Temporal variability of ¹³⁷Cs concentrations in coastal sediments off Fukushima

Large amounts of radiocesium were released into marine environments following the Fukushima Daiichi Nuclear Power Plant accident in March 2011. Released radiocesium influenced not only marine environment but also marine biota in Fukushima. Since marine biota as fisheries products is important for Japanese market, it is important to assess the distribution of radiocesium in coastal environment off Fukushima for safety concerns of radioactive contamination. Radiocesium concentrations in sediments are important for understanding fishing ground conditions and for proving the safety of fisheries products in Fukushima. In this study, monthly monitoring data collected from May 2011 to March 2020 were analyzed to describe the temporal variability of ¹³⁷Cs concentrations in coastal sediments off Fukushima (total of 3647 samples from eight lines at depths of 7-125 m off Fukushima, and three sites in Matsukawa-ura Lagoon). The ¹³⁷Cs concentration in sediment showed a decreasing trend, but our nonlinear model fitting suggested that this rate of decrease had slowed down. Additionally, ¹³⁷Cs concentrations were up to 4.08 times greater in shallow sampling sites (7, 10, 20 m depth) following heavy rainfall events (before five months vs. after five months), such as typhoons. These observations were consistent with increasing input from particulate ¹³⁷Cs fluxes from rivers and increasing dissolved ¹³⁷Cs concentrations in seawater. Finally, our numerical modeling suggested that riverine ¹³⁷Cs input could maintain ¹³⁷Cs concentrations in coastal sediment. These results indicate that riverine ¹³⁷Cs input following heavy rainfall events is the main factor for maintaining ¹³⁷Cs concentrations in coastal sediments near the Fukushima Daiichi Nuclear Power Plant.