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Workreport 2016-51



Compilation and Analysis of Hydrogeological Responses to Field Activities in Olkiluoto During 2013–2014


Pentti, E., Vaittinen, T.



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Groundwater flow characteristics provide essential input for the construction and safety assessment of a disposal facility for spent nuclear fuel. On the Olkiluoto site, hydraulic connections in the bedrock have been studied in deep drillholes by means of long-term pumping tests, various interference tests, and by interpreting the measured hydraulic heads. This report focuses on the assessment of measured hydraulic heads during 2013–2014.

Hydraulic heads have been measured both in open and in packed-off drillholes since 1991. The interpretation of the hydraulic connections is based on observed changes in hydraulic head distribution caused by certain investigation or construction activities on the site. Field activities may increase the head, e.g. drilling, or more typically decrease the head, e.g. flush pumping after drilling, difference flow logging with pumping, and both temporary and currently stable inflows into underground facilities caused by the construction of ONKALO.

Processing of the head observations has been developed by determining sectionspecific corrections for natural fluctuation of the groundwater. The objective of the corrections is to remove natural fluctuation of the groundwater table and sea level, tidal effect, and atmospheric pressure to improve detection of changes in hydraulic head caused by field activities. Time series of observations are compared to schedules of field activities and values for responses are calculated. In addition to temporary responses, long-term drawdown at the end of 2014 is estimated as well as reasons for changes in it during 2013–2014.

The temporary drawdowns during the studied period were mainly caused by leaks from the HZ20 system into the intersecting grouting holes of the shafts in the ONKALO, and to pumping events related to flow logging and groundwater sampling in various drillholes. The reaming of OL-KR7 caused rapid increases of head in the HZ20 system in 2014. Considering net changes of head within the period 2013–14, the long-term drawdown appears to have stabilised in drillholes intersecting HZ19 and HZ20, but is still slowly developing in zones HZ056, HZ099, and HZ21 that are hydraulically connected by OL-KR7.

On the basis of the head observations in packed-off drillholes, sub-horizontal hydraulic zones form a layered system at the ONKALO area corresponding to the current hydrogeological structure model. New observations gathered from the packedoff drillholes in the eastern area of the Olkiluoto Island support the current hypothesis of the relation between ductile deformation model and hydrogeological structure.


Disposal of spent nuclear fuel, groundwater flow connection, hydraulic head, hydraulic conductivity, packed-off drillhole.


WR 2016-51_web (pdf) (37.8 MB)


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