations (notably iron, manganese, sulfate) before, during, and after the flooding of the mining reservoirs of underground coal mines. Subsequently, it aims to model the flows and geochemical interactions within each reservoir to make reliable predictions. The main research question encompasses several associated research questions, including: How can existing 3D models (gallery networks, geology) be used to define a conceptual model of the structure and functioning of the mine and the aquifers feeding it? What hydrological and geochemical modeling approach should be adopted? How can the numerous available geochemical data, which are unevenly distributed in time and space, be effectively utilized? The experimental site supporting this research will be the flooded coal mine of Gardanne.

The practical outcomes of this research will include a methodology for defining a conceptual model of the structure and functioning of flooded coal mines, the selection of a modeling approach suited to the operational and societal challenges of these large groundwater reservoirs, and the optimization of monitoring networks by selecting the necessary and sufficient data for reliable predictions with the constructed models.

Keywords: Geochemical modelling, Coal mine, Post-mining, Flooded mine, Gardanne