Hydrochemistry in the Gotthard Base Tunnel (Central Alps) – Composition and evolution of deep groundwater in continental crust

Excavation of the Gotthard Rail Base Tunnel through the Central Alps opened fractured granite/gneiss and frequent fluid inflows provided access to the major water-rock-interactions (WRI) processes in orogenic crystalline upper crust. Construction of the 57 km long tunnel was divided into five different sections and was drilled from 1999 to 2010 (2011). Here we report data and observations from the long Faido- and Bodio sections of a combined length of 30 km. 152 water samples were collected at inflow points 550 to 2350 m below the surface. The exceptional samples and data provide a comprehension of the hydrochemical evolution and solute acquisition of deep groundwater in basement units. The Faido- and Bodio sections cut across a pile of nappes of granite, gneiss and schist of pre-Alpine age and across a narrow zone of Mesozoic meta-sediments. Rock temperature along the two sections varies from 20 °C to 47 °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. Total of dissolved solids (TDS) at inflow points, water-composition, and pH of fluids change dramatically along the two tunnel sections. Typically, TDS in granitic waters is low, waters are dominated by sodium (Na) and sulfate (SO 4 ), fluoride (F) is high, and pH reaches values of 10 and more. Although plentiful in the rocks, calcium (Ca), potassium (K), and magnesium (Mg) are low to very low in the fracture waters. Along the tunnel sections two high-TDS zones (> 3000 mg L −1 ) occur, reflecting the occurrence of outcrops of Triassic metasediments (Röti-Formation, Rauwacke, cellular dolomite). These rocks contain gypsum (anhydrite) and dolomite and are strongly karstified at surface outcrops and to depth of some 100 m. Water in the tunnel arriving from these areas are typically Ca-SO 4 dominated, have high TDS and a lower pH. They change the composition to a Na-Ca-SO 4 dominated water type when passing through the granitic rocks below. High fluoride concentrations of up to 16.5 mg L −1 are characteristic for most waters. As a result of high pH dissolved silica (SiO 2 ) reaches concentrations of up to c. 70 mg L −1 , especially in the Faido section. The waters are typically strongly supersaturated with respect to quartz. Fluoride and dissolved silica result from alteration of biotite (micas) forming clay minerals and chlorite. The newly formed solids remove Mg, Fe, and K almost quantitatively from the water. The data show distinctly that at least up to depths of 2.5 km below surface infiltrating pristine meteoric water evolves to a low TDS, high pH solution by interaction with gneiss and granite. High-TDS Ca-SO 4 waters in the tunnel result exclusively from waters that have passed through highly soluble and thus high permeable rocks near the surface.