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

Monitoring of 137Cs, 239+240Pu, and 90Sr in the marine environment of South Korea and their impact on marine biota: 10 years after the Fukushima accident

This study conducted an analysis of the behavior of radionuclides and assessment their radioactive risk based on seawater and seabed sediment samples gathered from the East, South, and Yellow Seas of South Korea over the period from 2011 to 2020. The distribution for each radionuclides in seawater obtained from the East, South, and Yellow Seas were similar. However, the concentrations of 137Cs and 239+240Pu in sediments from the East Sea were observed to be higher compared to those from the South and Yellow Seas. This variation can be attributed to differences in the ocean inflow, water column properties, and seabed characteristics among the seas around South Korea. There were no statistically significant differences between the radioactive concentrations of seawater and seabed samples collected before and after the Fukushima accident, and no areas with unusually high radiation levels were detected. Using the distribution coefficient (Kds) and the concentration ratio (CR) calculated from the 2011-to-2020 data, we evaluated the radiological impact on fish. The ERICA tool was utilized to assess these data, and indicated a negligible radiological risk from radioactivity in the seawater, seabed sediments, and marine biota in the South Korean Ocean.

Short review of plutonium applications for the sediment transport studies

The transport of sediments in surface waters is a natural process involving the relocation of the sediments themselves and the nutrients, pollutants and radionuclides associated with them. Plutonium isotopes have proven to be a useful tool for studying this process over several decades. In this article, we review the characteristics of the behaviour of plutonium in the water column and the main directions of its use to study the transport of sediments at different scales. The characteristic isotopic fingerprints of the sources and their known input functions, as well as the good reactivity of the particles, favour the widespread use of plutonium for the study of the chronology of sediments and deposits and for studies of the fate and migration pathways of sediments at different scales. While other radionuclides are losing relevance due to the short half-life the ²³⁹Pu and ²⁴⁰Pu will remain useable for a long time.

Spatio-temporal distribution of 239+240Pu in sediments of the China sea and adjacent waters

Data of ^239+240Pu activities and ²⁴⁰Pu/²³⁹Pu atom ratios in surface and core sediments of the China Sea and adjacent waters were collected. We examine a dataset of ^239+240Pu activities and ²⁴⁰Pu/²³⁹Pu ratios determined from surface sediments at 516 sites and 84 core sediment mainly across the China Sea and adjacent waters. For the first time the spatial distributions of the ^239+240Pu activities, the ²⁴⁰Pu/²³⁹Pu ratios and the Pacific Proving Ground (PPG) fraction in the China Sea and adjacent waters are fully presented at the same time. Four types of typical ^239+240Pu distribution with depth are commonly summarized: non-peak, pseudo single peak, single peak and multi peaks, which are based on the comprehensive analysis of the vertical distribution of ^239+240Pu in 84 sediment cores that had been studied in the China Sea and adjacent waters. Their occurrence probability are ∼15%, ∼4%, ∼67% and ∼11%, respectively. This is the dominant Pu source in seawater which was transported by the North Equatorial Current and Kuroshio Current and its extension into the China Sea and adjacent waters first from east to west, then from south to north. A sea area to the northeast of Taiwan Island and the Okinawa Trough, shows high ^239+240Pu activities and ²⁴⁰Pu/²³⁹Pu atom ratios spatial distribution trends, which are related to the intrusion of the Kuroshio Current carrying ^239+240Pu from the PPG nuclear weapon tests. The used two end-member mixing model suggests that global fallout and PPG close-in fallout are the main sources of Pu in most of the investigate areas. As the ²⁴⁰Pu/²³⁹Pu of global fallout is relatively constant, the change of ²⁴⁰Pu/²³⁹Pu ratios in surface sediments of the China Sea and adjacent waters are mainly controlled by the PPG close-in fallout input.

Determination, Separation and Application of 137Cs: A Review

In the context of the rapid development of the world's nuclear power industry, it is necessary to establish background data on radionuclides of different samples from different regions, and the premise of obtaining such basic data is to have a series of good sample processing and detection methods. The radiochemical analysis methods of low-level radionuclides ¹³⁷Cs (Cesium) in environmental and biological samples are introduced and reviewed in detail. The latest research progress is reviewed from the five aspects of sample pretreatment, determination, separation, calculation, application of radioactive cesium and the future is proposed.

