Speciation and leachability of radiocesium in the soil, sediment, and aquatic organisms surrounding Lake Onuma at Mt. Akagi

The 2011 accident at the Fukushima Daiichi Nuclear Power Plant resulted in the widespread release of radioactive cesium (radiocesium, including ¹³⁴Cs and ¹³⁷Cs) across Fukushima Prefecture and its neighboring regions. This study investigates temporal changes in radiocesium, focusing mainly on ¹³⁷Cs, concentrations and their speciation using a sequential extraction procedure in Lake Onuma, an isolated mountain lake, from 2011 to 2019. The ¹³⁷Cs concentrations in various environmental samples decreased markedly until 2014 and then reduced gradually after 2015. This trend may be attributable to the relatively long water residence time (∼2 years) in the lake, with limited upstream inflow and downstream discharge. The sequential extraction procedure revealed that insoluble forms of ¹³⁷Cs in lake sediment and the surrounding soils decreased more slowly than soluble forms, which predominated in phytoplankton (PP), zooplankton (ZP), and wakasagi smelt (Hypomesus nipponensis). Throughout the nine-year study period, ¹³⁷Cs concentrations in plankton exhibited the following order: PP (1850–390 Bq kg⁻¹) > ZP (395–69 Bq kg⁻¹), contrasting expectations regarding bioaccumulation patterns within the food chain, suggesting that the insoluble ¹³⁷Cs species in PP may originate from mineral-rich suspended solids near the lake sediment surface. These findings highlight the importance of continued monitoring and contribute to a deeper understanding of the long-term ecological impacts associated with the Fukushima Daiichi Nuclear Power Plant accident.