Transport paths of radiocesium and radium isotopes in the intermediate layer of the southwestern Sea of Okhotsk.

The spatial distributions of 134Cs, 137Cs, 226Ra, and 228Ra in/around the southwestern Sea of Okhotsk were examined in July 2019 and July 2021. Wide variations in the concentrations of these radionuclides were detected at the surface, including 0.2-0.7 mBq/L of 134Cs (decay-corrected to the date of the Fukushima Dai-ichi Nuclear Power Plant accident), which indicated a large mixing ratio between the Soya Warm Current and East Sakhalin Current/Okhotsk Sea Surface Water. The Intermediate Cold Water at depths of approximately 30-300 m was subjected to the effects of 226Ra-rich and 228Ra-poor intermediate (or deeper) seawater. Moreover, the 134Cs concentrations were maximum in 2021 (approximately 0.6 mBq/L), which most probably resulted from the increase in 134Cs concentrations in the southward dense shelf water along the eastern Sakhalin Island along with the effect in the Okhotsk Sea Intermediate Water originating from the western subarctic water (e.g., the East Kamchatka Current) in the Pacific Ocean.

Temporal variability of 137Cs concentrations in coastal sediments off Fukushima.

Large amounts of radiocesium were released into marine environments following the Fukushima Daiichi Nuclear Power Plant accident in March 2011. Released radiocesium influenced not only marine environment but also marine biota in Fukushima. Since marine biota as fisheries products is important for Japanese market, it is important to assess the distribution of radiocesium in coastal environment off Fukushima for safety concerns of radioactive contamination. Radiocesium concentrations in sediments are important for understanding fishing ground conditions and for proving the safety of fisheries products in Fukushima. In this study, monthly monitoring data collected from May 2011 to March 2020 were analyzed to describe the temporal variability of 137Cs concentrations in coastal sediments off Fukushima (total of 3647 samples from eight lines at depths of 7-125 m off Fukushima, and three sites in Matsukawa-ura Lagoon). The 137Cs concentration in sediment showed a decreasing trend, but our nonlinear model fitting suggested that this rate of decrease had slowed down. Additionally, 137Cs concentrations were up to 4.08 times greater in shallow sampling sites (7, 10, 20 m depth) following heavy rainfall events (before five months vs. after five months), such as typhoons. These observations were consistent with increasing input from particulate 137Cs fluxes from rivers and increasing dissolved 137Cs concentrations in seawater. Finally, our numerical modeling suggested that riverine 137Cs input could maintain 137Cs concentrations in coastal sediment. These results indicate that riverine 137Cs input following heavy rainfall events is the main factor for maintaining 137Cs concentrations in coastal sediments near the Fukushima Daiichi Nuclear Power Plant.

Radiocesium in Japan Sea associated with sinking particles from Fukushima Dai-ichi Nuclear Power Plant accident.

This study examined the temporal variations in radiocesium concentration associated with sinking particles in the northeastern Japan Sea between September 2010 and July 2012. We analyzed sediment trap samples from this period after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in March 2011. Cesium-134 was detected in samples collected between May and July 2011 at a depth of 1100 m (4.2-11 mBq g-dry-1) but not in other periods at 1100 m or deeper (3100 and 3500 m). These results confirmed the deposition of FDNPP-derived radiocesium on the surface water in the late April 2011, which rapidly sank with sinking particles to a depth of at least 1100 m, in the northeastern Japan Sea, about 40 days after the deposition in the North Pacific. If FDNPP-derived 137Cs was excluded, no seasonal changes were detected in the 137Cs activity concentration of the sinking particles, and the 137Cs activity concentration of the particles increased with increasing depth. Judging from the concentration of 137Cs of sinking particle and seasonal variation of total mass flux and organic matter content, the lithogenic particle seems to be important for radiocesium associated with sinking particles. These data also strongly suggest a difference in sinking features of particles between 2010-2011 and 2011-2012 deployments. Due to the existence of benthic front, shallow water (1100 m) and deep water (3500 m) are separated during 2010-2011 deployment, but in the winter of 2011-2012, this front disappeared and the particles in surface water seem to have sunk to the depth of 3100 m. The sinking velocity of the particles at 1100 m was estimated to be 33-62 m day-1, with a mean sinking velocity of 43 m day-1. These values were comparable to those estimated at depths shallower than 1000 m in the North Pacific after the FDNPP accident, or in the Mediterranean, North, and Black Seas after the Chernobyl accident.

