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 the Central Alps. The investigated tunnel section cuts across gneisses and schists of the pre-Alpine basement and across two narrow zones of meta-sediments. Rock temperature varies from 30 °C to 45 °C depending on the thickness of the overburden. The fracture water is of meteoric origin and acquires its composition exclusively by chemical interaction with the surrounding rocks along the flow path.
Water from inflow points in the basement of the Gotthard massif has typically a high pH of about 10 and total dissolved solids in the range of 100 to 300 mg L-1. Sodium is the prime cation of most waters. Although plentiful in the rocks, calcium, potassium, and magnesium are low to very low in the water. The anions associated with Na are carbonate/bicarbonate, sulfate, fluoride and chloride in widely varying proportions. High fluoride concentrations of up to 15.4 mg L-1 are characteristic for most waters. As a result of the high pH dissolved silica (SiO2) reached concentrations of up to 58 mg L-1 and represents 25 - 30 wt.% of the solutes.