Elevated natural radioactivity in undisturbed forest and mountain areas of arctic Norway - local geology, soil characteristics, and transfer to biota

This study deals with the geology in areas close to a large unexploited uranium deposit and the impact of bedrock characteristics on levels of radionuclides and other elements in soil and biota. Factors influencing soil inventory and ecosystem transfer are discussed, focussing on ²³⁸U, ²²⁶Ra, and ²¹⁰Pb. Field work was carried out in Salangen Valley in Northern Norway. Sampling stations for soil and biota covered different habitats - grassland, birch forest and low alpine heathland. The geological survey confirmed uranium-bearing minerals in granitic gneiss and pegmatites. There was large variation in the local occurrence of uranium, reflecting the irregular nature of the pegmatite. Activity concentrations of ²³⁸U, ²²⁶Ra, and ²¹⁰Pb in surface soil were elevated at sites close to U-enhanced bedrock, compared to sites with other types of bedrock. Particularly high soil levels were found for ²²⁶Ra and ²¹⁰Pb, whereas activity concentrations of ²³⁸U were more variable, depending of local soil characteristics. Levels of other natural radionuclides (⁴⁰K, ²³²Th) merely increased with soil mineral content, and concentrations of heavy metals were generally low at all sites. External dose rate (1 m above ground surface) was closely correlated with ²²⁶Ra levels in soil. Plant levels of ²³⁸U and ²²⁶Ra varied by several orders of magnitude depending on soil level and plant species, whereas plant levels of ²¹⁰Pb and ²¹⁰Po were largely affected by aerial fallout. Berries generally contained lower levels of ²³⁸U and ²²⁶Ra than green plant parts. As was the case for plants, the levels of ²³⁸U in earthworms were strongly correlated with the respective concentrations in the soil. Soil-to-plant transfer was markedly higher for ²²⁶Ra than for ²³⁸U. For both radionuclides, a positive correlation was found between concentration ratios of V. myrtillus (heath) and soil organic matter content. The ²³⁸U concentration ratios for earthworms were generally two orders of magnitude higher than for plants.