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.

Introduction

Plutonium (Pu), caused by atmospheric nuclear weapons tests,[1] nuclear power plants, nuclear accidents (e.g., Fukushima accident), and nuclear fuel reprocessing,[2,3] was mainly derived from the global fallout. Anthropogenic radionuclides 239Pu (T1/2 = 24 100 years) and 240Pu (T1/2 = 6560 years) have high radiological toxicity and long-term retention in the environment. It was estimated that approximately 15 000 TBq 239+240Pu was produced by atmospheric nuclear weapon tests, and approximately 12 500 TBq was globally distributed via the stratosphere.[4]239Pu and 240Pu with relatively long half-life have been used as tracers to analyze series processes of Pu in nature, including the sources of Pu, Pu inputs, and distribution, rebuilding the Pu historical events, migration of Pu in seas and land, and special impacts on the environment.[5−12] For example, the 240Pu/239Pu atom ratio can provide fingerprints for different sources,[13] and it is considered an excellent indicator for nuclear forensics. In detail, the weapons-grade Pu has the 240Pu/239Pu atom ratio of 0.01–0.07,[6] the Pu from nuclear reactor-grade has the 240Pu/239Pu atom ratio of 0.2–1.0,[14] the 240Pu/239Pu atom ratio from global fallout is approximately 0.178 ± 0.019,[15] while the Pu from the Pacific Proving Ground (PPG) in the Marshall Islands has the 240Pu/239Pu atom ratio of 0.30–0.36.[16,17] In addition, the Pu caused by the Fukushima accident (FNA) has the 240Pu/239Pu atom ratio of 0.30–0.38.[13,14]

In the past few years, many researchers have studied Pu in China. In mainland China, the activity of 239+240Pu in surface soils showed obvious latitude-dependent distribution, with maximum settlement occurring between 40°N and 50°N.[18] The average atom ratio of 240Pu/239Pu is 0.19, indicating that the Pu source in surface soils is mainly from global fallout.[18−20] It is well known that 45 nuclear tests had been conducted at the Lop Nor nuclear test site from 1964 to 1996, 23 of which were atmospheric nuclear tests, which is located in the northwestern part of China.[21] A 240Pu/239Pu atom ratio of 0.103 was found in sediments from Lake Sugan located ca. 500 km southeast of the Lop Nor test site.[22] A similar 240Pu/239Pu ratio (0.080) was found in a sediment core collected from Lake Bosten, which is the closest lake to the Lop Nor nuclear test site.[10] Moreover, a three-peak pattern vertical distribution of Pu in a sediment core was collected in Lake Chenghai, and one peak indicated that the Pu might have been derived from Chinese nuclear tests (CNTs) during the 1970s.[23] However, the source of Pu in Chinese marginal seas is complicated. Many previous studies on the 240Pu/239Pu atom ratios in Chinese marginal seas have found that these values are between the value of global fallout (0.178 ± 0.019) and the signature value (0.30–0.36) of the PPG, which indicates a finite fraction of Pu from PPG into Chinese marginal seas via some ocean currents (the North Equatorial Current and Kuroshio Current).[11,12,24] Even the 240Pu/239Pu atom ratios in some surface sediments of river estuaries in China are also higher than the value of global fallout but lower than the value of PPG, indicating that PPG and global fallout are mainly the sources of Pu in some river estuaries (Table ).[7,9,11]

Table 1

239+240Pu Activity and 240Pu/239Pu Atom Ratio of Different Sources in the Environment in China

