The complex functioning of a karstic aquifer: integrated methods for the characterisation of the Gran Sasso aquifer

Among aquifer systems, karst environments represent those with the most complex groundwater hydrodynamics and are one of the main resources of fresh water for human purposes. Understanding the groundwater flowpaths within karst aquifers, their functioning, and the protection and management of water resources are still relevant issues. In fact, in the last decades, many studies focusing on karst systems are growing to better understand their complex hydrodynamics. Groundwater in karst aquifers is a valuable resource in Mediterranean region but is vulnerable due to climate change and over-exploitation. Although groundwater has always shown excellent resilience in adapting to climate change, its careful assessment is required. Studying groundwater flows, recharge, and discharge processes in karst aquifers is crucial to enable sustainable water management. In a framework of increasing awareness of climate-change effects, such as infiltration reduction and aquifer resource depletion, it is important to update groundwater recharge assessment methods based on actual data in order to guarantee sustainable management of groundwater resources. The PhD research project aims to deepen the understanding of the Gran Sasso karst aquifer (Central Apennines, Italy) and it is included in the European project KARMA (Karst Aquifer Resources Availability and Quality in the Mediterranean Area). The KARMA project, granted under the PRIMA call (Horizon 2020 programme) of the European Union, is dealing with the achievement of substantial progress in the hydrogeological understanding and sustainable management of karst water resources in the Mediterranean area. The study realised during my PhD has been performed through a multidisciplinary approach, including hydrogeological, hydrogeochemical, and tracer test methods. The research sheds light not only on groundwater flow patterns and velocities but also delineates recharge zones for sustainable water resource management. The recharge evaluation of the Gran Sasso aquifer has been carried out considering the 2001–2020 monitoring period. It has been conducted using three different methods (Turc, Thornthwaite, and Aplis methods) showing consistently reliable results, confirming an average infiltration rate of about 600 mm/year. The results exhibit high variability in infiltration rates, ranging from over 800 mm/year to approximately 300 mm/year over two decades, emphasizing the need for continuous monitoring. The total recharge considers not only rainfall but also the contribution of snow melting on infiltration. In particular, the study focuses on the significant role of snowmelt in aquifer recharge, highlighting its sensitivity to climate variations. In this respect, the endorheic basin of Campo Imperatore plain (with an altitude of 1500-1900 m) plays a primary role, acting as a preferential zone for infiltration. The plain has therefore been taken into account both for the amount of snow coverage, given its proximity to the highest peaks, but also for being a preferential recharge zone for the aquifer. In fact, many of the main springs may have been fed by not only rainfall but also by snowmelt coming from the Campo Imperatore plain, with an estimation of about 75% of precipitation feeding the aquifer by infiltration. The reduction in snow cover due to climate change could severely impact the aquifer's recharge, reflecting the importance of water management in reducing vulnerability to climate change. The obtained results have raised a potential risk for groundwater resource depletion in mountainous aquifers and offer several insights and perspectives for the future management of groundwater resources. Furthermore, the study assesses the impact of altitude ranges on recharge, revealing the vulnerability of the 600–1000 m altitude range to precipitation variations. The identification of groundwater recharge areas, especially in complex karst aquifers, is crucial for resource protection. The conceptual hydrogeological model, based on the hydrogeological quantitative data and the geo-structural ones, finds possible confirmation through hydrogeochemical features. In fact, the application of chemical and isotopic methodologies makes it possible to obtain information on the evolutionary path of waters in aquifer sectors that cannot be easily monitored. Also, advanced integrated hypotheses on the origin, mixing, and chemical and physical processes that control underground dynamics have been inferred. In addition, isotopic analysis is fundamental for determining the average elevations of the recharge areas and, therefore, for confirming the extent of the hydrogeological basins. In detail, thanks to hydrogeological, hydrogeochemical, and isotopic data, a conceptual model of spring recharge has been proposed and subsequently validated by the tracer test. The tracer test has been carried out by using fluorescent dyes at the Vitella d'Oro spring, characterised by complex feeding patterns, fed both by the regional aquifer and by a local system exposed to karst features developed in the Rigopiano Conglomerates formation. This type of monitoring method is essential for the reconstruction of the hydrodynamics of the hydrogeological system of the area, thus recognizing the groundwater connections, directions of runoff, and areas of supply to the springs. The identification of groundwater flowpaths is the basis from which to begin to implement any type of intervention on an aquifer, to operate in a compatible way in the management of water resources, and for the evaluation of relations with the underground environment and with the entire hydrogeological structure. Future validation of transport processes in karst systems is essential to develop effective strategies for the management and protection of karst water resources. In summary, this multidisciplinary study enhances the understanding of the Gran Sasso karst aquifer, offering valuable insights for sustainable water resource management and providing a basis for future modelling and climate change impact assessments. The results highlight the importance of acquiring new hydrogeological data, particularly in assessing groundwater dynamics and recharge variability influenced by climatic and local factors. The improved knowledge will lead to a withdrawal optimization coupled with sustainability goals, aiming for active management able to balance human needs and environmental issues.