Transfer factors of radionuclides from soil to rice and wheat collected in Japan

Obtaining local soil-to-crop transfer factor (TF) is important because climates, soil types and vegetations influence on the factor. In this study, analyses of stable isotopes and natural radioisotopes in rice and wheat grains and the associated soils collected throughout Japan were carried out in order to obtain TF under equilibrium conditions. The studied samples had been collected for the measurement of global fallout 137Cs and 90Sr at National Institute of Agro-Environmental Sciences. Inductively coupled plasma mass spectrometry and inductively coupled plasma optical emission spectrometry were used to measure about 30 elements, including Cs, Sr, Th and U. The geometric-mean-TFs of Cs, Sr, Th and U for polished rice collected in Akita, Japan, were 0.0024, 0.0037, 0.0001 and 0.000053, respectively. The results indicated that TF-Cs was usually lower than TF-137Cs (geometric mean=0.0033). Possibly, the physico-chemical forms of stable Cs differed from those of global fallout 137Cs in the soil samples, that is, a part of the stable Cs was strongly fixed in the soil particles, while 137Cs was more mobile in the soil. However, the difference of TF-Cs and -137Cs was small, thus, the TF-Cs can be used for long-term transfer of 137Cs in the environment. The methodology would be applied to the other elements, though physico-chemical forms of natural and non-natural isotopes should be clarified