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Journal of Geographical Sciences >

2022 , Vol. 32 >Issue 4: 605 - 616

DOI: https://doi.org/10.1007/s11442-022-1963-z

Original article

Quantitative assessment of the spatial distribution of 239+240Pu inventory derived from global fallout in soils from Asia and Europe

  • CAO Liguo ,
  • ZHOU Zhengchao ,
  • WANG Ning ,
  • XIAO Shun , *
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  • School of Geography and Tourism, Shaanxi Normal University, Xi’an 710119, China
*Xiao Shun (1981-), PhD and Associate Professor, specialized in atmospheric pollution and environmental health. E-mail:

Cao Liguo (1986-), PhD, specialized in nuclide analysis and application of radioisotope. E-mail:

Received date: 2021-01-15

  Accepted date: 2021-08-12

  Online published: 2022-06-25

Supported by

Key Technologies Research and Development Program of Shaanxi Province(No.2021ZDLSF05-02)

National Natural Science Foundation of China(No.41901129)

National Natural Science Foundation of China(No.42101100)

The Natural Science Foundation of Shaanxi Province(No.2021JM-200)

The Natural Science Foundation of Shaanxi Province(No.2021JQ-313)

The Fundamental Research Funds for the Central Universities(No.GK202001003)

Fold

Abstract

Due to the atmospheric nuclear weapon tests carried out, terrestrial environments have been extensively contaminated by global fallout of plutonium (Pu) worldwide. The 249+240Pu inventories in soil profiles from undisturbed and flat forest or grasslands (reference sites) mainly from Europe and Asia are considered as important background for evaluating soil erosion. Thus, we conducted a literature survey over an area extending from 2.6°W to 148.9°E and from 53.2°S to 76.6°N, with the purpose of analyzing the spatial distribution of 239+240Pu inventories and possible controlling factors. The aim of this work was to derive an empirical model of 239+240Pu inventory based on currently available 239+240Pu data, precipitation and latitude. The results show that, in general, the latitudinal distribution of 239+240Pu inventory was consistent with the UNSCEAR reports. However, the 239+240Pu inventories are higher than the UNSCEAR data, especially in the Northern Hemisphere. In addition, close relationships (at the 0.01 significance level) were identified between 239+240Pu inventories and annual precipitation and latitude. An empirical formula was therefore developed to estimate 239+240Pu inventories in soils based on latitude and precipitation data. However, future research may require more data of measured 239+240Pu inventories in soil profiles that can be used to compare, validate and improve upon the accuracy of the inferred empirical equation.

Key words： spatial distribution; plutonium; global fallout; precipitation

Cite this article

CAO Liguo , ZHOU Zhengchao , WANG Ning , XIAO Shun . Quantitative assessment of the spatial distribution of 239+240Pu inventory derived from global fallout in soils from Asia and Europe[J]. Journal of Geographical Sciences, 2022 , 32(4) : 605 -616 . DOI: 10.1007/s11442-022-1963-z

