Vertical distribution and migration of global fallout Pu in forest soils in southwestern China

Source, preservation and re-suspension of 239,240Pu in a well dated peat core collected from northwest China

Owning to the high radiotoxicity in high concentrations, as well as the irreplaceability in quantifying soil erosion rates, demarcating the Anthropocene, and dating of sediment, anthropogenic 239,240Pu have drawn high attention. However, the source in specific areas, preservation characteristics in different environment media, and re-distribution process after the cessation of atmospheric nuclear weapons tests, have not been fully understood, which obscures the exact start year, temporal variation, and deposition flux of 239,240Pu in sedimentary records, and hinders the wide application of 239,240Pu in environment study. A sediment core from the Yiwu peat bog with dominance of atmospheric deposition in the source material, simple sedimentary environment, and high dust deposition flux, was collected to examine the 239,240Pu, and explore the source, preservation, and re-distribution process. The double peaks of 239,240Pu in 1952 CE and 1963 CE, as well as 240Pu/239Pu ratios of 0.163-0.190 with an average of 0.177 ± 0.010 confirmed that the 239,240Pu source originated predominantly from global fallout. The minimal vertical migration of plutonium in the Yiwu peat core was attributed to the near-neutral pH condition. The high inventory of 128 ± 7 Bq m-2, as well as the atypical negative correlation between 239,240Pu concentrations and organic matter content (r = - 0.79, P < 0.01), was attributed to the contribution of 239,240Pu re-suspended with dust from the neighboring Gobi Desert, particularly in the cold and dry years. The total re-suspended 239,240Pu was estimated to be 77 Bq m-2, exceeding the direct fallout level of 51 Bq m-2 during 1945-2016 CE. In this study, the specified deposition pattern of 239,240Pu after the cessation of atmospheric nuclear weapons was established, providing an important standard for multiple environmental studies, and the re-suspended amount of 239,240Pu in a typical arid area was quantified for the first time.

Quantitative estimation of dust transport in the desert region of northwest China by plutonium isotopes

Dust is an important source of atmospheric pollution, and quantitative estimation of desert dust transport is crucial for air pollution control. In this study, five typical sandy soil profiles in the Tengger Desert were collected and analyzed for 239,240Pu concentration and 240Pu/239Pu atomic ratios in order to identify the source of 239,240Pu in this area and explore the sedimentary characteristics of dust in different profiles. The results revealed that the concentrations of 239,240Pu in the soil profiles were between 0.002 and 0.443 mBq/g with an exception of the deep layer soil at one site. The measured atomic ratios of 240Pu/239Pu are at the global atmospheric fallout level with a mean of 0.184 ± 0.020, indicating that global fallout is the dominant source of plutonium in this region. The total inventories of 239,240Pu in the reference sites in this area were estimated to be 39.2-44.6 Bq/m2, this is in agreement with the value from the global fallout of atmospheric nuclear weapon tests at the similar latitude (30-40 °N: 42 Bq/m2). The estimated erosion rate in the erosion profile utilizing soil erosion intensity mode is 2491 t/km2/yr and the soil erosion depth is 9.86 cm, While, the stacking rate of the accumulation profile is 1383 t/km2/yr, and the depth of accumulation is estimated to be 5.48 cm. The difference between the erosion and accumulation profiles indicated that approximately 1107 t/km2/yr of dust was exported from the Gobi landform area of the Tengger Desert, which might be transported long distance in the downwind direction.

Level, distribution and sources of Np, Pu and Am isotopes in Peter the Great Bay of Japan sea

Transuranium elements such as Np, Pu and Am, are considered to be the most important radioactive elements in view of their biological toxicity and environmental impact. Concentrations of 237Np, Pu isotopes and 241Am in two sediment cores collected from Peter the Great Bay of Japan Sea were determined using radiochemical separation combined with inductively coupled plasma mass spectrometry (ICP-MS) measurement. The 239,240Pu and 241Am concentrations in all sediment samples range from 0.01 Bq/kg to 2.02 Bq/kg and from 0.01 Bq/kg to 1.11 Bq/kg, respectively, which are comparable to reported values in the investigated area. The average atomic ratios of 240Pu/239Pu (0.20 ± 0.02 and 0.21 ± 0.01) and 241Am/239+240Pu activity ratios (3.32 ± 2.76 and 0.45 ± 0.17) in the two sediment cores indicated that the sources of Pu and Am in this area are global fallout and the Pacific Proving Grounds through the movement of prevailing ocean currents, and no measurable release of Np, Pu and Am from the local K-431 nuclear submarine incident was observed. The extremely low 237Np/239Pu atomic ratios ((2.0-2.5) × 10-4) in this area are mainly attributed to the discrepancy of their different chemical behaviors in the ocean due to the relatively higher solubility of 237Np compared to particle active plutonium isotopes. It was estimated using two end members model that 23% ± 6% of transuranium radionuclides originated from the Pacific Proving Grounds tests, and the rest (ca. 77%) from global fallout.

