Study of the behavior of radiocesium in soil-water environment after the Fukushima accident

  • The contaminated territory of Fukushima Prefecture is characterized by an expansive and differentiated hydrographic network. The river catchments contaminated from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident became a long-term source of secondary contamination of water bodies by surface runoff and radiocesium flux to the Ocean. Moreover, surface runoff and river transport results in the transfer of radiocesium from contaminated evacuated areas to cleaner populated regions, and the settling of radiocesium in bottom sediments of river reservoirs and on floodplains (mostly during rainy seasons).
    After the Chernobyl accident in April 1986 large-scale investigations of radionuclide fate and transport were carried out and methodologies for parameterization, assessment and prediction of secondary contamination in water bodies and their catchments were developed. These approaches can now be tested against data on radiocesium's behavior in the environment after the Fukushima Dai-ichi NPP accident and applied to assess and predict water contamination in the Fukushima area.
    Climate and geographical conditions for Fukushima Prefecture of Japan and Chernobyl zone differ. For example, the catchments of the Chernobyl zone are flat and characterized by low slopes while Fukushima's watersheds are hilly with steep slopes. Annual precipitation also differs substantially, with annual averages of 1105 mm for Fukushima and 483 mm at Chernobyl.

  • Therefore, the primary objective of our work in the Institute of Environmental Radioactivity of Fukushima University is a quantitative assessment of radiocesium's solid-liquid distributions and migration in soils and sediments on contaminated watersheds of Fukushima, and their comparative analysis with corresponding data from Chernobyl's 30-km zone.
    At present, we carry out our research in the following directions:
    - Collection and analysis of data on radiocesium solid-liquid distribution, wash-off from the catchments and river transport in contaminated areas after FDNPP accident;
    - Field sampling and observations on irrigation ponds/reservoir and their catchments including soil cores sampling, water and suspended sediments collection with subsequent laboratory processing, gamma-spectrometric and chemical analysis;
    - Field observations on the Niida river catchment and floodplain which include investigation of radiocesium vertical distribution in soils and sediments, investigation of sediment deposition on the floodplain using artificial-lawn mats, collection and analysis of suspended matter in the river channel.

  • These data are of interest from the standpoint of possible redistribution of radiocesium within the watersheds of small rivers such as the Niida during flooding from heavy rains, particularly typhoons, and accumulation of the radionuclide on the river floodplain, specifically, the lower reaches where initial contamination was not very high. The radiocesium activity concentration associated with suspended matter were the same as freshly deposited floodplain sediments where samples were collected and therefore it provides an indication of radiocesium concentration changes in the sediment load along the river. The data are also valuable for decision-making and planning remediation and recovery measures on the contaminated areas, among them, clean-up of the floodplain portions.