1 Introduction

Plutonium (Pu) isotopes have been released into various environments since 1945 through above ground testing of nuclear weapons (Harley, 1980), nuclear fuel reprocessing, and the operation of and accidents involving nuclear power generation and other nuclear activities (Muramatsu et al., 2000; Onishi et al., 2007, Zheng et al., 2012; Froehlich et al., 2019). The nuclear weapon tests conducted in the early 1960s are the main source of Pu in the terrestrial environments, and it has been reported that the total amount of 239+240Pu released from these tests is estimated to be approximately 11 PBq (UNSCEAR, 2000).
Among the released Pu isotopes, the most abundant in the environment are 239Pu (half-life of 2.41 × 104 yr) and 240Pu (half-life of 6.65 × 103 yr). Much scientific and public concern has been shown to the evaluation of radiation risk associated with these two isotopes in the environment due to their strong radiological toxicity and long-term persistence (Yamamoto et al., 1999; Turner et al., 2003; Ketterer et al., 2004; Łokas et al., 2019). Other studies have focused on their migration behavior within the terrestrial environment (Bu et al., 2014; Ni et al., 2018; Zhang et al., 2018; Cao et al., 2019). In addtion, 239+240Pu can be served as a geochemical tracer for many oceanic processes (Yamada et al., 2012; Wu et al., 2014; Buesseler et al., 2018), where it can be applied to estimate sedimentation rates (Yamamoto et al., 1991; Zheng et al., 2008; Tims et al., 2010; Pan et al., 2011), and to assess soil erosion (Schimmack et al., 2002; Xu et al., 2015; Meusburger et al., 2016; Zhang et al., 2019).
The total 239+240Pu atmospheric deposition of and the inventory of 239+240Pu in soils can be considered as important background information for environmental tracing studies and radiation risk evaluation. It was reported that the distribution pattern for 239+240Pu displayed the highest levels of deposition in the Northern Hemisphere’s temperate latitudes and a minimum in the equatorial regions (Hardy et al., 1973). There is now a substantial body of information on fallout isotopes in the literature, including measurements of the actual fallout from across the globe, which document a significant increase in knowledge over that provided by United Nations Scientific Committee on the Effect of Atomic Radiation (UNSCEAR) reports. The deposition of Pu isotopes in mid-latitude region was higher than in other latitude bands because most of the atmospheric nuclear testing was carried out in these areas (UNSCEAR, 2000) (Figure 1). Many previous studies have focused on simulating the spatial distribution and characteristics of the atmospheric deposition and environmental sampling of 137Cs, another important radionuclide with the same source term as the Pu isotopes (Walling and He, 2000; Aoyama et al., 2006; Fukuyama et al., 2008; Furuichi et al., 2013; Zhang et al., 2015; Tagami et al., 2019). For instance, the spatial distribution of 137Cs in soil reference sites was quantified in Asia, where it seems there is a high correlation between the logarithm of global 137Cs fallout concentrations and latitude (Tagami et al., 2019). This study also concluded that it could be difficult to determine the 137Cs activity in the future in low latitude regions (20°S-20°N) because of its low concentrations in soils. Compared with radioactive 137Cs, the spatial distribution of 239+240Pu and its relationship with precipitation and latitude remained unresolved, although the average deposition of 239+240Pu for ten-degree latitude bands from 0-50° has been determined (Hardy et al., 1973; UNSCEAR 2000). In addition to the latitude, the geographical distributions of 137Cs and 239+240Pu inventories in soils were also controlled by precipitation (Zhang et al., 2015; Huang et al., 2018). However, the relationship between 239+240Pu inventory in soils and precipitation and latitude were not quantitatively investigated and analyzed especially in large scale.
Figure 1 Sites of nuclear weapons tests worldwide (redrawn from UNSCEAR, 2000)
The inventory of 239+240Pu in soil profiles where the reference site is flat and undisturbed (i.e., without erosion or deposition) theoretically equals the accumulative deposition of Pu. A majority of published works have compared the inventory of 239+240Pu in soils collected from undisturbed grasslands or forests with the values presented by the UNSCEAR reports within the same latitude band (UNSCEAR, 2000; Lee et al., 2001; Huh and Su, 2004; Xu et al., 2013; Ni et al., 2018; Zhang et al., 2019). However, most of the 239+240Pu inventories determined in soils were generally higher than those presented in the UNSCEAR reports (UNSCEAR, 2000), suggesting that the UNSCEAR may have underestimated the accumulative deposition of Pu isotopes from atmospheric nuclear testing.
Over the past decades, detection of the Pu isotopes were mainly performed using traditional α-spectrometry, which is a time-consuming process with low efficiency (Hirose, 2009; Bu et al., 2014; Cao et al., 2016). Therefore, the available database of 239+240Pu inventory in soil profiles and atmospheric deposition is quite limited, hence, little is known regarding the spatial distribution of the global fallout of Pu isotopes, particularly over large spatial scales (i.e., the Northern Hemisphere or globally). Recently, mass-spectrometric technique (ICP-MS, AMS and TIMS) has become more and more attractive and effective technique for the determination of Pu isotopes and their atom ratios. Besides, procedures for separation and purification of Pu isotopes have been greatly improved and developed (Qiao et al., 2009; Bu et al., 2014; Cao et al., 2016; Wang et al., 2017; Xing et al., 2018), thus more Pu isotopes data in soils can be available. In the present study, we summarize a the preliminary dataset of 239+240Pu inventories in soil profile from published literature mainly across Europe and Asia to quantify the spatial distribution of 239+240Pu, and to provide more reliable reference values of the 239+240Pu inventory for different latitude bands. From this, an empirical model for estimating its inventory in soils is derived. Based on previous studies, we attempt to provide a comprehensive assessment of the 239+240Pu inventory, which can be beneficial to decision-making processes in terms of reference site selection (i.e., soil erosion evaluation using 239+240Pu as a tracer) and risk management related to anthropogenic Pu contamination due to unexpected incidents of radionuclides release.

