Differentiating Fukushima and Nagasaki sourced plutonium from global fallout: Pu vs Cs in soils and biota

The release of plutonium (Pu) from the 2011 Fukushima accident has raised questions on how prevalent it is in the environment and how its cycling into the biosphere compares with that from the previous Nagasaki and global-fallout sources. Here, we report on systematic sampling and analysis of soils, earthworms, and wild boar as markers of Pu in the deposition areas near the Fukushima Daiichi Nuclear Power Station (FDNPS) and Nagasaki. Highly-sensitive Accelerator Mass Spectrometry (AMS) methods were used to distinguish the Pu sourced from the FDNPS accident, and Nagasaki-detonation, from worldwide fallout Pu. We primarily used 241Pu/239Pu atom ratios, as the other typically-used Pu measures (240Pu/239Pu atom ratios, activity concentrations) were less sensitive and did not distinguish the FDNPS Pu from background in most study samples. Near the FDNPS, results indicate that five years after the accident, 0.4% – 2% of the Pu in the local soils (0-5cm) had originated from the FDNPS releases, the remainder being from global fallout. The trace amounts of FDNPS Pu (e.g., 0.02-0.04 Bq kg-1239Pu estimated in local ~3km deposition) contrasted sharply with the 134+137Cs content which was about 106 times greater than background in the same samples. The accident also contributed new Pu of ~0.3% – 3% in earthworms and ~1% – 10% in wild boar near the FDNPS. The soil and wild boar data from across the study sites consistently indicate only low levels of new accident-Pu and do not support the concept of a substantial undiscovered deposit of Pu near the FDNPS. Unlike sparsely-taken individual soil samples that might miss a Pu hotspot, the wild boar samples represent the integration of uptake throughout their entire foraging areas. Near Nagasaki, our measurements in 2016 show a lasting legacy of Pu sourced from the 1945 detonation (~93% soils, ~88% earthworm, ~96% boar in samples <5km from the Nagasaki hypocentre; the remainder from global fallout). Even with these high percentages arising from the 1945 detonation, the Pu amounts at all study sites in Japan are comparable to background fallout levels elsewhere and are orders of magnitude lower than what remains near Chernobyl. At the study areas, the dose rates from Pu to organisms, as well as to potential human consumers of wild boar meat, have been only slightly elevated above background and are orders of magnitude lower than the dose potentials from the 134,137Cs in samples from near the FDNPS. The results demonstrate progress in increasing the sensitivity of AMS methods, including the use of 241Pu/239Pu atom ratios, to compare recent and past nuclear contamination events and suggest that the Nagasaki-detonation Pu will be distinguishable in the environment long after the FDNPP-accident Pu is not.