Surface distributions of 228Ra in seas and oceans globally: Implications for water circulation and contaminant transport.

Seawater movements are challenging to track and monitor for the transport of various soluble materials. Since the 1960s, naturally occurring radium isotopes (Ra), particularly 228Ra, with a half-life of 5.75 y, have provided unique insights into oceanic seawater circulations within a 30 y timescale. Since the 1980s, especially in the 2000s, frequent research expeditions and improvements in analytical techniques have enabled the determination of fine-scale lateral variations in 228Ra/226Ra ratio and 228Ra concentration. These results describe ocean-, sea-, and basin-scale seawater circulations and current mixing, including seasonal variations. Additionally, the source areas of Ra in seawater (i.e., coastal and shallow shelf areas) often overlap with areas containing contaminants released by human activities. Notably, the surface current systems inferred from the distribution of 228Ra closely explained the transport patterns of radiocesium derived from the Fukushima Dai-ichi Nuclear Power Plant. Databases of 228Ra/226Ra ratios and 228Ra concentrations have the potential to predict flow pathways and timescales for various soluble contaminants in ocean and sea environments.

Unique transport paths of 137Cs from the Indian to Southern Oceans.

To assess ocean-scale transport systems, we examined the latitudinal cross-sectional distribution of 137Cs activity concentrations in the Indian and Southern Oceans between December 2019 and January 2020 using low-background γ-spectrometry. At 0°-20°S, 137Cs concentrations exhibited a gradual decrease below the mixing layer (1-0.1 mBq/L). However, the concentrations steeply decreased toward the Southern Ocean along a transect of 30°-60°S (from 0.8 to 0.02 mBq/L) with minor vertical variation at each site. For the 137Cs inventories (0-800 m depth) from 15 to 600 Bq/m2, a maximum value was recorded at 30°S, indicating the downwelling of 137Cs as a reservoir for the Subantarctic Mode Water. The significantly low concentrations (0.02 mBq/L) at 60°S suggest minimal transport of 137Cs to the Southern Ocean. These findings assist in understanding 137Cs circulation patterns and provide valuable insights into the transport pathways of soluble contaminants.

Subarctic-scale transport of 134Cs to ocean surface off northeastern Japan in 2020.

We studied the spatiotemporal variations in 134Cs, 137Cs, and 228Ra concentrations at the sea surface off southeastern Hokkaido, Japan (off-Doto region) from 2018 to 2022 using low-background γ-spectrometry. The 134Cs concentrations in the off-Doto region, decay-corrected to the date of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, exhibited wide lateral variation each year (e.g., 0.7-1.1 mBq/L in 2020). By studying the 228Ra concentrations and salinity, this variation was explained based on the current mixing patterns. Furthermore, the 134Cs concentrations in the waters highly affected by the Oyashio Current (OYC) gradually increased from 2018 to 2020, and subsequently decreased in 2022. This implies that the water mass maximally contaminated with 134Cs was transported back to the side of the Japanese islands 10 years after the FDNPP accident along with counter-clockwise currents (e.g., the OYC) in the northern North Pacific Ocean. The 134Cs concentrations in the OYC-affected waters in the off-Doto region in 2020 were ~ 1/6 times those in the 134Cs-enriched core of waters off the western American Coast in 2015, which can be ascribed to dilution via spatial dispersion during subarctic current circulation. Overall, we elucidated the ocean-scale subarctic current systems in the northwestern North Pacific Ocean, including water circulation timespans.

Origin of surface water in the Southern Ocean: Implications of soluble radionuclide distributions.

Concentration data of soluble radionuclides in the southern South Indian Ocean and Southern Ocean transition zone are rare or insufficient for the study of its current system. We examined the lateral surface variations in soluble natural (226Ra and 228Ra) and anthropogenic (134Cs and 137Cs) radionuclide activity concentrations in the surface waters in this area from November 2021 to March 2022. The surface distributions of 226Ra and 137Cs concentrations were classified into Subantarctic Mode Water-, Antarctic Intermediate Water-, and Upper Circumpolar Deep Water (UCDW)-dominated areas along latitudinal band (40°S-65°S, 110°E-120°E). Notably, the highest 226Ra concentrations occurred along the longitudinal band (60°S-65°S, 40°E-120°E). Significantly lower 137Cs concentrations in the Southern Ocean than those in surface waters in other global oceans were observed along with depletion of 228Ra. Additionally, 226Ra and 137Cs concentrations appeared to show small variations between eastern and western areas (2.5-3.0 mBq/L and 0.06-0.03 mBq/L, respectively). Lateral profiles in the Southern Ocean are governed by a large contribution from deep/old waters (e.g., UCDW), with a small effect from southward transport of Subantarctic Mode Water.

