11:30 AM - 11:45 AM
[HCG24-10] Distribution of cesium-rich micro-particles in forest watershed at Fukushima
Keywords:FDNPP, radiocesium, radioactive particle, material transportation, watershed
The accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) in March 2011 released large amounts of radionuclides, including radioactive cesium, into the atmosphere and deposited them on land, including forests. Although forests have a lower priority for decontamination than residential areas and agricultural lands, radioactive materials may be transferred from forests to the living areas through rivers. Therefore, we have to monitor that transfer process. A CsMP (cesium-rich micro-particle) is one of the forms of radiocesium released into the environment. It is a fine particle composed mainly of SiO2 glass, with radiocesium incorporated inside. CsMPs have a higher radioactivity per unit mass than other forms of radiocesium. Hence, there are concerns about potential health risks, such as internal exposure, if people inhale CsMPs into their respiratory system. Although the chemical properties of CsMPs have been investigated in previous studies, there are few examples of investigations on the spatial distribution and dynamics of CsMPs in the environment.
In this study, we aimed to quantify CsMPs in forest ecosystems and their downstream areas and to clarify the distribution and transport characteristics of CsMPs. We investigated the following three, (1) topographical and geographic effects on the distribution of CsMPs on forest slopes, (2) vertical profile of CsMPs in forest soils, (3) migration of CsMPs through rivers.
The samples were collected from the Kamioguni River (and its tributaries) and its watershed in Date City, Fukushima, about 55 km northwest of the FDNPP, and from the Takase River and its watershed in Namie Town, Fukushima, about 8 km northwest of the FDNPP. Topsoil samples were collected from the surface layer 0-5 cm on a forest slope (10 x 10 m plot) upstream of each river. And there, 0-30 cm depth soil samples were collected. Also, about 10 L river water samples were collected during normal water conditions at points upstream, midstream, and downstream of each river. Sampling was conducted in the Kamioguni River watershed on August 12 and 13, 2021, and in the Takase River watershed on September 5 and 6, 2021. Soil samples were dried and then sieved through a 2 mm mesh sieve to remove gravel. River water samples were filtered to obtain suspended solids. Autoradiographic analysis was performed on soil and suspended solid to detect CsMPs and to quantify the radioactivity of CsMPs. Note that a radioactive particle with more than 0.01 Bq was defined as a CsMP in this study. In addition, the radiocesium radioactivities of bulk samples were determined using γ-ray spectrometry, and the measurement results were used for the quantification of CsMPs.
As a result of the surface soil study, there was a larger number of CsMPs upstream of Takase River than upstream of Kamioguni River. However, the RF value, the contribution of CsMP-derived radioactivity to the total radiocesium radioactivity, was about 1 % at each soil sample, and there were no geographical differences. Comparison with previous studies about the CsMP distribution implies CsMPs could flow out relatively more than other radioactive particles.
There was no significant difference in the vertical distribution of CsMPs and total radiocesium, indicating that there is no significant difference in the mobility during vertical transport between CsMPs and other forms of radiocesium. Comparison between the upstream of the Kamioguni River and the Takase River showed differences in the vertical distribution of both CsMPs and other forms of radiocesium, which may be caused by differences in physical properties such as the pore structure of the soil.
Several CsMPs were detected in suspended solid from the midstream of Takase River, suggesting that CsMPs are transferred through rivers even during normal water conditions in the Takase River watershed where CsMPs are abundant upstream. However, no CsMPs were detected in suspended solid from other survey points.
In this study, we aimed to quantify CsMPs in forest ecosystems and their downstream areas and to clarify the distribution and transport characteristics of CsMPs. We investigated the following three, (1) topographical and geographic effects on the distribution of CsMPs on forest slopes, (2) vertical profile of CsMPs in forest soils, (3) migration of CsMPs through rivers.
The samples were collected from the Kamioguni River (and its tributaries) and its watershed in Date City, Fukushima, about 55 km northwest of the FDNPP, and from the Takase River and its watershed in Namie Town, Fukushima, about 8 km northwest of the FDNPP. Topsoil samples were collected from the surface layer 0-5 cm on a forest slope (10 x 10 m plot) upstream of each river. And there, 0-30 cm depth soil samples were collected. Also, about 10 L river water samples were collected during normal water conditions at points upstream, midstream, and downstream of each river. Sampling was conducted in the Kamioguni River watershed on August 12 and 13, 2021, and in the Takase River watershed on September 5 and 6, 2021. Soil samples were dried and then sieved through a 2 mm mesh sieve to remove gravel. River water samples were filtered to obtain suspended solids. Autoradiographic analysis was performed on soil and suspended solid to detect CsMPs and to quantify the radioactivity of CsMPs. Note that a radioactive particle with more than 0.01 Bq was defined as a CsMP in this study. In addition, the radiocesium radioactivities of bulk samples were determined using γ-ray spectrometry, and the measurement results were used for the quantification of CsMPs.
As a result of the surface soil study, there was a larger number of CsMPs upstream of Takase River than upstream of Kamioguni River. However, the RF value, the contribution of CsMP-derived radioactivity to the total radiocesium radioactivity, was about 1 % at each soil sample, and there were no geographical differences. Comparison with previous studies about the CsMP distribution implies CsMPs could flow out relatively more than other radioactive particles.
There was no significant difference in the vertical distribution of CsMPs and total radiocesium, indicating that there is no significant difference in the mobility during vertical transport between CsMPs and other forms of radiocesium. Comparison between the upstream of the Kamioguni River and the Takase River showed differences in the vertical distribution of both CsMPs and other forms of radiocesium, which may be caused by differences in physical properties such as the pore structure of the soil.
Several CsMPs were detected in suspended solid from the midstream of Takase River, suggesting that CsMPs are transferred through rivers even during normal water conditions in the Takase River watershed where CsMPs are abundant upstream. However, no CsMPs were detected in suspended solid from other survey points.