Scientists from Japan’s National Institute of Radiological Sciences, Hirosaki University, and Peking University (pdf), May 2015 (emphasis added): Pu Distribution in Seawater in the Near Coastal Area off Fukushima… the amount of Pu isotopes directly released into the marine environment remains unknown. In the high level radioactive accumulated water collected at the FDNPP after the accident, high level radioactivities of Pu isotopes (ca. 10-3 Bq/mL) were detected. These values were 6 to 7 orders of magnitudes [1,000,000 – 10,000,000 times] higher than that of the seawater in the western North Pacific. In addition, a new study on Pu isotopes… suggested there was a potential sediment-borne Pu supply from Fukushima coastal rivers to the Pacific Ocean. Thus more attention should be paid to the contamination situation of Pu isotopes in the marine environment off Fukushima since the FDNPP accident… Pu isotopes in seawater… needs to be routinely investigated… There are two sampling sites close to the FDNP… 239+240Pu concentrations in seawater were reported in 2012-2014 and the range was from detection limit to 14 mBq/m3 except 31 mBq/m3 observed at T-2-1 site on 10 April 2014.
Scientists from Japan, Belgium, and French gov’t (pdf), 2015: Tracing the dispersion of contaminated sediment with plutonium isotope measurements in coastal catchments of Fukushima Prefecture — The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident led to important releases of radionuclides into the environment, and trace levels of plutonium (Pu) were detected in northeastern Japan… In this study, we measured Pu isotopic ratios in recently deposited sediments along rivers draining the most contaminated part of the inland radioactive plume… Results showed that the entire range of measured Pu isotopes (i.e. 239Pu, 240Pu, 241Pu, and 242Pu) were detected in all samples, although in extremely low concentrations. The 241Pu/239Pu atom ratios measured in sediment deposits (0.0017 – 0.0884) were significantly higher than the corresponding values attributed to the global fallout (0.00113 – 0.00008 on average in the Northern Hemisphere between 31-71 N)… These results demonstrate that this radionuclide has been transported relatively long distances… and deposited in rivers representing a potential source of Pu to the ocean.