Measurement of 240Pu/239Pu isotopic ratios in soils from the Marshall Islands using ICP-MS

Review of the sources and behaviors of plutonium isotopes in the atmosphere and ocean

Plutonium, as well as fission products such as 137Cs, had been released into the earth environment in 1945 after the first atmospheric nuclear explosion of plutonium bomb in the desert of New Mexico (USA, July 16) and later over Nagasaki (August 9), followed then by many other explosions. Thus, plutonium cycling in the atmosphere and ocean has become a major public concern as a result of the radiological and chemical toxicity of plutonium. However, plutonium isotopes and 137Cs are important transient tracers of biogeochemical and physical processes in the environment, respectively. In this review, we show that both physical and chemical approaches are needed to comprehensively understand the behaviors of plutonium in the atmosphere and ocean. In the atmosphere, plutonium and 137Cs attach with aerosols; thus, plutonium moves according to physical and chemical processes in connection with aerosols; however, since plutonium is a chemically reactive element, its behavior in an aqueous environment is more complicated, because biogeochemical regulatory factors, in addition to geophysical regulatory factors, must be considered. Meanwhile, 137Cs is chemically inert in aqueous environments. Therefore, the biogeochemical characteristics of plutonium can be elucidated through a comparison with those of 137Cs, which show conservative properties and moves according to physical processes. Finally, we suggest that monitoring of both plutonium and 137Cs can help elucidate geophysical and biogeochemical changes from climate changes.

Spatial distribution and modelling of 239+240Pu in the sediments and seawater columns of the South China Sea and Indian Ocean

In order to investigate the 239+240Pu potential influence in the ocean, and develop a new method for rapidly monitoring radioactive pollution, the 239+240Pu spatial distribution in the South China Sea (SCS) and the Indian Ocean (IND) sediments is analyzed by SF-ICP-MS (ELEMENT 2). The inventory-weighted mean activities of 239+240Pu were 0.413 ± 0.333 mBq/g, 0.128 ± 0.044 mBq/g, and 0.483 ± 0.606 mBq/g in the sediments of the SCS, eastern IND, and Arabian Sea, respectively. The 239+240Pu activity spatial distribution in the SCS sediments was influenced by the current, the vertical distribution of Pu in seawater, and the transport of particulate matter. The 239+240Pu activity spatial distribution in the IND sediments could be impacted by Antarctic Intermediate Water. The average of 240Pu/239Pu atomic ratios were 0.258 ± 0.034, 0.219 ± 0.031, and 0.212 ± 0.028 in the sediments of the SCS, eastern IND, and Arabian Sea, respectively. The 240Pu/239Pu atomic ratios in the SCS and IND indicate that Pu from the Pacific Proving Ground (PPG) is transported to the IND via the SCS internal current and transverse ocean currents within Indonesia. In addition, a seawater advection-dispersion equation (S-ADE) model is established based on the actual physical processes of radionuclides in the seawater column and well fitting results were obtained (R2 = 0.49 to 0.99). The 239+240Pu data and the geographic information from the sample site were used to correct the Pu distribution in the seawater. The calculated 239+240Pu mean concentrations in the surface seawater were 2.465 mBq/m3 and 2.205 mBq/m3 for the SCS and the eastern IND seawater, respectively, and the result is consistent with the previous measurements. Then, the 239+240Pu stored in the study area of SCS and eastern IND was estimated to be approximately 1.0-1.4% of the global ocean based on the model. This study provides a useful model for guiding and designing future monitoring of pollution by anthropogenic Pu and other isotopes.

Spatial distribution, source identification, and transportation paths of plutonium in the Beibu Gulf, South China Sea

To investigate the spatial distribution and source of plutonium isotopes in the Beibu Gulf, surface sediments were collected and analyzed using sector field inductively coupled plasma mass spectrometry (SF-ICP-MS). The activities of 239+240Pu in surface sediments ranged from 0.012 to 0.451 mBq/g (mean: 0.171 ± 0.138 mBq/g, n = 36), indicating a decreasing trend in a counterclockwise direction from the southern bay mouth. The counterclockwise decreasing trend in the south of the bay mouth is similar to the current in the Beibu Gulf. The 240Pu/239Pu atom ratios in surface sediments ranged from 0.156 to 0.283 (mean: 0.236 ± 0.031, n = 36), slightly higher than that of the global fallout value of 0.18. This suggests that the Pu in the Beibu Gulf was a combination of global fallout and Pacific Proving Ground (PPG). The average contribution of the plutonium (Pu) derived from the PPG in the sediment was estimated to be 52 % ± 24 %.

Phytotoxicity of radionuclides: A review of sources, impacts and remediation strategies

Various anthropogenic activities and natural sources contribute to the presence of radioactive materials in the environment, posing a serious threat to phytotoxicity. Contamination of soil and water by radioactive isotopes degrades the environmental quality and biodiversity. They persist in soils for a considerable amount of time and disturb the fauna and flora of any affected area. Hence, their removal from the contaminated medium is inevitable to prevent their entry into the food chain and the organisms at higher levels of the food chain. Physicochemical methods for radioactive element remediation are effective; however, they are not eco-friendly, can be expensive and impractical for large-scale remediation. Contrastingly, different bioremediation approaches, such as phytoremediation using appropriate plant species for removing the radionuclides from the polluted sites, and microbe-based remediation, represent promising alternatives for cleanup. In this review, sources of radionuclides in soil as well as their hazardous impacts on plants are discussed. Moreover, various conventional physicochemical approaches used for remediation discussed in detail. Similarly, the effectiveness and superiority of various bioremediation approaches, such as phytoremediation and microbe-based remediation, over traditional approaches have been explained in detail. In the end, future perspectives related to enhancing the efficiency of the phytoremediation process have been elaborated.

Radioactivity research in mosses from typical Karst Regions in Leye Tiankeng, Southern China

Radionuclides in environmental ecosystems have ecotoxicity and health impact on human and environment, so radioactive contamination has always been one of the global concerns. This study mainly focused on the radioactivity of mosses collected from the Leye Tiankeng Group in Guangxi. The activities of ^239+240Pu measured by SF-ICP-MS and ¹³⁷Cs measured by HPGe in moss and soil samples are as follows: 0-2.29 Bq/kg in mosses and 1.5-11.9 Bq/kg in soils for ¹³⁷Cs, and 0.025-0.25Bq/kg in mosses and 0.07-0.51Bq/kg in soils for ^239+240Pu. The range of ²⁴⁰Pu/²³⁹Pu atom ratios (0.201 in mosses and 0.184 in soils) and ^239+240Pu/¹³⁷Cs activity ratios (0.128 in mosses and 0.044 in soils) indicated that the ¹³⁷Cs and ^239+240Pu in study area were mainly contributed by global fallout. ¹³⁷Cs and ^239+240Pu showed similar distribution in soils. However, their behaviors in mosses were quite different due to the differences in the growth environment of mosses. The transfer factors of ¹³⁷Cs and ^239+240Pu from soil to moss varied in different growth stages and specific environments. A weak positive correlation among ¹³⁷Cs, ^239+240Pu in mosses and soil-derived radionuclides suggested that resettlement was predominant here. The negative correlation between ⁷Be, ²¹⁰Pb and soil-derived radionuclides indicated that ⁷Be and ²¹⁰Pb came from atmospheric components, while the weak correlation between them suggested that their specific sources were different. The Cu and Ni were moderately enriched in mosses here due to the use of agricultural fertilizers, At the same time, Zn was at a high level in the Lilang area, where transportation was more developed.

