Review of Survey activities 2015: Pre-Quaternary rocks and sediments with a high level of radioactivity in Denmark

Abstract

© 2016 GEUS. Geological Survey of Denmark and Greenland Bulletin 35, 31–34. Open access: www.geus.dk/publications/bull The pre-Quaternary sediments and rocks in Denmark generally have a low content of radioactive minerals and elements. Uranium, thorium and radium are built into mineral structures or are, for example, adsorbed on the surface of clay minerals, Fe-minerals or organic material. Radon (222Rn) is a radioactive noble insoluble gas with a half-life of 3.8 days. It belongs to the uranium (238U) decay chain where radon is formed from radium (226Ra). When Rn is formed by radioactive decay from Ra, the emanation process sends part of the radon produced into the pore spaces of rocks and soils. From here, the radon can enter and accumulate in buildings. The source of the radioactive materials in Danish sediments and rocks is primarily from weathered Precambrian crystalline rocks from Norway, Sweden, Finland and the Danish island of Bornholm. Physical and chemical weathering disintegrates these rocks and rivers transport the material into the Danish–Norwegian and Danish–Polish sedimentary basins. Several studies have analysed and described the radioactive content of Danish sediments and crystalline rocks (e.g. Damkjær & Korsbech 1985, 1988; Gravesen et al. 1996, 1999; Gravesen & Jakobsen 2010) and investigations have demonstrated a relationship between sediments and rocks and Rn levels in Danish buildings (Andersen et al. 2001). This paper addresses the radioactive content of sediments and rocks with the highest radioactive levels in Denmark and the highest recorded radon emanations: Precambrian crystalline rocks on Bornholm and Late Paleocene clays in north-western Jylland (Fig 1). The data were collected by Gravesen et al. (1999) at the Geological Survey of Denmark and Greenland (GEUS) with the aim of characterising and mapping Rn in Danish rocks and sediments.