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.

Plutonium in sediments of the Eastern Guangdong coast-its sources and their contribution

The ^239+240Pu activities and ²⁴⁰Pu/²³⁹Pu atom ratios of surface sediments from the Eastern Guangdong coast (EGDC) were determined by sector field ICP-MS in order to examine the sources of plutonium (Pu) and quantify their contributions. The ^239+240Pu activities in the EGDC ranged from 0.113 to 0.451 Bq kg^-1, with an average of 0.225 ± 0.090 Bq kg^-1 (n = 17). Consistently high ²⁴⁰Pu/²³⁹Pu atom ratios, ranging from 0.218 to 0.274 (average = 0.254 ± 0.014, n = 17), indicate a non-global fallout Pu source in the EGDC. The horizontal distribution of the ²⁴⁰Pu/²³⁹Pu atom ratios in the EGDC sediment suggests the non-global fallout Pu is sourced from close-in fallout from the Pacific Proving Grounds (PPG). Using a simple two end-member mixing model, we calculated the relative proportions of Pu from the PPG and global fallout in the EGDC to be 57 ± 9 % and 43 ± 9 %, respectively. Moreover, from the well-defined relationship between ^239+240Pu activity and total organic carbon content in sediments and a two end-member mixing model using δ¹³C, we further calculated the Terr-global fallout (riverine input) and Mar-global fallout (direct atmospheric deposition) to be 11 ± 2 % and 32 ± 6 %, respectively. Finally, from the activity levels and atom ratios of Pu isotopes in the EGDC, we established a baseline for future use in environmental risk assessment related to nuclear power plant operations.

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.

Distribution and migration of 239,240Pu in soil profiles in North China

The source, vertical distribution and migration behavior of plutonium in five soil profiles (from forest, grassland and desert areas) in northern China were investigated. The average ²⁴⁰Pu/²³⁹Pu atomic ratio of 0.184 ± 0.022 observed in these samples is in good agreement with the reported value of global fallout, suggesting that the global fallout is the major source of plutonium in northern China. The ^239,240Pu inventories in five soil profiles ranges from 43.3 Bq/m² to 175 Bq/m², lying in the reported range for global fallout in the similar latitude band. The effective convection velocity (0.04-0.16 cm/y) and effective dispersion coefficient (0.13-0.41cm²/y) of plutonium in different soil profiles derived using the CDE model varies significantly, attributed to multi-factors including location, topography, climate and soil types. The results showed that the migration of plutonium in grassland soil is significantly slow compared to other type of soil, especially desert soil.

An overview of plutonium isotopes in soils, China: Distribution, spatial patterns, and sources

Plutonium (Pu) is an anthropogenic radionuclide which has drawn significant attentions due to its radiotoxicity, and the sources of plutonium linked with nuclear accidents and contaminations. The ²⁴⁰Pu/²³⁹Pu atom ratio is source dependent and can be used as a fingerprint to determine the sources of radioactive contaminant. However, the distribution and sources of plutonium in soils of China have not yet been systematically studied at a national scale up to date. The distribution, spatial patterns, and sources of plutonium in soils of China were discussed in this work. The concentrations of ^239,240Pu are in the range of 0.002-4.824 mBq/g with a large variation, and the ^239,240Pu concentrations in surface soils increase with the increasing latitude, which affects by multi-factors such as organic matter and particle size, etc. The inventories of ^239,240Pu are in the range of 7.31-554 Bq/m². The weighted average of ²⁴⁰Pu/²³⁹Pu atom ratios (0.180 ± 0.004) in all surface samples is good agreement with the ratio of global fallout (0.180 ± 0.014) of the nuclear weapons tests, this indicate that the major source of plutonium in China is global fallout. However, among some sites, distinctly lower ²⁴⁰Pu/²³⁹Pu atom ratio compared to the global fallout values were observed in the northwest China, indicating a significant contribution from other source besides the global fallout. Furthermore, the spatial clustering patterns of hot spots (high values) and cold spots (low values) for plutonium showing the clear associations with nuclear tests, especially the Chinese Lop Nor nuclear weapons tests (CNTs) and the Semipalatinsk nuclear weapons tests (STS). Radioactive material including plutonium from the STS or CNTs was transported by the prevailing westerlies to the northwest China. This review about the fingerprints and distribution of plutonium in soils of China will help researchers to establish a reference database for future radiation risk assessment and environmental radioactive management.

