Peculiarities of behavior in “soil–water” environment of radiocesium in contaminated area after the accident at Fukushima Dai-ichi NPP

Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in March 2011 led to extensive environmental contamination by several radionuclides, particularly 134Cs (half-life Т1/2=2.06 years) and 137Cs (Т1/2=30.17 years). This has rekindled the interest in the behavior of radiocesium in the environment, particularly given the geoclimatic conditions of Japan. This review paper summarizes key findings of post-Fukushima studies of radiocesium fate and transport in soil-water environment and challenges for the future. The studies after the Fukushima accident have clearly demonstrated that behaviour of accidentally released radiocesium in the environment is governed by speciation in fallout and site-specific environmental characteristics. The Fukushima-derived 137Cs is found to be strongly bound to soil and sediment particles, which reduces the potential bioavailability of this radionuclide. Up to 80% of the deposited 137Cs on the soil of the contaminated area were reported to be incorporated in hot glassy microparticles (CsMPs) insoluble in water. These particles decompose in the environment very slowly, and long-term radiocesium leaching from these particles is a challenge for future studies. In Fukushima contaminated areas the high annual precipitation and steep slopes are conducive to significant erosion and intensive r-Cs wash-off especially during devastating typhoons. Typhoons Etou in 2015 and Hagibis in 2019 demonstrated a pronounced redistribution of 137Cs on river watersheds and floodplains, and natural self-decontamination occurred in some cases. Understanding mechanisms and prediction of radiocesium long-term dynamics and seasonality in water bodies, as well as its remobilization from river-transported sediments at the interface between freshwater and marine water in estuaries is important challenge for contemporary radioecology.