Cesium-137 and 137Cs/133Cs atom ratios in marine zooplankton off the east coast of Japan during 2012-2020 following the Fukushima Dai-ichi nuclear power plant accident.

We measured the concentrations of cesium isotopes (133Cs, 134Cs, and 137Cs) in zooplankton samples collected in waters off the east coast of Japan from May 2015 to June 2020. By combining these data with those obtained previously from May 2012 to February 2015, we evaluated the long-term impacts of the Fukushima Dai-ichi Nuclear Power Plant accident on marine zooplankton. Relatively high 137Cs concentrations in zooplankton, exceeding 10 Bq/kg-dry weight, were sporadically observed until June 2016, regardless of year or station. After May-June 2017, 137Cs concentrations decreased to below 1 Bq/kg-dry at most stations, and by May 2020, concentrations were below 0.5 Bq/kg-dry except those off Fukushima Prefecture. Since the accident, the 137Cs/133Cs atom ratios of zooplankton samples were higher than those of ambient seawater until 2019, but in May-June 2020 the ratios matched those of seawater except off Fukushima Prefecture. Highly radioactive particles were not detected in zooplankton samples by autoradiography using imaging plates after May-June 2017, although they were before. Therefore, the persistence of elevated 137Cs/133Cs ratios in zooplankton relative to seawater for nine years after the accident was probably due to the incorporation of highly radioactive particles (cesium-bearing particles or clay-mineral aggregates with highly adsorbed radiocesium) onto/into zooplankton for several years after the accident. However, since at least May-June 2017, these elevated ratios have likely been caused by small highly radioactive particles (or larger particles disaggregated into small pieces) entering the ocean from land via rivers or directly discharged from the Fukushima Nuclear Power Plant. Microplastics enriched with radiocesium with higher 137Cs/133Cs ratios than seawater may have also contributed 137Cs to the zooplankton.

Transuranic nuclides Pu, Am and Cm isotopes, and 90Sr in seafloor sediments off the Fukushima Daiichi Nuclear Power Plant during the period from 2012 to 2019.

The 238Pu, 239+240Pu, 241Am, 242Cm, 243+244Cm and 90Sr concentrations in seafloor surface sediments collected at three sampling stations off the Fukushima Daiichi Nuclear Power Plant (FDNPP) site during the period from 2012 to 2019 were determined to elucidate the impacts of the FDNPP accident onto their concentrations in coastal sediments and to discuss the sources of the measured radionuclides. The 239+240Pu, 241Pu and 241Am concentrations and 240Pu/239Pu atom ratios in a sediment core were also determined to allow comparison of their inventories between this study and previously reported values and to identify the Pu sources. The 238Pu, 239+240Pu, 241Am and 90Sr concentrations showed no remarkable temporal variations; no significant increases in concentrations after the FDNPP accident were observed; these concentrations were comfortably within the previously reported concentration range; and no detectable 242Cm and 243+244Cm amounts were observed in surface sediments. The observed 238Pu/239+240Pu activity ratios were approximately two orders of magnitudes lower than those for the damaged FDNPP reactor core inventories and the observed values in terrestrial samples after the accident. The 239+240Pu, 241Pu and 241Am inventories in the sediment core were 389 ± 5, 503 ± 33 and 214 ± 3 Bq m-2, respectively. The 239+240Pu inventory was about an order of magnitude greater than the expected cumulative deposition density of global fallout from atmospheric nuclear weapons testing due to an enhanced scavenging effect. The 240Pu/239Pu atom ratios in the sediment core ranged from 0.239 to 0.246 with a mean value of 0.242 ± 0.002; these ratios were clearly greater than the mean global fallout ratio of 0.18. The results for 238Pu/239+240Pu activity ratios and 240Pu/239Pu atom ratios reflected a mixture of global fallout and Pacific Proving Grounds (PPG) close-in fallout Pu rather than Fukushima accident-derived Pu. The sediment column inventory for 239+240Pu originating from the PPG close-in fallout was calculated as 166 Bq m-2, which corresponded to 43% of the total inventory. A significant amount of the PPG-derived Pu has been transported by ocean currents and then preferentially scavenged in the coastal waters of Japan.

Circulation paths of 134Cs in seawater southwest of Japan in 2018 and 2019.

The spatial variations of low-level 134Cs concentrations (activities) in seawater off the Japanese Archipelago, particularly in the eastern East China Sea (ECS), in 2018 and 2019 were examined. The 134Cs concentrations, decay-corrected to the date of the Fukushima Dai-ichi Nuclear Power Plant accident, in seawaters were 0.5-2.0 mBq/L. High 134Cs concentrations (1.1-2.0 mBq/L) of the Kuroshio Current subsurface water (densities of 25-26σθ) in the eastern ECS could indicated the contribution of the subtropical mode water from the Pacific Ocean side, and total column inventories were 330-426 Bq/m2. In contrast, as indicated by the same 134Cs concentration level at the surface of the eastern ECS and Sea of Japan, larger portions of the subsurface waters remained in the ECS and Yellow Sea side in response to the existence of the shallow Tsushima Strait.

Radiocesium in the swash zones off the coast of the Japan Sea.

Radiocesium concentrations were measured in seawater and sediment samples collected in the swash zones in Ishikawa and Niigata prefectures, off the coast of Japan Sea opposite to the side where TEPCO Fukushima dai-ichi Nuclear Power Plant (FDNPP) is located in September 2016 and August 2017, five to six years after the accident. Cs-134 in the seawater samples was detected, suggesting the intrusion of FDNPP-derived radiocesium in both swash zones. FDNPP-derived radiocesium was appeared to be transported by the Tsushima Warm Current. In the surface sediments only 137Cs was detected during the sampling period. We could not find out the presence of the FDNPP-derived radiocesium in the corresponding sediment on the swash zones; however, detected radiocesium in those sediments was assumed to be influenced by 137Cs of FDNPP-derived radiocesium little for Ishikawa area and some for Niigata area.