Inferring the chemical form of 137Cs deposited by the Fukushima Dai-ichi Nuclear Power Plant accident by measuring 137Cs incorporated into needle leaves and male cones of Japanese cedar trees

Abstract

We hypothesized that the water-soluble (ionic) and water-insoluble (stable) radiocesium from the initial fallout of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident was distributed in various proportions in the surrounding areas and that this distribution was reflected in the trees that suffered deposition from the initial fallout. This study attempted to evaluate local variations in the chemical form of 137Cs derived from the initial fallout of the FDNPP accident and whether its chemical form affected the radiocesium concentration in the tissues currently growing in trees, even after the initial fallout ceased. For these estimations, the ratio between the 137Cs concentration in Cryptomeria japonica needle leaves in the tree crown, which existed before the FDNPP accident and subsequently directly exposed to the initial fallout (137Cspre-accident N), and the amount of 137Cs in the initial fallout itself (137Csfallout) was determined (137Cspre-accident N/137Csfallout) at 66 sites. In addition, the 137Cs ratios between the male cones produced in 2012 (137Csmale cone) and needle leaves that had elongated in the spring of 2011 (137Cs2011N) was determined at 82 sites (137Csmale cone/137 Cs2011N). Most of the sites with lower 137Cspre-accident N/137Csfallout ratios were distributed in eastern Fukushima, relatively close to the Pacific Ocean coastline. Lower 137Cspre-accidentN/137Csfallout and higher 137Csmalecone/137Cs2011N were found to be associated with higher proportions of 137Cs in ionic forms. These observations are consistent with the hypothesis, and likely reflect regional variations in the chemical form of the deposited radiocesium.