Fukushima Decontamination Affects Downstream Sediment and Cs-137 Discharge into the Ocean - 23300

Government-led decontamination has played an important role in reducing radiation risks and recovering Cs-137 contaminated soil in Fukushima, but its long-term downstream impacts remain unknown. Here we combine government decontamination data, high-resolution satellite imagery, and contemporaneous river monitoring results to provide the first comprehensive assessment of the impact of decontamination from 2013 to 2018. The results show that erosivity intensified during the decontamination period from 2013 to 2016, but began to decrease during the subsequent revegetation period. River suspended sediment (SS) dynamics responded well to this change in erodibility, with SS loading during the 2016 one-year flood increasing by approximately 237% compared to 2013. Sediment contributions from decontaminated areas increased gradually with decontamination (up to 75.7%), causing a rapid decrease in particulate Cs-137 concentrations. After decontamination, sediment yield and sediment contribution from the decontaminated area decreased due to rapid vegetation recovery, but downstream normalized Cs-137 fluxes did not change significantly. These results highlight the unsustainable downstream effects of upstream decontamination and emphasize the important role of vegetation restoration in regulating the persistence of these effects. (authors)