The impact of radioactive cesium-bearing microparticles on Cs transfer factor of brown rice on the paddy field in the first year of resumption of farming

Radioactive cesium-bearing microparticles (CsMPs) were insoluble glassy matrixes derived from Fukushima Daiichi Nuclear Power Plant accident. CsMPs have greater radioactive cesium (Cs) concentration per unit mass than other particles adsorbing Cs such as clay and organic particles (i.e., particulate Cs). In addition, the size of CsMPs was about a few µm. CsMPs may move around the environment as in the case of particulate Cs. Therefore, there is a concern for an internal exposure when CsMPs may enter the living organisms. Furthermore, previous studies have shown that CsMPs may attach to soil and crops, and locally increase Cs concentration of them. On the other hand, the difference of particulate Cs and CsMPs is Cs desorption from those particles. For particulate Cs, some Cs may electrically adsorb onto clay planar site and functional groups, therefore, their Cs may be exchanged with other cations, desorbed, and adsorbed by the crop through the roots. However, Cs in the CsMPs is included in in soluble grassy matrixes. In other words, it is considered that Cs in the CsMPs may be difficult for crops to absorb. Transfer factor (TF) is used to evaluate transfer of Cs from soils to crops. However, the conventional TF is a simple ratio of Cs concentrations between soils and crops. This suggests that conventional TF may not be able to accurately assess Cs transfer from soils to crops, since Cs in the soils contains a mixture of Cs with different crop availability. Since there are many contaminated forest areas in Fukushim, it is necessary to evaluate secondary contamination due to the inflow of Cs and CsMPs from these areas into farmland after decontamination.

In this study, we investigated the changes of Cs and CsMPs deposition in the paddy field in the first year of resumption of farming after the accident. Furthermore, we evaluate the contribution of CsMPs on particulate Cs to grasp the impact of CsMPs on TF of brown rice.

As a result, the accumulation of Cs and CsMPs increased in the entire field of this study in the period from plowing to harvest. The deposition of Cs per unit area increased in the center and the four corners of the paddy field other than inlet and outlet. This may be due to the deposition of suspended matter in areas with relatively litte water movement. On the other hand, there was no clear trend for CsMPs deposition in the paddy filed. This is because CsMPs were only detected in some of the soil samples, and the amounts varied depending on the soil. The Cs concentration in brown rice was less than 100 Bq kg^-1-FW in all samples. This indicated that safe rice could be produced on this study site after decontamination. Furthermore, there was no significant difference between TF values with and without the contribution of CsMPs to Cs concentration in the soil. This suggested that there was no problem to evaluate TF using conventional methods for the paddy filed in Fukushima Prefecture.