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Olkiluoto Island, which is located in SW Finland, has been selected as the deep geological repository for the high-level nuclear waste generated by the nuclear power plants operated by the Finnish power companies TVO and Fortum, and the repository is expected to become operational in the early 2020s. Posiva is an expert organisation responsible for the management and implementation of the nuclear waste repository at Olkiluoto and is owned by TVO and Fortum. Posiva submitted a construction licence application to the Finnish government in December 2012 and an integrated description of the Olkiluoto site, including its geology, was a part of the background material. After their review, in early 2015, the Radiation and Nuclear Safety Authority in Finland (STUK) gave a positive statement to the Ministry of Employment and the Economy: "the final disposal facility designed by Posiva can be built to be safe".Posiva has continued updating the multidisciplinary site description, which is aiming at the last licencing stage of the repository - the application of the operating licence. Understanding the geology of Olkiluoto is a substantial part of the site description work. It is based on data from hundreds of outcrops, tens of kilometers of drill core, extensive tunnel mapping programme and a wide variety of geophysical investigations. The Palaeoproterozoic bedrock in Olkiluoto consists of variably migmatised supracrustal high-grade metamorphic rocks: migmatized meta-pelites, meta-arenites and intermediate, pyroclastic metavolcanites. They are intruded by Paleoproterozoic felsic, granitic–tonalitic plutonic rocks and granitic pegmatoids, and diabase dikes. The rocks were metamorphosed simultaneously with the different phases of ductile deformation. Hydrothermal alteration at Olkiluoto can be subdivided into two different modes on the basis how the fluids have proceeded in the rock mass: fracture- or veinlet-controlled alteration and pervasive or disseminated alteration. The brittle deformation history of Olkiluoto can be reconstructed through paleostress inversion of fault-slip data from outcrops and drill cores and from K-Ar ages if fault gouge illites: seven distinct paleostress states can be identified, spanning in time from ca. 1.7 Ga to 1.0 Ga.

Over the past 20 years a considerable amount of work has been carried out to establish a palaeohydrogeological understanding of the Olkiluoto site and surrounding area, and to integrate this knowledge into the hydrogeochemical and hydrogeological descriptive and modelling programmes. This has involved not only a wide range of well established disciplines such as geology, hydrogeology and hydrochemistry, but also the extraction and determination of rock matrix porewaters by out-diffusion, a relatively new approach in crystalline rock. This required a sophisticated laboratory based input, not only to extract and analyse the porewaters, but also to take into consideration any effects associated to, for example, connected physical porosity and/or geochemical porosity in the rock matrix. In general, there is a good integrated understanding of the Olkiluoto site in terms of the geology, mineralogy, hydrology, hydrochemistry and the overall palaeohydrogeochemical model. The Olkiluoto site has had a complex geological and environmental history from Precambrian to the Quaternary as shown by fluid inclusions in quartz grains and fracture calcites. The Quaternary time period has been dominated by a large climatic variation of cold glacial cycles with temperate interglacials and sea-level changes, all of which have contributed to the hydrogeochemical evolution at the Olkiluoto site. All data indicate that infiltration of aerobic water has systematically been limited to few metres depth in the bedrock at Olkiluoto. Today at about the -300 m elevation level, there exists a distinct change in groundwater chemistry and mean residence time including a redox divide supported by a significant reduction in both the intensity and transmissivity of the water connected fracture networks. These indicate that long term stability (over the time span of glacial cycles) and sufficient buffering capacity of the water-rock system against aerobic infiltration, has dominated continuously until present times at Olkiluoto. Investigation results from matrix porewaters and fracture groundwaters indicate at least five to six different end-member water types that have contributed to current groundwater compositions. Salinity in these end-member waters varies from fresh water to highly saline brine and they seem to represent source waters from different environmental conditions, i.e. meteoric and marine waters from glacial to warm, and humid to arid climates. There are, however, several areas of uncertainty that have been highlighted during the present study that need to be resolved. For example, attempts to integrate the hydrogeology, groundwater chemistry and porewater chemistry, and their relationship to the palaeoevolution of the Olkiluoto site before and since the start of last glaciation, have encountered some difficulties. These include the choice of initial boundary conditions for the hydrogeological modelling, the necessity to consider alternative scenarios to explain present groundwater conditions, the choice of model input parameter values for diffusion and matrix pore diffusivity, and resolving potential problems associated with sampling and analysis of the porewaters. Some uncertainties are also related to the experimentally derived porewater concentrations. For example, one explanation to salinity differences between matrix porewaters and fracture groundwaters may be anion exclusion which has been interpreted in several laboratory experiments based on Olkiluoto and Finnish rock samples. Presently plans and studies are underway to address the many uncertainties that have resulted from this study and additional corroborative (or otherwise) data should be available in the near future.