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.

Fukushima-derived radiocesium in the western subarctic area of the North Pacific Ocean, Bering Sea, and Arctic Ocean in 2019 and 2020.

We measured dissolved radiocesium (134Cs and 137Cs) in surface seawater collected in the western subarctic area of the North Pacific Ocean, Bering Sea, and Arctic Ocean in 2019 and 2020. The radiocesium released from the accident of the Fukushima Dai-ichi nuclear power plant (FNPP1) in 2011 was still observed in these areas (∼2 Bq m-3 decay-corrected to the date of the accident). In 2019/2020, the FNPP1-derived radiocesium concentrations in the Bering Sea and the Chukchi Sea, which is a marginal sea of the Arctic Ocean connecting the Bering Sea to the Arctic Ocean, were within the range of those observed in 2017/2018. On the other hand, the FNPP1-derived radiocesium was detected in the Arctic Ocean farther north of the Chukchi Sea in 2019/2020 for the first time. This was probably derived from the long-range transport of the FNPP1-derived radiocesium from the North Pacific coastal area of Japan to the Arctic Ocean through the Bering Sea during the past decade. The transport of the FNPP1-derived radiocesium from the Bering Sea to the western subarctic area in 2019/2020 is not clear, which implies the retainment of the FNPP1-derived radiocesium within the Bering Sea.

Seasonal variations in marine polycyclic aromatic hydrocarbons off Oki Island, Sea of Japan, during 2015-2019.

Concentrations of 13 phase-partitioned polycyclic aromatic hydrocarbons (PAHs) in seawater were monitored monthly off Oki Island, Japan, during 2015-2019 to elucidate seasonal variations, main source, and transport pathways of PAHs in the southwestern Sea of Japan. Total PAH (dissolved plus particulate) concentrations in surface seawater at 36°09.0'N, 133°17.3'E (site OK) were in the range 0.49-9.36 ng L-1 (mean 2.77, SD 2.05 ng L-1) with higher levels in summer-autumn, an order of magnitude lower than those in the East China Sea during 2005 and 2009-2011 and about one-third of those recorded in the Sea of Japan in 2008 and 2010. The main sources of dissolved and particulate PAHs were combustion products. Increasing dissolved PAH levels during July-October indicate that the area around southern Oki Island is impacted by PAH-rich summer continental-shelf water transported by the Tsushima Warm Current flowing from the East China Sea.

Unique current connecting Southern and Indian Oceans identified from radium distributions.

We examined the spatial variations in 226Ra and 228Ra (activities) concentrations from the surface to a depth of 830 m in the Indian and Southern Oceans from December 2019 to January 2020. 226Ra concentrations at the surface increased sharply from 30° S to 60° S along a ~ 55° E transect (1.4-2.9 mBq/L), exhibiting small vertical variations, while 228Ra decreased southward and became depleted in the Southern Ocean. These distributions indicated the ocean-scale northward lateral transport of 226Ra-rich and 228Ra-depleted currents originating from the Antarctic Circumpolar Current (ACC). 226Ra concentrations indicated that the fractions of the ACC at depths of 0-800 m decreased from 0.95 to 0.14 between 60° S and 30° S. The ACC fractions in the subantarctic western Indian Ocean were higher than those previously reported in the eastern Indian region, indicating preferential transport of the ACC. The fractions obtained were approximately equivalent to those in the western Indian Ocean in the 1970s. This could be attributed to the minimal southward shift of the polar front due to global warming over the last 50 years.

Factors controlling dissolved 137Cs activities in coastal waters on the eastern and western sides of Honshu, Japan.