Suspended Particle-Water Interactions Increase Dissolved 137Cs Activities in the Nearshore Seawater during Typhoon Hagibis.

Distributions of 137Cs in dissolved and particulate phases of the downstream reaches of seven rivers and adjacent nearshore and offshore waters as far as ∼60 km south of the Fukushima Dai-ichi nuclear power plant (FDNPP) were studied during the high-river-flow period (June-September 2019) and during the period of October 2019 after typhoon Hagibis. Dissolved 137Cs activities in nearshore water were higher than those in rivers and offshore waters, and this distribution was more intensified after the typhoon, indicating the desorption of 137Cs from riverine suspended particles in addition to the ongoing release of contaminated water from the FDNPP and re-entry of radiocesium via submarine groundwater discharge. This scenario is also supported by the reduction of distribution coefficient (Kd) from a geometric mean value of 5.5 × 105 L/kg in rivers to 9.8 × 104 L/kg in nearshore water. The occupation of desorbed 137Cs to the dissolved activity of this nuclide in nearshore water was estimated to be 0.7%-20% (median: 9.7%) during the high-river-flow period, increasing to 1.4%-66% (32.3%) after the typhoon, suggesting that the desorption during the flood period such as typhoons further contributes to the increase in dissolved 137Cs levels in nearshore water.

Radiocesium in North Pacific coastal and offshore areas of Japan within several months after the Fukushima accident.

We measured activity concentrations of radiocesium (134Cs and 137Cs) in seawater samples collected in North Pacific coastal and offshore areas of Japan within several months after the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident in March 2011, including archived seawater samples whose radiocesium concentrations were previously reported to be below detection limits. By merging 329 new data with published results, we succeeded in reconstructing the temporal changes in activity concentrations and inventories of FNPP1-derived radiocesium in the coastal and offshore areas within several months after the accident for the first time. 137Cs directly-discharged from the FNPP1 was transported eastward within the coastal area about 250 km from the FNPP1 during two months after the accident due to complex movements of coastal surface currents. The eastward speed was calculated to be about 5 cm s-1. Eastward transport of 137Cs to the offshore area more than 600 km away from the FNPP1 along the north flank of the Kuroshio Extension Current was faster (about 9 cm s-1) and probably more dominant in the eastward transport. The total inventory of directly-discharged 137Cs in early April 2011 was estimated to be 3-6 PBq approximately, which agrees with the smaller estimates in previous studies (2-6 PBq).

Intrusion of Fukushima-derived radiocaesium into subsurface water due to formation of mode waters in the North Pacific.

The Fukushima Dai-ichi Nuclear Power Plant accident in March 2011 released radiocaesium ((137)Cs and (134)Cs) into the North Pacific Ocean. Meridional transects of the vertical distribution of radiocaesium in seawater were measured along 147 °E and 155 °E in October-November 2012, 19 months after the accident. These measurements revealed subsurface peaks in radiocaesium concentrations at locations corresponding to two mode waters, Subtropical Mode Water and Central Mode Water. Mode water is a layer of almost vertically homogeneous water found over a large geographical area. Here we show that repeated formation of mode water during the two winter seasons after the Fukushima accident and subsequent outcropping into surface water transported radiocaesium downward and southward to subtropical regions of the North Pacific. The total amount of Fukushima-derived (134)Cs within Subtropical Mode Water, decay-corrected to April 2011, was estimated to be 4.2 ± 1.1 PBq in October-November 2012. This amount of (134)Cs corresponds to 22-28% of the total amount of (134)Cs released to the Pacific Ocean.

Radioactive cesium dynamics derived from hydrographic observations in the Abukuma River Estuary, Japan.

