Research papersUsing noble-gas and stable-isotope data to determine groundwater origin and flow regimes: Application to the Ceneri Base Tunnel (Switzerland)

- Tunnel drilling offers a unique opportunity to study deep waters in mountain regions.
- We collected water from inflows in the Ceneri Base Tunnel for geochemical analyses.
- We investigated the flow dynamics and the origin of water in fractured rocks.
- A conceptual hydrogeochemical model has been developed for the investigated massif.

Tunnel drilling provides a unique opportunity to sample and study deep groundwaters that are otherwise difficult to access. Understanding deep groundwater flow is of primary importance in assessing the possible impacts of tunnelling on hydrogeological systems. During this study, water was sampled for noble-gas analysis from tunnel inflows in the AlpTransit Ceneri Base Tunnel (Canton Ticino, southern Switzerland), which passes through an area mainly characterized by metamorphic rocks (gneiss). Furthermore, water was sampled from springs located in the same geological environment.Based on the measurement of noble-gas concentrations and isotope ratios, tritium concentrations, the stable isotope composition of hydrogen (δ2H) and oxygen (δ18O), and the concentrations of major ions in the water, a conceptual hydrogeological model was established for this case study that allowed the most probable origin of the groundwaters sampled at different locations to be determined. The measured abundances of 3He, 4He, and 20Ne allow the geochemical characterization of old groundwaters strongly enriched in terrigenic helium of crustal origin and the identification of mixing with water that circulates preferentially through cataclastic structures. Noble-gas concentrations and isotope ratios as well as tritium are useful proxies for the characterization of faults that may be critical for tunnel drilling because of their active hydrogeological role and their influence on the mechanics of the rocks.