A 3D Geological Framework for Conceptual Hydrogeological Modelling of a Structurally Complex Aquifer System: Insights from the SE Tagus–Sado Basin, Iberian Pyrite Belt, Portugal

Groundwater is a critical resource for ecological sustainability and socio-economic development, but it is increasingly threatened by climate change and anthropogenic pressures. In southern Europe, particularly in Portugal, declining groundwater availability has become a major concern in recent years. This study focuses on the southeastern sector of the Tagus–Sado Basin Left Bank Aquifer System (TSLBA), a major regional aquifer system exhibiting signs of increasing water stress. This study develops a three-dimensional geological model to improve understanding of the aquifer’s hydrogeological behaviour. The model integrates regional geological mapping, data from 90 mineral exploration boreholes (up to ~800 m deep), and geophysical surveys, including gravity, magnetometry, and resistivity data. These datasets were used to reconstruct the Cenozoic Basin and the underlying Palaeozoic basement structures. The detailed 3D model provides insights into the spatial configuration of Cenozoic and Palaeozoic formations and Alpine and Variscan fault zones. The model successfully captures the groundwater flow dynamics of the aquifer system and its relationships with the adjacent ones. The results indicate that groundwater flow within the aquifer system is influenced by surface hydrology in the shallow units and by structural geological features at depth, leading to compartmentalised flow regimes and complex hydraulic interactions between aquifers.