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Workreport 2017-21



Slug tests in PP-, PVP-, and HP-holes at Olkiluoto in 2016


Pentti, E.



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As a part of the program for the final disposal of spent nuclear fuel, Posiva Oy investigates the hydrological conditions at the Olkiluoto Island. Monitoring of hydraulic conductivity by annual slug tests in shallow investigation holes started in 2002. In summer 2016, measurements were carried out in holes OL-PP39, ‑PVP4A, ‑PVP4B, ‑PVP41A, ‑PVP41B, ‑PVP42A, ‑PVP42B, ‑HP1, ‑HP2, and ‑HP4. The length of OL-PP39 is 13.7 m, and the eight test sections (1 m) are located in the bedrock. Depths of the measured PVP- and HP-tubes range between 3.4–12 m, and the test sections (1, 2 or 3 m) are located in the overburden.

The measurements were carried out by using the slug test technique with equipment renewed in 2010. In the slug test, the hydraulic head in the borehole is abruptly changed either by pouring water into the hole or by lowering the pressure sensor. Both methods were applied in the 2016 tests. The hydraulic conductivity is interpreted from the recovery of the water level. This report presents the field measurements and their interpretation. The interpretation has been done using the Hvorslev’s method, and for reference, conductivity has also been calculated according to the Thiem’s equation.

According to the results, hydraulic conductivity in OL-PP39 ranges from 3.3×10−10 m/s to 5.3×10−6 m/s and in the PVP- and HP-tubes from 6.7×10−7 m/s to 1.4×10−5 m/s. The observed range is quite similar to the preceding measurements in 2002 and 2004–2015. In general, the results are consistent with the results obtained in the earlier measurements. However, results from the sections of OL-PP39 with a very low conductivity show quite a large variation.

Most of the 2016 results agree well with hydraulic conductivity interpreted from the pre-pumping done in connection with the groundwater sampling, but in a few cases the difference is about an order of magnitude.


Hydraulic conductivity, slug test, disposal of spent nuclear fuel, hydrology


WR 2017-21_web (pdf) (10.4 MB)


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