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SEDIMENTS AND RADON – A DANGEROUS COMBINATION? A CASE STUDY FROM KINSARVIK, NORWAY

V.Valen 1 , O.Soldal 2 , T.Strand 3 , C.L.Jensen.3 and A.V.Sundal 4
1 InterConsult Group ASA, P.box. 69 Sandsli, N-5861 Bergen, Norway 2 Instanes as, Storetveitveien 96, N-5072 Bergen 3 Norwegian Radiation Protection Authority, P.box 55, N-1332 Østerås 4 Department of Geology, University of Bergen, N-5020 Bergen,

ABSTRACT
In Kinsarvik, Norway, extraordinary high radon levels in dwelling units are revealed. The bedrock
geology was expected to give the answer to why the levels are so high. However, the uranium or
radium content is not especially high 123 . Thus, we believe the explanation to the high radon
consentration to be related to the unconsolidated sediments 4 .
The main part of the sediment deposit is a considerable endmoraine which was generated at the end
of last glacial 5 . Since the land was pressed down due to the heavy load of ice, the sea level was
much higher than today and reached almost the top of the endmoraine at 110 metres above sea
level. This produced a capping of finer sediments above the much coarser moraine material with
high permeability and a zone of more than 20 m above the groundwater level (phreatic zone).
To understand how radon was transported in the ground in this approximately 0.2 km 2 area, we used
C-39 alpha track detectors buried in 23 different localities. The detectors were sealed inside thin
plastic bags to prevent immediately overexposure and were exposed for up to 3 days. This was
repeated five times in different seasons and at different air temperatures.
The results show distinct seasonal variability (Figure 1), implying a profound soil air movement in
the coarse moraine. There is a movement of relatively warm soil air towards the higher areas during
winter giving rise to a high radon content of the ground in the topographical elevated areas. In the
summertime, the process is reversed, giving rise to a high radon content in lower parts of the area.
This transport of air in the ground entails a more extensive range from where radon can be
transported to housings.
Results from a comprehensive study of radon in dwellings in the same area shows the same
profound seasonal variability 6 . The buildings in the lower part of this moraine ridge show higher
values during summertime than wintertime. The buildings in the elevated parts of the area show the
opposite.
The conclusions of our research are that sediment permeability and size of the phreatic zone have a
major influence on the radon content of housings. And, dependent on type of sediments, there can
be a significant seasonal fluctuation of radon content in the ground. Thus, calculations of yearly
average exposure to radon should not solely be based on measurements during winters. A radon
potential mapping must also include both type of sediments and topography.