Impacts of the Fukushima nuclear accident on the China Seas: Evaluation based on anthropogenic radionuclide 137Cs

The Kuroshio radiocesium stream

The Fukushima accident released short-lived Cs-134 and longer-lived Cs-137 to the ocean. The amount, although substantial, is much less than that produced during the atomic bomb tests 60 years ago. But, the latter has received little attention. Here we found only Cs-137 in waters near the origin of the Kuroshio. The speed of the Kuroshio current generally decreases with water depth, yet, the Cs-137 activity increases with depth to reach a subsurface maximum of 2.4 Bq m^-3. As a result, a core of high Cs-137 flux, or a radiocesium stream, exists at approximately 200-400 m in depth. In total, the Kuroshio transports about 1 PBq year^-1 Cs-137 northward between 121 and 123°E, 1000 times more than the 0.73-1.0 TBq year^-1 discharged to the ocean at Fukushima in 2016-2018.

90Sr in seawater of the East China Sea: Inventory, new potential source, and environmental implications

⁹⁰Sr is useful for tracing water mass movement in oceans. We collected a suit of seawater samples from the East China Sea (ECS) in the May 2011 to investigate the spatial and vertical distribution of ⁹⁰Sr and to understand its transportation and fate. To understand the sources and transportation of ⁹⁰Sr more clearly, published ¹³⁷Cs data from the same cruise were used to obtain the ⁹⁰Sr/¹³⁷Cs activity ratios. The results showed that ⁹⁰Sr activities were controlled by the circulation system of the ECS, with high values in coastal regions and low values in oceanic waters. The plume with a high ⁹⁰Sr/¹³⁷Cs ratio showed that in late spring, the Changjiang Diluted Water could flow southeastward and extend to 126-127° E, which is farther than the previously known value of 124° E. The high ⁹⁰Sr/¹³⁷Cs ratios (1.35 ± 0.62) and a long effective half-life of ⁹⁰Sr (20.0 ± 0.3 y) in the ECS surface water revealed that ⁹⁰Sr is surplus in comparison with ¹³⁷Cs. However, historical variations in the ⁹⁰Sr/¹³⁷Cs ratio seem to preclude the simple explanation that riverine input causes a ⁹⁰Sr surplus in the ECS. Groundwater discharge with a high ⁹⁰Sr but very low ¹³⁷Cs may be a new potential source. However, it is difficult to quantify the contribution of groundwater discharge at present, and more detailed studies are required in this regard. Additionally, we compiled ⁹⁰Sr and ¹³⁷Cs water column inventory data in the western North Pacific and found that the cumulative fallout onto the ocean was different from that on land in the 20-40° N band.

Distribution and budget of 137Cs in the China Seas

Cesium-137 is one of the most abundant anthropogenic radionuclides released by atmospheric nuclear testing and nuclear accidents, and accordingly it may significantly impact the health of humans and marine environmental eco-systems. Documenting the distribution and inventory of 137Cs is thus a crucial task. In this study, we collected a large number of datasets with field observations of 137Cs in the China Seas, in order to provide an in-depth understanding of 137Cs budgets and distributions. The activity and inventory of 137Cs in China Seas' sediments showed large spatial variations, related to the 137Cs source, sedimentation rates and the mineral composition of sediments. The 137Cs concentration in sediments decreased with distance from the shore, generally tracing the distribution of sedimentation rates. High 137Cs inventories in the water column indicated a high solubility and long mean residence times. The mean residence times of 137Cs in the China Seas were determined to be 45.6 ± 3.8 years for the South China Sea (SCS), 36.8 ± 3.1 years for the East China Sea (ECS), and 12.0 ± 1.0 years for the Yellow Sea (YS). A 137Cs mass balance suggests that oceanic input from the north Pacific is the dominant 137Cs source to the China Seas, contributing about 96.9% of this substance. Furthermore, the bulk of 137Cs remains dissolved in the SCS water column, while 137Cs is mostly deposited to the sediments of the ECS and the YS. This new compilation of the activity level and inventory of 137Cs help to establish background levels for future 137Cs studies in the China Seas.

