Discovery of radiocesium-bearing microparticles in river water and their influence on the solid-water distribution coefficient (Kd) of radiocesium in the Kuchibuto River in Fukushima

We found four radiocesium-bearing microparticles (CsMPs) with high cesium (Cs) radioactivity in suspended particles collected from the Kuchibuto River in Fukushima by filtering water during 2011-2016. The CsMPs were identified by autoradiography and subsequently were separated from other suspended particles by the "multiple wet separation method" using a NaI scintillation counter. The present four CsMPs contained 0.426-2.827 Bq of Cs-137 and had a chemical composition similar to that of particles released during the accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP). The activity ratio of Cs (Cs-134/Cs-1(37)) suggests that they originated from Unit 2 or 3 of the FDNPP. The ratio of the radioactivity of the separated CsMPs to the total radiocesium on the filters ranged from 0 to 46%. Moreover, we calculated the radioactivities of CsMPs with lower radioactivity (0.1 to 0.4 Bq Cs-137) determined by autoradiography. When smaller CsMPs were included, the ratio of the radioactivity of the CsMPs to the total radioactivity of radiocesium on the filters ranged from 1.3 to 67%. It has been previously suggested that the solid-water distribution coefficient (K-d) of radiocesium in rivers is apparently increased due to the possible presence of CsMPs in the solid phase because the water solubility of radiocesium in CsMPs is small. However, this study reveals that higher K-d values in rivers in Fukushima compared with those in Chernobyl cannot be explained by the contribution of CsMPs alone. The temporal variation of the ratio of radioactivity of CsMPs to the total radioactivity of radiocesium in river water after the FDNPP accident is also discussed.