[en] The Olkiluoto island rose from the Baltic Sea 2500 to 3000 years ago. The layered sequence of groundwaters can be related to climatic and shoreline changes from modern tune through former Baltic stages to the deglaciation phase about 10 000 years ago and even to preglacial times. Fresh groundwater is found to the depth of about 150 metres, brackish between 100 and 400 metres, deeper groundwaters are saline. At the depth of 500 meters, the content of Total Dissolved Solids (TDS) varies between 10 and 25 g/l. The most saline waters at depths greater than 800 metres have TDS values between 30 and 75 g/l. These deep saline waters seem to have been undisturbed during the most recent glaciation and even much longer in the past. Today fresh water infiltrating at the surface gradually displaces brackish and saline groundwater in the bedrock. Due to the still ongoing postglacial land uplift, Olkiluoto is likely to become an inland site with brackish or fresh groundwater at the depth of 500 metres within the next 10 000 years. During the construction and operation phases groundwater will be drawn into the repository from the surrounding bedrock. As a consequence, more saline groundwaters, presently laying 100 to 200 metres below the repository level, may rise to the disposal level. After the closing of the repository the salinity distribution will gradually return towards the natural state. During the glacial cycle groundwater salinity may increase, for example, during freezing of groundwater into permafrost, when dissolved solids concentrate in the remaining water phase, and in a situation where deep saline groundwaters from under the centre of the glacier are pushed to the upper parts of the bedrock at the periphery of the glacier. The most significant open issue related to saline groundwater is the performance of the tunnel backfill which in the BS-3 concept has been planned to consist of a mixture of crushed rock and 10-30% of bentonite. Saline groundwater may significantly decrease the swelling pressure and increase the hydraulic conductivity of such a backfill. The most promising alternative backfill options are natural mixed-layer clay (Friedland clay) and crushed rock backfill combined with special sealing structures. It is recommended that for a repository to be constructed at the depth of about 500 metres at Olkiluoto, all engineered barriers should be designed to perform properly at groundwater salinities ranging from fresh water to 35 g/l. Geochemistry and salinity of groundwater will be a key area in the further characterisation of Olkiluoto, in supporting research, as well as in performance assessment. Posiva will participate in studies and large-scale experiments on the performance of bentonite-based as well as alternative backfill and buffer materials in the projects to be launched within the 5th Framework Programme of the European Commission and in the Prototype Repository in the Hard Rock Laboratory at Aespoe. (orig.)