Olkiluoto Island, which is located in SW Finland, has been selected as the deep geological repository for the high-level nuclear waste generated by the nuclear power plants operated by the Finnish power companies TVO and Fortum, and the repository is expected to become operational in the early 2020s. Posiva is an expert organisation responsible for the management and implementation of the nuclear waste repository at Olkiluoto and is owned by TVO and Fortum. Posiva submitted a construction licence application to the Finnish government in December 2012 and an integrated description of the Olkiluoto site, including its geology, was a part of the background material. After their review, in early 2015, the Radiation and Nuclear Safety Authority in Finland (STUK) gave a positive statement to the Ministry of Employment and the Economy: "the final disposal facility designed by Posiva can be built to be safe". In 2016, the Finnish Government granted a licence to Posiva for the construction of a final disposal facility for spent nuclear fuel.Posiva has continued updating the multidisciplinary site description, which is aiming at the last licencing stage of the repository - the application of the operating licence. Understanding the geology of Olkiluoto is a substantial part of the site description work. It is based on data from hundreds of outcrops, tens of kilometers of drill core, extensive tunnel mapping programme and a wide variety of geophysical investigations. The Palaeoproterozoic bedrock in Olkiluoto consists of variably migmatised supracrustal high-grade metamorphic rocks: migmatized meta-pelites, meta-arenites and intermediate, pyroclastic metavolcanites. They are intruded by Paleoproterozoic felsic, granitic–tonalitic plutonic rocks and granitic pegmatoids, and diabase dikes. The rocks were metamorphosed simultaneously with the different phases of ductile deformation. Hydrothermal alteration at Olkiluoto can be subdivided into two different modes on the basis how the fluids have proceeded in the rock mass: fracture- or veinlet-controlled alteration and pervasive or disseminated alteration. The brittle deformation history of Olkiluoto can be reconstructed through paleostress inversion of fault-slip data from outcrops and drill cores and from K-Ar ages if fault gouge illites: seven distinct paleostress states can be identified, spanning in time from ca. 1.7 Ga to 1.0 Ga.

The rocks of Olkiluoto can be divided into two major classes: 1) supracrustal high-grade metamorphic rocks including various migmatitic gneisses, tonalitic-granodioriticgranitic gneisses, mica gneisses, quartz gneisses and mafic gneisses, and 2) igneous rocks including pegmatitic granites and diabase dykes. The migmatitic gneisses can further be divided into three subgroups in terms of the type of migmatite structure: veined gneisses, stromatic gneisses and diatexitic gneisses. On the basis of refolding and crosscutting relationships, the metamorphic supracrustal rocks have been subjected to polyphased ductile deformation, consisting of five stages, the D2 being locally the most intensive phase, producing thrust-related folding, strong migmatisation and pervasive foliation. In 3D modelling of the lithological units, an assumption has been made, on the basis of measurements in the outcrops, investigation trenches and drill cores, that the pervasive, composite foliation produced as a result of polyphase ductile deformation has a rather constant attitude in the ONKALO area. Consequently, the strike and dip of the foliation has been used as a tool, through which the lithologies have been correlated between the drillholes and from the surface to the drillholes. In addition, the largest ductile deformation zones and tectonic units are described in 3D model. The bedrock at the Olkiluoto site has been subjected to extensive hydrothermal alteration, which has taken place at reasonably low temperature conditions, the estimated temperature interval being from slightly over 300°C to less than 100°C. Two types of alteration can be observed: firstly, pervasive alteration and secondly fracturecontrolled alteration. Clay mineralisation and sulphidisation are the most prominent alteration events in the site area. Sulphides are located in the uppermost part of the model volume following roughly the foliation and lithological trend. Kaolinite is also mainly located in the uppermost part, but is not controlled by lithological trend. Another member of the clay system, illitisation, consists of few distinct volumes, which are located deeper, and are spatially associated with site-scale thrust faults. The fault zones at Olkiluoto are mainly SE-dipping thrust faults formed during contraction in the last stages of the Fennian orogeny, approximately at 1800 Ma ago and were reactivated in several deformation phases, as indicated by fault-slip data and K-Ar age determinations. In addition, NE-SW striking strike-slip faults are also common. Fault zone intersections from drillholes, the ONKALO access tunnel and outcrops have been correlated by the application of slickensides orientations, mise-à-lamassemeasurements, eletctromagnetic soundings, 3D seismics and VSP-reflectors, resulting the model of 179 brittle fault zones.