Hydrochemical and isotopic (d2H, d18O, 3H) characterization of fracture water in crystalline rock (Grimsel, Switzerland)

Knowledge on water circulation through crystalline rock is of key interest for radioactive waste disposal, geothermal projects or groundwater supply. This study presents first results of long-term monitoring of fracture groundwater chemical and isotope compositions combined with structural data on fracture networks developed in the crystalline rocks at the Grimsel Test Site (GTS), Switzerland. Groundwater circulation in the two major lithologies, the Central Aar Granite (CaGr) and the Grimsel Granodiorite (GrGr) is mainly associated to NE-SW trending brittle deformation structures. Within the different lithologies groundwater chemical and water isotope (delta18O, delta 2H) compositions remain constant over the 2 years monitoring period. This suggests the absence of seasonal variations at the sampling depth of 420–520 m below surface. Differences in chemical and isotope composition occur, however, between groundwater collected from the CaGr and GrGr and can be related to differences in infiltration area, flow paths, residence time and water-rock interaction in the two rock units. The delta 2H and delta 18O signatures indicate a meteoric origin from the same moisture sources under present-day climatic conditions and the absence of measurable 3H suggests residence times > 65 yrs for groundwater from both units