Soil-to-crop Transfer Factors of Radium in Japanese Agricultural Fields

Radium-226 (Ra), an α emitter with a half-life of about 1600 y, is a natural decay product of U. Radium-226 is of special interest because it is an important radionuclide for the assessment of radioactive waste disposal. This radionuclide can reach humans through several transfer paths in the environment. Once Ra is taken into the human body by ingestion of food and water or inhalation, it can distribute into bone where it has a long biological half-life; exposure to Ra can cause cancers and other body disorders. Therefore, its longterm management is required and understanding of Ra behavior in the environment is important. In mathematical models that are used for long-term radiological assessment, environmental transfer parameters are needed. Among them, the soil-to-crop transfer factor (TF) is a key parameter that directly affects the internal radiation dose assessment for the ingestion pathway. TF is defined as the ratio of radioactivity concentration in plant to radioactivity concentration in soil. Although Ra is present in the environment, due to its low concentration in crops, TFs that have been obtained from agricultural fields are limited. In many cases, therefore, TFs used in such models were from the Technical Report Series 364 (TRS-364) compiled by IAEA. These data were obtained in temperate zones mainly from Europe and North America, and thus, the numbers of TFs for rice and crops native to Japan were limited. Yunoki et al. measured TF-Ra for spinach and Chinese cabbage, but they collected their samples from only two fields. Recently, Sasaki et al. tried to measure crop TF-Ra, however, most sample concentrations were under the detection limit, and only two hulled rice sample data were available from their study. Thus, it is necessary to collect TFs of Ra nationwide. In this study, we determined the concentrations of Ra in upland field crops (leafy vegetables, onion, potato, and so on) and associated soils collected from 45 locations throughout Japan in order to obtain TFs. We also measured alkaline earth metal concentrations to compare their behavior with Ra, which is the last member of this group and whose lighter members, Mg and Ca, are plant nutrients. 2. Experimental