Analysis of the vertical distribution of 137Cs and 239,240Pu in waterlogged and non-boggy soils by the sequential extraction method

Mobile capabilities of ¹³⁷Cs, ^239,240Pu and some stable element physicochemical forms were studied in soil layers at a depth of 6-8 cm (maximum concentrations of radionuclides) of non-boggy and waterlogged soil cores sampled on the shores of Lake Bedugnis (Lithuania). Soil samples were acidic with small amount of clay (muscovite). The radionuclide activity concentrations were about 2.5 times higher in non-boggy soil. It was explained by different conditions of deposition of radioactive fallout at these sampling sites and density of the soil samples. The value of the exchangeable fraction of radionuclides is shown to be an indicator of their migration capabilities in the soil. Under anaerobic conditions in waterlogged soil (stagnant pore water conditions) and due to the presence of clay admixture, ∼90% of ¹³⁷Cs was concentrated in the residual fraction and its mobility was low. ^239,240Pu was concentrated mainly in organic matter with rather large exchangeable fractions (∼9.6-∼13.9%). Under oxidizing conditions in non-boggy soil (dynamic pore water conditions in the case of rain), ^239,240Pu was mostly concentrated in the oxide fraction. Its exchangeable fractions were less than those in waterlogged soil and, respectively, its mobility was lower. In non-boggy soils, exchangeable fractions of ¹³⁷Cs were large and varied in the range of 10.1-12.2%, which indicated its high mobility. In the case, ¹³⁷Cs adsorption by clay materials was reduced and its residual fraction did not exceed 71.3%. The obtained data show that in the area of Lake Bedugnis, the migration capabilities of ¹³⁷Cs and ^239,240Pu change in antiphase.

Distributions of fallout 137Cs, 239+240Pu and 241Am in a soil core from South Central China

The source and vertical distribution of ¹³⁷Cs,^239+240Pu and ²⁴¹Am activity concentrations in a soil core from Hunan Province, China were investigated. The maximum ¹³⁷Cs and ^239+240Pu activity concentrations were 15.45 ± 0.76 mBq/g and 0.819 ± 0.066 mBq/g, respectively. While the maximum ²⁴¹Am activity concentration in samples obtained from the core was 0.341 ± 0.019 mBq/g. The ²⁴⁰Pu/²³⁹Pu atom ratio and the ¹³⁷Cs/^239+240Pu activity ratio were 0.183 ± 0.011 and 19.5 ± 1.8, respectively, and both were consistent with the characteristic value of global fallout. The integrated ²⁴¹Am/^239+240Pu activity ratio for global fallout was also re-estimated. The measured ²⁴¹Am/^239+240Pu activity ratio (average 0.43 ± 0.07) in the samples was very close to the estimated value (0.45), which suggested their ²⁴¹Am also came from the global fallout. Regarding the vertical distribution of ¹³⁷Cs, ^239+240Pu and ²⁴¹Am in these red soil samples, all these radionuclides had higher concentrations in upper layers of several centimeters of soil while they had slightly lower concentrations in lower soil layers down to 30 cm. Vertical distributions of ¹³⁷Cs/^239+240Pu and ²⁴¹Am/^239+240Pu activity ratios indicated the migration velocity was Am ≈ Pu > Cs. The intrinsic chemical properties of the radionuclides as well as soil type and properties (acidic, nutrient-deficient and low in organic matter and cation exchange capacity) might be reasons for the differences in their migration behaviors.

Plutonium isotopes in the Qinghai-Tibet Plateau: Sources, distribution, and their environmental behaviors

Due to the high radiotoxicity in high concentrations, plutonium isotopes have drawn high attentions in the consideration of radiation risk, their sources, level, environmental behaviors, including deposition, retention and migration behaviors. However, such research in the Qinghai-Tibet Plateau is still missing, where is deemed as an environmental sensitive area. ^239,240Pu in surface soil collected from the Qinghai-Tibet Plateau were determined for the first time in this work. The concentrations of ^239,240Pu are in the range of 0.0176-1.95 Bq/kg, falling into the reported ranges in the background areas from the similar latitude belt. The ²⁴⁰Pu/²³⁹Pu atomic ratio range was measured to be 0.146-0.225, which is similar with the global fallout values. Both indicate that the global fallout is the major source of plutonium in this region, and the low plutonium level will not cause any radiation risk so far. Based on the statistical analysis of the possible parameters (organic content, moisture content, average annual precipitation, altitudes, topography and human activity), the large variations of ^239,240Pu concentrations were mainly attributed to the retention process related factors including soil organic content and human activity disturbances. While, the deposition related factors including the average annual precipitation, altitudes, topography made insignificant influence on the spatial distribution of ^239,240Pu concentrations due to the low ^239,240Pu concentrations in atmosphere, less wet deposition amount and insignificant re-suspended amount. The highest ^239,240Pu concentrations of 0.805-1.95 Bq/kg were mainly due to the good retention condition in the sampling sites with higher soil organic content and less human activity disturbances.