Vertical distribution of plutonium isotopes from the floodplain and lacustrine sediments in Poyang Lake, China

Anthropogenic radionuclides deposited in sediments have been used for environmental radiation risk valuation as well as source identification. In this study, we investigated the vertical distribution of plutonium (Pu) isotopes and ²⁴⁰Pu/²³⁹Pu atom ratios in both floodplain and lacustrine sediments in Poyang Lake. The ^239+240Pu activity concentrations in floodplain sediment cores were found to range from 0.002 to 0.085 Bq kg^-1, with a maximum value at the subsurface layer. The activity in lacustrine sediment cores was from 0.062 to 0.351 Bq kg^-1 with a mean of 0.138 ± 0.053 Bq kg^-1. The inventory of 43.15 Bq m^-2 in lacustrine sediment core is comparable to the average value of global fallout expected at the same latitude. The average ²⁴⁰Pu/²³⁹Pu atomic ratios (0.183 ± 0.032) for sediment cores indicated that the global fallout is the major source of Pu in the studied region. The results are of great significance to the further understanding of sources, records, and environmental impacts of regional nuclear activities in the environment.

Temporal evolution of plutonium concentrations and isotopic ratios in the Ukedo - Takase Rivers draining the Difficult-To-Return zone in Fukushima, Japan (2013-2020)

In 2011, the Fukushima Dai-Ichi Nuclear Power Plant (FDNPP) accident released significant quantities of radionuclides into the environment. Japanese authorities decided to progressively reopen the Difficult-To-Return Zone after the decontamination of priority reconstruction zones. These areas include parts of the initially highly contaminated municipalities located to the north of the FDNPP, including Namie Town, an area drained by the Ukedo and Takase Rivers. Eleven years after the accident, research focused on the spatial distribution of plutonium (Pu) and radiocesium (Cs) isotopes at contrasted individual locations. To complement previous results, the current research was conducted on flood sediment deposits collected at the same locations after major flooding events during eleven fieldwork campaigns organised between 2013 and 2020 at the outlet of the Ukedo and Takase Rivers (n = 22). The results highlighted a global decrease of the Pu and ¹³⁷Cs contents in sediment with time during the abandonment phase in the region, from 2013 (238.20 fg g^-1) to 2020 (4.28 fg g^-1). Furthermore, based on the analysis of the ²⁴⁰Pu/²³⁹Pu isotopic ratios, the plutonium transiting these rivers (range: 0.166 - 0.220) essentially originated from the global fallout (0.180 ± 0.014 (Kelley et al., 1999)). Sediment showed contrasted properties in the two investigated rivers, which is likely mainly the result of the occurrence of Ogaki Dam on upper sections of the Ukedo River as it strongly impacts the material supply from this river to the Pacific Ocean. A statistical analysis highlighted the strong correlation between Pu activity concentrations and ¹³⁷Cs activities in both rivers, confirming that both radionuclides are transported with a similar pathway. Despite it was detected early after the accident (2011-2013), the current research demonstrates that plutonium originating from FDNPP is no longer detected in these rivers draining the Difficult-To-Return Zone at the onset of the reopening of the area to its former inhabitants.

Plutonium Signatures in a Dated Sediment Core as a Tool to Reveal Nuclear Sources in the Baltic Sea

Plutonium distribution was studied in an undisturbed sediment core sampled from the Tvären bay in the vicinity of the Studsvik nuclear facility in Sweden. The complete analysis, including minor isotopes, of the Pu isotope composition (²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu, ²⁴²Pu, and ²⁴⁴Pu) allowed us to establish the Pu origin in this area of the Baltic Sea and to reconstruct the Studsvik aquatic release history. The results show highly enriched ²³⁹Pu, probably originating from the Swedish nuclear program in the 1960s and 1970s and the handling of high burn-up nuclear fuel in the later years. In addition, the ²⁴⁴Pu/²³⁹Pu atomic ratio for the global fallout period between 1958 and 1965 is suggested to be (7.94 ± 0.31)·10^-5. In the bottom layer of the sediment, dated 1953-1957, we detected a higher average ²⁴⁴Pu/²³⁹Pu ratio of (1.51 ± 0.11)·10^-4, indicating the possible impact of the first US thermonuclear tests (1952-1958).

240Pu/239Pu signatures allow refining the chronology of radionuclide fallout in South America

Atmospheric nuclear tests (1945-1980) have led to radioactive fallout across the globe. French tests in Polynesia (1966-1974) may influence the signature of fallout in South America in addition to those conducted by USA and former USSR until 1963 in the Northern hemisphere. Here, we compiled the ²⁴⁰Pu/²³⁹Pu atom ratios reported for soils of South America and conducted additional measurements to examine their latitudinal distributions across this continent. Significantly lower ratio values were found in the 20-45° latitudinal band (0.04 to 0.13) compared to the rest of the continent (up to 0.20) and attributed to the contribution of the French atmospheric tests to the ultra-trace plutonium levels found in these soils. Based on sediment cores collected in lakes of Chile and Uruguay, we show the added value of measuring ²⁴⁰Pu/²³⁹Pu atom ratios to refine the age models of environmental archives in this region of the world.

Sequential scavenging and measurement of seawater radiocesium concentrations and plutonium isotopic ratios offshore Fukushima

The scientific interest in radiocesium and plutonium found in the oceans and seas has increased enormously in the past years as a consequence of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident and is expected to be ongoing due to many unresolved questions. Hence, continuous development of new and verification of old analytical methods should be at the top of the list of the community, working on the topic. In this study, we processed and analyzed several seawater samples, collected in different time frames (2011-2015) from the North Pacific Ocean offshore Fukushima, to determine their radiocesium activities, ¹³⁴Cs/¹³⁷Cs activity ratios and ²⁴⁰Pu/²³⁹Pu isotopic ratios using the sequential scavenging method, gamma spectrometry and accelerator mass spectrometry (AMS). The observed radiocesium levels in seawater (0.07-0.042 Bq L^-1) clearly indicated that the investigated region remained impacted by releases from the damaged power plant even after four years after the accident. Regarding plutonium, its successful separation from large volume seawater samples was confirmed by detection of ²⁴⁰Pu by AMS. However, several problems emerged during the analyzes, which we tried to address with the use of additional methods (e.g., measurements of uranium by ICPMS). The efficiencies of the applied methods and other issues are also discussed.

Plutonium isotopes in the North Western Pacific sediments coupled with radiocarbon in corals recording precise timing of the Anthropocene

Plutonium (Pu) has been used as a mid-twentieth century time-marker in various geological archives as a result of atmospheric nuclear tests mainly conducted in 1950s. Advancement of analytical techniques allows us to measure ²³⁹Pu and ²⁴⁰Pu more accurately and can thereby reconstruct the Pacific Pu signal that originated from the former Pacific Proving Grounds (PPG) in the Marshall Islands. Here, we propose a novel method that couples annual banded reef building corals and nearshore anoxic marine sediments to provide a marker to precisely determine the start of the nuclear era which is known as a part of the Anthropocene. We demonstrate the efficacy of the methods using sediment obtained from Beppu Bay, Japan, and a coral from Ishigaki Island, Japan. The sedimentary records show a clear Pu increase from 1950, peaking during the 1960s, and then showing a sharp decline during the 1970s. However, a constantly higher isotope ratio between ²³⁹Pu and ²⁴⁰Pu suggest an additional contribution other than global fallout via ocean currents. Furthermore, single elevations in ²⁴⁰Pu/²³⁹Pu provide supportive evidence of close-in-fallout similar to previous studies. Coral skeletal radiocarbon displays a clear timing with the signatures supporting the reliability of the Beppu Bay sediments as archives and demonstrates the strength of this method to capture potential Anthropocene signatures.

