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.

Effects of the nuclear disaster on marine products in Fukushima: An update after five years.

Original data (134Cs and 137Cs, and sampling location) of marine products in Fukushima Prefecture monitored during 2011-2015 (n = 32,492) were analyzed to present an updated detailed description of radiocesium contamination after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident and to examine taxon/habitat-specific decreasing trends in different areas. Furthermore, marine species data presented by the Tokyo Electric Power Company (TEPCO) during 2012-2015 (n = 5458) were analyzed to evaluate the decreasing trends of 137Cs inside and outside (within a 20 km radius) of the FDNPP port. Monitoring results by Fukushima Prefecture show that percentages of samples higher than the Japanese regulatory limit of 100 Bq kg-1-wet (>RL%) were higher, whereas those below the detection limit (RL% and

Effects of the nuclear disaster on marine products in Fukushima.

After the release of huge amounts of radionuclides into the ocean from the devastated Fukushima Dai-ichi Nuclear Power Plant (FDNPP), safety concerns have arisen for marine products in Fukushima Prefecture. As of October 2012, we had inspected the radionuclide ((131)I, (134)Cs and (137)Cs) concentrations in 6462 specimens within 169 marine species collected off the coast of Fukushima Prefecture from April 2011. Only two species exceeded the Japanese provisional regulatory limit for (131)I (2000 Bq/kg-wet) immediately after the FDNPP accident. In 2011 and 2012, 63 and 41 species respectively exceeded the Japanese regulatory limit for radioactive Cs (100 Bq/kg-wet). The overall radioactive Cs concentrations of the total marine products have decreased significantly. However, the time-series trends of radioactive Cs concentrations have differed greatly among taxa, habitats (pelagic/demersal), and spatial distributions. Higher concentrations were observed in shallower waters south of the FDNPP. Radioactive Cs concentrations decreased quickly or were below detection limits in pelagic fishes and some invertebrates, and decreased constantly in seaweed, surf clams, and other organisms. However, in some coastal demersal fishes, the declining trend was much more gradual, and concentrations above the regulatory limit have been detected frequently, indicating continued uptake of radioactive Cs through the benthic food web. The main continuing source of radioactive Cs to the benthic food web is expected to be the radioactive Cs-containing detritus in sediment. Trial fishing operations for several selected species without radioactive Cs contamination were commenced in Soma area, 50 km north of the FDNPP, from June 2012. Long-term and careful monitoring of marine products in the waters off Fukushima Prefecture, especially around the FDNPP, is necessary to restart the coastal fishery reliably and to prevent harmful rumors in the future.