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

摘要

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 U-238, Ra-226, and (210) Pb. 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 U-238, Ra-226, and Pb-210 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 Ra-226 and Pb-210, whereas activity concentrations of U-238 were more variable, depending of local soil characteristics. Levels of other natural radionuclides (K-40, Th-232) 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 Ra-226 levels in soil. Plant levels of U-238 and Ra-226 varied by several orders of magnitude depending on soil level and plant species, whereas plant levels of Pb-210 and Po-210 were largely affected by aerial fallout. Berries generally contained lower levels of U-238 and Ra-226 than green plant parts. As was the case for plants, the levels of U-238 in earthworms were strongly correlated with the respective concentrations in the soil. Soil-to-plant transfer was markedly higher for Ra-226 than for U-238. For both radionuclides, a positive correlation was found between concentration ratios of V. myrtillus (heath) and soil organic matter content. The U-238 concentration ratios for earthworms were generally two orders of magnitude higher than for plants.