Global Fallout Technetium-99 Distribution and Behavior in Japanese Soils

Abstract

In this study, concentrations of global fallout 99Tc in twenty-one surface soil samples collected in Japan were determined by inductively coupled plasma mass spectrometry. The range of 99Tc concentrations in rice paddy field, upland field and other soils were determined as 6 - 88, 4 - 8, and 7 - 29 mBq/kg-dry, respectively. The highest average activity ratios of99Tc/137Cs were found in paddy field soils followed by upland field and other soils. The activity ratios in paddy field samples were one order of magnitude higher than the theoretical one estimated from nuclear fission yield, which was presently calculated as 3.3 × 10-4. Not only the activity ratio in deposition samples, but also mechanisms of 99Tc accumulation in paddy field soils from irrigation of surface water, can explain the high 99Tc/137Cs activity ratios in soils. The results of vertical distributions of 99Tc and 137Cs in undisturbed soils showed that the migration rate of 99Tc would be slightly faster than that of 137Cs, but most of 99Tc would still be retained in the 0 - 20 cm layer from the surface.