During drilling of the Gotthard Base Tunnel through the Central Alps the exposed fractured rocks and the frequent water inflows provided a deep insight into regional hydrogeological processes in orogenic crystalline basements. Here we report data from the 10 km long central Sedrun section. 211 water samples were collected from frequent inflow points at 900 to 2350 m below the surface. The singular samples and data provide a comprehension of the deep hydrochemical cross-section through...