Assessment of radionuclide and metal contamination in a thorium rich area in Norway

Abstract

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.