Environmental Radioactivity Research Network Center

First Assessment of the Downstream Impacts of Post-Fukushima Decontamination
– Temporary Increase in Sediment Discharge Mitigated by Reduced Radiocesium Concentrations and Vegetation Recovery –

The release of radioactive materials resulting from nuclear activities and nuclear accidents poses serious threats to both the environment and the economy. Following the accident at the Fukushima Daiichi Nuclear Power Plant, large quantities of radioactive materials containing radiocesium, which has a relatively long half-life, were released into the environment. Government-led decontamination successfully reduced radiation risks by removing radiocesium-contaminated surface soil. However, the long-term impacts of these decontamination activities on downstream environments had remained unclear.

Large-scale decontamination was carried out from 2012 in areas centered on the difficult-to-return zones of Fukushima Prefecture. During the decontamination process, approximately 5 cm of topsoil and vegetation were removed and replaced with uncontaminated soil, resulting in a temporary reduction in land cover. Over the following years, vegetation gradually recovered, restoring land cover in the decontaminated areas. Understanding how these dramatic land-cover changes affected sediment transport and radiocesium dynamics is essential for evaluating environmental sustainability.

In this study, the researchers comprehensively evaluated how land-cover changes caused by agricultural decontamination influenced suspended sediment and particulate ¹³⁷Cs transport in the Niida River catchment, Fukushima Prefecture (Figure 1), where large-scale farmland decontamination was conducted.

Maps showing the progression of decontaminated areas were created using official government decontamination records. Vegetation recovery was monitored using the Normalized Difference Vegetation Index (NDVI) derived from satellite imagery. To assess river responses, long-term field observations covering both the decontamination period (2013–2016) and the natural recovery period (2017–2018) were conducted. River discharge, turbidity (measured at 10-minute intervals), and particulate ¹³⁷Cs concentrations were continuously monitored at four observation sites, including two upstream and two downstream locations.

By integrating these long-term monitoring data with remote sensing analyses, the study systematically clarified how long-term land-cover changes in upstream decontaminated areas affected downstream river environments and the transport of ¹³⁷Cs to the Pacific Ocean.

Although reducing radiation exposure was the highest priority of the Fukushima decontamination program, the large-scale land modification required careful consideration of downstream environmental impacts. The study found that downstream impacts were relatively short-lived and limited because of both the substantial reduction in ¹³⁷Cs concentrations following decontamination and the rapid recovery of vegetation promoted by Fukushima’s humid climate. At the same time, the findings demonstrate that large-scale land modification does not necessarily minimize downstream impacts under all conditions. When similar remediation measures are considered for radioactive or other environmental contaminants in the future, it will be important to evaluate local vegetation recovery potential in advance and implement appropriate revegetation measures to minimize impacts on downstream environmental sustainability.

Paper Information

Title:Persistent impact of Fukushima decontamination on soil erosion and suspended sediment
Authors:Bin Feng, Yuichi Onda*, Yoshifumi Wakiyama, Keisuke Taniguchi, Asahi Hashimoto, Yupan Zhang
Journal:Nature Sustainability
Published:July 14, 2022
DOI:10.1038/s41893-022-00924-6

Related Links

• Press Release (PDF) (This link will open the University of Tsukuba website.)
  Principal Investigator　
  Yuichi Onda
  Professor
  Faculty of Life and Environmental Sciences / Center for Research in Isotopes and Environmental Dynamics (CRiED)
  University of Tsukuba