sample locations (core name)	240Pu/239Pu atom ratios	239+240Pu activities (mBg/g)	references
Lake Qinghai (2006QH-1/2/3)	(0.169–0.228)ab	0.010–6.993	(22)
	0.038–0.159b
Lake Bosten (06BS2-1/3, 07BS10-2)	(0.080–0.219)c	0.002–2.209	(10)
Poyang Lake	0.185–0.192 (0.187 ± 0.004)d	0.104–0.665	(25)
South China Sea (surface sediment)	0.246–0.281	0.157–0.789 (0.187 ± 0.004)d	(11)
South China Sea (A8)	0.247–0.312 (0.286 ± 0.011)d	0.036–1.772 (0.871 ± 0.016)d	(11)
Northern North Yellow Sea (surface sediment)	∼0.18	0.022–0.515	(26)
East China Sea (surface sediment)	0.158–0.297 (0.238 ± 0.036)d	0.048–0.492 (0.188 ± 0.119)d	(12)
Bohai Bay (surface sediment)	0.172–0.236 (0.201 ± 0.015)d	0.103–0.987 (0.497 ± 0.269)d	(24)
Liaodong Bay (surface sediment)	0.173–0.241 (0.190 ± 0.014)d	0.052–0.978 (0.343 ± 0.276)d
Pearl River estuary (surface sediment)	0.186–0.244 (0.214 ± 0.023)d	0.026–0.137 (0.072 ± 0.003)d	(11)
Yangtze River estuary (SC07)	0.238 ± 0.007	0.716 ± 0.030	(9)
Liao River coastal zone (Z-9, LH-10/15/18, DP-2/4 LT-2)	0.173–0.215 (0.188 ± 0.049)d	0.103–0.978 (0.294 ± 0.024)d	(27)
Loess Plateau	0.186 ± 0.017	110 (Bq/m2)	(28)
North of China (40°N–50°N) (surface soil)	0.19	0.01–2.66	(18)
Central China (25°N–40°N) (surface soil)		0.01–0.62
South of China (<25°N) (surface soil)		0.03–0.17
Lop Nor (surface soil)	0.155–0.286	0.10–0.83	(20)
coastal areas of China	0.186 ± 0.021	0.002–0.670	(29)
Pearl River Basin	0.165–0.221 (0.183 ± 0.021)d	0.011–3.322 (0.424 ± 0.065)d	present work
WGC	0.163–0.221 (0.194 ± 0.018)d	0.033–4.676 (0.609 ± 0.028)d	present work
Jinjiang Basin	0.168–0.222 (0.193 ± 0.017)d	0.091–0.586 (0.374 ± 0.057)d	present work
Hainan Province rivers	0.167–0.172 (0.169 ± 0.021)d	0.034–0.076 (0.056 ± 0.061)d	present work
rivers of Xinjiang Province	0.183–0.234 (0.208 ± 0.015)d	0.051–0.242 (0.159 ± 0.024)d	present work

The sediment layer above the 1964 peak.

The sediment layer below the 1964 peak.

One abnormal 240Pu/239Pu atom ratio of 0.080 found at the 239+240Pu activity peak in the core of 07BS10-2.

Average mean values.

Although there have been some studies on the transportation and distribution of Pu in China, there is less information on the Pu in Chinese river basins. In this study, we selected several surface sediments of river basins in southern China, including the Pearl River Basin, coastal rivers in western Guangdong Province (WGC), the Jinjiang Basin and the Minjiang Basin (Figure ). We did a series of tests, including the 239+240Pu activity, the 240Pu/239Pu atom ratio, the content of clay, and the content of total organic carbon (TOC) of these samples, aiming (1) to investigate the distribution of 239+240Pu activity and 240Pu/239Pu atom ratio; (2) to identify the source of Pu and to fill in the gap of Pu in the major river systems in southern China; (3) to identify the function of Pu transport in river basins and to provide the basic background data for the planned and operational nuclear power plants in Chinese river basins; and (4) to analyze some factors (the content of clay, TOC, heavy metals) that affect the 239+240Pu activity.

Figure 1

Map of sampling sites along river basins in southern China.[24]

The Pearl River is the second longest river in China and the Pearl River Basin is comprised of four tributaries: the Dongjiang River, Beijiang River, Xijiang River, and some rivers of the Pearl River Delta (PRD). The coastal rivers in western Guangdong Province (WGC) mainly refer to the rivers that flow into the sea from the west of the Pearl River estuary to the Leizhou Peninsula in Guangdong Province. The Jinjiang basin originates from Daiyun Mountain in central Fujian Province, Dongxi and Xixi tributaries are on the upper reach, and the Xixi tributary is the source of this basin. The Minjiang basin is the largest river flowing into the East China Sea in Fujian Province, which is composed of three main tributaries: Jianxi, Futunxi, and Shaxi, and Shaxi is the source of the basin. Hainan Island is a tropical island with three major rivers on the island, namely, Wanquan River in the eastern part of the island, Nandu River in the northern part of the island, and Changhua River in the midwest part of the island.

In this study, we used the basin partitioning function of the GeoMap app to divide the main river basins in our study region (the dark red lines in Figures and 3). However, we divided WGC into the Pearl River Basin because of the partition. (The individual watershed system of WGC is less affected by the upstream Xijiang and Pearl River Basins.) Moreover, our samples were selected from the main streams and tributaries of these river basins, representing all the river basins (Figure ).

