Sediments and radon – a dangerous combination? a case study from kinsarvik, Norway

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 high123. Thus, we believe the explanation to the high radon consentration to be related to the unconsolidated sediments4.
The main part of the sediment deposit is a considerable endmoraine which was generated at the end of last glacial5. 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 km2 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 variability6. 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.