Temporal variation in radiocesium concentrations in waters of the Natsui and Same Rivers, South Fukushima Prefecture, Japan from 2011 to 2020

In order to determine the long-term impacts of radiocesium contamination on ecosystems in watersheds and coastal areas following the Fukushima Daiichi Nuclear Power Plant accident in March 2011, it was important to monitor the transport behavior of radiocesium from an early stage. In this study, conducted from July 2011 to October 2020, we carried out field research along the Natsui and Same rivers running through watersheds in the south of Fukushima Prefecture, Japan, in which there had been a relatively low accumulation of radiocesium. We found that under normal flow conditions, the total (dissolved + particulate phase) activity of ¹³⁷Cs decreased with increasing time following the accident. However, the water samples collected after rain events showed higher activity of up to 895 mBq l^-1 with a higher percentage (>92% of total) of the particulate phase. These findings indicate that radiocesium deposited on the ground surface is predominantly transported in the particulate phase from watersheds to rivers via precipitation. The decontamination process, which was performed in the farmland during December 2014 to March 2015 and in forest during February 2013 to April 2014, was small effects of the transport of ¹³⁷Cs. Under normal flow conditions, total ¹³⁷Cs activity was largely determined by the suspended solids (SS) concentration and/or ¹³⁷Cs concentration in the SS. (authors)