Figure 2

(a) Distribution of 239+240Pu activity and (b) the 239+240Pu activities in surface sediments of river basins in southern China. The blue lines are main streams (b), and the dark lines are river networks (a). In (b), the unfilled triangles are planned nuclear power plants and the filled triangles are operational nuclear power plants. (1) Bailong nuclear power plant (NPP), (2) Taishan NPP, (3) Baisha NPP, (4) Zhaoqing NPP, (5) Shaoguan NPP, (6) Huizhou NPP, (7) Lufeng NPP, (8) Haifeng NPP, (9) Jieyang NPP, (10) Zhangzhou NPP, (11) Yanjiashan NPP, (12) Ruijin NPP, and (13) Sanming NPP.

Figure 3

240Pu/239Pu atom ratios in surface sediments of river basins in southern China.

Results and Discussion

The analytical results of the 239+240Pu activities and 240Pu/239Pu atom ratios in the surface sediments of river basins in southern China are summarized in Table S1.

239+240Pu Activities

The 239+240Pu activities in surface sediments collected from river basins in southern China ranged from 0.01 to 4.676 mBq/g. Therein, a wide range of 239+240Pu activities was found in the surface sediments of the Pearl River Basin at 0.011–3.322 mBq/g (mean: 0.242 ± 0.065 mBq/g). While a sample (E31) (3.322 ± 0.499 mBq/g) collected from Dongjiang estuary (Pearl River Basin) had the highest 239+240Pu activity (Figures and 2) in samples of Pearl River Basin, and most of the samples collected from the Pearl River Basin had similar 239+240Pu activities to the 239+240Pu activities found in the Pearl River estuary (0.026–0.137 mBq/g) (Table S1).[11] Moreover, similar 239+240Pu activities were observed in surface sediments near the Yangtze estuary.[7] The samples collected from WGC had 239+240Pu activities from 0.033 to 4.676 mBq/g (mean: 0.609 ± 0.028 mBq/g). Only one sample (W73), closed to the South China Sea (Figures and 2), had the highest 239+240Pu activity (4.676 ± 0.075 mBq/g) in WGC samples. Different from the surface sediments collected from the Pearl River Basin and WGC, the samples from Jinjiang basin had 239+240Pu activities ranging from 0.091 to 0.586 mBq/g, with an average of 0.374 ± 0.057 mBq/g. Unfortunately, there was only one sample that was collected from Minjiang basin (E40), with a 239+240Pu activity of 0.029 ± 0.011 mBq/g. The samples collected from rivers in Hainan Province had the 239+240Pu activities from 0.034 to 0.076 mBq/g (mean: 0.056 ± 0.016 mBq/g).

In Figure , the distribution of 239+240Pu activity presented an increasing trend from the upstream and midstream to the estuary, and some higher values appeared in some estuaries. For example, in the Pearl River Basin, the average 239+240Pu activity was approximately 0.173 ± 0.040 mBq/g in the upstream region, approximately 0.479 ± 0.047 mBq/g in the midstream region, and 0.560 ± 0.082 mBq/g in the downstream region. Similar to the Jinjiang basin, the average 239+240Pu activities in the upstream and downstream regions were approximately 0.302 ± 0.051 and 0.417 ± 0.037 mBq/g, respectively. However, the 239+240Pu activities in the Jinjiang basin were generally higher than those in the Pearl River Basin. Many factors might affect the distribution of 239+240Pu activities, such as contents of TOC and particle sizes of samples, riverine input, and estuary experiences (river runoff, tide bores, coastal currents, and anthropogenic activities (dam-building, oil exploration, and nuclear power plants)).[27,30] Furthermore, the higher 239+240Pu activities in some estuaries show the transport function of Pu in these river basins, where Pu is transported from the Chinese continent to estuaries then to seas by river basins, and the soil erosion in southern China might have also contributed to this trend. The Pu isotopes absorbed by soil particles enter rivers along with surface runoff,[31,32] and the Pu together with riverine mass finally lead to resuspension and redeposition of bottom sediments in some areas, particularly the estuary, coastal zone, marginal sea, and bottom of the ocean.[33] Remarkably, the higher 239+240Pu activities in these estuaries might be affected not only by the source of Pu from the upstream of these river basins but also by the source of Pu from the open Pacific.[34]

