Incorporation into organically bound tritium and the underground distribution of HTO applied to a simulated rice field

In order to study the incorporation into organically bound tritium (OBT) of HTO deposited onto a paddy field during the growing season of rice, a solution of HTO was applied to the water surface of the culture box in a greenhouse 1 d before, and at 5 different times after, transplanting rice seedlings. The OBT concentration in the mature plant varied with the application time by factors of 5–25 depending on plant parts. Soil-to-plant transfer factors (m2 kg−1-dry) of HTO for OBT in straw and hulled seed were in the ranges 4.0×10−5 – 2.1×10−4 and 1.3×10−5 – 3.3×10−4, respectively. The highest was achieved at the application time when each part grew most actively. Mixing HTO with the topsoil tended to increase OBT production. The HTO concentration in percolating water reached its maximum 10–25 d after each application and then decreased slowly. The fractions of applied HTO that were leached out of the boxes were 8.0–28.4% depending on application times. One week after harvest, the depth profile of HTO in the top 30 cm of soil was investigated showing that less than 4% of applied HTO remained there. It was estimated that 70–90% of applied HTO escaped to the air via evapotranspiration.