An overview of current knowledge concerning the inventory and sources of plutonium in the China Seas

This study reviews the current understanding of the inventory and sources of plutonium (Pu) in the marine environment adjacent to China. The ^239+240Pu inventory in the China Seas was found to have large spatial variations. The quantity in sediments decreases away from the shore, generally tracing the sedimentation rate distribution. High ^239+240Pu inventories indicated that Pu in the water column was easily scavenged since Pu has a high particle affinity. Indeed, substantially higher ²⁴⁰Pu/²³⁹Pu atom ratios were observed in the sediment and seawater of the China Seas than are found in global fallout. We thus clarified that Pu sources in the China Seas were from both global fallout and the Pacific Proving Grounds (PPG) in the Pacific Marshall Islands. Plutonium from the latter source is transported into the China Seas through the North Equatorial Current (NEC) and Kuroshio. Using a two end-member mixing model, we revealed that the contribution of Pu from the PPG accounts for over 40% of the Pu in the East China Sea (ECS) and South China Sea (SCS), and less than 20% of the Pu in the Yellow Sea (YS). The distributions and isotopic composition of Pu in the China Seas indicate strong scavenging of Pu in the ECS and high Pu accumulation in the SCS. This information on the inventory and isotopic composition of Pu helps to establish a background for the future study of Pu in the China Seas.

Distribution of 137Cs in the Bohai Sea, Yellow Sea and East China Sea: Sources, budgets and environmental implications

The transport processes and fate of ¹³⁷Cs in eastern Chinese seas (ECSs) that consists of the Bohai, Yellow, and East China Seas, have not been well established. In this study, we measured the concentrations of ¹³⁷Cs in the ECSs water and surface sediments during 2013-2014. Combined with a number of published ¹³⁷Cs inventory data from drainage basins and sediment accumulation rates in the ECSs, the distribution, sources and budgets of ¹³⁷Cs in the ECSs were investigated. The ¹³⁷Cs activity in the water column and surface sediments ranged from 0.03 to 1.92 Bq/m³ and from 0.30 to 5.22 Bq/kg, respectively. No ¹³⁴Cs signal was observed, suggesting that the Fukushima accident had limited impact on the ECSs during the investigation period. Mass balance of ¹³⁷Cs suggests that at least 7.4 × 10¹² Bq/y of ¹³⁷Cs imported into the ECSs from the Northwestern Pacific that accounts for 0.7% of the ¹³⁷Cs transported by the Kuroshio Current, and this value is 5.2 times higher than the sum of atmospheric fallout and total riverine input. The apparent half-lives of ¹³⁷Cs are estimated to be 15.1 y for the ECS and 7.7 y for the YS. The vertical profiles in the continental shelf edge and the Yangtze River Estuary reveal that the upwelling of the Kuroshio Subsurface Water is the main mechanism of ¹³⁷Cs import into the ECSs. The high level of ¹³⁷Cs in oceanic water masses and the low level of ¹³⁷Cs in riverine and coastal waters make ¹³⁷Cs a good indicator for tracing water mass movement and interaction. In addition, good correlation between ¹³⁷Cs activity and mean grain size (φ) indicates that ¹³⁷Cs can serve as an effective tracer to track dispersal pathways of fine sediments in river-dominated marginal seas.

Sources and scavenging of plutonium in the East China Sea

The ²⁴⁰Pu/²³⁹Pu atom ratio and ^239+240Pu activity of seawater in the East China Sea (ECS) was measured in order to examine the Pu sources and elaborate Pu scavenging process. High ²⁴⁰Pu/²³⁹Pu atom ratios (0.187-0.243, average = 0.221 ± 0.017) in the surface water and water column were observed during 2011, implying of non-global fallout Pu sources. The distribution of ²⁴⁰Pu/²³⁹Pu atom ratio in the ECS was in agreement with the introduction pathway of the Kuroshio, showing a decreasing trend away from the outer shelf. An even higher ²⁴⁰Pu/²³⁹Pu atom ratios (0.243-0.263, average = 0.253 ± 0.007) were observed in the Kuroshio, indicating the non-global fallout Pu signal from the Pacific Proving Grounds (PPG). Using a two end-member mixing model, the Pu source from the PPG contribution was calculated to be 36 ± 9% in the ECS seawater. The ^239+240Pu activities of surface seawater were in the range of 2.00-2.95 mBq m^-3 in the ECS. The spatial distribution of ^239+240Pu activity in the surface seawater showed an increasing trend from the outer shelf to the nearshore. Moreover, ^239+240Pu inventory of water column at the station DH23 in the ECS was calculated to be ~0.29 Bq m^-2, which was 1-3 orders of magnitude lower than the estimates of sediment cores in the ECS shelf (9-407 Bq m^-2). Such differences were determined by the high degree Pu scavenging efficiency in the ECS and high Pu input carried by terrestrial sediments from the Yangtze River. Finally, both ²⁴⁰Pu/²³⁹Pu atom ratios and ^239+240Pu activities were identical before and after the Fukushima nuclear accident (FNA), suggesting that the impact of the FNA on the ECS was negligible.

