Comparison of numerical approaches for groundwater flow modelling within mining environment

In groundwater flow modelling within mining environments, three main numerical approaches are commonly used: i) the Equivalent Porous Media (EPM), according to which the rock mass and its discontinuities are represented as a continuum; ii) the Discrete Fracture Network (DFN), where hydraulic properties are assigned to natural and/or anthropic discontinuities; iii) the hybrid approach, a combination of the above. To evaluate the advantages and limitations of each method, a selected mine located in the Metalliferous Hills (Colline Metallifere; GR), Tuscany Region, Italy, was selected as a case study. The mine is part of a five-unit complex, joined by a level at −200 m asl, that was active for pyrite extraction until 1981. It was selected due to the availability of hydrogeological and operational data covering a time span of almost 100 years, from around 1920 to 2024. Currently, two 3d transient numerical models have been developed with FEFLOW 10, following the hybrid and EPM approaches. In the former, 1-D anthropic discontinuities (tunnels, shafts, and wells) and 2-D natural discontinuities (major faults) have been represented as discrete features, while in the latter they are implemented as boundary conditions and zones with specific hydrogeological properties, respectively. Hence, the main difference between the two modelling approaches is the representation of geological and anthropic elements. Both models have been successfully used to simulate three managed flooding events that occurred in the period 1997-2014, during which groundwater levels in the mine were raised from −140 m asl to the current −95 m asl by controlling pumping rates of the dewatering system wells active 24/7. At this stage of the research project, the main objective is to evaluate which model better fits observations during managed flooding events. The best fitting model will be used to run predictive simulations of the planned dewatering systems shutdown, which will raise groundwater levels from the current −95 m asl to 70 m asl, where a drainage tunnel is located. In the next stage of the research project, both models will be compared with a DFN model, to evaluate which approach is the most representative of the hydrogeological conditions of the study area and suitable for applications of similar case studies.