Distributions and impacts of plutonium in the environment originating from the Fukushima Daiichi Nuclear Power Plant accident: An overview of a decade of studies

This paper reviews the current knowledge on plutonium (Pu) isotopic composition (the atom or activity ratios) and activity concentrations of ²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu, and ²⁴¹Pu resulting from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in 2011. In this critical review, we document the characteristic values of Pu atom or activity ratios (fingerprints) and present their spatial distributions around the FDNPP site. Based on multiple Pu fingerprints (²³⁸Pu/^239+240Pu activity ratio, ²⁴⁰Pu/²³⁹Pu atom ratio, and ²⁴¹Pu/²³⁹Pu atom ratio), we clarify that Pu contamination from the FDNPP accident occurred in a restricted terrestrial area, while Pu in the Northwest Pacific Ocean is still predominately sourced from the Pacific Proving Grounds (PPG) and global fallout. Using a simple two end-member mixing model, we calculate average contributions of Pu from the FDNPP accident of 13 ± 20% (n = 180) in soil samples, 55 ± 32% (n = 38) in leaf litter samples, and 67 ± 26% (n = 129) in air dust/black substances. In the marine environment, the PPG source average contributions are 45 ± 15% (n = 76) in seawater and 42 ± 12% (n = 48) in sediments. The spatial distributions of Pu atom or activity ratios based on existing studies suggest that: 1) in the terrestrial region investigated 80 km northwest of the FDNPP site, the Pu contamination is mainly observed in an area within a 50 km distance, and 2) in the terrestrial region investigated 60 km southwest of the FDNPP site, the Pu contamination is mainly observed in an area within a 30 km distance. Studies of Cs-bearing radioactive particles indicate that Pu occurs as Pu oxide, and the fuel fragments containing Pu that were released from the reactors to the surrounding environment are associated with micron-scale Cs-bearing radioactive particles. We note that the fractionation between Pu and other radionuclides occurred after release. These new findings about the Pu fingerprints around the FDNPP site will help researchers to establish a reference background database for future environmental risk assessment and geochemical study there.

Plutonium isotopes in the northwestern South China Sea: Level, distribution, source and deposition

The spatial distribution of plutonium isotopes (²³⁹Pu, ²⁴⁰Pu) in the surface sediments collected from the northwestern South China Sea (SCS) in 2018 was investigated. The ^239,240Pu concentrations in surface sediments vary from 0.048 to 0.960 mBq/g (with mean of 0.282 ± 0.242 mBq/g) depending on the geographical feature of the sampling location such as the river estuary, continental shelf, slope and deep basin. Higher ²⁴⁰Pu/²³⁹Pu atomic ratios (0.24-0.31) in the surface sediment of the SCS compared to the global fallout value of 0.18 were observed, this is attributed to the input of close-in fallout of the Pacific Proving Ground (PPG) transported by the North Equatorial Current and Kuroshio Current to the northern SCS. The contribution of the PPG derived plutonium in the SCS sediment was estimated to be 39%-78% using a simple two-end member mixing model based on the measured ²⁴⁰Pu/²³⁹Pu atomic ratios in the sediment. Besides the soluble ^239,240Pu level in seawater, load of suspended particulate matter from the river runoff and biological debris, hydrographic and hydrodynamic conditions are key parameters influencing the deposition process of plutonium to the sediment.

Distribution of plutonium isotopes in soils between two nuclear test sites: Semipalatinsk and Lop Nor

Plutonium (Pu) has attracted attention as an environmental tracer due to its radiotoxicity and the possibility of sources linked with nuclear accidents in recent years. Plutonium isotopes (^239,240Pu) were detected at trace levels in soils collected from the Xinjiang region located between the Semipalatinsk nuclear test site and China's Lop Nor nuclear test site. Little is known regarding the spatial variation of ^239,240Pu in soils from this region. This study reports the use of Sector Field Inductively Coupled Plasma Mass Spectrometry (SF-ICP-MS) methods to distinguish between Pu isotopes derived from global fallout and nuclear weapon tests. We found that the ^239,240Pu activity concentrations ranged from 0.035 to 1.338 mBq/g; the ²⁴⁰Pu/²³⁹Pu atomic ratios were 0.157-0.223 with a weighted average of 0.180 ± 0.002, corresponding with the expected average global fallout ratio of 0.180 ± 0.014. This indicated that global fallout is the major source of Pu in the study region. The ^239,240Pu inventories in these soils ranged from 23.67 to 222.7 Bq/m², corresponding with those from other areas in China and other countries within the latitude range. Our Pu isotope data was supplemented with other published Pu data for soils collected in the vicinity of the Semipalatinsk nuclear test site and Lop Nor nuclear test site. Results indicate that ^239,240Pu inventories and ²⁴⁰Pu/²³⁹Pu atomic ratios in soils exhibit large variations with distance from the Semipalatinsk nuclear test site. High deposition and accumulation of Pu, and low ²⁴⁰Pu/²³⁹Pu ratios were observed in close-in fallout and downwind regions of the Semipalatinsk nuclear test site and China's Lop Nor nuclear test site.

Isotopic signatures of plutonium and uranium at Bikar atoll, northern Marshall Islands

We present plutonium (Pu) and uranium (U) isotopic fingerprints (or signatures) in environmental samples collected at Bikar Atoll. Bikar is the second -most northern atoll of the Republic of the Marshall Islands, and therefore an important reference point to evaluate the extension of the regional fallout from the Pacific Proving Grounds (PPG) in Bikini and Enewetak Atolls. Previous studies have shown that regional fallout from atmospheric nuclear weapon testing (NWT) in Bikini and Enewetak has resulted in elevated levels of fallout radionuclides in this atoll. In order to optimally interpret the isotopic fingerprints, we compare our results with data obtained in eleven certified reference materials, representing different contamination sources. As well as ²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu, ²³⁸U and ²³⁵U, this study also encompasses less commonly reported radionuclides such as ²⁴²Pu, ²⁴⁴Pu and ²³⁶U. We show the importance of combining numerous fingerprints for improved assessment of the source of a nuclear contamination. In samples from Bikar, Pu and U isotope ratios were found to vary within narrow ranges. Pu and U fingerprints suggest that regional fallout from the Castle Bravo test in March 1954 was the main source of the contamination. This was further confirmed by two different age dating approaches that estimated 1954 as the year of the contamination. We demonstrate that use of an exponential function to approximate the yield of heavy radionuclides in thermonuclear explosions with increasing mass is a valid approach for estimating the age of a contamination. We show that, if sufficient radionuclide activity concentration measurement results with low uncertainties are available, this method is robust.

Assessment of residual radionuclide levels at the Bokak and Bikar Atolls in the northern Marshall Islands

The Bikar and Bokak Atolls, located in the northern Marshall Islands, are extremely isolated and consist of pristine marine and terrestrial ecosystems. Both atolls may have experienced significant radioactive deposition following the nuclear weapon testing conducted at Bikini and Enewetak proving grounds. Here we report activity concentrations of artificial radionuclides (²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu, ²⁴¹Am, ¹³⁷Cs and ⁹⁰Sr) in marine and terrestrial samples collected from Bikar and Bokak Atolls. Artificial radionuclides in soil from the Majuro Atoll are also reported and form a radiological baseline against which the levels at the other atolls can be compared. We observed low levels of artificial radionuclides in soil from Majuro and Bokak, but significantly higher levels in soil from Bikar. The residual radioactivity in the Bikar environment is comparable to the levels previously reported for other nearby atolls, including Taka and Utrik, but lower than for Rongerik, Rongelap, Bikini and Enewetak. An analysis of ²⁴⁰Pu/²³⁹Pu isotope ratios and estimations of the dates of contamination from ²⁴¹Am/²⁴¹Pu activity ratios both indicated that the Bikar Atoll was contaminated mainly by radioactive fallout from the Castle Bravo test in 1954. We compare the results of our measurements at Bikar and Bokak to data from other atolls in the Marshall Islands and to regions of the world affected by both global and regional fallout from atmospheric nuclear weapons testing and nuclear accidents.

