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.

Desorption Behavior of Fukushima-derived Radiocesium in Sand Collected from Yotsukura Beach in Fukushima Prefecture.

Beach sand samples were collected along a coastal area 32 km south of the Fukushima Daiichi Nuclear Power Plant (FDNPP) in Fukushima Prefecture, Japan, 5 years after the FDNPP accident. Desorption experiments were performed on the sand samples using seawater in a batch experimental system to understand the forms of existence of radiocesium in sand and their desorption behavior in a coastal environment. The percentage of radiocesium desorption decreased exponentially with an increase in the number of desorption experiments for the four sand samples, with 137Cs radioactivity from 16 to 1077 Bq kg-1 at surface and deeper layers from three sites. Total desorption percentage ranged from 19 to 58% in 12 desorption experiments. The results indicate that the weak adsorption varies with the sampling sites and their depth layer. To understand the desorption behavior of radiocesium in the sand samples, the desorption experiments were performed for a sand sample by using natural and artificial seawater, and NaCl solution in the presence and absence of KCl. The 137Cs desorption from the sand collected at a depth of 100 - 105 cm from the ground surface (137Cs radioactivity 1052 ± 25 Bq kg-1) was 0.1% by ultrapure water, 3.7% by 1/4 seawater and 7.1% by 1/2 seawater, 2.2% by 470 mM NaCl solution (corresponding to a similar concentration of seawater) and 10 - 12% by seawater, artificial seawater and 470 mM NaCl + 8 mM KCl solution. These results indicate that about 10% of radiocesium adsorbed on the sand is mainly desorbed by ion exchange of potassium ion in seawater, though the concentration of major cation, or sodium ion, in seawater makes a small contribution on 137Cs desorption from the sand samples.