2 Materials and methods

The inventory data (Bq/m2) of 239+240Pu in soil cores were collated from undistributed sites, and the annual precipitation for the sampling sites was obtained covering the period 1973-2019. The median value was adopted if only the range of the precipitation was provided in the literature. The data used for statistical analysis primarily came from published articles as well as UNSCEAR reports. The dataset was selected and built based on the following three criteria: (1) the location of the sampling sites and annual precipitation were clearly given; (2) that global fallout is the dominant source of Pu isotopes in soils, or it can be neglected if the site received close-in fallout Pu; (3) the unit of the 239+240Pu inventory is provided in terms of Bq/m2. If the sampling year was not addressed, as was the situation in some cases, we assumed that the sampling year was two years before the article’s submission. Considering the long half-lives of 239Pu and 240Pu, correcting for the decay of the two radionuclides was unnecessary and has not been considered in this study.

3 Results and discussion

3.1 Spatial distribution of 239+240Pu inventories in soil profiles

Following the data selection criteria listed in Section 2.1, a total of 150 available data points were collected to create the 239+240Pu inventory dataset from soil profiles worldwide. The detailed sampling information is provided in Table S1. The study region extends from 2.6°W to 148.9°E and from 53.2°S to 76.6°N, covering 34 countries. The continents and regions included are the Asia (Hardy et al., 1973; Sha et al., 1991; Lee et al., 1996; Lee et al., 2001; Huh and Su, 2004; Quang et al., 2004; Zheng et al., 2009; Dong et al., 2010; Xu et al., 2013; Bu et al., 2014; Meusburger et al., 2016; Ni et al., 2018; Cao et al., 2019; Zhang et al., 2019), Europe (Hardy et al., 1973; Cigna et al., 1987; Bunzl and Kracke, 1988; Bunzl et al., 1994; Hölgye and Filgas 1995; Jia et al., 1999; Komasa 1999; Riekkinen and Jaakkola 2001; Michel et al., 2002; Hölgye et al., 2004; Zollinger et al., 2015; Bouisset et al., 2018; Meusburger et al., 2018), North America (Hardy et al., 1973; Price 1991; Ketterer et al., 2004), South America (Hardy et al., 1973), Africa (Hardy et al., 1973), and Oceania (Hardy et al., 1973; Hoo et al., 2011; Froehlich et al., 2019).
Figure 2 presents the spatial distribution of the 239+240Pu inventories. Some sampling sites may not have accurate location information (e.g., longitude), hence these have not been included in Figure 2. Notably, most previous studies of 239+240Pu inventories in soils were conducted in Asia and Europe, where there is more available data compared to other regions. As shown in Figure 2, the values of the 239+240Pu inventories range from 10.5 Bq/m2 to 410 Bq/m2, with the maximum value in Taiwan, China (Huh and Su, 2004) and the minimum value in Shanxi, China (Sha et al., 1991).
Figure 2 Spatial distribution of 239+240Pu inventories in soil profiles
In Asia, the values for 239+240Pu inventories are much higher at lower latitudes (Tropics, 0-30°N) than those at higher latitudes (mid-latitudes, 30°N-60°N), the latitudinal patterns of which are somewhat opposite to the UNSCEAR reports (UNSCEAR, 2000). Of great interest is that there is a decreasing trend in the 239+240Pu inventories from coastal to inland regions in both Europe and Asia. Differences in precipitation, vegetation, and topography, as well as the 239+240Pu concentration in the atmosphere at the time of deposition, all affected the