Radioactivity Dynamics in River System
ＱWill heavy rain cause the runoff of deposited cesium? 【Radioactive cesium migration caused by torrential rain in October 2019: Middle to lower river basin】
ＡWe studied air dose rates and soil deposition situation observed before and after the Kanto-Tohoku Torrential Rain in September 2015 and the East Japan Typhoon in October 2019 (Typhoon No. 19).
As a result, we found that dose rates decreased at all sites surveyed due to sand deposition. This is presumably because soils with low cesium concentrations were washed away from river channel walls were deposited and shielded the ground, leading to lower air dose rates.
(1) Kamimano River (Koike temporary storage site): Air dose rate and soil deposition
- Although some areas in the upper basin of the Kamimano River have higher dose rates than the area around the temporary storage site, the estimated origin of most of the soil deposited near the temporary storage site is the small river.
→ The origin of gravel is also the small river. There is little soil deposition on the upstream side of the junction with the small river.
- The small river basin from which the deposited soil came is mostly forests.
Air dose rates on the dry riverbed of (2) Ota River and (3) Odaka River
After the typhoon, air dose rates significantly decreased, reducing the area with 0.4 μSv/h or above (green to orange area).
●Origin and characteristics of the deposited soil
- Air dose rates decreased in areas of soil deposition because the deposition of soils with low cesium concentrations shielded the original ground surface.
- Why soils with low cesium concentrations are deposited.
- The deposited sand is not the surface soil of forests but decomposed granite soil, which is relatively large in grain size and resembles beach sand.
- Decomposed granite soil is mainly washed away from places vulnerable to erosion, such as gullies, which work as channels in forests.
Note: A gully is a deep ditch that is formed through the scouring of running water.
- Gullies are more vulnerable to erosion during flooding because deeper soils are exposed.
- Deeper soils contain little cesium.
- It is assumed that future soil runoff associated with heavy rain will consist mainly of deeper soils that contain little cesium.
- Will heavy rain cause the runoff of deposited cesium? 【Radioactive cesium migration caused by torrential rain in October 2019: Upper river basin】
- Do dose rates in riverbeds increase after heavy rains? 【Changes of air dose rate in heavy rain】
- Does the concentration of radioactive cesium in river water increase after rainfall? 【Changes of the concentration of 137Cs in heavy rain】
- How much amount of radiocesium discharges into the ocean?
- Dose the radiocesium discharge ratio from rivers depend on the local environment?