From geological complexity to applied models: integrating structural geology and hydrogeology for groundwater management in Mediterranean settings

Groundwater systems in Mediterranean regions are commonly characterized by geological complexity, spatial heterogeneity, and temporal variability driven by both natural processes and human pressures. In such contexts, applied hydrogeological analyses often rely on simplified conceptual and numerical models that inadequately represent the geological controls governing groundwater flow, storage, and vulnerability, often underestimating the uncertainty inherent to geological systems. This contribution argues that geological interpretation, particularly structural and stratigraphic analysis, should be regarded as a central component of groundwater assessment, and that its effectiveness is significantly enhanced when coupled with geochemical and multi-isotopic investigations explicitly aimed at reducing model uncertainty. We present an integrated methodological framework developed through several research projects and case studies in Sardinia (Italy), a region representative of many Mediterranean environments affected by water scarcity, groundwater salinization, and diffuse contamination. The proposed approach combines detailed geological reconstruction, structural analysis, long-term monitoring data, hydrogeological modeling, and targeted geochemical and isotopic analyses to address geological complexity and to constrain conceptual and numerical models while acknowledging data limitations and scale-dependent uncertainty. The selected case studies include (i) coastal alluvial aquifers affected by salinization processes, where sedimentary architecture and structural features exert a primary control on seawater intrusion pathways; (ii) intensively cultivated plains impacted by nitrate contamination, where the interaction between shallow aquifers, unsaturated zone processes, and geological heterogeneity controls contaminant transport and attenuation; and (iii) volcanic aquifers, characterized by permeability contrasts and recharge mechanisms linked to lithological variability and fracture networks. In all cases, stable water isotopes ( ¹⁸O and  ²H) are used to identify recharge sources, seasonal signals, evaporation effects, and mixing between different groundwater bodies, providing constraints that help discriminate among alternative conceptual models. Tritium measurements offer additional insight into groundwater residence times and the contribution of recent recharge, while nitrate isotopes ( ¹⁵N–NO₃⁻,  ¹⁸O–NO₃⁻) support the interpretation of contamination sources and transformation processes under uncertain boundary conditions. The results highlight how structural elements influence preferential flow paths depending on their internal architecture and spatial arrangement, while stratigraphic heterogeneities and depositional geometries strongly affect groundwater flow patterns and system responses to stresses such as pumping or prolonged droughts. Geochemical and isotopic data act as independent, process-based constraints that reduce non-uniqueness in model calibration and improve the robustness of scenario analysis, particularly where monitoring networks are sparse or unevenly distributed. A key aspect of the proposed framework is the explicit recognition that geological and hydrogeological models are inherently based on incomplete and scale-dependent data and should therefore be considered as working hypotheses rather than definitive representations. By integrating geological interpretation, monitoring data, and isotopic tracers within a multi-evidence framework, uncertainty is not eliminated but managed transparently, supporting risk-informed and adaptive groundwater management strategies. This study emphasizes that the effectiveness of groundwater management in complex Mediterranean environments depends on both the quality of geological understanding embedded within applied models and the explicit treatment of uncertainty. Strengthening the dialogue between structural geology, hydrogeology, geochemistry, and monitoring practices is crucial for moving from reactive approaches toward more proactive and sustainable groundwater governance.