Temporal trend of 240Pu/239Pu atom ratios in water columns in the Western North Pacific Ocean and its marginal seas

The ²⁴⁰Pu/²³⁹Pu atom ratios were determined by inductively coupled plasma mass spectrometry for seawater samples from the Okinawa Trough in the western North Pacific Ocean margin to identify their Pu sources and to elucidate the temporal variability in atom ratios in the water columns in the western North Pacific Ocean and its marginal seas. The ²³⁹Pu, ²⁴⁰Pu and ^239+240Pu water column inventories were 12.2 ± 0.5, 10.5 ± 0.5 and 22.7 ± 0.7 Bq m^-2, respectively. The observed ²⁴⁰Pu/²³⁹Pu ratios were clearly greater than the mean global fallout ratio of ∼0.18 with a mean value of 0.236 ± 0.014. The observed greater atom ratios confirmed the presence of close-in fallout Pu from nuclear testing at the Pacific Proving Grounds (PPG). The relative percentage of the PPG-derived Pu to global fallout Pu was estimated as 41 ± 10%. The ^239+240Pu water column inventory originating from the PPG close-in fallout was also calculated as 9.2 ± 0.4 Bq m^-2. The temporal variability in the mean ²⁴⁰Pu/²³⁹Pu atom ratios in the water columns in the western North Pacific Ocean and its marginal seas was obtained through comparison of the present study values and the previously reported values, and it had no significant temporal variation with a mean value of 0.237 ± 0.004 during the observation period of 1984-2015. The ²³⁹Pu and ²⁴⁰Pu derived from the two different sources of global fallout and the close-in fallout at the PPG were homogenized in the water masses in the studied areas.

Distributions of 239Pu and 240Pu Concentrations and 240Pu/239Pu Atom Ratios and 239+240Pu Inventories in a Water Column in the Eastern Indian Ocean: Transport of Pacific Proving Grounds-Derived Pu via the Indonesian Throughflow

The ^239+240Pu concentrations and ²⁴⁰Pu/²³⁹Pu atom ratios in seawater from the eastern Indian Ocean were determined to identify their Pu sources and to propose the transport pathway of Pacific Proving Grounds (PPG)-derived Pu into the studied area. This is the first study by anyone on these Pu atom ratios in the Indian Ocean. In the West Australia Basin, the ^239+240Pu concentration was 2.89 mBq m^-3 in the surface water and increased with depth; a subsurface maximum was identified at 200 m depth and then decreased gradually with depth; its water column inventory was 32.8 Bq m^-2. The inventory-weighted mean ²⁴⁰Pu/²³⁹Pu atom ratios were 0.208 in the South Australia Basin, 0.226 in the Perth Basin, 0.242 in the West Australia Basin, 0.232 in the Bay of Bengal, and 0.225 in the Andaman Sea. The obtained ²⁴⁰Pu/²³⁹Pu ratios were clearly greater than the mean global fallout ratio of 0.18. These high atom ratios proved the presence of close-in fallout Pu from PPG nuclear tests. The relative contribution of global and PPG fallouts was evaluated using the two-end-member mixing model. The ^239+240Pu inventories originating from the PPG fallout were calculated as 2.9-14.9 Bq m^-2, which corresponded to 20-46% of the total ^239+240Pu inventory. A significant amount of the PPG-derived Pu has been transported to the eastern Indian Ocean. The proposed transport pathway accounting for the high ²⁴⁰Pu/²³⁹Pu ratio is the transportation of PPG-derived Pu by the North Equatorial Current followed by the Mindanao Current, Indonesian Throughflow, and then spreading over the Indian Ocean by its surface circulation system.

A record of fallout 239Pu and 240Pu at World Heritage Bathurst Harbour, Tasmania, Australia

This study presents the first measurements of anthropogenic plutonium (²³⁹Pu and ²⁴⁰Pu) concentrations and atom ratios (²⁴⁰Pu/²³⁹Pu) for Tasmania, in sediment collected from Bathurst Harbour, in the Tasmanian Wilderness World Heritage Area, Australia. The weighted mean ²⁴⁰Pu/²³⁹Pu atom ratio measured at this site was 0.172 ± 0.007 which is consistent with published data from mainland Australia and global and Southern Hemisphere averages. The ²⁴⁰Pu/²³⁹Pu atom ratios ranged between 0.11 and 0.21 with the earliest recorded ²⁴⁰Pu/²³⁹Pu atom ratios being the lowest, suggesting an influence of low atom ratio fallout from nuclear testing in Australia. Post-moratorium fallout ²⁴⁰Pu/²³⁹Pu atom ratios were consistent with other records. Lead-210 (²¹⁰Pb) sediment chronologies indicate sediment accumulation rates have increased since the early part of the 19th century at this location.

Contribution of close-in fallout from the French atmospheric tests in inventories of 137Cs, 241Am and plutonium (238, 239, 240) in Gambier Islands (French Polynesia) - Signatures of stratospheric fallout in the Southern Hemisphere

The inventories of ¹³⁷Cs (503 ± 34 Bq.m^-2), ²⁴¹Am (11.3 ± 1.2 Bq.m^-2), ²⁴¹Pu (33.7 ± 3.4 Bq.m^-2), ²³⁸Pu (6.82 ± 0.87 Bq.m^-2) and ^239+240Pu (113.0 ± 5.9 Bq.m^-2), sum of ²³⁹Pu (100 ± 11 Bq.m^-2) and ²⁴⁰Pu (14.5 ± 1.7 Bq.m^-2), in the Gambier archipelago (23°S) of the French Polynesia, are well higher the global fallout at this latitude, in unequal proportions for the different radionuclides. (²⁴⁰Pu/²³⁹Pu)_AR (AR: atomic ratio) of 0.0394 ± 0.0062, and (²⁴¹Pu/²³⁹Pu)_AR of (2.03 ± 0.39)10^-4, confirm that the overwhelmingly dominant source of these radionuclides comes from local fallout during the 1970s of the French atmospheric tests of Moruroa and Fangataufa located nearly 400 km from Gambier. The signatures of the local fallout were deduced from the excess of its inventory in ¹³⁷Cs and from the mixing lines established from the signatures of the global fallout, some of the test sites and the isotopic ratios measured in Gambier. Signatures obtained are 2.0 ± 0.4 for ¹³⁷Cs/^239+240Pu, 0.045 ± 0.008 for ²³⁸Pu/^239+240Pu, 0.031 ± 0.009 for ²⁴¹Am/^239+240Pu, 0.092 ± 0.027 for ²⁴¹Pu/^239+240Pu, 0.0163 ± 0.0049 for (²⁴⁰Pu/²³⁹Pu)_AR, (0.78 ± 0.23)10^-4 for (²⁴¹Pu/²³⁹Pu)_AR. The concordance of the mixing lines of the [(²⁴⁰Pu/²³⁹Pu)_AR, (²⁴¹Pu/²³⁹Pu)_AR] and the linear regression of these ratios measured in the stratosphere (40°S) during the 1970s, indicates that the signatures of the close-in deposition are also those of the stratospheric injections of the French tests. The signatures of stratospheric fallout in the Southern Hemisphere were evaluated by considering that the fission energy of these injections represents 11% and that of the Northern Hemisphere represents 89% and that the isotopic ratios of stratospheric injections remained the same over the period 1970-1974. The activity ratios deducted are 21.9 ± 0.1 in ¹³⁷Cs/^239+240Pu, 0.11 ± 0.05 in ²³⁸Pu/^239+240Pu, 1.03 ± 0.12 in ²⁴¹Pu/^239+240Pu and 0.35 ± 0.04 in ²⁴¹Am/^239+240Pu. The associated atom ratios are 0.157 ± 0.011 for (²⁴⁰Pu/²³⁹Pu)_AR and (8.33 ± 0.48)10^-4 for (²⁴¹Pu/²³⁹Pu)_AR. These signatures appear to be consistent with the results of the inventories at Hiva Oa, located more than 1,000 km north of both French test sites, and with those found in the Australian continent, in regions not impacted by UK-test debris. The proportions of close-in tropospheric fallout from the French tests are about 90% in Gambier. They represent a proportion in the inventories of 40% for the ¹³⁷Cs, 60% for ²⁴¹Am and in the range between 80 and 90% for Pu isotopes.