Meteoric 10Be, 137Cs and 239+240Pu as Tracers of Long- and Medium-Term Soil Erosion—A Review

Isotopes of meteoric 10Be, 137Cs, 239+240Pu have been proposed as a soil redistribution tracer and applied worldwide as an alternative method to classical field-related techniques (e.g., sediment traps). Meteoric 10Be provides information about long-term soil redistribution rates (millennia), while 137Cs and 239+240Pu give medium-term rates (decades). A significant progress in developing new models and approaches for the calculation of erosion rates has been made; thus, we provide a global review (n = 59) of research articles to present these three isotopes (meteoric 10Be, 239+240Pu and 137Cs) as soil erosion markers in different environments and under different land-use types. Understanding the dynamics and behaviours of isotopes in the soil environment is crucial to determine their usefulness as soil erosion tracers; thus, we discuss the chemical–physical behaviour of meteoric 10Be, 137Cs and 239+240Pu in soils. The application of these isotopes sometimes has strong limitations, and we give suggestions on how to overcome them or how to adapt them to a given situation. This review also shows where these isotopic methods can potentially be applied in the future. A lack in knowledge about soil redistribution rates exists particularly in loess-dominated areas where the tillage system has changed or in areas with strong wind erosion.

Distribution of Pu isotopes and 210Pb in the Bohai Sea and Yellow Sea: Implications for provenance and transportation

Particle-reactive radionuclides are useful for tracing sediment dynamics in marginal seas. We collected a suite of surface sediment samples in May 2014 from the Bohai Sea (BS) and Northern Yellow Sea (NYS) to observe the spatial distribution of Plutonium (Pu) isotopes and ²¹⁰Pb activities. ^239+240Pu activities ranged from 0.001 to 0.288 and 0.040-0.269 Bq kg^-1 in BS and NYS surface sediments, respectively. ²¹⁰Pb_ex shows a significant correlation with ^239+240Pu (r = 0.84, p < 0.01) that suggested these two nuclides were scavenged to the same grade. ²⁴⁰Pu/²³⁹Pu atom ratios in BS (0.173-0.256) and NYS (0.196-0.275) were slightly higher than the global fallout value of 0.18 and lower than the Pacific Proving Ground (PPG) value of 0.36, indicating that some fraction of Pu originating from the PPG was capable of being transported to the BS and NYS. Mass balance results showed that 41% of ^239+240Pu (8.9 × 10⁹ Bq yr^-1) and 18% of ²¹⁰Pb (2.4 × 10¹² Bq yr^-1) in the NYS originated in the oceanic input. In the BS, 63% of ²¹⁰Pb originated from atmospheric deposition and 84% of ^239+240Pu originated from riverine input. Using Pu and ²¹⁰Pb as tracers, we estimate that (1.8-2.6) × 10⁸ t yr^-1 and (3.6-3.8) × 10⁸ t yr^-1 of sedimentary particles could be transported from the BS to the NYS and from the NYS to the Southern Yellow Sea, respectively. Furthermore, the ²²⁶Ra/²³⁸U activity ratio distribution suggested that sedimentary particles derived from the Yellow River could be transported to the middle of the BS and coastal areas of the NYS.

Plutonium in Southern Yellow Sea sediments and its implications for the quantification of oceanic-derived mercury and zinc

The spatial distributions of mercury (Hg) and zinc (Zn) concentration and the isotopic composition of plutonium (Pu) were investigated in surface sediments and sediment cores collected from the Southern Yellow Sea (SYS) during May 2014. The variation of the ²⁴⁰Pu/²³⁹Pu atom ratio (0.18-0.31) in the surface sediments of the SYS clearly indicated a signal of close-in fallout input from the Pacific Proving Ground (PPG). The buried ^239+240Pu in the sediment of the SYS was estimated to be (4.7 ± 0.5) × 10¹⁰ Bq y^-1 during the period from 2011 to 2014, of which ∼33% (1.5 × 10¹⁰ Bq y^-1) was derived from the PPG by long-range transport via ocean currents (e.g., the North Equatorial Current and Kuroshio Current). The concentrations of Hg and Zn varied from 0.003 to 0.067 mg kg^-1 and from 43.9 to 137 mg kg^-1, respectively, and exhibited positive correlations with the ^239+240Pu activity both in the surface sediments (0-1 cm) and upper layers (7 cm) of the sediment cores. Therefore, by using Pu as a tracer, we estimated that the oceanic input contributed 2.0 tons y^-1 of Hg and 1.0 × 10³ tons y^-1 of Zn to the SYS sediments between 2011 and 2014, which accounted for 33% and 3% of total buried Hg and Zn, respectively. These findings indicate that environmental pollution control should also consider the oceanic contribution of some pollutants. The results of the present work help to elucidate the biogeochemical cycling of trace metals in marginal seas, and are helpful for managing environmental pollution in marine environments.

