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Recent publications

Workreport 2019-6



Hydrogeological Structure Model of the Olkiluoto Site in 2015


Vaittinen, T., Ahokas, H., Nummela, J., Pentti, E., Paulamäki, S.



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As part of the programme for the final disposal of spent nuclear fuel, a hydrogeological structure model containing the hydraulically significant zones of Olkiluoto Island has been compiled. The structure model describes the deterministically identified site scale zones that are considered to dominate the groundwater flow. Typically transmissivity of the modelled zones is in the order of 10−5 – 10−6 m2/s and wide range hydraulic connectivity has been interpreted. In addition, available information on local repository-scale hydrogeological zones has been gathered.

The main objective of the study is to provide the geometry and the hydraulic properties of the zones and the less fractured rock between the zones to be used in the numerical modelling of groundwater flow and geochemical transport in the safety assessment. Also, these zones should be taken into account in the design of the repository layout and in the construction of the disposal facility as they might have a long-term impact on the evolution of the site and the safety of the spent fuel repository.

The following issues have been taken into account as the hydrogeological structure model has been updated since 2003: 1) high fracture transmissivities enabling groundwater flow, 2) anomalous low heads indicating connection to a discharge area, 3) requirements related to numerical groundwater flow simulations, i.e. hydraulic connectivity, and 4) extensive brittle deformation zones or anomalous geophysical features close to the planned repository facilities indicating possible groundwater flow routes. As a new issue are 5) monitoring sections, where head has decreased below the measurable elevation due to inflow into the ONKALO.

The previous hydrogeological structure model was compiled in 2010 and updated version presented here is based on data available at the end of 2014. The updating was based on new hydrogeological observations and a systematic approach covering all drillholes to assess measured fracture transmissivities typical of the site-scale hydraulically significant zones. New data consisted of head observations and interpreted pressure and flow responses caused by temporary field activities and ONKALO leakages. The analysis of head observations focuses on cross-hole observations to determine the continuity and the extent of the zones. Essential background data for the modelling included the ductile deformation model and the site scale brittle deformation zones modelled in the Geological Site Model version 3.0. The geological model combines both geological and geophysical investigation data on the site.

As a result of the modelling campaign, 16 site-scale zones were included in the hydrogeological structure model. Compared with the previous model, two brittle fault zones and one former possible hydrogeological zone are included in the model. For the modelled zones, both the zone intersections, describing the fractures with high transmissivity and hydrogeological influence zones, including possibly connected fractures with lower transmissivity, are provided. To characterise the hydraulic properties of the bedrock between site-scale zones, hydraulic connections interpreted as repository-scale zones are reported, as well. Compared to the site-scale zones, hydraulic connections of the repository-scale zones are interpreted less extensive.

The hydraulic properties of the zones and the bedrock were quantified for numerical flow simulation purposes. Both the drillhole-specific transmissivities and the geometric means of the measured transmissivities as a function of depth are provided for the zones and the hydraulic conductivity for the less fractured rock as a function of depth was assessed. Evaluation of the fracture transmissivities and the upgraded interpretation of the hydrogeological influence zones caused minor changes in the zone transmissivities.


Hydrology, transmissivity, hydraulic conductivity, hydrogeological zone, groundwater flow, disposal of spent nuclear fuel


WR 2019-06_web (pdf) (25.2 MB)


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