Phytoremediation of radionuclides in soil, sediments and water

Soil and water contaminated with radionuclides threaten the environment and public health during leaks from nuclear power plants. Remediation of radionuclides at the contaminated sites uses mainly physical and chemical methods such as vitrification, chemical immobilization, electro-kinetic remediation and soil excavation, capping and washing being among the preferred methods. These traditional technologies are however costly and less suitable for dealing with large-area pollution. In contrast to this, cost-effective and environment-friendly alternatives such as phytoremediation using plants to remove radionuclides from polluted sites in situ represent promising alternatives for environmental cleanup. Understanding the physiology and molecular mechanisms of radionuclides accumulation in plants is essential to optimize and improve this new remediation technology. Here, we give an overview of radionuclide contamination in the environment and biochemical characteristics for uptake, transport, and compartmentation of radionuclides in plants that characterize phytoextraction and its efficiency. Phytoextraction is an eco-friendly and efficient method for environmental removal of radionuclides at contaminated sites such as mine tailings. Selecting the most proper plant for the specific purpose, however, is important to obtain the best result together with, for example, applying soil amendments such as citric acid. In addition, using genetic engineering and optimizing agronomic management practices including regulation of atmospheric CO₂ concentration, reasonable measures of fertilization and rational water management are important as well. For future application, the technique needs commercialization in order to fully exploit the technique at mining activities and nuclear industries.

Local and global trace plutonium contributions in fast breeder legacy soils

Trace-level plutonium in the environment often comprises local and global contributions, and is usually anthropogenic in origin. Here, we report estimates of local and global contributions to trace-level plutonium in soil from a former, fast-breeder reactor site. The measured ²⁴⁰Pu/²³⁹Pu ratio is anomalously low, as per the reduced ²⁴⁰Pu yield expected in plutonium bred with fast neutrons. Anomalies in plutonium concentration and isotopic ratio suggest forensic insight into specific activities on site, such as clean-up or structural change. Local and global ²³⁹Pu contributions on-site are estimated at (34 ± 1)% and (66 ± 3)%, respectively, with mass concentrations of (183 ± 6) fg g^-1 and (362 ± 13) fg g^-1. The latter is consistent with levels at undisturbed and distant sites, (384 ± 44) fg g^-1, where no local contribution is expected. The ²⁴⁰Pu/²³⁹Pu ratio for site-derived material is estimated at 0.05 ± 0.04. Our study demonstrates the multi-faceted potential of trace plutonium assay to inform clean-up strategies of fast breeder legacies.

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.

Use of natural and artificial radionuclides to determine the sedimentation rates in two North Caucasus lakes

The specific activities of natural (²¹⁰Pb, ²²⁶Ra, and ²³²Th) and artificial (¹³⁷Cs, ^239,240Pu, and ²⁴¹Am) radionuclides in the sediments of two North Caucasus lakes were determined. The two lakes, Lake Khuko and Lake Donguz-Orun, differ in their sedimentation conditions. Based on the use of unsupported ²¹⁰Pb_ex and both Chernobyl-derived and bomb-derived ¹³⁷Cs as chronological markers, it was established that the sedimentation rates in Lake Khuko over the past 55-60 y did not exceed 0.017 cm y^-1. Sedimentation rates in Lake Donguz-Orun were found to be more than an order of magnitude higher. In the latter case, the sedimentation rates for the period from 1986 to the present were over 1.5 times higher than they were for the period 1963-1986. The differences in sedimentation rates were due to differences in the rates of denudation of their respective catchment areas. The specific activities of artificial radionuclides (¹³⁷Cs, 2600 Bq kg^-1; ^239,240Pu, 162 Bq kg^-1; and ²⁴¹Am, 36 Bq kg^-1) and their ratios in the sediments of Lake Khuko show that their deposition was mainly due to global stratospheric fallout of technogenic radionuclides associated with nuclear bomb testing during 1954-1963-rather than fallout from the Chernobyl accident. Several factors, including the mode of precipitation, features of the surface runoff, and location of Lake Khuko, were responsible for the accumulation of artificial radionuclides.

240Pu/239Pu atom ratios in water columns from the North Pacific Ocean and Bering Sea: Transport of Pacific Proving Grounds-derived Pu by ocean currents

The ²⁴⁰Pu/²³⁹Pu atom ratios in seawater from the northern North Pacific Ocean and Bering Sea were determined to elucidate the temporal variability in ^239+240Pu inventory, to identify their Pu sources and to propose the transport pathway of Pacific Proving Grounds (PPG)-derived Pu into the studied area. Water column ^239+240Pu inventory in the Bering Sea was lower than that reported for a comparable station in the Geochemical Ocean Sections Study, indicating that the Bering Sea inventory has been decreasing at the average rate of 2.6 Bq m^-2 yr^-1. The ²⁴⁰Pu/²³⁹Pu atom ratios ranged from 0.222 to 0.255 with a mean value of 0.235 in the northern North Pacific Ocean and from 0.215 to 0.237 with a mean value of 0.224 in the Bering Sea. The obtained ²⁴⁰Pu/²³⁹Pu ratios were clearly higher than the mean global fallout ratio of 0.18. These high atom ratios proved the presence of close-in fallout Pu from nuclear weapons testing at the PPG. The relative contribution of the global fallout and the PPG fallout were evaluated by using the two-end-member mixing model. The ^239+240Pu inventories originating from the PPG fallout were calculated as 17.9-25.4 Bq m^-2 in the northern North Pacific Ocean and 11.6-16.6 Bq m^-2 in the Bering Sea, which corresponded to 38-54% and 31-45% of the total ^239+240Pu inventory in the water column in the northern North Pacific Ocean and Bering Sea, respectively. A significant amount of the PPG-derived Pu has been transported to the northern North Pacific Ocean and Bering Sea. The proposed transport pathway accounting for the high ²⁴⁰Pu/²³⁹Pu ratio in the Bering Sea water column is the transportation of PPG-derived Pu by the North Equatorial Current followed by the Kuroshio Current, Kuroshio Extension, North Pacific Current and subsequently the Alaska Current, and then the Alaskan Stream.

