Using the activity of naturally occurring radionuclides to identify the contribution of the Al industry to the atmosphere and plants

International audienceAlumina production from bauxite ore gives rise to a large amount of liquid and solid waste since the production of 1 kg of alumina involves the same weight of solid residue, the well-known red muds enriched in iron and heavy metals. Up to now, without any valorization of by-products, the red muds have been stored in artificial ponds which are a potential source of pollutants for the surroundings due to the uplift of red dust under strong wind conditions. The goal of the current work is to perform an in situ study of the transfer of red mud micro particles in the atmosphere and in the terrestrial environment around the red mud disposition basins. To this end, the activity concentration of NORM (Naturally Occurring Radioactive Materials such as U-238, decay products and Th-232) determined in plant leaves (Quercus robur and leaf vegetables) and in grains (wheat) is compared with the activity of the atmospheric micro particles, both taken in the environment of the red mud disposition basins (100-1,500 m). The activity ratios of NORM in vegetation and atmospheric micro particles are also compared to the ratios of potential sources of dust, such as soil and, above all, the red mud and bauxite emitted from the basins and from the piles, respectively.The increase in Th-232 and U-238 activity concentration in a few tree leaves (by a factor of 9 and 4, respectively) and in some aerosol samples is accompanied by a decrease in U-238/Th-232 activity ratio for these matrices. Low U-238/Th-232 is also observed in bauxite and red mud. This suggests that the airborne particles emitted by bauxite piles and red mud basins first contaminate the atmosphere and then the leafs surfaces, after deposition. Locally produced foodstuffs at a distance greater than 1,000 m from the ponds do not show any excess radionuclides, suggesting the low influence of airborne particles from the alumina production. Further variations in the Po-210/Pb-210 ratio in the plants and the atmospheric micro particles are consistent with the variation over time of these radionuclides coming in all likelihood from the atmosphere rather than from the aluminum industry