In addition, the activity of 239+240Pu in surface soils in southern China (<25°) was at 0.03–0.17 mBq/g, with an average of 0.07 mBq/g.[18] Obviously, the 239+240Pu activities in surface sediments in these river basins were higher than those in surface soils in southern China (<25°) (except for samples in Hainan rivers), which was mostly because the Pu isotopes precipitate with materials in river basins. And there is input of soils into these river basins due to deposition, resuspension, and erosion processes, which could also be interpreted as “source-sink”.[23] Moreover, similar to these river basins in southern China, an increasing number of research results of Pu in Chinese marginal seas has indicated that the 239+240Pu activities in seawater and ocean sediments are evidently higher than those in terrestrial soils in China, which could also be explained by source-sink.[18,24,35,36]

240Pu/239Pu Atom Ratio

The 240Pu/239Pu atom ratio can be used as fingerprint to determine the source of Pu. For example, the 240Pu/239Pu atom ratio of global fallout was suggested to be 0.178 ± 0.019 in the Northern Hemisphere (0–30°N).[15] Unlike the variations in 240+239Pu activities, the 240Pu/239Pu atom ratios in surface sediments in the Pearl River Basin were 0.165–0.212 (Table S1). The 240Pu/239Pu atom ratios in WGC ranged from 0.163 to 0.221. Samples from the Jinjiang basin had 240Pu/239Pu atom ratios of 0.167–0.222, and the 240Pu/239Pu atom ratio in the Minjiang basin was 0.182 ± 0.016. The 240Pu/239Pu atom ratios in the Hainan Province rivers (individual watershed system) ranged from 0.167 to 0.172, with an average of 0.169 ± 0.021, which were well within the value of global fallout (0.178 ± 0.019, 0–30°N).

Therefore, most of these 240Pu/239Pu atom ratios in surface sediments agreed well with the global fallout value (0.178 ± 0.019, 0–30°N) (Figures and 4). These results were also consistent with previous studies on the 240Pu/239Pu atom ratios in surface soils in southern China.[18−20] Moreover, the major source of Pu in these surface soils was mainly global fallout. In central and northern China, similar 240Pu/239Pu atom ratios have also been reported.[18,31,37] However, there was a relatively higher 240Pu/239Pu atom ratio (0.222 ± 0.011) in a surface sediment sample (E25) upstream of Jinjiang basin, presenting a special “outlier” of Pu pollution in China (abnormally high 240Pu/239Pu atom ratio), which might be because of the higher TOC content in this sample (Table S1) (Figures and 5). Unfortunately, there was only data on the 240Pu/239Pu atom ratio of one river surface sediment sample, and it is not yet clear whether this abnormal point was widespread or merely a sporadic outlier of Pu pollution.

Figure 4

Relationship between 239+240Pu activity and 240Pu/239Pu atom ratio found in surface sediments of river basins in southern China. The mean level of the atom ratio is significantly higher than the mean value of global fallout ratio 0.178 (P = 0.0065), but significantly lower than 0.197, the upper limit of global fallout ratio (P < 0.0001).

Figure 5

TOC content in surface sediments of river basins in southern China.

Therefore, these values implied that these places might have received Pu with high atom ratio (such as Pu from high-yield thermonuclear devices) that some samples collected from estuaries of Jinjiang basin and some places in WGC (which were closed to the South China Sea) had higher 240Pu/239Pu atom ratios than the global fallout value (0.178 ± 0.019, 0–30°N) (Figures and 4). In addition, the Pu in the western Pacific with high atom ratio is mostly from PPG in the Marshall Islands, where large-scale nuclear tests were carried out during 1952–1958. Moreover, due to the North Equatorial Current, the Pu from PPG with high atom ratio (0.30–0.36) enter into China marginal seas and flow into coastal estuaries along the Kuroshio Current and its extension currents (Figure S2).[24,34,35,38] Previous published data and some other upcoming data from Liu et al. also observed similar higher 240Pu/239Pu atom ratios in the Yangtze River estuary, which indicated that the sources of Pu mainly consisted of global fallout and the PPG.[7,9] The Pu from PPG showed intense scavenging with sediments in these estuaries, which led to a higher value of the 240Pu/239Pu atom ratio. Consequently, we could explicitly confirm that the source of Pu in these estuaries was mainly from global fallout and the PPG, while the source of Pu in inner river basins was from global fallout.

