Jelena Mrdakovic Popic

The present study was done in the Fen Complex, a Norwegian area rich in naturally occurring radionuclides, especially in thorium (²³²Th). Measurement of radioactivity levels was conducted at the decommissioned iron (Fe) and niobium (Nb) mining sites (TENORM) as well as at the undisturbed wooded sites (NORM), all open for free public access. The soil activity concentrations of ²³²Th (3280–8395 Bq kg⁻¹) were significantly higher than the world and the Norwegian average values and exceeded the Norwegian screening level (1000 Bq kg⁻¹) for radioactive waste, while radium (²²⁶Ra) was present at slightly elevated levels (89–171 Bq kg⁻¹). Terrestrial gamma dose rates were also elevated, ranging 2.6–4.4 μGy h⁻¹. Based on long-term surveys, the air concentrations of thoron (²²⁰Rn) and radon (²²²Rn) reached 1786 and 82 Bq m⁻³, respectively. Seasonal variation in the outdoor gamma dose rates and Rn concentrations was confirmed. Correlation analyses showed a linear relationship between air radiation levels and the abundance of ²³²Th in soil. The annual outdoor effective radiation doses for humans (occupancy 5 h day⁻¹) were estimated to be in the range of 3.0–7.7 mSv, comparable or higher than the total average (summarized indoor and outdoor) exposure dose for the Norwegian population (2.9 mSv year⁻¹). On the basis of all obtained results, this Norwegian area should be considered as enhanced natural radiation area (ENRA).

The Fen Central Complex in southern Norway, a geologically well investigated area of magmatic carbonatite rocks, is assumed to be among the world largest natural reservoirs of thorium (²³²Th). These rocks, also rich in iron (Fe), niobium (Nb), uranium (²³⁸U) and rare earth elements (REE), were mined in several past centuries. Waste locations, giving rise to enhanced levels of both radionuclides and metals, are now situated in the area. Estimation of radionuclide and metal contamination of the environment and radiological risk assessment were done in this study. The average outdoor gamma dose rate measured in Fen, 2.71 μGy h⁻¹, was significantly higher than the world average dose rate of 0.059 μGy h⁻¹. The annual exposure dose from terrestrial gamma radiation, related to outdoor occupancy, was in the range 0.18–9.82 mSv. The total activity concentrations of ²³²Th and ²³⁸U in soil ranged from 69 to 6581 and from 49 to 130 Bq kg⁻¹, respectively. Enhanced concentrations were also identified for metals, arsenic (As), lead (Pb), chromium (Cr) and zinc (Zn), in the vicinity of former mining sites. Both radionuclide and heavy metal concentrations suggested leaching, mobilization and distribution from rocks into the soil. Correlation analysis indicated different origins for ²³²Th and ²³⁸U, but same or similar for ²³²Th and metals As, Cr, Zn, nickel (Ni) and cadmium (Cd). The results from in situ size fractionation of water demonstrated radionuclides predominately present as colloids and low molecular mass (LMM) species, being potentially mobile and available for uptake in aquatic organisms of Norsjø Lake. Transfer factors, calculated for different plant species, showed the highest radionuclide accumulation in mosses and lichens. Uptake in trees was, as expected, lower. Relationship analysis of ²³²Th and ²³⁸U concentrations in moss and soil samples showed a significant positive linear correlation.