Source evaluation of 137Cs in foodstuffs based on trace 134Cs radioactivity measurements following the Fukushima nuclear accident.

We performed gamma-ray analysis to determine the amount of radioactive cesium-134 (134Cs) and cesium-137 (137Cs) in 259 foodstuffs five years after the Fukushima nuclear accident of 2011. Using measurements of trace 134Cs radioactivity, we investigated the contribution ratio of 137Cs derived from the Fukushima accident on 2011 and pre-Fukushima. The median detected concentration of radiocesium (134Cs + 137Cs) in foodstuffs was 0.33 Bq/kg-raw, a much lower radioactivity than the Japanese regulatory limit. However, a few samples had particularly high radioactivity, including some dried mushrooms sold in Iwate Prefecture that had a 137Cs radioactivity concentration as high as 441 Bq/kg. Our analysis showed that 75.5% of the 137Cs detected in these mushrooms originated from the Fukushima accident, and 24.5% was originated before the Fukushima event. Our study clarified the 137Cs contamination in 75 of all 259 food samples before and after the Fukushima nuclear accident, showing that not only mushrooms but also fish had been contaminated before the Fukushima accident.