Soil-to-Plant Transfer Factors of Stable Elements and Naturally Occurring Radionuclides: (2) Rice Collected in Japan

The critical paths of radionuclides and the critical foods in Japan are different from those in European and North American countries because agricultural products and food customs are different. Consequently, safety assessment in Japan is required to consider rice and vegetables as the critical foods. In this study, we measured soil-to-plant transfer factors (TFs) for rice using naturally existing elements as analogues of radionuclides under equilibrium conditions. Rice and associated soil samples were collected from 50 sampling sites throughout Japan and TFs of 36 and 34 elements for brown rice and white rice, respectively, were calculated on dry weight basis.

Probability distributions of TFs of elements for brown rice and white rice were a log normal type. Except for As and Mo, the TFs for brown rice were usually lower than those for wheat and barley especially for K, Fe, Sr (t-test: p < 0:05) and slightly lower than those for Ba, rare earth elements, Th and U. When the TFs of brown rice were compared with those of white rice, TFs of Th and U were higher in white rice, but, for many other elements, the TFs were lower in white rice than those in brown rice. Comparison of the data with previously reported values for Sr and Cs showed that the TFs of stable Sr and Cs were close to the respective values of fallout ⁹⁰Sr and ¹³⁷Cs observed in field experiments. Thus, the TFs of naturally existing elements can be used as TFs of those radionuclides which have been in contact with the environment for a long time and have reached equilibrium conditions.