Sources of plutonium isotopes and 137Cs in coastal seawaters of Liaodong Bay and Bohai Strait, China and its environmental implications

In order to investigate the sources of plutonium in seawaters of Liaodong Bay and Bohai Strait, China, surface seawater samples were collected and analyzed for ^239+240Pu and ¹³⁷Cs by radiochemical separation combined with ICP-MS and γ-spectrometry, respectively. A large variation of ^239+240Pu activities was observed, ranging from 1.993 to 29.677 mBq/m³ in the Liaodong Bay and from 0.932 to 10.183 mBq/m³ in the Bohai Strait. ¹³⁷Cs activities showed little variation between the investigated locations, which was attributed to the different behavior characteristics of ^239+240Pu and ¹³⁷Cs in seawaters. The ²⁴⁰Pu/²³⁹Pu atom ratios showed a significant variation between the Liaodong Bay (0.185) and Bohai Strait (0.225), indicating that Pu derived from the Pacific Proving Grounds might have been transported to the Bohai Strait but not yet to the Liaodong Bay, which could further provide valuable reference for evaluating the seawater exchange cycle between the Bohai Sea and Yellow Sea.

Spatial and vertical distribution of radiocesium in seawater of the East China Sea

The ¹³⁷Cs activity in surface water of the East China Sea (ECS) was 0.66-1.36 Bq m^-3 during May of 2011. The low activities were observed in the Changjiang Estuary and Zhejiang-Fujian coast and high activities were observed in the south offshore and Kuroshio Current pathway, suggesting that the influence from the current system in the ECS. The ¹³⁴Cs were undetectable (<0.03 Bq m^-3) and the contribution of the Fukushima accident to ECS is estimated to be below 3%; hence it is negligible during the investigation period. Using the vertical profiles of ¹³⁷Cs in the ECS, the mass balance is obtained, which suggests that the oceanic input dominates the ¹³⁷Cs source in the ECS. ¹³⁷Cs is potentially useful to trace water mass movement in the ECS. Our study provides comprehensive baseline of ¹³⁷Cs in the ECS for evaluation of the possible influence of the nuclear power plants in the future.

Impacts of Fukushima Daiichi Nuclear Power Plant accident on the Western North Pacific and the China Seas: Evaluation based on field observation of 137Cs

After the Fukushima nuclear accident (FNA), ¹³⁷Cs activities of seawater in the Western North Pacific (WNP) and China Seas were determined in order to examine whether ¹³⁷Cs derived from FNA across the Kuroshio was still existed and if there were any indications of FNA's impact. High ¹³⁷Cs activities at 200-500m were observed in the south of Kuroshio during 2015, indicating ¹³⁷Cs from FNA could cross the Kuroshio. Surface ¹³⁷Cs activities were on average 1.43±0.42Bqm^-3 for WNP, 1.11±0.14Bqm^-3 for South China Sea (SCS) and 1.10±0.29Bqm^-3 for East China Sea (ECS) during 2011-2015. The activities and inventories of ¹³⁷Cs were almost identical before and after the FNA, indicating the impacts of FNA on the WNP and China Seas were minor. The effective environmental half-lives of ¹³⁷Cs in surface seawater were firstly estimated to be 14.4±1.7yrs for ECS and 16.9±2.1yrs for SCS.