The distributions of dissolved 137Cs in river, nearshore, and offshore waters on the east and west coasts of the Japanese island of Honshu were studied in 2018-2021, 7-10 years after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. On the east side along the north western North Pacific (Fukushima Prefecture), estuarine processes, including desorption from riverine particles and dissolution into pore water from riverine particles that had settled to the seafloor, contributed to the maintenance of high dissolved 137Cs activities in nearshore and offshore waters. A survey and mass-balance calculation in a semi-enclosed estuarine area, the Matsukawa-ura, in the northern part of Fukushima, provided convincing evidence that rivers contributed to the influx of 137Cs to coastal waters. In contrast, the extremely low activities of dissolved and particulate 137Cs in the Tedori River of Ishikawa Prefecture on the western side of Japan along the Japan Sea suggested that inputs of riverine 137Cs made a negligible contribution to the increase of dissolved 137Cs activities in the nearshore and offshore waters. The relatively high dissolved 137Cs activities observed in the offshore waters of the Japan Sea were due to movement of FDNPP-derived 137Cs into the Japan Sea via the Tsushima Warm Current. Mechanisms controlling the distributions of 137Cs activities in coastal waters of the eastern and western sides of Japan therefore differ.

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 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.

Temporal Variations of Polycyclic Aromatic Hydrocarbons in the Seawater at Tsukumo Bay, Noto Peninsula, Japan, during 2014-2018.

Concentrations of phase-partitioning 13 polycyclic aromatic hydrocarbons (PAHs) in seawater were investigated in the Tsukumo Bay, Noto Peninsula, Japan, during 2014-2018, to improve the understanding of the environmental behavior of PAHs in the coastal areas of the Japan Sea. Total PAH (particulate plus dissolved) concentrations in surface seawater were in the range 0.24-2.20 ng L-1 (mean 0.89 ng L-1), an order of magnitude lower than the mean values observed in the Japan Sea in 2008 and 2010. Although the PAH contamination levels during 2014-2018 were significantly lower than those in the East China Sea, the levels increased from 2014 to 2017 and were maintained at the higher level during 2017-2018. The main sources of particulate and dissolved PAHs during 2014-2018 were combustion products, of which the former were more influenced by liquid fossil-fuel combustion and the latter by biomass or coal combustion. The increase in particulate PAH concentrations in October-December during 2014-2018 was due to the impact of PAH-rich airmasses transported from the East Asian landmass in the northwesterly winter monsoon winds. The increase in dissolved PAH levels during July-September in 2014, 2016, 2017, and 2018 indicates that the Tsukumo Bay is possibly impacted by the PAH-rich summer continental shelf water transported by the Coastal Branch of the Tsushima Warm Current, which flows into the Japan Sea from the East China Sea.

Low levels of Fukushima Dai-ichi NPP-derived radiocesium in marine products from coastal areas in the Sea of Japan (2012-2017).

Radiocesium concentrations in marine biota in coastal areas of the Sea of Japan were < ~0.005-0.02 Bq/kg-wet and ~0.01-0.18 Bq/kg-wet for 134Cs and 137Cs, respectively (2012-2017). The biota-seawater concentration factors were ~25-100, which approximately agreed with those of 137Cs recorded before FDNPP accident. The low levels of 134Cs in marine biota were likely taken up from ambient seawaters. The total of radiocesium concentrations is now equivalent to that in the 1990s based on the ambient water data.

Preliminary investigation of 222Rn in the Yakumo Wind-hole, an algific talus deposits, from Izumo, southwest Honshu, Japan.

The Yakumo Wind-hole in southwest Japan formed by landslip, and it is known as a cold air blowhole. This wind-hole consists of two parts, which have complementary relationships in regard to the flow of air, namely, topographically upper and lower holes that can be characterized as a warm wind-hole (WWH) and cold wind-hole (CWH), respectively. We carried out a preliminary investigation of radon behavior in the Yakumo Wind-hole. The data showed remarkable seasonal change from high 222Rn concentrations reaching to 7.6 ±â€¯0.1 kBq/m3 in the warm season (mid-May to October) to low 222Rn concentrations in the cold season (December to early May) at the CWH. The threshold in the regional atmospheric temperature was estimated as 16.2 °C for the beginning and 17.1 °C for the ending periods of air blow with higher 222Rn concentrations. These seasonal changes in 222Rn were not only associated with the dynamic convection caused by temperature differences in and out of the talus, but were also related to the relative humidity of air that is blown out. High 222Rn concentrations were formed in the high humidity environment, and the humidity may possibly be associated with melting ice. According to the known information on 222Rn behavior in relation to humidity, a radon trap in the growing ice in spring and in the melted water in summer are suggested. This study revealed that 222Rn measurements are a useful tool to understand the air dynamics in the talus.

