Miljøkonsekvenser knyttet til radionuklider og sporelementer i det thoriumrike Fensfeltet i Norge
Prescribed subject of the trial lecture:
Natural and artificial radioactivity in important food-chains
Time and place for the trial lecture and the public defence:
Tuesday May13th 2014 in auditorium J106, Soil building. Trial lecture starts at 12:15
Professor Lindis Skipperud, Department of Environmental Sciences, NMBU (main supervisor)
Professor Brit Salbu, Department of Environmental Sciences, NMBU (co-supervisor)
Professor Terje Strand, Oslo (co-supervisor)
Professor Peter Stegnar, Joef Stefan International Postgraduate School (IPS), Ljubljana, Slovenia
Dr. Per Roos, Senior Scientist, Technical University of Denmark, Center for Nuclear Technologies (DTU NUTECH), Roskilde, Denmark
Committee administrator: Associate Professor Elin Gjengedal, Department of Environmental Sciences, NMBU.
The doctoral thesis is available for public review at the UMB library.
Thesis number 2014:26, ISSN 1503-1667, ISBN 978-82-575-1196-8
The present work was initiated as an integrated ecological and human impact assessment whose main objectives were to assess the possible radionuclide and trace elements contamination of the Fen Complex environment, impact on biota and radiation doses to humans due to outdoor radiation exposure. The Fen Complex area which is comprised of both legacy NORM and undisturbed 232Th-rich sites served as a natural laboratory where environmental compartments and biota could be investigated in the natural state, under realistic conditions. With respect to the specific Fen area and previously published data, the main focus of this work was on radionuclides such as 232Th and uranium (238U) and their progenies, as well on trace elements such as arsenic (As), chromium (Cr), cadmium (Cd) and lead (Pb). To assess the impact of radionuclides and trace elements on the ecosystem and humans, information was needed regarding the characterization, mobility and biological uptake of radionuclides and trace elements, as well as their different exposure pathways. These aspects were studied and presented in five scientific papers on which this thesis is based on. The first paper (Paper I) describes the initial screening of the Fen Complex. It included the analysis of radionuclides and trace elements in samples of soil, rock, water and plants. The high gamma dose rates in outdoor air were recorded. Based on the obtained data, radionuclides in soil were inhomogeneously distributed and ˝hot spots˝ with high levels of radionuclides (up to about 7000 Bq/kg 232Th and 150 Bq/kg 238U) and elevated gamma dose rates (up to 10 μGy/h) were identified. ˝Hot spots˝ were observed within legacy NORM (former mining) sites, and also in some undisturbed surrounding 232Th-rich sites. The initial ERICA impact assessment demonstrated that dose rates for certain terrestrial organisms were higher than the adopted screening level (10 μGy/h), suggesting the need for more refined analysis. In addition to the previously published literature, the data presented in Paper I assisted in defining future directions of investigation, the choice of sites for more detailed surveys, biota selection and in defining aspects of human exposure that would be investigated. Binding mechanisms in soil determine the potential mobility and bioavailability of radionuclides and trace elements, and soil fractionation is therefore essential for assessing their behaviour in the environment. In the present work, mobility analysis of the investigated elements, based on the results of sequential extractions, was performed (Paper II). Soil fractionation showed that the majority of 232Th and As were irreversibly bound in the soil as they were only leached by concentrated HNO3 at elevated temperature. With respect to 232Th, the result was in accordance with its chemical nature, established low mobility and tendency to be tightly bound in soil fractions. Uranium and trace elements (Cr, Cd, Pb, Ni, Cu and Zn) were found to be potentially more mobile and associated with pH-sensitive soil phases, redox-sensitive amorphous soil phases and organic soil compounds. Multivariate statistical data analysis provided the link between soil chemical and physical parameters and output data from the sequential extractions. Further mobility investigation was performed by determining the distribution coefficients (Kd). The Kd (232Th) and Kd (238U) suggested elevated dissolution and mobility at legacy NORM sites, especially at decommissioned Nb mining site (346 and 100 L/kg for 232Th and 238U, respectively), while higher sorption of radionuclides was demonstrated at undisturbed 232Th-rich site (10672 and 506 L/kg for 232Th and 238U, respectively). Earthworms were chosen as biota representative for a detailed analysis of radionuclide and trace element uptake and chronic exposure to radiation (Paper III). Tissues of four different earthworm species, including epigeic and endogeic species, were analyzed and the results were linked to total soil concentrations, bioavailable or