Speciation of naturally occurring radionuclides in Mediterranean soils: bioavailabilty assessment

Knowledge of soil-to-plant transfer processes is a key element that can have a significant health impact. Much effort has been taken to characterize the speciation of anthropogenic radionuclides released into the environment. However, the information about naturally occurring radionuclides is scarce. This work evaluate the potential risks of transference, that is, the bioavailability of the ^234,238U, ²²⁶Ra, ^228,230,232Th, and ²¹⁰Po in three different soils collected in Mediterranean ecosystems. Chemical speciation of these radionuclides was carried out according to two different methods, Pavlotskaya and a modification of Tessier's protocol. Most of these radionuclides were associated to fractions strongly bound to soil particles and not able to be transferred. Increasing concentrations of U and Th extracted with increasing volume of NH₄OAc 1 M were observed, until it reached saturation. Readily bioavailable fraction in both methods (either exchangeable or water soluble + exchangeable) decreased in the following order: ²²⁶Ra > ^234,238 U > ^228,230,232Th > ²¹⁰Po. It was found that < 3% of the natural radionuclide concentration in soil are readily bioavailable for plant uptake in this region of Spain, and the resulting human health risk is negligible from natural radionuclide ingestion.