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.

Characteristic of Pu from urban wetland and lacustrine sediments in Suzhou Industrial Park, China

In this study, plutonium activity concentrations in the urban wetlands and lacustrine sediment of Suzhou Industrial Park (SIP) are studied for the first time. Results show ^239+240Pu activity concentrations in the wetland surface soils of SIP range from 0.035 to 0.426 mBq/g and the ²⁴⁰Pu/²³⁹Pu atom ratio ranges from 0.171±0.024 to 0.226±0.049. Judging from the atom ratio of ²⁴⁰Pu/²³⁹Pu, the main source of Pu in the wetland is global fallout. The correlations of Pu between organic matter and heavy metals are also studied. The correlation coefficients show Pu has significant positive correlations with Cu, Sn and Pb but negative correlation with As. Unlike distributions of Pu in other places, Pu in SIP has weak correlation with organic matter content. A sediment core from Lake Yangcheng is also analyzed to investigate the historical record of Pu deposition. The atom ratios of each layer in the sediment core indicate the area is mainly influenced by global fallout. Using Pu as a discrete-time maker, the deposition rate in Lake Yangcheng is 0.396±0.019 cm/yr. The calculated inventory of ^239+240Pu is 58.5 Bq/m², which is in the range of inventories of the corresponding latitudes according to UNSCEAR.

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 and Source Identification of Pu in River Basins in Southern China

The ^239+240Pu activities and ²⁴⁰Pu/²³⁹Pu atom ratios in surface sediments from the major river basins in southern China were analyzed to investigate the distribution and source of Pu. We clarified that the ^239+240Pu activities in these river basins were very similar, however, only the ^239+240Pu activities in the Jinjiang Basin were generally higher than other samples. Because of river transport function, the distribution of ^239+240Pu activities in these river basins presented an increasing trend from the upstream region to the estuary. According to the ²⁴⁰Pu/²³⁹Pu atom ratios, the Pu source in the inner river basins might be from global fallout, and the Pu in river estuaries might be from the global fallout and the Pacific Proving Grounds (PPG) in the Marshall Islands. Using a mass balance of the Pu model, we quantified in the Pearl River Estuary and the Pu contribution from the Pearl River Basin to Pu inventory was 13 ± 5%. These data not only filled in a knowledge gap of Pu in these river basins but also served as background data for Pu contamination from a nuclear reactor. Also, there are several planned and operating nuclear power plants in these river basins and these data could provide some indications for dealing with nuclear accidents in different parts of river basins in the future. In this study, we also analyzed some factors that would affect the distribution of ^239+240Pu activities; however, only total organic carbon (TOC) content and the heavy metal As had a positive correlation with the ^239+240Pu activity.

137Cs and 239+240Pu in the Bohai Sea of China: Comparison in distribution and source identification between the inner bay and the tidal flat

We investigated artificial radionuclides (^239+240Pu and ¹³⁷Cs) in surface sediments and sediment cores collected from the Bohai Bay and the tidal flat of the Liaodong Bay, China. Increasing trends for ^239+240Pu activities and ²⁴⁰Pu/²³⁹Pu atom ratios (˃0.18) were observed from land to sea and from north to south in the Bohai Bay. This spatial pattern implied that the scavenging process between riverine sediments and Pacific Proving Ground (PPG) source Pu transported by the currents such as Yellow Sea Warm Current had occurred in the Bohai Bay. In contrast, relatively lower ²⁴⁰Pu/²³⁹Pu atom ratios in the tidal flat of Liaodong Bay were due to the mixing process between more global out and less PPG source Pu. The riverine Pu contributions to the total global fallout Pu in the Haihe River (32.8%) were much lower than those in the Yangtze River estuary (77%-80%), indicating better soil conservation in the Haihe River Catchment.