Spatial distribution and migration of 239+240Pu in Chinese soils

The migration of radionuclides is a critical threat to the soil and groundwater environment. This study investigates highly radiological toxic ^239+240Pu in 647 surface soils and 66 soil cores in China. First, the spatial distributions of ^239+240Pu activities and ²⁴⁰Pu/²³⁹Pu ratios are presented in Chinese surface soils. Second, four different types of vertical distribution of ^239+240Pu, namely 45.4%, 40.9%, 9.1% and 4.5% of Chinese soil cores proportions are integrated using statistical analysis. Furthermore, different soil types are accompanied by different ^239+240Pu vertical distributions, which relate closely to the ^239+240Pu migration. Finally, based on the Convection Dispersion Equation (CDE) model, the present work find that the apparent convection velocities of ^239+240Pu are ranging from 0.00032 ± 0.00031 cm a^-1 to 0.473 ± 0.083 cm a^-1. As shown by the four typical vertical activity distribution of ^239+240Pu in soil cores, the deepened activity maximum value position of ^239+240Pu implies a fast migration rate or apparent convection velocity. This study, for the first time, suggests a significant linear correlation (R² = 0.847) between the depth of ^239+240Pu maximum value (cm) and the apparent convection velocity (v, cm a^-1). We hypothesize that v usually does not exceed 0.5 cm a^-1 in the CDE model. A significant linear correlation is also identified between apparent dispersion coefficient (D, cm² a^-1) and v² in Chinese soil cores from the CDE model. It will provide an important reference for harmful heavy metal migration research in the future.

Sources and Variability of Plutonium in Chinese Soils: A Statistical Perspective with Moving Average

We investigated the different sources and their corresponding impact areas of Pu in Chinese surface soils to illustrate the state-of-the-art of the sources, levels and distributions of 240Pu/239Pu atom ratios as well as 239+240Pu activity concentrations in China. For the first time a moving average strategy in combination with statistical analysis was employed to partition geographic areas in China based on the reported 240Pu/239Pu atom ratio and 239+240Pu concentration data from public literature. During the partitioning, the median (MED) of the dataset was basically employed as a criteria in place of the commonly used arithmetic average (AM). Concisely, three areas were partitioned according to the different influences of Pu from the Lop Nor (LNTS) and Semipalatinsk (STS) test sites and the global fallout. The partitioned Ternary area (80° E–105° E, 35° N–50° N) was supposed to have multiple sources of Pu including the STS and LNTS besides the global fallout, which was characterized with slightly lower 240Pu/239Pu atom ratios (MED = 0.174) as well as elevated 239+240Pu concentrations (MED = 0.416 mBq/g). Meanwhile, the Binary area (35° N–45° N, 100° E–115° E) was considered to have received the extra contribution from the high-yield nuclear tests at the LNTS besides the global fallout, resulting in the highest 240Pu/239Pu atom ratios (MED = 0.200) across China. The remaining area was marked as the Unitary area, where it only received the exclusive contribution of global fallout. Furthermore, through the statistical analysis of the 240Pu/239Pu data in the Unitary area, we recommended a value of 0.186 ± 0.021 (AM ± SD) as a representative or area-specific 240Pu/239Pu atom ratio baseline to characterize the global fallout derived Pu in Chinese soils.

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.

Distinctive distribution and migration of global fallout plutonium isotopes in an alpine lake and its implications for sediment dating

This work investigated plutonium (Pu) isotopes in sediment cores collected from an alpine lake (Lake Heinongpo with 3779 m above sea level) in Southwestern China. ²⁴⁰Pu/²³⁹Pu atom ratios in all sediment samples showed the typical global fallout values of ∼0.18 without any influences from other Pu contaminant sources. ^239+240Pu activities with surface and subsurface maximums followed by exponential decline with sediment depth were respectively observed in the two sediment cores. The distinctive depth distributions of ^239,240Pu in the lake sediments was attributed to the very slow sediment deposition rate due to the lack of terrestrial sediment input, while the alpine snowmelt input was the primary source of Pu in the lake sediments in addition to the direct atmospheric deposition. The total Pu inventory was estimated to be 56.3 ± 1.4 and 63.9 ± 0.8 Bq/m² respectively in the two sediment cores. The generally higher Pu inventory in the Lake Heinongpo compared with other reported lakes in similar latitude should be mainly attributed to their different Pu input passages. The advection-diffusion equation was further applied to evaluate the downward migration of Pu isotopes in the sediment cores and predict the future evolution of Pu distribution in the sediment cores. The fitted results indicated that the diffusion effect controlled the downward migration of Pu in the sediments, but this diffusive migration will not prevent the peak of global fallout Pu in undisturbed sediment cores from being a valuable time marker for sediment dating.

