Uranium, thorium and their decay products are present in trace amounts in all rocks on Earth. Weathering, mechanisms of soil formation and soil-plant transfers lead to the presence of these radionuclides in all the components of the environment and, through the food-chain transfers, they are also present in animals and men. Thus, the objective of this study consists in improving the knowledge on the levels and the variability of the activities of uranium, thorium and their decay products in various foodstuffs and on their sources and transfers.
This in-situ study is based on the geological variability of the studied sites (granitic and volcanic zones of the Massif central, granitic massif of the Vosges and the alluvial Rhône valley) where various foodstuffs are sampled (vegetables, cereals, meat, eggs and dairy products). The possible sources of radionuclides (irrigation waters and soils for plants; water, food and soils for animals) are also sampled in order to study their contribution to the measured activities in the foodstuffs.
The results obtained present high variability of the activities in plants (238U: 8-120 mBq.kg-1 fresh in lettuces; 12-169 mBq.kg-1 fresh in wheat), less pronounced in animal products (238U: 1.7-9.7 mBq.kg-1 fresh in poultry meat; 0.48 1.30 mBq.kg-1 fresh in eggs). For plants (wheat and lettuce), the main radionuclide source seems to be the crop soils. Irrigation water, when it has significant uranium concentrations (>30 mBq.L-1), can also significantly contribute. The effect of soil particle resuspension and their adhesion to plant surface seems to be important in some cases. The deposit of atmospheric particles mainly concerns 210Pb. For the activities in animal products, negative correlations were found with the activities in animal foodstuff, for all the radionuclides (238U: r²=0.96; 232Th: r²=0.77). On the other hand, positive correlations were highlighted between 232Th activities in meat and those of the soil potentially ingested by animals (r²=0.96) which reveals a significant contribution of the soil to thorium activity. Water contribution to uranium activity in meat and eggs is an area worth further researches. Thus, this study of the possible sources of radionuclides highlights the importance of their role in the understanding of the radionuclide transfers to foodstuffs.