Eight-year variations in atmospheric radiocesium in Fukushima city

After the Fukushima nuclear accident, atmospheric Cs-134 and Cs-137 measurements were taken in Fukushima city for 8 years, from March 2011 to March 2019. The airborne surface concentrations and deposition of radiocesium (radio-Cs) were high in winter and low in summer; these trends are the opposite of those observed in a contaminated forest area. The effective half-lives of 137Cs in the concentrations and deposition before 2015 (0.754 and 1.30 years, respectively) were significantly shorter than those after 2015 (2.07 and 4.69 years, respectively), which was likely because the dissolved radio-Cs was discharged from the local terrestrial ecosystems more rapidly than the particulate radio-Cs. In fact, the dissolved fractions of precipitation were larger than the particulate fractions before 2015, but the particulate fractions were larger after 2016. X-ray fluorescence analysis suggested that biotite may have played a key role in the environmental behavior of particulate forms of radio-Cs after 2014. However, the causal relationship between the seasonal variations in particle size distributions and the possible sources of particles is not yet fully understood. The current study also proposes an evaluation method of the consistency of a numerical model for radio-Cs resuspension and suggests that improvements to the model are necessary.

Automated summary

After the Fukushima nuclear accident, atmospheric measurements of Cs-134 and Cs-137 were conducted in Fukushima city from March 2011 to March 2019. The study found that airborne surface concentrations and deposition of radiocesium were higher in winter and lower in summer, opposite to the trends observed in a contaminated forest area. The effective half-lives of 137Cs before 2015 were significantly shorter than those after 2015, possibly due to the faster discharge of dissolved radio-Cs from local ecosystems compared to particulate radio-Cs. Biotite was suggested to play a key role in the behavior of particulate radio-Cs after 2014. However, the relationship between seasonal variations in particle size distributions and the sources of particles is not fully understood.