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.

Vertical profiles of Fukushima Dai-ichi NPP-derived radiocesium concentrations in the waters of the southwestern Okhotsk Sea (2011-2017).

We examined the vertical 134Cs and 137Cs concentration profiles in the southwestern Okhotsk Sea in 2011, 2013, and 2017. In June 2011, atmospheric deposition-derived 134Cs from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) was detected at depths of 0-200 m (0.06-0.6 mBq/L). In July 2013, 134Cs detected at depths of 100-200 m (∼0.05 mBq/L) was ascribed to the transport of low-level 134Cs-contaminated water and/or the convection of radioactive depositions (<0.03 mBq/L at depths of 0-50 m). In July 2017, 134Cs was detected in water samples at depths above 300 m (0.03-0.05 mBq/L), and the inventory, decay-corrected to the FDNPP accident date, exhibited its maximum value (85 Bq/m2) during this period. Combining temperature-salinity data with the concentrations of global fallout-derived 137Cs led to a plausible explanation for this observation, which is a consequence of re-entry of FDNPP-derived radiocesium through the Kuril Strait from the northwestern North Pacific Ocean to the Okhotsk Sea and subsequent mixing with the south Okhotsk subsurface layer until 2017.

Temporal variations in (134)Cs and (137)Cs concentrations in seawater along the Shimokita Peninsula and the northern Sanriku coast in northeastern Japan, one year after the Fukushima Dai-ichi Nuclear Power Plant accident.

Ninety-six seawater samples were collected between May 2011 and March 2012 at 6 sites along the Shimokita Peninsula and the northern Sanriku coast, 250-450 km north of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP). Cesium-134 and (137)Cs concentrations were determined by low-background γ-spectrometry. During May-June 2011, (134)Cs and (137)Cs concentrations in surface waters decreased from 1.0-2.8 to 0.7-1.5 mBq/L and from 2.1-3.9 to 1.9-3.0 mBq/L, respectively. These decreases were due to diffusion and advection in the ocean after atmospheric input of the FDNPP-derived radionuclides. However, in July-August 2011, the concentrations of both radionuclides in the water samples collected on the Pacific side of the Shimokita Peninsula and the northern Sanriku coast exhibited 30-50-fold increases (∼40 mBq/L for (134)Cs and ∼50 mBq/L for (137)Cs) over concentrations observed at these sampling sites in June 2011 in contrast to the gradual decreases in the concentrations on the Tsugaru Strait side of the Shimokita Peninsula. These results suggest that radiocesium-contaminated waters offshore in the Pacific Ocean were transported to coastal regions along the Pacific side of the Shimokita Peninsula and the northern Sanriku coast by ocean currents.

Opening of the closed water area and consequent changes of ²²8Ra/²²6Ra activity ratios in coastal lagoon Nakaumi, southwest Japan.

In Lake Nakaumi, the second largest coastal lagoon in Japan, artificially closed (Honjyo) area, which was left untouched for 28 years, was partly opened in May, 2009. (228)Ra/(226)Ra ratio of waters in Honjyo area and Lake Nakaumi showed a well-tuned seasonal variation exhibiting high value in summer. After the opening event, however, the (228)Ra/(226)Ra ratios in the Honjyo water showed an unclear seasonal variation in both surface and deep water. This opening event caused the change of active movement of lake and marine water.

Spatial variations of low levels of ¹³4Cs and ¹³7Cs in seawaters within the Sea of Japan after the Fukushima Dai-ichi Nuclear Power Plant accident.

Our method based on low background γ-spectrometry enabled the measurement of low radiocesium concentrations in only 20 L of seawater. In May 2011 after deposition of radiocesium, (134)Cs concentration in surface water within the Sea of Japan was confirmed to be significantly small (<0.1-1 mBq/L) by the method. The concentration was not detected (<0.1 mBq/L) below 50 m depth. The Fukushima-derived radiocesium migrated from the surface water of the Sea of Japan without advection to below the thermocline.