Origin and residence time of salinity in the Äspö groundwater system

The origin of salinity within the Äspö groundwater system is investigated by combining interpretations of conservative dissolved ions and of stable isotope ratios in water. The interpretation concludes that the groundwater salinity results from a mixing between Baltic Sea water intrusion and a deep seated saline groundwater of marine origin. This conclusion supports the geochemical model developed for the Äspö site. The residence time of the deep salinity is assessed by comparing the 36Cl content of dissolved salt at different depths and the secular equilibrium value of the host rock. The 36Cl of deepest levels corresponding to the highest salinity, is in equilibrium with rock, suggesting a penetration of the deep salinity into the host rock more than 1.5 Ma ago.