Deep hydrochemical section through the Central Alps: evolution of deep water in the continental upper crust and solute acquisition during water–rock-interaction along the Sedrun section of the Gotthard Base Tunnel

Abstract Drilling of the Gotthard Rail Base Tunnel through Central Alps from 2005 to 2010 opened up fractured basement units and frequent water inflows provided access major fluid-rock interactions processes in orogenic crystalline upper crust. Construction 57 km long tunnel was divided into five different sections. Here we report data observations 10 central Sedrun section 211 samples were collected at inflow points 900 2350 m below surface. The exceptional provide a comprehension hydrochemical evolution solute acquisition deep groundwater units. investigated drilled steeply dipping rock vertical fracture systems high angle. It cuts across granite, gneiss schist pre-Alpine two narrow zones meta-sediments. Rock temperature along varies 30 45 °C depending on thickness overburden. is meteoric origin acquires its composition exclusively by chemical interaction with surrounding rocks flow path. Water Massif has typically pH about total dissolved solids range 100 300 mg L −1 . Sodium prime cation most waters. Although plentiful rocks, calcium, potassium magnesium are low very water. anions associated Na carbonate/bicarbonate, sulfate, fluoride chloride widely varying proportions. High concentrations 15.4 characteristic for As result silica (SiO 2 ) reached 58 represents 25—30 wt% solutes. recharge provides O CO processes. solutes derive dissolution feldspar (Na + , SiO 2aq ), oxidation sulfides sulfate (SO 4 2− alteration biotite (F − fluid inclusions brittle deformation (Cl ). formed during interaction, mainly zeolites, chlorite (and other clay minerals) secondary Fe-minerals, remove Mg, Fe, K almost quantitatively results hydrolysis silicates. distinctly show that within depth interval 1.0–2.5 surface continental evolves TDS, pH, sodium carbonate solution granite infiltrating pristine water, snow rain.