Determination of ultra-trace level plutonium isotopes in soil samples by triple-quadrupole inductively coupled plasma-mass spectrometry with mass-shift mode combined with UTEVA chromatographic separation

Although triple-quadrupole inductively coupled plasma-mass spectrometry (ICP-MS/MS) has become an attractive technique for the measurement of long-lived radionuclides, the abundance sensitivity, isobaric and polyatomic ions interferences seriously restrict the application. The spectral peak tailing and uranium hydrides (UH⁺, UH₂⁺) from ²³⁸U have a serious influence on the accurate measurement of ²³⁹Pu and ²⁴⁰Pu, especially for the ultra-trace level plutonium isotopes in the higher uranium sample. A new method was developed using ICP-MS/MS measurement in mass-shift mode with collision-reaction gas combined with a chemical separation procedure. As O₂ readily converted Pu⁺ ion to PuO₂⁺, while disassociated the interfering diatomic ions of interfering elements (U, Pb, Hg, Tl, etc.), the interferences from these elements were completely eliminated if plutonium was detected as PuO₂⁺ at the m/z more than 270. By the mass filter in MS/MS mode combined with O₂ as reaction gas the lower peak tailing of ²³⁸U⁺ (<5 × 10^-12) was significantly suppressed. By this way, the ²³⁸UO₂H⁺/²³⁸UO₂⁺ atomic ratio was reduced to 4.82 × 10^-9, which is significantly lower than that of other collision-reaction gas modes. Interferences from Pb, Hg and Tl polyatomic ions were also completely eliminated. Thus, accurate measurement of ultra-trace level ²³⁹Pu in high uranium sample solutions with the ²³⁹Pu/²³⁸U concentration ratio of 10^-10 was achieved by the mass-shift mode with 0.15 mL/min O₂/He + 12.0 mL/min He as collision-reaction gas, and high elimination efficiency of uranium interferences up to 10¹⁴ can be obtained by combination with the chemical separation using a single UTEVA resin column. The developed method can be applied to accurately determine the fg level ²³⁹Pu in high uranium samples, such as large-size deep seawater, deep soil and sediment, uranium debris of nuclear fuel.

The level, distribution and source of artificial radionuclides in surface soil from Inner Mongolia, China

Mid- and long-half-life artificial radioisotopes in the earth's surface environment are of great concern to the environmental radiation risk assessment. As nuclear fuel and fission products, ²³⁹Pu, ²⁴⁰Pu, ²⁴¹Am, ⁹⁰Sr and ¹³⁷Cs in soils from Inner Mongolia of China were analyzed with a modified systematic separation procedure combined with ICP-MS and LSC measurements, to study the level, distribution and source of artificial radionuclides in the region. The radioactivity and inventory of ¹³⁷Cs (0.26-28.3 Bq/kg, 0.5-5.4 kBq/m²), ^239+240Pu (0.05-1.26 Bq/kg, 20-229 Bq/m²), ²⁴¹Am (0.036-0.35 Bq/kg, 11-81 Bq/m²) and ⁹⁰Sr (1.2-7.6 Bq/kg, 0.39-1.7 kBq/m²) all lie in the range of the global fallout. Vertical distributions of these radionuclides were examined for two soil core samples SC14025 and SC14038, and great differences were observed between these two sample locations. For SC14025 where little human disturbance to soil occurred, both ¹³⁷Cs and ^239+240Pu have a subsurface activity maximum followed by an exponential decay. Fittings base on CDE model gives a small downward migration velocity of about 0.097 cm/y for both Pu and ¹³⁷Cs. Source identification for SC14025 and SC14038 soil cores with ²⁴⁰Pu/²³⁹Pu (average of 0.180 ± 0.017 and 0.164 ± 0.035, respectively), ¹³⁷Cs/^239+240Pu (average of 25.3 ± 0.6 and 25.6 ± 3.0, respectively) and ²⁴¹Am/^239+240Pu (average of 0.56 ± 0.08 and 0.60 ± 0.09, respectively) ratios consistently indicated that anthropogenic radionuclides in Xilingol region are mostly from the global fallout of atmospheric nuclear weapons tests in the last century. According to the geographical distribution of the radioactivity level, the high radioactivity level in the east of Inner Mongolia probably results from enhanced deposition by the blocking of the Great Khingan Range.

Vertical distribution of Pu in forest soil in Qinghai-Tibet Plateau

The ^239+240Pu and ¹³⁷Cs activities and ²⁴⁰Pu/²³⁹Pu atom ratios in surface soil and soil core collected from Qinghai-Tibet Plateau were investigated. The maximum of ^239+240Pu and ¹³⁷Cs activity concentrations in five soil cores were found in upper layers and have a same trend: the maximum followed by exponential decline. The ²⁴⁰Pu/²³⁹Pu atom ratio ranged from 0.160 ± 0.009 to 0.212 ± 0.012, clarified that the plutonium mainly came from the global fallout and the contribution of Lop Nor test sites was negligible. In addition, the vertical profiles of radionuclides (²¹⁰Pb, ²³⁸U, ²³²Th, ²²⁶Ra, ⁴⁰K) have been studied and a high correlation has been found between them. The correlations of Pu between organic matter (OM) and heavy metals were also studied. Person Correlation Coefficients revealed Pu had significant positive correlations with organic matter and Cd, Cu, Co, Zn, but negative correlation with Tl. The results have important implication for further understanding of the sources, records and environmental impacts of global and regional nuclear activities, which expanded the database of Pu activity level and atom ratio in Chinese soil and established a foundation for future environmental risk assessment.

