Rock-avalanche deposit causes extreme high indoor radon concentrations in Kinsarvik, Ullensvang municipality, Western Norway

The radon problem in Kinsarvik, Ullensvang municipality in Western Norway, is among the most severe in the world. Annual average indoor radon concentrations as high as 56,000 Bq/m 3 have been measured. The problem affects >100 houses, where average yearly indoor radon concentrations of 4340 Bq/m 3 has been reported. Indoor radon concentrations vary during the year, which has been explained by thermally induced flows of radon-bearing air through a porous ice-margin deposit. Updated geological knowledge and new mapping methods (LIDAR data) have allowed for a re-examination of the problem. There is significant evidence that does not fit with the established geological understanding that classifies the Kinsarvik deposit to be an ice-marginal moraine. Instead, the landforms and internal structures of the deposit are characteristic of a rock-avalanche deposit. The LIDAR data suggests a rockslide deposit of approximately 50 million m 3. Airborne and ground gamma-ray spectroscopy show elevated uranium content in the granitic bedrock north and east of Kinsarvik. These uranium-bearing rocks are also found in an open pit in the Kinsarvik deposit. Terrestrial cosmogenic nuclide dating of four surface boulders shows an average age of 10,900 ± 600 years. This suggests the rock avalanche fell immediately after the last deglaciation of Western Norway. Our results support that the radon problem in Kinsarvik is caused by uranium bearing bedrock and a porous rock-avalanche deposit that emanates radon, which in turn exhibits an alternating, temperature driven flow direction.