Large quantities of radioactive materials were released into the air and the ocean as a result of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, caused by the 2011 Tohoku earthquake and the subsequent major tsunami off the Pacific coast. There is much concern about radioactive contamination in both the watershed of the Abukuma River, which flows through Fukushima Prefecture, and its estuary, where it discharges into the sea in Miyagi Prefecture. We investigated radioactive cesium dynamics using mixing diagrams obtained from hydrographic observations of the Abukuma River Estuary. Particulate radioactive cesium dominates the cesium load in the river, whereas the dissolved form dominates in the sea. As the salinity increased from <0.1 to 0.1-2.3, the mixing diagram showed that dissolved radioactive cesium concentrations increased, because of desorption. Desorption from suspended particles explained 36% of dissolved radioactive cesium in estuarine water. However, the dissolved and particulate radioactive cesium concentrations in the sea decreased sharply because of dilution. It is thought that more than 80% of the discharged particulate radioactive cesium was deposited off the river mouth, where the radioactive cesium concentrations in sediment were relatively high (217-2440 Bq kg(-1)). Radioactive cesium that was discharged to the sea was transported southward by currents driven by the density distribution.

Use of otolith for detecting strontium-90 in fish from the harbor of Fukushima Dai-ichi Nuclear Power Plant.

To clarify the level of contamination with radioactive cesium (radiocesium) discharged from Fukushima Dai-ichi Nuclear Power Plant (FDNPP), three fish species caught in the main harbor of FDNPP were subjected to γ-ray analysis. The concentration of radiocesium in muscle differed among individual fish, even those of similar size of the same species, and showed little relation to the standard length of fish. The maximum concentration of radiocesium (202 kBq/kg wet) was detected from fat greenling samples. A comparison to data from outside the port indicated that the level of radiocesium contamination inside the port was higher than that outside. We found that ß-rays were emitted from otoliths of fishes caught in the port of FDNPP. ß-ray intensities were correlated with the concentrations of radiocesium in muscles of the three fish species. In Japanese rockfish, the ß-ray count rates from otoliths were significantly correlated with the concentration of radiocesium and (90)Sr in the whole body without internal organs of Japanese rockfish. However, no ß-rays were detected from brown hakeling samples collected around FDNPP, suggesting that the detection of ß-rays from otoliths may indicate living in the main harbor of FDNPP.

Exposure of a herbivorous fish to ¹³4Cs and ¹³7Cs from the riverbed following the Fukushima disaster.

Ayu Plecoglossus altivelis, a herbivorous fish, is an important fishery resource and key component of the foodweb in many Japanese streams. Radionuclide contamination of this species is likely transferred to higher trophic levels, include humans, in the food chain. After the Fukushima accident in March 2011, ayu were exposed to highly contaminated silt while feeding on algae attached to the riverbed stones. To understand the route by which herbivorous fish are exposed to radionuclides, the activity concentrations of sum of (134)Cs and (137)Cs (radiocesium) were analyzed in riverbed samples (algae and silt) and in the internal organs and the muscle of ayu in five river systems in the Fukushima Prefecture between summer 2011 and autumn 2013. Although there was a positive correlation between the radiocesium activity concentrations in the muscle and the internal organs of ayu, the median activity concentration in the muscle was much lower than those in the internal organs. The activity concentrations of radiocesium in the riverbed samples and the internal organs and the muscle of ayu were correlated with contamination levels in soil samples taken from the watershed upstream of the sample sites. The results of the generalized linear mixed models suggest that the activity concentrations in both the internal organs and the muscle of ayu declined over time. Additionally, the activity concentrations in the internal organs were correlated with those in the riverbed samples that were collected around the same time as the ayu. The activity concentrations in the muscle were correlated with ayu body size. Our results suggest that ayu ingest (134)Cs and (137)Cs while grazing silt and algae from the riverbed, and a part of the (134)Cs and (137)Cs is assimilated into the muscle of the fish.

Radiocesium contamination of greenlings (Hexagrammos otakii) off the coast of Fukushima.