Until now, both the Pu isotope activity and the Pu atom ratio in surface sediments of river basins in southern China has shown the baseline and can fill in the knowledge gap of Pu, which has hitherto existed in the major river systems in southern China. These data can also provide a reference for Pu transport and pollution control in other river basins in China and basic background data for the planned and operational nuclear power plants in Chinese river basins (data for Pu pollution prevention and control for nuclear power plant activities, nuclear accidents (like the Fukushima Daiichi Nuclear Power Plant), and previous nuclear weapons tests). If a nuclear accident happens in the operational nuclear power plants in the coastal zones, it would affect the entire surrounding coastal area and spread to other seas with the currents.[36,39,40] If a nuclear accident happened in the inner river basins, it might cause Pu pollution of the whole river system and river sediments throughout the catchments and affect the whole downstream and coastal zones. As a result, the 240Pu/239Pu atom ratios surrounding ca. 10–20 km of nuclear power plants would increase, and the atom ratios downstream would also deviate from the average value of the global fallout.

Mass Balance of Pu in the Pearl River Estuary

According to the above discussion results, the source of Pu in the Pearl River Estuary (PRE) was affected not only by the Pu of riverine input (rivers of the Pearl River Basin) but also by the Pu from PPG. To calculate the contributions of Pu from different sources, we used a simple mass balance model, which is summarized in Figure . In this balanced model, the surface area of the Pearl River Estuary was estimated to be 8.6 × 109 m2 based on Google Earth. In this study, we estimated the 239+240Pu inventory to be 360 Bq/m2 in the PRE,[11,21] and the total sedimentary deposition of Pu was calculated to be 3.096 × 1012 Bq. The previous study has reported that the contributions to the PRE from the PPG were approximately 30 ± 5%.[11] Therefore, the Pu from PPG in the PRE was calculated to be 7.74 × 1011–10.84 × 1011 Bq, and the rest of the Pu was from the global fallout of approximately 2.01 × 1012–2.32 × 1012 Bq (70 ± 5%), which included the Pu of riverine input (rivers of the Pearl River Basin) and the Pu of direct deposition. Unfortunately, we did not directly analyze the 239+240Pu inventory in the Pearl River Basin, so we used the value of the global fallout of 36–88.5 Bq/m2 between 20°N and 30°N to calculate the riverine input.[19,21] Previous studies have reported that the residence time of Pu in river basins is 800–3000 years,[41,42] and the Pearl River Basin is the second largest river basin in China and contains many dams. Therefore, we took 3000 years as the residence time of Pu in the Pearl River Basin. The following equation was used to calculate the Pu of the Pearl River Basin input[43]where Ad is the area of the catchment (4.4 × 1011 m2), If is the 239+240Pu inventory (36–88.5 Bq/m2), and fe is the fraction of Pu eroded in the catchments each year (fe = ln2*3000 years = 2.3 × 10–4 years). The Pu from the Pearl River Basin was calculated to be 3.64 × 109–8.96 × 109 Bq/year, and the total Pu input derived from the Pearl River Basin was 2.33 × 1011–5.73 × 1011 Bq during 64 years (1952–2016). The Pu of direct deposition was approximately 14.39 × 1011–20.89 × 1011 Bq. Therefore, the Pu percentages from riverine input and direct deposition contributing to the Pu source in the PRE were approximately 13 ± 5% and 57 ± 5%, respectively.

Figure 6

Mass balance of Pu in the Pearl River Estuary.

From the calculated results, we suggested that the major source of Pu in the PRE was from the Pu of direct deposition of global fallout. In contrast, the Pu from river input accounted for a small proportion to the entire inventory of Pu in the PRE, which might indicate that the soil erosion in the Pearl River Basin was not very serious. However, 77–80% of the Pu in the Yangtze River estuary was from the Yangtze River input,[44] which was nearly eight times higher than that in the PRE. Thus, we should pay more attention to the Yangtze River and try our best to keep healthy environment systems for the Pearl River Basin.

Factors Associated with the Distribution of 239+240Pu Activity

The 239+240Pu activities in surface sediments of river basins in southern China was much higher than that in surface soils, and this might be associated with the result of multiple factors, including (1) the organic matter contents and particle sizes of surface sediments of river basins, (2) the variable climatic conditions (rainfall, humidity, wind, etc.), for example, wind and rain have competing effects on the amounts of resuspended plutonium-containing material collected,[45,46] (3) estuary experiences (such as river runoff, tide bores, currents, and anthropogenic activities (dam-building, oil exploration, nuclear power plants)),[47] and (4) the material composition of the surface sediments.

