Improved soil-to-plant transfer factors for 99Tc and 79Se in natural and agricultural ecosystems

Geological Disposal Facilities (GDFs) are the preferred option for the disposal of high- and medium-level nuclear waste but environmental assessments for GDFs are complex. Models of transfer into the biosphere for radioisotopes that occur in nuclear waste rely on estimated Transfer Factors (TFs) that often have high levels of uncertainty and only exist for a few species. Here, using two key radioisotopes found in nuclear waste, we show that taxonomic analyses and phylogenetically based trait prediction (PTP) can be used to both reduce uncertainty in current estimates of soil-to-plant TF and to predict them for the many species with no measurements. We grew 61 species of plants selected to provide a phylogenetically informed sample, measured their uptake of ⁹⁹Tc and ⁷⁵Se, and reconstructed their possible evolutionary relationships using gene sequence information. The uptake of Tc and Se isotopes by plants was correlated, and for Tc was more similar within plant groups than between them and included significant taxonomic and phylogenetic influences. We use these findings to suggest improved soil-to-plant Transfer Factors (TFs) for⁹⁹Tc and ⁷⁹Se. We suggest that the approaches we used might be useful for a range of radionuclides, in both improving current estimates of TF and for predicting TFs to plants and, perhaps, to other biota. Such improvements might be useful not only for environmental assessments of nuclear waste disposal but also the environmental regulation of a nuclear industry being expanded in several nations to help meet targets for reducing global CO₂ emissions.