In the early 1980s, a smaller system was developed by the British Geological Survey for operation in small ships in nearshore waters 11. ![]() Since the late 1950s, systems have been developed in at least ten countries, with a main breakthrough, such as technological advance of the battery and downsizing of the electronic circuit, emerging in the 1970s 10. These conventional methods of sediment sampling are insufficient for understanding the distribution characteristics of 137Cs off the coast of Fukushima.Ī towed gamma-ray detection system is useful for determining the radiocesium distribution in surface sediment. These evaluation values are based on limited monitoring data, such as grab sampling and short core sampling data of several dozens. Moreover, radiocesium, which flows from rivers to seas, increases during flooding events, such as typhoons 9. Otosaka (2017) estimated that > 43% of 137Cs in the surface sediment around the FDNPP was transported to deeper sediment layers via vertical mixing of sediments from March 2011 to the end of 2015 8. There is a larger amount of deposited radiocesium in seabed sediment near river mouths than that in pelagic areas, and radiocesium is distributed deeply throughout the seafloor 7. Approximately 1–2% of the 137Cs in the sea (0.2 ± 0.05 PBq) was deposited onto the sediment 6. The total 137Cs released from the FDNPP was 15 to 18 PBq 5. Visualization of its distribution in coastal areas will provide information about the behavior mechanisms of environmental contaminants by interpreting radiocesium as an environmental tracer. Radiocesium in sediment should be monitored to evaluate the behavior of radiocesium in the environment and its effect on aquatic organisms. The concentration of radiocesium in the seawater has decreased since the accident ten years ago 5. Regarding radiocesium ( 134Cs and 137Cs), which has a relatively long half-life, certain national institutes are monitoring the seawater and bottom sediment off the coast of Fukushima to assess the radiocesium situation 4. The terrestrial deposition of these radionuclides has been verified through continuous radiation survey 2, 3. Large quantities of volatile radionuclides were released into the atmosphere and the hydrosphere following the March, 2011 accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP), which belongs to Tokyo Electric Power Company Holdings Inc. Finally, the 137Cs concentration decreased with time in consideration of such uncertainty. The map generated using the towed radiation survey depicted the 137Cs concentration distribution as the position resolution of a 1 km mesh. ![]() ![]() Validation of the created map showed that it was comparable with actual sediment core samples. The characteristics of the formation of 137Cs anomaly at the estuaries were analyzed using a contour map of 137Cs concentration combined with water depth. The characteristics of the radiocesium distribution were examined considering the seafloor topography and a geological map, which were obtained via acoustic wave survey. The uncertainty of the radiocesium map was evaluated via comparison with a large amount of sediment core sample data. In this study, the radiocesium distribution in the surface sediment around the FDNPP was visualized as a radiocesium concentration map using periodical survey data from a towed gamma-ray detection system. Monitoring of radiocesium in sediment is important for evaluating the behavior of radiocesium in the environment and its effect on aquatic organisms. Large quantities of volatile radionuclides were released into the atmosphere and the hydrosphere following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident on March, 2011.
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