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.

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.

Level, distribution and sources of plutonium in the coastal areas of China

Plutonium is an important radioactive element in view of its biological toxicity and environmental impact. The two most important isotopes of plutonium, ²³⁹Pu and ²⁴⁰Pu, in the surface soil from the coastal area of China were determined using radiochemical separation combined with inductively coupled plasma mass spectrometry (ICP-MS) measurement. The average of the measured ²⁴⁰Pu/²³⁹Pu atomic ratios in all these samples is 0.186 ± 0.021, which is in good agreement with the ratio of global fallout of the nuclear weapons tests. This indicates that the major source of plutonium in this region is global fallout, and there is no measurable release of plutonium from any nuclear power plant along the coast of China. The ^239,240Pu concentrations in all 71 soil samples range from 0.002 mBq/g to 0.670 mBq/g, which fall in the range of the reported values in the background area not directly influenced by nuclear activities. The anthropogenic disturbance of sampling sites, vegetation coverage of land, precipitation rate and organic substance content and erosion of soil have significant influence on the level of plutonium in the surface soil.

The 240Pu/239Pu atom ratio in Chinese soils

The ²⁴⁰Pu/²³⁹Pu atom ratio is a very effective tool for the identification of the origin of plutonium (Pu) in the soil environment. We examine a dataset of ²⁴⁰Pu/²³⁹Pu atom ratios determined from surface and core soils at 240 sites across China. The data were compiled from 18 separate literature sources from the last 20 years. For the first time the spatial distribution (3 latitude bands and 7 natural regions) of the weighted average ²⁴⁰Pu/²³⁹Pu atom ratios in Chinese soils is investigated. An area to the West of Xining City, shows a weighted average ²⁴⁰Pu/²³⁹Pu atom ratio of 0.167 ± 0.002, lower than that of average global fallout, which likely arises from the addition of local fallout radionuclides from the Chinese nuclear weapon tests at Lop Nor between 1964 and 1980. The Yumen and Jiuquan areas of Northwest China in particular show evidence of very low ratio material from the Chinese nuclear weapon tests. Excluding the impacted area around the test site the weighted average ²⁴⁰Pu/²³⁹Pu ratio of 0.182 ± 0.002 suggests that global fallout is the main source of Pu in most Chinese soils.

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.

Distinctive distributions and migrations of 239+240Pu and 241Am in Chinese forest, grassland and desert soils

The vertical distributions and downward migrations of the global fallout derived ^239+240Pu and ²⁴¹Am in diverse types of Chinese soils (forest, grassland and desert) were studied. The mean ^239+240Pu and ²⁴¹Am activity concentrations in the investigated soil cores were 0.28-0.69 mBq/g and 0.13-0.37 mBq/g, respectively, while the accumulative inventories were 61.53-138.99 Bq/m² for ^239+240Pu and 28.29-61.05 Bq/m² for ²⁴¹Am. The convection-dispersion equation (CDE) was used to calculate the migration parameters of ^239+240Pu and higher apparent dispersion coefficients (D) were observed for the acidic forest soils compared with the alkaline grassland and desert soils; meanwhile a compartment model was employed to compare the migration of ^239+240Pu and ²⁴¹Am in successive soil layers which showed that the migration behaviors of ^239+240Pu and ²⁴¹Am were rather similar; both velocities were less than 0.3 cm/y in diverse types of soils and these findings were compatible with those of short-term laboratory simulation experiments in China.

Inventory and vertical distribution of 137Cs, 239+240Pu and 238Pu in soil from Raivavae and Hiva Oa, two French Polynesian islands in the southern hemisphere

