One nuclear fuel reprocessing facility is currently operating in Japan, and preparations are underway to open a larger reprocessing facility. About 30 MBq of ¹²⁹I is currently released annually from the former reprocessing facility (in 2002); this amount is not considered to be excessive. However, ¹²⁹I is an important nuclide in radiation monitoring because of its extremely long half-life (15.7 million years). Therefore, it is important to forecast the movement of ¹²⁹I over a large area emanating from the site of a malfunction or accident. Using non-radiological ¹²⁷I, we have tried to estimate the effect that farmland in the Tonegawa River watershed in Ibaraki prefecture, particularly, paddy fields, would have on the movement of ¹²⁹I moving in tandem with water throughout a large area. In Japan cultivated acreage accounts for 12.6% of the landmass . Of that, 54.5% is paddy fields. The Tonegawa River (catchment area, 16,840 km²) on the Ibaraki and Chiba Prefecture boundary and Lake Kasumigaura (Nishiura and lake area, 172 km²) are the tap-water sources for the southern part of Ibaraki and the northern part of Chiba Prefecture and the Tokyo metropolitan area; they are located 30-80 km apart to the west and the south of the currently operating reprocessing facilities. Ibaraki has the highest percentage of land under cultivation of any prefecture in Japan, and the Tonegawa River watershed, which contains Lake Kasumigaura, occupies more than 50% of the prefecture. In the Tonegawa River watershed in Ibaraki Prefecture, the rates of paddy field and upland field are 19.5% and 12.7%, respectively. The experimental field of National Institute for Agro-Environmental Sciences (NIAES) was in the Tonegawa River watershed. The ¹²⁷I concentration in ponding water during irrigation at the NIAES field was 11.8 µg L^-1, higher than that of precipitation, 3.0 µg L^-1, or that of irrigation water, 10.9 µg L^-1. The ¹²⁷I concentrations in soil water at depths of 0.2 m and 0.5 m during irrigation were 18.8 and 16.7 µg L^-1, respectively. The concentration of the outlet water from underdrainage at a depth of 1 m during irrigation was 2.4 µg L^-1. Much ¹²⁷I from a paddy field flows toward land surface water when large amounts of overflow water from ponding water or outlet water from underdrainage in shallow depth originate. The existence of rivers of which ¹²⁷I concentration during the irrigation period is clearly higher than that during non-irrigation period shows that much ¹²⁷I flows toward the land surface water from paddy field. The ¹²⁷I concentration in Lake Kasumigaura exceeded 10 µg L^-1; therefore, the ¹²⁷I concentration in irrigation water in the NIAES field was also high because it was taken from Lake Kasumigaura. As one of the reasons of high ¹²⁷I concentration in the lake water, the influence of the circulation use was thought. In other words, the irrigation water from Lake Kasumigaura is used for the paddy field, and the river water, which leads to high concentrations of ¹²⁷I in the paddy field, returns to Lake Kasumigaura. Moreover, the water table in the paddy field near Lake Kasumigaura was high even during the non-irrigation period. Therefore, the redox potential of the soil was low and the concentration of ¹²⁷I in the ponding water was high (15 µg L^-1). This seems to be another reason for the high concentration of ¹²⁷I in Lake Kasumigaura. Seeing in a mid/long term, it is thought that rough movement of ¹²⁹I that floats on the atmosphere, and descends with rain is similar to that of ¹²⁷I. Therefore, when the reprocessing facility is abnormally driven by any chance and ¹²⁹I is released in the environment, as described here, a paddy field promotes the accumulation of ¹²⁹I in Lake Kasumigaura. However, it is possible that the outflow control to land surface water from the paddy field could be managed if a malfunction were of a short duration. During normal operations of a reprocessing facility, when ¹²⁹I is barely detectable in the Tonegawa River watershed, it could be assumed that the amount of ¹²⁷I in the environment increases in a paddy field, and the buffer effect in ¹²⁹I pollution is improved. On the other hand, ¹²⁷I concentration in well water at a depth of 15 m under the paddy field at NIAES was higher, though it was a little, than those in an adjoining upland field and forest plot; therefore, the possibility that ¹²⁷I could leach into groundwater from the paddy field was indicated. There was groundwater where  ¹²⁷I concentration exceeded 10 µg L^-1 under the paddy field in the Tonegawa River watershed. On upland field and forest plot, the ¹²⁷I concentration in the groundwater was low, and rather it was necessary to think about the problem in the aspect of accumulation of iodine in the soil.