Fukushima-derived radiocesium fallout in Hawaiian soils

Abstract

Several reactors at the Fukushima Dai-ichi Nuclear Power Plant suffered damage on March 11, 2011, resulting in the release of radiocesium (134Cs and 137Cs), as well as other radionuclides, into the atmosphere. A week later, these isotopes were detected in aerosols over the state of Hawai'i and in milk samples analyzed on the island of Hawai'i. This study estimated the magnitude of cesium deposition in soil, collected in 2015–2016, resulting from atmospheric fallout. It also examined the patterns of cesium wet deposition with precipitation observed on O'ahu and the island of Hawai'i following the disaster. Fukushima-derived fallout was differentiated from historic nuclear weapons testing fallout by the presence of 134Cs and the assumption that the 134Cs to 137Cs ratio was 1:1. Detectable, Fukushima-derived 134Cs inventories ranged from 30 to 630 Bq m−2 and 137Cs inventories ranged from 20 to 2200 Bq m−2. Fukushima-derived cesium inventories in soils were related to precipitation gradients, particularly in areas where rainfall exceeded 200 mm between March 19 and April 4, 2011. This research confirmed and quantified the presence of Fukushima-derived fallout in the state of Hawai'i in amounts higher than predicted by models and observed in the United States mainland, however the activities detected were an order of magnitude lower than fallout associated with historic sources such as the nuclear weapons testing in the Pacific. In addition, this study showed that areas of highest cesium deposition do not overlap with densely populated or agriculturally used areas.