Atmospheric nuclear weapons tests carried out by the United States, the former Soviet Union, the United Kingdom, France and China between 1945 and 1980 resulted in radioactive fallout over the earth's surface of long-lived radionuclides, such as ¹³⁷Cs, ^239+240Pu and ²³⁸Pu that could be detected more than 50 years after their production. In addition, the burnup in the upper atmosphere of a thermoelectric generator fueled by ²³⁸Pu, SNAP-9A, contributed to the inventory of ²³⁸Pu deposited on the ground. In order to estimate the deposition densities of ¹³⁷Cs, ^239+240Pu and ²³⁸Pu in French Polynesia, we collected undisturbed soil samples up to 30 cm deep at eight sites in two islands (Hiva Oa, 139°W - 10°S and Raivavae, 148°W - 24°S) in 2015-2016. The top 0-10 cm of the soil cores were sliced into five 2-cm layers and the bottom 10-30 cm into four 5-cm layers for gamma spectrometry and alpha spectrometry measurements. We found that more than 50% of the radioactive inventories are still contained within the first 10 cm and that the average vertical migration velocities of ¹³⁷Cs and Pu are less than 0.2 cm y^-1. The average accumulated depositions, deduced from the profile measurements, are 236 ± 11 Bq.m^-2 and 313 ± 39 Bq.m^-2 for ¹³⁷Cs, 12.1 ± 1.5 Bq.m^-2 and 22.1 ± 1.7 Bq.m^-2 for ^239+240Pu, and 1.23 ± 0.46 Bq.m^-2 and 1.58 ± 0.60 Bq.m^-2 for ²³⁸Pu, in Hiva Oa and Raivavae, respectively. The ²³⁸Pu/^239+240Pu ratios are 0.102 ± 0.050 at Hiva Oa and 0.072 ± 0.033 at Raivavae. Both values are higher than the ratio in nuclear weapons tests fallout estimated to be 0.016 in 2016 (Hardy et al., 1973), because of the contribution of ²³⁸Pu fallout from SNAP-9A, which is latitude dependent. The ¹³⁷Cs/^239+240Pu ratios, 19.5 ± 3.2 at Hiva Oa and 14.2 ± 2.8 at Raivavae are in the lower part of the range of values observed in other regions of the world.

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.

Plutonium Partitioning Behavior to Humic Acids from Widely Varying Soils Is Related to Carboxyl-Containing Organic Compounds

In order to examine the influence of the HA molecular composition on the partitioning of Pu, ten different kinds of humic acids (HAs) of contrasting chemical composition, collected and extracted from different soil types around the world were equilibrated with groundwater at low Pu concentrations (10^-14 M). Under mildly acidic conditions (pH ∼ 5.5), 29 ± 24% of the HAs were released as colloidal organic matter (>3 kDa to <0.45 μm), yet this HA fraction accounted for a vast majority of the bound Pu, 76 ± 13% on average. In comparison, the particulate HA fraction bound only 8 ± 4% on average of the added Pu. The truly dissolved Pu fraction was typically <1%. Pu binding was strongly and positively correlated with the concentrations of organic nitrogen in both particulate (>0.45 μm) and colloidal phases in terms of activity percentage and partitioning coefficient values (logK_d). Based on molecular characterization of the HAs by solid state ¹³C nuclear magnetic resonance (NMR) and elemental analysis, Pu binding was correlated to the concentration of carboxylate functionalities and nitrogen groups in the particulate and colloidal phases. The much greater tendency of Pu to bind to colloidal HAs than to particulate HA has implications on whether NOM acts as a Pu source or sink during natural or man-induced episodic flooding.

Association of Plutonium isotopes with natural soil particles of different size and comparison with 137Cs

Information on how plutonium (Pu) isotopes associate with natural soil particles of different size is very important for the interpretation involved in the application of Pu isotopes as soil erosion tracers. This work investigated the association of Pu isotopes with different particle size fractions of natural soils and compared it with that of ¹³⁷Cs. Ten bulk soils collected from two different areas were separated into different particle size fractions by a combination of wet sieving and centrifugation techniques and the sub-samples were analyzed for ¹³⁷Cs, ²³⁹Pu and ²⁴⁰Pu. Results showed that the concentrations of both ^239+240Pu and ¹³⁷Cs increase with decreased particle sizes and are closely related to the specific surface areas of soil particles, which demonstrated a similar preferential association of Pu with finer soil particles as ¹³⁷Cs. The activity ratios of ^239+240Pu/¹³⁷Cs in soil fractions increasing with increased particle size further indicated a less preferential transport of Pu with fine particles compared to ¹³⁷Cs. These results not only highlight the suitability of Pu isotopes as soil erosion tracers, but also provide useful information for assessing the migration behavior of Pu in contaminated environments.

Plutonium, 137Cs and uranium isotopes in Mongolian surface soils

Plutonium (²³⁸Pu and ^239,240Pu), ¹³⁷Cs and plutonium activity ratios (²³⁸Pu/^239,240Pu) as did uranium isotope ratio (²³⁵U/²³⁸U) were measured in surface soil samples collected in southeast Mongolia. The ^239,240Pu and ¹³⁷Cs concentrations in Mongolian surface soils (<53 μm of particle size) ranged from 0.42 ± 0.03 to 3.53 ± 0.09 mBq g^-1 and from 11.6 ± 0.7 to 102 ± 1 mBq g^-1, respectively. The ²³⁸Pu/^239,240Pu activity ratios in the surface soils (0.013-0.06) coincided with that of global fallout. The ²³⁵U/²³⁸U atom ratios in the surface soil show the natural one. There was a good correlation between the ^239,240Pu and ¹³⁷Cs concentrations in the surface soils. We introduce the migration depth to have better understanding of migration behaviors of anthropogenic radionuclides in surface soil. We found a difference of the migration behavior between ^239,240Pu and ¹³⁷Cs from ¹³⁷Cs/^239,240Pu - ¹³⁷Cs plots for the Mongolian and Tsukuba surface soils; plutonium in surface soil is migrated easier than ¹³⁷Cs.

