Behaviour of Iodine-129 in Rice Paddy Fields

Transfer factors of iodine from soil to rice were obtained by laboratory experiments using 125I tracer. Two typical soil types in Japan, Andosol and Gray lowland soil, were used. The transfer factor (TF) is defined as ’concentration of the nuclide in a plant organ at harvest’ divided by ’concentration of the nuclide in dry soil’. The TFs for brown (hulled) rice were 0.006 for Andosol and 0.002 for Gray lowland soil. The TFs for different organs of rice plants decreased in the order of blade > stem > rachis > unhulled rice >> brown rice.The concentration of iodine in soil solution under flooded conditions varied with time during cultivation. The iodine concentration in rice plants seemed to be influenced by the soil solution.The effect of removal of 1–129 from paddy fields by harvesting rice plants was also modelled. Even assuming continuous deposition of 1–129 onto the field, annual harvesting of the blades and stems of rice plants could effectively reduce the amount of the nuclide in the root zone.