Radionuclides in the environment around the uranium mines in Guangxi, China

Uranium and plutonium are both poisonous radioactive elements, which are very harmful to human health and environment. Therefore, it is of great significance to study the distribution of ²³⁸U concentration and ^239+240Pu activity in the uranium mine surrounding soils. We have collected some surface soil sediments within 2 km of two uranium mines and a solid waste management center in Guangxi Province. The ²³⁸U concentration in these study areas is in the range of 1.44-83.91 mg/g, and the ²³⁸U concentration in the A uranium mine surrounding surface soils is higher than that in the B uranium mine and the solid waste management center. While the B uranium mine and the solid waster management center don't pollute the surrounding soils because the ²³⁸U concentrations in their surrounding soils are similar to the average ²³⁸U concentration in the soil. The ^239+240Pu activities in soil samples collected around the two uranium mines and the solid waste management center are close ranged from 0.06 mBq/g to 0.51 mBq/g. Moreover, the ²⁴⁰Pu/²³⁹Pu atom ratios in our study samples are ranged from 0.15 to 0.23, which indicate the Pu may come from the global fallout. In addition, we study heavy metals in our collected samples, only heavy metal Tl has weak positive correlations with ²³⁸U concentrations and ^239+240Pu activities. And there is a weak positive correlation between ²³⁸U concentrations and ^239+240Pu activities.

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.

Background gamma radiation and soil activity measurements in the northern Marshall Islands

From 1946 to 1958, the United States tested 67 nuclear weapons in the Marshall Islands, a remote constellation of atolls in the Pacific Ocean that was then a US trust territory. Two atolls, Bikini and Enewetak, were used as ground zero for the tests, which caused unprecedented environmental contamination and, for the indigenous peoples of the islands, long-term adverse health effects. In addition to the populations of Bikini and Enewetak, the people of Rongelap and Utirik were also affected by radioactive fallout from the largest nuclear test the United States has ever conducted, the Bravo test held March 1, 1954. This article presents a picture of current radiological conditions by examining external gamma radiation and soil radionuclide activity concentrations.

We report on measurements of external gamma radiation on 9 islands in 4 atolls in the northern Marshall Islands, all of which were affected by the US nuclear testing program from 1946 to 1958 (Enjebi, Ikuren, and Japtan in Enewetak Atoll; Bikini and Enyu in Bikini Atoll; Naen in Rongelap Atoll; and Aon, Elluk, and Utirik in Utirik Atoll). We also report americium-241, cesium-137, plutonium-238, and plutonium-239,240 activity concentrations in the soil samples for 11 islands in 4 northern atolls (Enewetak, Japtan, Medren, and Runit in Enewetak Atoll; Bikini and Enyu in Bikini Atoll; Naen and Rongelap in Rongelap Atoll; and Aon, Elluk, and Utirik in Utirik Atoll) and from Majuro Island, Majuro Atoll in the southern Marshall Islands. Our results show low external gamma radiation levels on some islands in the Enewetak Atoll and Utirik Atoll, and elevated levels on Enjebi Island in the Enewetak Atoll, on Bikini Atoll, and on Naen Island in the Rongelap Atoll. We perform ordinary kriging on external gamma radiation measurements to provide interpolated maps. We find that radionuclides are absent from all Majuro soil samples, and that they are present at highest activity concentrations in samples from Runit and Enjebi islands (Enewetak Atoll), Bikini Island (Bikini Atoll), and Naen Island (Rongelap Atoll). We contextualize all results by making comparisons between islands and to various standards, as well as to regions of the world affected by nuclear accidents. We also discuss implications for informed decision-making by the Marshallese and local atoll governments and their people on issues pertaining to island resettlement.

240Pu/239Pu and 242Pu/239Pu atom ratios of Japanese monthly atmospheric deposition samples during 1963-1966

Global fallout plutonium isotopic ratios from the 1960s are important for the use of Pu as environmental tracers. We measured the ²⁴⁰Pu/²³⁹Pu and ²⁴²Pu/²³⁹Pu atomic ratios of monthly atmospheric deposition samples collected in Tokyo and Akita, Japan during March 1963 to May 1966. To our knowledge, our results represent the first data measured for actual atmospheric deposition samples collected continuously during the 1960s. Both atomic ratios increased rapidly from March 1963 to June 1963, followed by a gradual increase until September 1963. Then, both ratios declined with a half-life of approximately 5.6 months. The observed temporal changes of the ratios were likely caused by the upper-stratospheric input of nuclear debris from high-yield atmospheric nuclear weapon testing during 1961–62, followed by its downward transport to the troposphere.

Isotopic composition and source of plutonium in the Qinghai-Tibet Plateau frozen soils

The 239+240Pu activities and 240Pu/239Pu atom ratios in the frozen soils of the Yellow River Source Area (YRSA) were determined to examine the Pu source and evaluate its environmental risk. The 239+240Pu activities of surface frozen soils in the YRSA, ranging from 0.053 to 0.836 mBq g-1, are comparable to those observed in China elsewhere (0.005-1.990 mBq g-1). The 240Pu/239Pu atom ratios of surface soils in the YRSA are in the range of 0.168-0.201 (average = 0.187 ± 0.012, n = 6), comparable to the global fallout of 0.180 ± 0.014. Based on the latitudinal and spatial distribution of Pu isotopic composition, I clarified that the Pu source is mainly from global fallout at present. The activity levels of Pu in the YRSA do far not cause a Pu toxicity to the downstream drinking water even the frozen soil begins to melt and release Pu to the Yellow River. However, since close-in fallout from Lop Nor where the Chinese nuclear tests were carried out during 1964-1980, high deposition and accumulation of Pu was observed in the Chinese soil cores through synthesizing an expanded Pu dataset, which alerts us it is necessary to further monitor the Pu activity levels in the YRSA soil cores to ensure the safety of downstream drinking water. Finally, I point out that information on Pu isotopes would help in establishing a baseline for future environmental risk assessment.

Sources and distribution of 241Am in the vicinity of a deep geologic repository

The detection, distribution, and long-term behavior of ²⁴¹Am in the terrestrial environment at the Waste Isolation Pilot Plant (WIPP) site were assessed using historical data from an independent monitoring program conducted by the Carlsbad Environmental Monitoring & Research Center (CEMRC), and its predecessor organization the Environmental Evaluation Group (EEG). An analysis of historical data indicates frequent detections of trace levels of ²⁴¹Am in the WIPP environment. Positive detections and peaks in ²⁴¹Am concentrations in ambient air samples generally occur during the March to June timeframe, which is when strong and gusty winds in the area frequently give rise to blowing dust. A study of long-term measurements of ²⁴¹Am in the WIPP environment suggest that the resuspension of previously contaminated soils is likely the primary source of americium in the ambient air samples from WIPP and its vicinity. Furthermore, the ²⁴¹Am/^{239 + 240}Pu ratio in aerosols and soils was reasonably consistent from year to year and was in agreement with the global fallout ratios. Higher than normal activity concentrations of ²⁴¹Am and ²⁴¹Am/^{239 + 240}Pu ratios were measured in aerosol samples during 2014 as a result of February 14, 2014 radiation release event from the WIPP underground. However, after a brief spike, the activity concentrations of ²⁴¹Am have returned to the normal background levels. The long-term monitoring data suggest there is no persistent contamination and no lasting increase in radiological contaminants in the region that can be considered significant by any health-based standard.

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.

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.