Marine ecological risk assessment methods for radiation accidents

Ecological risk assessment (ERA) is a powerful technical tool that can be used to analyze potential and extreme adverse environmental impacts. With the rapid development of nuclear power plants in coastal areas around the world, the establishment of approaches and methodologies for marine ERA with a focus on radiation accidents is an urgent requirement for marine environmental management. In this study, the approaches and methodologies for ERA pertaining to marine radiation accidents (MRA) are discussed and summarized with applications in case studies, such as the nuclear accident in Fukushima, Japan, and a hypothetical accident in Daya Bay, China. The concepts of ERA and Risk Degree of ERA on MRA are defined for the first time to optimize the ERA system. The results of case studies show that the ERA approach and methodology for MRA are scientifically sound and effective in both the early and late stage of MRAs along with classic ERA Approach and the ERICA Integrated Approach. The results can be useful in the decision-making processes and the risk management at the beginning of accident as well as the ecological restoration after the accident.

Assessment of radiological risk for marine biota and human consumers of seafood in the coast of Qingdao, China

This paper reports the levels of (226)Ra, (232)Th, (40)K and (137)Cs in the edible parts of 11 different marine species collected from the Qingdao coast of China. The activities of (226)Ra, (232)Th and (40)K ranged from 0.08±0.03 to 1.65±0.60 Bq kg(-1) w.w., 0.09±0.02 to 1.44±0.10 Bq kg(-1) w.w., 26.89±1.25 to 219.25±5.61 Bq kg(-1) w.w., respectively. Artificial (137)Cs was undetectable or close to the detection limit in the biota sampled. To link radioactivity to possible impact on health, we calculated radiation doses to both the marine biota and human beings. We showed that doses in all cases were dominated by naturally occurring (40)K and that (137)Cs doses were negligible compared with (40)K-derived doses. The total doses to marine biota ranged between 16.55 and 62.41 nGy h(-1) among different biota species, which were below the benchmark level of aquatic organism. The committed effective dose to humans through seafood consumption varied from 10.55 to 36.17 μSv y(-1), and the associated lifetime cancer risks ranged from 5.93E-05 to 9.49E-05 for different age and gender groups. Both the dose and cancer risk to humans were at the acceptable range. Despite the significant amount of radionuclides released as a result of the Fukushima accident, their impact on the seafood in Qingdao coast appears to be negligible based on our measurements of concentrations of radionuclide activity and internal dose estimates.

Fukushima nuclear accident: preliminary assessment of the risks to non-human biota

This study assesses the 'radio-ecological' impacts of Fukushima nuclear accident on non-human biota using the ERICA Tool, which adopts an internationally verified methodology. The paper estimates the impacts of the accident on terrestrial and marine biota based on the environmental data reported in literature for Japan, China, South Korea and the USA. Discernible impacts have been detected in the marine biota around Fukushima Daiichi nuclear power plant. This study confirms that the Fukushima accident had caused heavier damage to marine bionts compared with terrestrial flora and fauna, in Japan.

Isotopic composition and distribution of plutonium in northern South China Sea sediments revealed continuous release and transport of Pu from the Marshall Islands

The (239+240)Pu activities and (240)Pu/(239)Pu atom ratios in sediments of the northern South China Sea and its adjacent Pearl River Estuary were determined to examine the spatial and temporal variations of Pu inputs. We clarified that Pu in the study area is sourced from a combination of global fallout and close-in fallout from the Pacific Proving Grounds in the Marshall Islands where above-ground nuclear weapons testing was carried out during the period of 1952-1958. The latter source dominated the Pu input in the 1950s, as evidenced by elevated (240)Pu/(239)Pu atom ratios (>0.30) in a dated sediment core. Even after the 1950s, the Pacific Proving Grounds was still a dominant Pu source due to continuous transport of remobilized Pu from the Marshall Islands, about 4500 km away, along the North Equatorial Current followed by the transport of the Kuroshio current and its extension into the South China Sea through the Luzon Strait. Using a simple two end-member mixing model, we have quantified the contributions of Pu from the Pacific Proving Grounds to the northern South China Sea shelf and the Pearl River Estuary are 68% ± 1% and 30% ± 5%, respectively. This study also confirmed that there were no clear signals of Pu from the Fukushima Daiichi Nuclear Power Plant accident impacting the South China Sea.