The use of stable Cs and Sr as proxies for the estimation of radionuclide soil-plant transfer factors

Transfer factors are key inputs for modelling the transport of radionuclides in the environment and for assessment of risk to humans and wildlife. However, for many combinations of soil conditions, plant species (products), and radionuclides, such information is limited. The use of the transfer factors of stable elements in the absence of information on their radioactive isotopes is quite common. The question of the legality of such an approach arises. For this purpose, work was carried out to establish the relationship between the accumulation coefficients of radionuclides (137Cs and 90Sr) and their stable isotopes. The research covered the dominant soil types of the Ukrainian Polissia and the main crops. It is shown that the transfer factors of stable cesium and strontium, calculated from their total content in the soil, are significantly lower than the corresponding coefficients for their radioactive isotopes and cannot be used to estimate the corresponding transfer factors of radioisotopes. The use of such estimates can lead to a significant underestimation of the transfer factors of radionuclides and, accordingly, human exposure doses. In the absence of information on the bioavailability of 90Sr, the transfer factors of stable strontium for productive organs of plants, calculated for the exchangeable or acid-soluble fractions of the element in soil, can be used to estimate the corresponding transfer factors of radiostrontium. For a conservative estimate, it is recommended to use the transfer factors of the exchangeable fraction of stable strontium.