Sedimentary record of plutonium in the North Yellow Sea and the response to catchment environmental changes of inflow rivers

Plutonium (Pu) isotopes were first determined in surface and core sediment samples collected from the northern North Yellow Sea (NYS) to elucidate their source terms and deposition process as well as the response to catchment environmental changes of inflow rivers. ²⁴⁰Pu/²³⁹Pu atom ratios in all sediments showed the typical global fallout value of ∼0.18 without any influences from the nuclear weapons tests conducted recently in the North Korea or early in the Pacific Proving Ground. The large variation of ^239+240Pu activities (0.022-0.515 mBq/g) observed in surface sediments should be mainly attributed to the re-suspension and transportation of fine sediments influenced by the Liaonan Costal Current. Based on the two ^239+249Pu depth profiles with easily observed onset fallout levels (1952) and global fallout peaks (1963), ^239+240Pu served as a valid time mark in the coastal sedimentary system. Riverine input Pu contributed only 15-27% to the total global fallout inventory (92.5-108.8 Bq/m²) in the northern NYS, much lower than that in the Yangtze River estuary (77-80%), indicating a better soil conservation in the northeast China due to higher forest coverage compared to the Yangtze River's drainage basin. The increase of riverine input Pu after 1980s reflected the more intense soil erosion degree caused by the land use and cover change due to the increment of human activities in the northeast China at the same period. Our results demonstrated that plutonium is a good indicator for studying sedimentary process and its response to the environment in the coastal area.

Determination of ultra-low level plutonium isotopes (239Pu, 240Pu) in environmental samples with high uranium

In order to measure trace plutonium and its isotopes ratio (²⁴⁰Pu/²³⁹Pu) in environmental samples with a high uranium, an analytical method was developed using radiochemical separation for separation of plutonium from matrix and interfering elements including most of uranium and ICP-MS for measurement of plutonium isotopes. A novel measurement method was established for extensively removing the isobaric interference from uranium (²³⁸U¹H and ²³⁸UH₂⁺) and tailing of ²³⁸U, but significantly improving the measurement sensitivity of plutonium isotopes by employing NH₃/He as collision/reaction cell gases and MS/MS system in the triple quadrupole ICP-MS instrument. The results show that removal efficiency of uranium interference was improved by more than 15 times, and the sensitivity of plutonium isotopes was increased by a factor of more than 3 compared to the conventional ICP-MS. The mechanism on the effective suppress of ²³⁸U interference for ²³⁹Pu measurement using NH₃-He reaction gases was explored to be the formation of UNH⁺ and UNH₂⁺ in the reactions of UH⁺ and U⁺ with NH₃, while no reaction between NH₃ and Pu⁺. The detection limits of this method were estimated to be 0.55 fg mL^-1 for ²³⁹Pu, 0.09 fg mL^-1 for ²⁴⁰Pu. The analytical precision and accuracy of the method for Pu isotopes concentration and ²⁴⁰Pu/²³⁹Pu atomic ratio were evaluated by analysis of sediment reference materials (IAEA-385 and IAEA-412) with different levels of plutonium and uranium. The developed method were successfully applied to determine ²³⁹Pu and ²⁴⁰Pu concentrations and ²⁴⁰Pu/²³⁹Pu atomic ratios in soil samples collected in coastal areas of eastern China.

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.

Establishing rapid analysis of Pu isotopes in seawater to study the impact of Fukushima nuclear accident in the Northwest Pacific

In order to assess the impact of the Fukushima derived Pu isotopes on seawater, a new analytical method to rapidly determine Pu isotopes in seawater by SF-ICP-MS including Fe(OH)₂ primary co-precipitation, CaF₂/LaF₃ secondary co-precipitation and TEVA+UTEVA+DGA extraction chromatographic separation was established. High concentration efficiency (~100%) and high U decontamination factor (~10⁷) were achieved. The plutonium chemical recoveries were 74–88% with the mean of 83 ± 5%. The precisions for both ²⁴⁰Pu/²³⁹Pu atom ratios and ^239+240Pu activity concentrations were less than 5% when 15 L of seawater samples with the typical ^239+240Pu activity of the Northwest Pacific were measured. It just needs 12 hours to determine plutonium using this new method. The limit of detection (LOD) for ²³⁹Pu and ²⁴⁰Pu were both 0.08 fg/mL, corresponding to 0.01 mBq/m³ for ²³⁹Pu and 0.05 mBq/m³ for ²⁴⁰Pu when a 15 L volume of seawater was measured. This method was applied to determine the seawater samples collected 446–1316 km off the FDNPP accident site in the Northwest Pacific in July of 2013. The obtained ^239+240Pu activity concentrations of 1.21–2.19 mBq/m³ and the ²⁴⁰Pu/²³⁹Pu atom ratios of 0.198–0.322 suggested that there was no significant Pu contamination from the accident to the Northwest Pacific.

Sources and accumulation of plutonium in a large Western Pacific marginal sea: The South China Sea

In order to examine the sources of plutonium (Pu) and elaborate its scavenging and accumulation processes, ²⁴⁰Pu/²³⁹Pu atom ratios and ^239+240Pu activities in the water column of the South China Sea (SCS) were determined and compared with our previously reported data for the sediments. Consistently high ²⁴⁰Pu/²³⁹Pu atom ratios that ranged from 0.184-0.250 (average=0.228±0.015), indicative of non-global fallout Pu sources were observed both in the surface water and at depth during 2012-2014. The spatial distribution of the ²⁴⁰Pu/²³⁹Pu atom ratio in the SCS showed a decreasing trend away from the Luzon Strait, which was very consistent with the introduction pathway of the Kuroshio Current. The Kuroshio had an even heavier Pu isotopic ratio ranging from 0.250-0.263 (average=0.255±0.006), traceable to the non-global fallout Pu signature from the Pacific Proving Grounds (PPG). Using a simple two end-member mixing model, we further revealed that this PPG source contributed 41±17% of the Pu in the SCS water column. The ^239+240Pu activities in the SCS surface seawater varied from 1.59 to 2.94mBqm^-3, with an average of 2.34±0.38mBqm^-3. Such an activity level was ~40% higher than that in the Kuroshio. The distribution of ^239+240Pu in the surface seawater further showed a general trend of increase from the Kuroshio to the SCS basin, suggesting significant accumulation of Pu within the SCS. The ^239+240Pu inventory of the water column in the SCS basin at the SEATS station with a total depth of ~3840m was estimated to be ~29Bqm^-2, which was substantially higher than the sediment core estimates made for the SCS basin (3.75Bqm^-2) but much lower than the sediment core estimates made for the shelf of the northern SCS (365.6Bqm^-2). Such differences were determined by the lower scavenging efficiency of Pu in the SCS basin compared to the northern SCS shelf.

Vertical distributions and source identification of the radionuclides 239Pu and 240Pu in the sediments of the Liao River estuary, China

Activity concentration of plutonium (Pu) and its isotopic compositions are extensively used for measuring transport processes of Pu and identifying its source. We investigated the spatial distribution characteristics of ^239+240Pu activity concentrations and ²⁴⁰Pu/²³⁹Pu atom ratio in several sediment cores collected from the Liao River coastal zone. Additionally, we calculated the ^239+240Pu inventories and based on the ²⁴⁰Pu/²³⁹Pu atom ratio to trace Pu source. The activity concentrations of ^239+240Pu in surface sediments of the Liao River estuary ranged between 0.103 ± 0.008 and 0.978 ± 0.035 mBq/g, with an average of 0.294 ± 0.024 mBq/g. The ²⁴⁰Pu/²³⁹Pu atom ratios, ranging from 0.173 ± 0.047 to 0.215 ± 0.061 (mean: 0.188 ± 0.049 (1σ)), were consistent with global fallout value, which indicates the global atmospheric fallout is the main source of Pu in sediment cores from the both sides of Liao River estuary. As for the tidal flat core LT-2, the mean ²⁴⁰Pu/²³⁹Pu atom ratio, slightly higher than that of the global fallout value, was 0.217 ± 0.050. Such pattern of Pu isotopic compositions indicated that Pu on the tidal flat in the Liao River estuary is sourced from a combination of global fallout and close-in fallout from the PPG by ocean currents transporting. And by using a two end-member mixing model, the results indicate the relative contribution of the PPG close-in fallout to core LT-2 is round 27% and 73% can be attributed to global fallout and river input. Therefore, these results clearly indicate that the direct global fallout is the main source of Pu in the Liao River estuary.

