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Workreport 2018-27

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Name:

Compilation and Analysis of Hydrogeological Responses to Field Activities in Olkiluoto During 2015-2016

Writer:

Pentti, E., Vaittinen, T.

Language:

English

Page count:

304

Summary:

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 2015–2016.

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 section-specific 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 2016 is estimated as well as reasons for changes in it during 2015–2016.

The temporary drawdowns during the studied period were mainly caused by pumping activities and other events in the OL-KR drillholes, such as pumping during PFL flow logging, groundwater sampling, and pauses in the long-term pumping of OL-KR6. In comparison with previous years, very few groundwater leaks into the ONKALO occurred with an effect on monitored head.

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 hydrogeological structure model updated in 2015.

Keywords:

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

File(s):

WR 2018-27_web (pdf) (16.5 MB)


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