Chemical and isotopic investigations (δ18O, δ2H, 3H, 87Sr/86Sr) to define groundwater processes occurring in a deep rock slide in flysch

Deep-seated landslides are complex systems; in many cases, multidisciplinary studies are necessary to unravel the key hydrological features that can influence their evolution over space and time. This paper aims to investigate the deep-seated Berceto landslide (BL) (Northern Apennines of Italy) to define the origin and geochemical evolution of groundwater (GW), identify the hydrological boundary of the slope system and highlight the GW flow paths, transit time and transfer modalities inside the landslide body. This research is based on a multidisciplinary approach that involves monitoring groundwater level (GWL), obtaining analyses of water chemistry and stable and unstable isotopes (δ18O - δ2H, 3H, 87Sr/86Sr), performing soil leaching tests, geochemical modeling (PHREECQ) and principal component analysis (PCA). The results of δ18O - δ2H and 87Sr/86Sr analyses show that GW recharge in the BL occurs from local rainwater and that external contributions from the local stream can be excluded. In the landslide body, two GW hydrotypes (i.e., Ca-HCO3 and Na-HCO3) are identified, while PHREEQC and PCA confirm that the chemical features of GW depend on water-rock interaction processes occurring inside the landslide. The 3H content suggests a recent origin for GW and appears to highlight mixing between shallow and deep GW aliquots. The 3H content and GWL confirm that shallow GW is mainly controlled by a mass transfer mechanism. The 3H analyses with GWL indicates that only deep GW is controlled by a pressure transfer mechanism; this mechanism is probably the main responsible for the kinematics of the landslide