Plutonium in the atmosphere: A global perspective

A number of potential source terms have contributed plutonium isotopes to the atmosphere. The atmospheric nuclear weapon tests conducted between 1945 and 1980 and the re-entry of the burned SNAP-9A satellite in 1964, respectively. It is generally believed that current levels of plutonium in the stratosphere are negligible and compared with the levels generally found at surface-level air. In this study, the time trend analysis and long-term behavior of plutonium isotopes (^239+240Pu and ²³⁸Pu) in the atmosphere were assessed using historical data collected by various national and international monitoring networks since 1960s. An analysis of historical data indicates that ^239+240Pu concentration post-1984 is still frequently detectable, whereas ²³⁸Pu is detected infrequently. Furthermore, the seasonal and time-trend variation of plutonium concentration in surface air followed the stratospheric trends until the early 1980s. After the last Chinese test of 1980, the plutonium concentrations in surface air dropped to the current levels, suggesting that the observed concentrations post-1984 have not been under stratospheric control, but rather reflect the environmental processes such as resuspension. Recent plutonium atmospheric air concentrations data show that besides resuspension, other environmental processes such as global dust storms and biomass burning/wildfire also play an important role in redistributing plutonium in the atmosphere.

Continuous transport of Pacific-derived anthropogenic radionuclides towards the Indian Ocean

Unusually high concentrations of americium and plutonium have been observed in a sediment core collected from the eastern Lombok Basin between Sumba and Sumbawa Islands in the Indonesian Archipelago. Gamma spectrometry and accelerator mass spectrometry data together with radiometric dating of the core provide a high-resolution record of ongoing deposition of anthropogenic radionuclides. A plutonium signature characteristic of the Pacific Proving Grounds (PPG) dominates in the first two decades after the start of the high yield atmospheric tests in 1950's. Approximately 40-70% of plutonium at this site in the post 1970 period originates from the PPG. This sediment record of transuranic isotopes deposition over the last 55 years provides evidence for the continuous long-distance transport of particle-reactive radionuclides from the Pacific Ocean towards the Indian Ocean.

Measurement of background gamma radiation in the northern Marshall Islands

We report measurements of background gamma radiation levels on six islands in the northern Marshall Islands (Enewetak, Medren, and Runit onEnewetak Atoll; Bikini and Nam on Bikini Atoll; and Rongelap on Rongelap Atoll). Measurable excess radiation could be expected from the decay of (137)Cs produced by the US nuclear testing program there from 1946 to 1958. These recordings are of relevance to safety of human habitation and resettlement. We find low levels of gamma radiation for the settled island of Enewetak [mean = 7.6 millirem/year (mrem/y) = 0.076 millisievert/year (mSv/y)], larger levels of gamma radiation for the island of Rongelap (mean = 19.8 mrem/y = 0.198 mSv/y), and relatively high gamma radiation on the island of Bikini (mean = 184 mrem/y = 1.84 mSv/y). Distributions of gamma radiation levels are provided, and hot spots are discussed. We provide interpolated maps for four islands (Enewetak, Medren, Bikini, and Rongelap), and make comparisons to control measurements performed on the island of Majuro in the southern Marshall Islands, measurements made in Central Park in New York City, and the standard agreed upon by the United States and the Republic of the Marshall Islands (RMI) governments (100 mrem/y = 1 mSv/y). External gamma radiation levels on Bikini Island significantly exceed this standard (P = <<0.01), and external gamma radiation levels on the other islands are below the standard. To determine conclusively whether these islands are safe for habitation, radiation exposure through additional pathways such as food ingestion must be considered.

Ultra-trace determination of (90)Sr, (137)Cs, (238)Pu, (239)Pu, and (240)Pu by triple quadruple collision/reaction cell-ICP-MS/MS: Establishing a baseline for global fallout in Qatar soil and sediments

The development of practical, fast, and reliable methods for the ultra-trace determination of anthropogenic radionuclides (90)Sr, (137)Cs, (238)Pu, (239)Pu, and (240)Pu by triple quadruple collision/reaction cell inductively coupled plasma mass spectrometry (CRC-ICP-MS/MS) were investigated in term of its accuracy and precision for producing reliable results. The radionuclides were extracted from 1 kg of the environmental soil samples by concentrated nitric and hydrochloric acids. The leachate solutions were measured directly by triple quadrupole CRC-ICP-MS/MS. For quality assurance, a chemical separation of the concerned radionuclides was conducted and then measured by single quadrupole-ICP-MS. The developed methods were next applied to measure the anthropogenic radionuclides (90)Sr, (137)Cs, (238)Pu, (239)Pu, and (240)Pu in soil samples collected throughout the State of Qatar. The average concentrations of (90)Sr, (137)Cs, (238)Pu, (239)Pu, and (240)Pu were 0.606 fg/g (3.364 Bq/kg), 0.619 fg/g (2.038 Bq/kg), 0.034 fg/g (0.0195 Bq/kg), 65.59 fg/g (0.150 Bq/kg), and 12.06 fg/g (0.103 Bq/kg), respectively.

Source term estimation and the isotopic ratio of radioactive material released from the WIPP repository in New Mexico, USA

After almost 15 years of operations, the Waste Isolation Pilot Plant (WIPP) had one of its waste drums breach underground as a result of a runaway chemical reaction in the waste it contained. This incident occurred on February 14, 2014. Moderate levels of radioactivity were released into the underground air. A small portion of the contaminated underground air also escaped to the surface through the ventilation system and was detected approximately 1 km away from the facility. According to the source term estimation, the actual amount of radioactivity released from the WIPP site was less than 1.5 mCi. The highest activity detected on the surface was 115.2 μBq/m(3) for (241)Am and 10.2 μBq/m(3) for (239+240)Pu at a sampling station located 91 m away from the underground air exhaust point and 81.4 μBq/m(3) of (241)Am and 5.8 μBq/m(3) of (239+240)Pu at a monitoring station located approximately 1 km northwest of the WIPP facility. The dominant radionuclides released were americium and plutonium, in a ratio that matches the content of the breached drum. Air monitoring across the WIPP site intensified following the first reports of radiation detection underground to determine the extent of impact to WIPP personnel, the public, and the environment. In this paper, the early stage monitoring data collected by an independent monitoring program conducted by the Carlsbad Environmental Monitoring & Research Center (CEMRC) and an oversight monitoring program conducted by the WIPP's management and operating contractor, the Nuclear Waste Partnership (NWP) LLC were utilized to estimate the actual amount of radioactivity released from the WIPP underground. The Am and Pu isotope ratios were measured and used to support the hypothesis that the release came from one drum identified as having breached that represents a specific waste stream with this radionuclide ratio in its inventory. This failed drum underwent a heat and gas producing reaction that overpowered its vent and lifted its lid to allow release of waste into the underground air.

Sources of plutonium in the atmosphere and stratosphere-troposphere mixing

Plutonium isotopes have primarily been injected to the stratosphere by the atmospheric nuclear weapon tests and the burn-up of the SNAP-9A satellite. Here we show by using published data that the stratospheric plutonium exponentially decreased with apparent residence time of 1.5 ± 0.5 years, and that the temporal variations of plutonium in surface air followed the stratospheric trends until the early 1980s. In the 2000s, plutonium and its isotope ratios in the atmosphere varied dynamically, and sporadic high concentrations of ^239,240Pu reported for the lower stratospheric and upper tropospheric aerosols may be due to environmental events such as the global dust outbreaks and biomass burning.