Environmental Transfer of Carbon-14 in Japanese Paddy Fields

Abstract

It has been recognized that carbon-14 (14C) is one of the dominant radionuclides affecting dose from transuranic (TRU) wastes. This radionuclide has a decay half-life of 5,730 years, and 14C organic materials have very low sorption properties to clay and rock in the environment, which raises some concerns about the releases of 14C to the biosphere from radioactive waste repositories. For the safety assessment of TRU waste disposal, we studied the behavior of 14C in rice paddy field soils. We also determined key parameters such as soil–soil solution distribution coefficients (K ds) and soil-to-rice plant transfer factors (TFs) of 14C in the field soils. The TFs were obtained in laboratory and field experiments. In our laboratory experiments, we used [1,2-14C] sodium acetate as a source of 14C because it has been suggested that low molecular weight organic-14C compounds are released from metallic TRU wastes. The results showed that 14C-bearing sodium acetate in irrigated paddy soils was rapidly decomposed by indigenous bacteria. Although some of the 14C was assimilated into the bacterial cells, most of the 14C was released into the air as gaseous compounds. The main chemical species of 14C gases was 14CO2, and a part of the released 14CO2 gas was used by rice plants during photosynthesis. Only a negligible amount of 14C was absorbed through the roots. Therefore, the contamination of rice plants is mainly caused by gasification of 14C, and microorganisms are responsible for driving this process. The activity of microorganisms is a key issue in the behavior of 14C in paddy fields.