Chemical evolution of waters during long term interaction with granitic rocks in northern Switzerland

As part of an assessment of crystalline rock as a potential host for a nuclear waste repository, water samples were collected from more than 50 locations from the crystalline basement where it is under sedimentary cover in Northern Switzerland and where it is at the surface in the Black Forest. These samples describe the changing chemistry of water in an extended flow system from dilute recharge waters far from chemical equilibrium with its host rock to saline waters at temperatures of 50 to 100°C with residence times far in excess of 105 a that have reached chemical equilibrium with their host rock. This unique set of samples allows an analysis of the chemical evolution of granitic waters from surficial waters far from equilibrium to almost equilibrated waters. Mobile ions, rare gases and isotopic data are used to estimate the extent of reaction between waters and their host rock. The evolution of controlled elements is interpreted as a function of this extent of reaction. Silica contents correspond to approximate equilibrium even in the recharge waters. The relative concentrations of Ca and especially Mg are significantly lower in the borehole waters than in recharge waters indicating a trend towards equilibrium. The Na/K ratios correspond to equilibrium only in the most evolved, older, waters which are shown to be at full equilibrium.