We measured the radiocesium ((134)Cs and (137)Cs) contamination of 236 greenlings (Hexagrammos otakii) off the coast of Fukushima Prefecture in Japan, following the accident at the Fukushima Daiichi Nuclear Power Plant in March 2011. The radiocesium concentrations of greenlings caught approximately 40 km south of the power plant were significantly higher than those of greenlings caught approximately 50 km north of the power plant. The radiocesium concentrations of greenlings caught in southern waters were significantly higher in shallow than in deep waters. Meanwhile, two outlier specimens of greenlings with higher (137)Cs concentrations, 16,000 Bq/kg-wet on 1 August 2012 and 1,150 Bq/kg-wet on 8 May 2013, were caught approximately 20 km from the power plant. Our calculations suggest that the probability of two such outlier specimens being found off the coast of Fukushima is exceedingly low. By contrast, extremely contaminated greenlings were frequently caught in the power plant port (geometric mean of (137)Cs = 17,364 Bq/kg-wet). Our results suggest that the two outlier greenlings with higher (137)Cs concentrations migrated from the power plant port. Continued close monitoring of radiocesium concentrations in the area should be done to ensure the safety of food supplies.

Five-minute resolved spatial distribution of radiocesium in sea sediment derived from the Fukushima Dai-ichi Nuclear Power Plant.

The spatial distributions of radiocesium concentration in sea sediment to a core depth of 14 cm were investigated in the offshore region from the Fukushima Prefecture to the northern part of the Ibaraki Prefecture in February and July 2012, at a spatial resolution of 5 min of latitude and longitude. The concentrations in the area south of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) were generally higher than those in the area north of it. In the southern area, a band of especially high concentration with a width about 20 km was present in the region shallower than 100 m, and a narrow minimal concentration band was found along the 200-m isobaths. In more than half of all cases, the vertical core profiles of radiocesium concentration generally showed an exponential decreasing trend with depth. However, in the area north of the FDNPP, where the radiocesium concentrations tended to be very low, radiocesium concentrations that had similar or larger magnitude compared with those of the most-surface layer were often found in deeper layers. Relatively good correlations were found between radiocesium concentrations and grain sizes of the most-surface sediment. The vertical profile of radiocesium concentration also had a relationship with grain size. In other case, the radiocesium concentration in the sediment seems to have had a dependence on the radiocesium concentration in bottom seawater, suggesting that the quantity of radiocesium supplied and the grain size were major factors determining the spatial distribution pattern of the radiocesium concentration after the FDNPP accident.

Southwest intrusion of 134Cs and 137Cs derived from the Fukushima Dai-ichi nuclear power plant accident in the Western North Pacific.

Enormous quantities of radionuclides were released into the ocean via both atmospheric deposition and direct release as a result of the Fukushima Dai-ichi Nuclear Power Plant (FNPP) accident. This study discusses the southward dispersion of FNPP-derived radioactive cesium (Cs) in subsurface waters. The southernmost point where we found the FNPP-derived (134)Cs (1.5-6.8 Bq m(-3)) was 18 °N, 135 °E, in September 2012. The potential density at the subsurface peaks of (134)Cs (100-500 m) and the increased water column inventories of (137)Cs between 0 and 500 m after the winter of 2011-2012 suggested that the main water mass containing FNPP-derived radioactive Cs was the North Pacific Subtropical Mode Water (NPSTMW), formed as a result of winter convection. We estimated the amount of (134)Cs in core waters of the western part of the NPSTMW to be 0.99 PBq (decay-corrected on 11 March 2011). This accounts for 9.0% of the (134)Cs released from the FNPP, with our estimation revealing that a considerable amount of FNPP-derived radioactive Cs has been transported to the subtropical region by the formation and circulation of the mode water.

(137)Cs concentration in zooplankton and its relation to taxonomic composition in the western North Pacific Ocean.

To study the role of zooplankton in the transport of (137)Cs in the ocean, zooplankton samples were collected in October 2005 and June 2006 in the western North Pacific Ocean. The peak zooplankton biomass was observed in the surface layer, and gelatinous plankton was more abundant in October 2005 than in June 2006 reflecting exchange of water masses. The concentrations of (137)Cs in zooplankton varied from 11 to 24 mBq kg wet(-1) and were higher in October 2005 than in June 2006. The elevated abundance of gelatinous zooplankton probably led to higher concentration of (137)Cs in zooplankton in October 2005. Annual export fluxes of (137)Cs by ontogenetic vertical migrant copepods were estimated to be 0.8 and 0.6 mBq m(-2) year(-1) at 200 and 1000 m depths, respectively; this suggested that transport of (137)Cs by zooplankton may be no trivial pathway.