Constraints on Evolution and Residence Time of Geothermal Water in Granitic Rocks at Grimsel (Switzerland)

Thermal spring water (17–28°C) occurring in crystalline rocks at the Grimsel area, Switzerland, represents a mixture of an old geothermal component with a surface-derived, young cold water component. Comparison of chemical and isotope data of cold and thermal spring water collected in 1991/92 and 2014/15 allows deciphering the residence time and evolution of the geothermal component. Accounting for changes in atmospheric input for δ18O, δ2H, 3H and 14C, a meteoric origin from similar moisture source(s) for the cold and geothermal component is proposed, a 3H-PF model age of about 7 years for the cold component and infiltration of the geothermal component before 30 ka during an interglacial cycle. Deep circulation of the geothermal component is inferred from water temperatures of 110–120 °C and the uptake of geogenic CO2 and/or CH4