extractable soil fractions, and root and litter concentrations in order to predict the favourable environmental pool for uptake. Variability in individual tissue concentrations of radionuclides was observed to be high as previously demonstrated in scientific literature for heavy metals in earthworms. Differences in uptake between four investigated earthworm species, but also between species collected at legacy NORM and undisturbed 232Th-rich sites were demonstrated. Higher transfer was observed for 238U (TF = 0.09 – 0.25) than for 232Th (TF = 0.03 – 0.08). Radiological dose rates (2.2 – 11.9 μGy/h), obtained by ERICA modelling, were higher than those generally experienced by terrestrial organisms (0.01 – 0.7 μGy/h) in the soil with background radionuclide concentrations. However, no radiation risk could be predicted since the obtained doses were much lower than the internationally (IAEA, 1992; UNSCEAR 2008; US DOE 2002) adopted levels of ionizing radiation below which no measurable population effects would occur (40 and 400 μGy/h). Further study of biota exposure was performed on nine wild plant species. A wide range of plants was included, with different uptake modes taken into account, and including both roots and aboveground plant parts (Paper IV). Plant tissue concentrations of radionuclides were only slightly enhanced (up to 50 and 5 Bq/kg of 232Th and 238U, respectively), and comparable to the values reported in literature. The levels of trace elements were within the reference range for plants. Roots appeared to be a natural barrier to radionuclide entry into plants. This is illustrated by the finding that the activity concentrations were higher by a factor of 25 in roots than in the aboveground plant parts. Thus, the transfer factors for plants were actually lower (4·10-5 – 1·10-2 for 232Th and 1·10-4 – 4·10-2 for 238U) than expected from the observed total soil concentrations (about 16000 Bq/kg 232Th and 900 Bq/kg 238U). Based on the ERICA calculation, dose rates up to 23 μGy/h (in moss and lichen) were predicted. Previous studies on human health risk in the Fen area have demonstrated that the annual exposure doses were among the highest in Europe (about 14 mSv). The majority of these investigations focused on the doses of indoor gamma radiation and radon (222Rn), as well as on the doses received via ingestion of food and water. In the conclusions presented in several papers, the need for investigating the contribution of outdoor exposure to total exposure doses, as well for measuring the 220Rn concentration in air was highlighted. This information focused our investigation into outdoor 220Rn, 222Rn and terrestrial gamma radiation (Paper V). Compared to the world average, high outdoor gamma dose rates (about 10 μGy/h), high 220Rn (up to 5000 Bq/m3) and moderate 222Rn (up to 200 Bq/m3) concentrations in the air were recorded in the Fen area. Levels of these parameters correlated with the distribution of radionuclides in the bedrock. Due to the high uncertainty when 220Rn is used to calculate the exposure doses, the annual outdoor doses (0.10 – 1.54 mSv) were obtained by summarizing the doses from terrestrial gamma radiation and 222Rn doses. However, when variations in the exposure times and the 220Rn dose (calculated using the equilibrium factor F from an earlier study in the Fen) are accounted for, an increase in the annual outdoor doses for over 10 mSv for at least some people can be expected.
Based on the overall results, the high concentrations of radionuclides in the soil, the high levels of terrestrial gamma radiation and the high outdoor levels of 220Rn and 222Rn were observed at both legacy NORM and undisturbed 232Th-rich sites in the Fen Complex. However, due to the relatively low mobility of radionuclide 232Th, no significant transport into investigated biota, such as earthworms and plants, was demonstrated so that low radiation doses and no elevated risk were predicted using the ERICA tool. However, the question of synergistic, additive or antagonistic actions of radionuclides and trace elements in biota of the Fen Complex remains. In the analysis of human outdoor exposure, high terrestrial gamma radiation was demonstrated to have a major impact on the magnitude of the received doses. Outdoor exposure doses higher than 10 mSv/y, for at least a certain group of people under specific exposure scenarios, were estimated as possible. Due to the high outdoor 220Rn concentrations, the contribution of 220Rn to the total dose of outdoor exposure could not be excluded, although quantification of such a contribution with the present 220Rn data set is connected to a large uncertainty. With respect to the current Norwegian legislation, an intervention should be considered at the legacy NORM site Søve. Although, there is little that could be done to change the doses at undisturbed high radioactivity sites, it would be reasonable, where possible, to avoid house construction, to restrict the time spent for recreation and to limit the use of the materials with elevated radioactivity.