spatial distribution of the 239+240Pu inventories (Huh and Su, 2004). Indeed, the controlling factors of the inventory of 239+240Pu in soil profiles depend on the spatial scales of investigation. For large spatial scales (i.e., global or continental scales), precipitation could be the dominant factors influencing the spatial distribution of 239+240Pu inventory. The annual precipitation in eastern Asia is higher than that in western Asia, displaying a decreasing trend from southeast to northwest Asia (Figure 3). Although uncertainty remains, the spatial distribution of annual precipitation is generally consistent with the declining trend of the 239+240Pu inventories from coastal to inland regions across Asia (Li et al., 2010; Cao and Pan 2014; Zhang et al., 2020).
Figure 3 Annual precipitation from 1960 to 2017 based on Global Precipitation Climatology Center (GPCC) data
Compared with Asia, the variance of 239+240Pu inventories in different reference sites is less across Europe. However, we still found slight downward trend in the 239+240Pu inventories collected from the western parts (coastal region) to the eastern parts (inland region), indicating the prominent influence of prevailing westerlies on the spatial distribution of 239+240Pu (Swiatek, 2011). Much of Europe is covered by the Great European Plain, a major topographic feature in Europe that may contribute to the low variance of 239+240Pu inventories in the soil.
When it comes to the latitudinal distribution of global fallout 239+240Pu, the value of the deposition density was the largest in the 40°-50°N band, and decreases gradually northward and southward (UNSCEAR, 2000). In this study, the geometric mean inventory of 239+240Pu in each 10-degree band is shown in Table 1. The inventory was within one order of magnitude in each band, the minimum and maximum inventories ranging from one third to six times the average value. The latitudinal distribution pattern of 239+240Pu was in good agreement with the UNSCEAR reports (UNSCEAR, 2000). However, the 239+240Pu inventories shown here were notably higher than the results in the UNSCEAR data (UNSCEAR, 2000), especially for the Northern Hemisphere. For instance, the average value of the inventory data determined in this work was 91 Bq/m2 across the 40°-50°N band, while the UNSCEAR reported a mean value of 58 Bq/m2 (UNSCEAR, 2000). As suggested by Aoyama et al. (2006), the UNSCEAR could have underestimated the 137Cs deposition density and thus the total inventory particularly in higher latitude areas (20°N to 50°N). Similarly, to the assessment of the 137Cs inventories, we argue that the underestimation of the 239+240Pu inventories was probably due to the uniform distribution model employed in the UNSCEAR reports. Note that the spatial sampling points in the UNSCEAR reports were too coarse to capture the true variance of the 239+240Pu inventories for different regions, particularly around lower latitude areas (10° N to 30° N, Aoyama et al., 2006; Tagami et al., 2019).
Table 1 Average latitudinal distribution and range of the 239+240Pu inventories, compared to that from UNSCEAR (1993, 2000)
Latitude band Number 239+240Pu inventory (Bq/m2) UNSCEA, 1993, 2000
Geometric mean Range
Northern
Hemisphere		 70°-80°N 2 14 12-15
60°-70°N 5 54 31-115
50°-60°N 9 57 40-101
40°-50°N 40 91 27-138 58
30°-40°N 42 60 11-139 42
20°-30°N 14 67 36-410 36
10°-20°N 3 61 8-148 22
0°-10°N 1 5
Southern
Hemisphere		 0°-10°S 5 11 3-19
10°-20°S 6 7 4-10
20°-30°S 8 15 7-23
30°-40°S 10 16 8-28
40°-50°S 4 13 7-25 16
50°-60°S 1 7