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.

(236)U and (239,)(240)Pu ratios from soils around an Australian nuclear weapons test site

The isotopes (236)U, (239)Pu and (240)Pu are present in surface soils as a result of global fallout from nuclear weapons tests carried out in the 1950's and 1960's. These isotopes potentially constitute artificial tracers of recent soil erosion and sediment movement. Only Accelerator Mass Spectrometry has the requisite sensitivity to measure all three isotopes at these environmental levels. Coupled with its relatively high throughput capabilities, this makes it feasible to conduct studies of erosion across the geographical extent of the Australian continent. In the Australian context, however, global fallout is not the only source of these isotopes. As part of its weapons development program the United Kingdom carried out a series of atmospheric and surface nuclear weapons tests at Maralinga, South Australia in 1956 and 1957. The tests have made a significant contribution to the Pu isotopic abundances present in the region around Maralinga and out to distances ∼1000 km, and impact on the assessment techniques used in the soil and sediment tracer studies. Quantification of the relative fallout contribution derived from detonations at Maralinga is complicated owing to significant contamination around the test site from numerous nuclear weapons safety trials that were also carried out around the site. We show that (236)U can provide new information on the component of the fallout that is derived from the local nuclear weapons tests, and highlight the potential of (236)U as a new fallout tracer.

Pu isotopes in soils collected downwind from Lop Nor: regional fallout vs. global fallout

For the first time, soil core samples from the Jiuquan region have been analyzed for Pu isotopes for radioactive source identification and radiological assessment. The Jiuquan region is in downwind from the Lop Nor Chinese nuclear test (CNT) site. The high Pu inventories (13 to 546 Bq/m²) in most of the sampling locations revealed that this region was heterogeneously contaminated by the regional fallout Pu from the CNTs. The contributions of the CNTs to the total Pu in soils were estimated to be more than 40% in most cases. The ²⁴⁰Pu/²³⁹Pu atom ratios in the soils ranged from 0.059 to 0.186 with an inventory-weighted average of 0.158, slightly lower than that of global fallout. This atom ratio could be considered as a mixed fingerprint of Pu from the CNTs. In addition, Pu in soils of Jiuquan region had a faster downward migration rate compared with other investigated places in China.

Plutonium concentration and isotopic ratio in soil samples from central-eastern Japan collected around the 1970s

Obtaining Pu background data in the environment is essential for contamination source identification and assessment of environmental impact of Pu released from the Fukushima Daiichi nuclear power plant (FDNPP) accident. However, no baseline information on Pu isotopes in Fukushima Prefecture has been reported. Here we analyzed 80 surface soil samples collected from the central-eastern Japan during 1969–1977 for ^239+240Pu activity concentration and ²⁴⁰Pu/²³⁹Pu atom ratio to establish the baseline before the FDNPP accident. We found that ^239+240Pu activity concentrations ranged from 0.004 –1.46 mBq g⁻¹, and ²⁴⁰Pu/²³⁹Pu atom ratios varied narrowly from 0.148 to 0.229 with a mean of 0.186 ± 0.015. We also reconstructed the surface deposition density of ²⁴¹Pu using the ²⁴¹Pu/²³⁹Pu atom ratio in the Japanese fallout reference material. The obtained results indicated that, for the FDNPP-accident released ²⁴¹Pu, a similar radiation impact can be estimated as was seen for the global fallout deposited ²⁴¹Pu in the last decades.

Plutonium as a tracer for soil erosion assessment in northeast China

Soil erosion is one of the most serious environmental and agricultural problems faced by human society. Assessing intensity is an important issue for controlling soil erosion and improving eco-environmental quality. The suitability of the application of plutonium (Pu) as a tracer for soil erosion assessment in northeast China was investigated by comparing with that of 137Cs. Here we build on preliminary work, in which we investigated the potential of Pu as a soil erosion tracer by sampling additional reference sites and potential erosive sites, along the Liaodong Bay region in northeast China, for Pu isotopes and 137Cs. 240Pu/239Pu atomic ratios in all samples were approximately 0.18, which indicated that the dominant source of Pu was the global fallout. Pu showed very similar distribution patterns to those of 137Cs at both uncultivated and cultivated sites. 239+240Pu concentrations in all uncultivated soil cores followed an exponential decline with soil depth, whereas at cultivated sites, Pu was homogenously distributed in plow horizons. Factors such as planted crop types, as well as methods and frequencies of irrigation and tillage were suggested to influence the distribution of radionuclides in cultivated land. The baseline inventories of 239+240Pu and 137Cs were 88.4 and 1688 Bq m(-2) respectively. Soil erosion rates estimated by 239+240Pu tracing method were consistent with those obtained by the 137Cs method, confirming that Pu is an effective tracer with a similar tracing behavior to that of 137Cs for soil erosion assessment.