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.