Temporal Variation and Source Analysis of Radiocesium in an Urban River after the 2011 Nuclear Accident in Fukushima, Japan

抄録

The release of radiocesium from the Fukushima Dai-ichi Nuclear Power Plant caused environmental contamination. We analyzed sources, behavior, and temporal trends of radiocesium by 1-year monitoring in an urban river, the Ohori River. The concentrations of ¹³⁷Cs in both particulate and dissolved phases decreased to ~ 16% within 1 year. The partition coefficient in dry weather was estimated to be 3.2 × 10⁴ L/kg. ¹³⁷Cs concentrations in the particulate phase were higher during wet weather than during dry weather on the basis of both suspended solids (SS) weight and liquid volume. The ratios of ¹³⁷Cs concentration to deposited ¹³⁷Cs in the Ohori River (e.g., composite: 0.26 m²/kg-SS in June - July 2012) were higher than those in the Abukuma River, probably because of differences in land use. Source analysis by a chemical mass balance method showed that 22% of ¹³⁷Cs came from river water in dry weather, 39% from river sediments, and 39% from road dust highlighting urban surface deposits such as road dust as major sources of ¹³⁷Cs in the river water even 2 years after the accident, and that the wash-off of these deposits increased the ¹³⁷Cs concentration in the river during wet weather.