17:15 〜 18:45
[AGE28-P03] 森林土壌における斜面および鉛直方向の高濃度放射性セシウム含有微粒子の分布
キーワード：福島、セシウム137、高濃度放射性セシウム含有微粒子
Introduction
Large amounts of radionuclides were released into the environment due to the accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) in March 2011. CsMPs (radioactive cesium-rich microparticles) are one of the physicochemical forms of radiocesium (Cs) released due to the FDNPP accident. Because CsMPs have high Cs concentration and insoluble glassy material, Cs in CsMPs can remain in the environment for a relatively long time. Previous studies have shown that CsMPs moves through the atmosphere and rivers, and they locally increase Cs concentrations in river water, soils, insects, and crops. It is one of the important issues for reconstruction to understand the migration and effects of CsMPs after the FDNPP accident. Since forest areas are difficult to fully decontaminate, a lot of radionuclides may remain. Furthermore, there is little information about the distribution of CsMPs in forest areas. In this study, we investigated the distribution of CsMPs in the slope and vertical directions of forest soil in Fukushima Prefecture in order to acquire fundamental knowledge for the CsMPs discharge from forests.
Samples and Methods
1. Collecting of soil surface samples in the slope direction
Soil samples were collected from two forest catchment, Site A and Site B (about 11km and 55km northwest of FDNPP, respectively). Site A is located in the difficult-to-return zone. Litter sample and soil surface sample with depth of 0-5 cm were collected at appropriate intervals based on topography on transects established from the valley bottom to the ridge. Cs concentration (137Cs and 134Cs) in the sample was measured using a germanium semiconductor detector. CsMPs were identified using the autoradiography method based on previous study.
2. Collecting of soil column samples
Litter and soil column samples with depth of 0-30cm were collected from 5 study sites. The sampling sites were selected from locations near the FDNPP and to the northwest. After taking the samples back to the laboratory, the soil column sample was cut into pieces at the predetermined depths. Cs concentration and CsMPs in the samples were measured in the same method as the samples of slope direction survey.
Results and Disucussions
1. CsMPs distribution in the slope direction
137Cs concentrations per unit area in the stream bed was lower than in upper slopes. This result indicated that 137Cs adsorbed on the soil near the stream might be discharged with the sediment. CsMPs showed a similar topographic distribution pattern as 137Cs. These results suggested that CsMPs and the other forms of 137Cs had similar behavior on forest slopes. The average proportion of Cs derived from CsMPs to Cs in the soil of each watershed was less than 2%. This result suggested that CsMPs have no significant effect on air dose rates and Cs concentration of the soil in forest slopes. However, there was a large variation in the contribution ratio at the relatively low contaminated catchment (i.e., Site B), and CsMPs could be locally significant radiation sources there.
2. CsMPs distribution in the vertical direction
CsMPs were mostly distributed in the surface layer of soil. Similar to the distribution of 137Cs in the depth direction reported in previous studies, the concentration of Cs derived from CsMPs in the soil decreased exponentially in the depth direction. There was no significant difference in the centroid of concentration in the depth direction between 137Cs and Cs derived from CsMPs at the four study sites. In the remaining one study site, the centroid of Cs concentration derived from CsMPs was located closer to the soil surface layer than 137Cs. These results indicate that suggest that CsMPs is located in the surface layer of forest soil and may be easily moved due to the effects of soil erosion.
This work was supported by the following funds: (1) JSPS Grant-in-Aid for Scientific Research (20H00435), (2) The Nippon Life Foundation, and (3) FY2022 Sumitomo Foundation.
Large amounts of radionuclides were released into the environment due to the accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) in March 2011. CsMPs (radioactive cesium-rich microparticles) are one of the physicochemical forms of radiocesium (Cs) released due to the FDNPP accident. Because CsMPs have high Cs concentration and insoluble glassy material, Cs in CsMPs can remain in the environment for a relatively long time. Previous studies have shown that CsMPs moves through the atmosphere and rivers, and they locally increase Cs concentrations in river water, soils, insects, and crops. It is one of the important issues for reconstruction to understand the migration and effects of CsMPs after the FDNPP accident. Since forest areas are difficult to fully decontaminate, a lot of radionuclides may remain. Furthermore, there is little information about the distribution of CsMPs in forest areas. In this study, we investigated the distribution of CsMPs in the slope and vertical directions of forest soil in Fukushima Prefecture in order to acquire fundamental knowledge for the CsMPs discharge from forests.
Samples and Methods
1. Collecting of soil surface samples in the slope direction
Soil samples were collected from two forest catchment, Site A and Site B (about 11km and 55km northwest of FDNPP, respectively). Site A is located in the difficult-to-return zone. Litter sample and soil surface sample with depth of 0-5 cm were collected at appropriate intervals based on topography on transects established from the valley bottom to the ridge. Cs concentration (137Cs and 134Cs) in the sample was measured using a germanium semiconductor detector. CsMPs were identified using the autoradiography method based on previous study.
2. Collecting of soil column samples
Litter and soil column samples with depth of 0-30cm were collected from 5 study sites. The sampling sites were selected from locations near the FDNPP and to the northwest. After taking the samples back to the laboratory, the soil column sample was cut into pieces at the predetermined depths. Cs concentration and CsMPs in the samples were measured in the same method as the samples of slope direction survey.
Results and Disucussions
1. CsMPs distribution in the slope direction
137Cs concentrations per unit area in the stream bed was lower than in upper slopes. This result indicated that 137Cs adsorbed on the soil near the stream might be discharged with the sediment. CsMPs showed a similar topographic distribution pattern as 137Cs. These results suggested that CsMPs and the other forms of 137Cs had similar behavior on forest slopes. The average proportion of Cs derived from CsMPs to Cs in the soil of each watershed was less than 2%. This result suggested that CsMPs have no significant effect on air dose rates and Cs concentration of the soil in forest slopes. However, there was a large variation in the contribution ratio at the relatively low contaminated catchment (i.e., Site B), and CsMPs could be locally significant radiation sources there.
2. CsMPs distribution in the vertical direction
CsMPs were mostly distributed in the surface layer of soil. Similar to the distribution of 137Cs in the depth direction reported in previous studies, the concentration of Cs derived from CsMPs in the soil decreased exponentially in the depth direction. There was no significant difference in the centroid of concentration in the depth direction between 137Cs and Cs derived from CsMPs at the four study sites. In the remaining one study site, the centroid of Cs concentration derived from CsMPs was located closer to the soil surface layer than 137Cs. These results indicate that suggest that CsMPs is located in the surface layer of forest soil and may be easily moved due to the effects of soil erosion.
This work was supported by the following funds: (1) JSPS Grant-in-Aid for Scientific Research (20H00435), (2) The Nippon Life Foundation, and (3) FY2022 Sumitomo Foundation.