Characterization of the main karst aquifers of the middle Valseriana (northern Italy) based on isotopic and hydrogeochemical data: Nossana and Ponte del Costone springs

Valseriana (Northern Italy) is a valley within the Orobian Pre-Alps of Bergamo Province, and it is characterized by high water availability due to the combination of its carbonate environment and an elevated average precipitation rate (about 2000 mm/year). In this context, the catchments of the main karst springs (Nossana and Ponte del Costone) become of strategic importance for the water exploitation and the related domestic service of the city of Bergamo and the municipalities in the area. This study elaborated the results of chemical and isotopic analyses of waters sampled during the campaign conducted between May 2018 and July 2019, thanks to the collaboration between the Università degli Studi di Milano and the company of the integrated water service (UniAcque S.p.a.). A total of 37 points was sampled from natural karst spring, river waters, wells, karst caves, and water of the Val del Riso mine. The chemical composition of the waters was described by applying the hierarchical cluster analysis technique to group them into uniform hydrochemical groups based on the values of major cations and anions, alkalinity (CaCO3), temperature, and CO2 partial pressure. The results of the stable isotope analysis indicate a dependence of the recharge on meteoric waters (the calculated study area regression water line is δ2H=7.3δ18O+7.2) and a secondary evaporation effect in the recharge area due to the morphology of the territory, markedly mountainous. The two karst systems can be simplified by a hierarchical flow system model, dynamically controlled by karst network development and infiltration water. Besides, the water reservoirs of the main springs were dated using the 3H/3He technique obtaining values of approximately 10 years for the Nossana spring and 30 years for the Ponte del Costone water system. The comparisons with a previous 2015 campaign indicate that the water reserve is not undergoing renewal. This leads to the need to continue to investigate whether this is a temporary phase or a warning sign. The study was also supported by the ENeRAG – Excellency Network Building for Comprehensive Research and Assessment of Geofluids project funded by the European Union’s Horizon 2020 research and innovation program under grant agreement No 810980.