3.2 Qualifying the relationship between 239+240Pu inventory and precipitation and
latitude

To investigate the impact of precipitation on 239+240Pu inventories in soil profiles, the 239+240Pu data collated for this study were plotted with respect to the precipitation at the sampling sites (Figure 4). It has been documented that within latitudinal bands, the flux of a fallout radionuclide could be highly correlated with the amount of precipitation (Owens et al., 1997). The relationship between 239+240Pu inventories and precipitation in the Northern Hemisphere was examined and the results suggest a close linear relationship between these two variables, with a correlation coefficient of 0.659 (significant at 0.01 level). We speculate that in the Northern Hemisphere, wet deposition (e.g., rainfall) could be the dominant factor that contributes to the scavenging of global fallout 239+240Pu from the atmosphere. In the Southern Hemisphere, however, the correlation between precipitation and 239+240Pu inventory was much weaker possibly due to sampling issues and land cover. It is well known that compared with the Northern Hemisphere, much less atmospheric nuclear testing was conducted in the south, while limited data dealing with 239+240Pu inventories was collected from here.
Figure 4 239+240Pu inventories in soil profiles plotted against precipitation in the Northern Hemisphere
To make a comparison with the radionuclide 137Cs distribution, we reassessed the 239+240Pu inventories and latitudes of the sampling points within 0-50° with the results being presented in Figure 5. It was found that there was a good correlation (significant at 0.01 level, R = 0.648) between 239+240Pu inventories and latitude, and the inventory increased exponentially with the latitude within 0-50° band. Similar observations were previously reported by Tagami et al. (2019), who found that the 137Cs inventories in soils rose exponentially with increasing latitude in Asia, which indicated that the inventories of 137Cs and 239+240Pu varied with latitudes in the same way. It is known that Pu, together with the other actinides, is highly non-volatile, and its transport mostly takes place through the dispersion of micrometric particles from nuclear fuel and fission and activation products, while Cs is volatile and its mobilization requires relatively low temperatures (Sandalls et al., 1993). Although the chemical properties of 137Cs and 239+240Pu are obviously different, it is interesting to conclude that global fallout leads to both of them being deposited, with the 137Cs and 239+240Pu inventories being well correlated with latitude in the same way. However, more attention should be paid to the mean tropospheric residence time of stratospherically injected plutonium and cesium, the yields and materials making up the nuclear fuel of each atmospheric nuclear test, and future global atmospheric patterns. These factors are of considerable importance when clarifying different types of correlations and spatial distributive characteristics of 239+240Pu and 137Cs inventories.
Figure 5 239+240Pu inventories in soil profiles plotted against latitudes
As a close correlation between the inventories of Pu isotopes and the amount of precipitation and latitude has been identified, precipitation and latitude can be used to estimate 239+240Pu inventory in soils by the empirical formula using the method of least squares. The simulated three- dimensional distribution of 239+240Pu inventories with respect to annual precipitation and latitude is presented in Figure 6 based on the following statistical function:
Val (x,y) = A1+ A2·x + A3·y + A4·x2+ A5·x·y + A6·y2+ A7·x3+ A8·x2·y + A9·x·y2+ A10·y3
where Val (x, y) is an inventory of 239+240Pu (Bq/m2), x is the degree of latitude, and y is the annual precipitation (mm). Based on the least squares regression method, the parameters {A1-10} in Eq (1) are given as follows: (A1: -8.597, A2: -0.6273, A3: 0.04115, A4: 0.06532, A5: 0.001333, A6: -4.051e-0.5, A7: -0.008162, A8: 2.67e-06, A9: -4.525e-07, A10: 1.087e-08), which would minimize the sum of squared residuals in estimating 239+240Pu inventory.
Using the empirical formula, the resulting correlation coefficient (R) is high with a value of 0.793 and significance at the 0.01 level. This formula is useful for the estimation of
239+240Pu inventories in soils for sites based on their precipitation and latitude, which may provide information for the evaluation of soil erosion and for the budget of the 239+240Pu inventories in lake systems. Note that our function was created based on limited data, hence further research requires more 239+240Pu inventory measurements that may improve the accuracy and suitability of the obtained empirical formula. Furthermore, the formula needs to be further validated by observations of determined 239+240Pu inventory of soil profiles in undisturbed areas, especially in Asia and Europe.
Figure 6 Spatial distribution of 239+240Pu inventory with precipitation and latitude

4 Conclusion

In the present work, we investigated the spatial distribution of 239+240Pu inventories in soil cores from undistributed sites (reference areas) worldwide by reviewing the data presented in the literature. The latitudinal distribution pattern of 239+240Pu inventories was in line with the UNSCEAR reports (UNSCEAR, 2000). However, the 239+240Pu inventories shown here were higher than the UNSCEAR results (UNSCEAR, 2000), especially in Northern Hemisphere. Also, we qualified the relationship between 239+240Pu inventories and precipitation and latitude and derived an empirical formula (binary multi-degree equation) that could be applied to estimate 239+240Pu inventory based on precipitation and latitude data. More 239+240Pu inventory data from soil profiles worldwide is required for future studies to validate and improve upon the accuracy of the obtained empirical formula.

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