Overview of geological and hydrogeological conditions of the Äspö hard rock laboratory site

The site of the underground facility, the Äspö Hard Rock Laboratory, was excavated at a depth of 450 m below the island of Äspö and has been extensively investigated by geological, hydrogeological and hydrochemical methods as part of the geoscientific research for disposal of nuclear waste in Sweden. The geological history of the area dates back to 1.85 Ga and is dominated by granitoids belonging to the Trans-Scandinavian Igneous Belt but also includes basic sheets and xenoliths and dikes of fine-grained granite. Seven tectonic episodes, giving rise to fracture mineralization, are recognised. The major discontinuities and fracture zones were characterised from surface investigations before the tunnel construction work started. These structural features were also identified in the tunnel and are, as predicted, the major water conducting features. Sets of open fractures in the NNW–NW and N–S directions and the brittle fine-grained granite are other important water conductors. Groundwater flow modelling shows that the location of Äspö island has a major impact on the current distribution of groundwater salinity due to varying hydraulic/boundary/conditions in the late and post glacial period.