In addition to direct Pu isotope measurements, some correlation coefficients can be used to estimate the degree of contamination of the territory by plutonium isotopes. For example, previous studies have already reported correlations between the activity of plutonium isotopes and the other radionuclides (144Ce, 103,106Ru, 137Cs, etc.) to estimate the content of plutonium isotopes in the environment and other radioactive fallout.[48−51] The activity of radiocesium is related to clay-sized sediment and material enriched in organic matter.[52,53] However, there is no significant relationship between 137Cs concentrations and TOC in sediments.[54] Although there are many correlations between Pu and Cs,[13,55] Pu is easily adsorbed on refractory oxides, colloidal particles, and organic matter,[56] and Pu particles with strong activity are more easily transferred to sediments by sedimentary particles; thus, Pu has a higher reserve in sediments.[57] Therefore, we paid special attention to the influence of material composition in surface sediments, such as TOC content, grain size, clay content, and heavy metals. The test results of the material composition are summarized in Table S1.

Previous studies reported less information about the content of TOC and the 239+240Pu activity. In this study, the distribution of TOC showed that the estuaries had higher contents of TOC in addition to some other sporadic areas of the river basins. These estuaries also had higher 239+240Pu activity (Figure ), which was consistent with the distribution of TOC. However, not all sporadic areas showed such characteristics, which might be due to the complexity of the river environment (such as climatic variation and human activities), and our study river basins were too broad to agree with all special sporadic areas. In other words, there was a certain correlation between the 239+240Pu activity and the content of TOC in surface sediments of river basins (Figure ).

The study on the relationship between natural soil particles and Pu isotope has indicated that Pu isotopes are easy to associate with finer particles because of their large specific surface areas.[44,58] In Figure S3, particle components of surface sediments in these river basins implied that these sediments existed in different grain sizes and mainly contained coarse particles (>150 μm) (Table S1). However, because of the smaller size of particles absorbing a higher content of Pu isotopes, in this study, the particle size of these samples was not the factor affecting the 239+240Pu activities in these river basins.[59] It was also noteworthy that 239+240Pu activities were essentially uncorrelated with particle components. This result might be due to the less content of clay particles in study samples.

Because heavy metals are difficult to be removed from water by self-purification, they are considered as a kind of pollutants with high ecological significance in environment.[60,61] Although Pu isotope testing techniques are developing rapidly, some properties of Pu isotopes are not well known.[37,62−64] We had some reported data on some heavy metals (V, Cr, Co, Ni, Cu, Zn, Ga, As, Cd, Sn, Tl, Pb, Mn) in the surface sediments that were collected from our study river basins in southern China.[65]Figure S4 clearly showed that most heavy metals were unrelated to the distribution of 239+240Pu activity. Only heavy metal As demonstrated consistent associations with the distribution of 239+240Pu activity, and the correlation coefficient was 0.69 (Figure S5). This correlation might mean that samples with higher As content would have higher 239+240Pu activity.

In addition, the 239+240Pu activities in surface sediments of river basins were significantly higher than those in surface soils in southern China, suggesting that there are other important factors besides particle components, particle sizes, organic matter, and heavy metal contents that control Pu activity in surface sediments in our investigated areas. These factors could include the chemical properties of the sediment (pH, other inorganic ion concentrations), root water uptake, and biological effect.[66] The influence of these factors on the activity of 239+240Pu in the surface sediments of the basin remains to be further studied in detail.

Conclusions

The spatial distribution of the 239+240Pu activity in surface sediments confirmed that it was nonuniform for 239+240Pu activity in major river basins in southern China. Due to riverine transport of masses of Pu isotopes into seas and the effect of Pu from the open Pacific, estuaries reserved higher 239+240Pu activities, and the spatial distribution of 239+240Pu activity showed an increasing trend from the upstream and midstream regions to the estuaries of these river basins. The distribution of atom ratios implied that the Pu source of the inner river basins was mainly global fallout, while the Pu sources in river estuaries were a mix of global fallout and the PPG. The mass balance model revealed that the major source of Pu in the PRE was from direct deposition (57 ± 5%), and the Pu input to the Pearl River Basin was only 13 ± 5%. By analyzing the particle components of surface sediments, we concluded that only the content of TOC and the heavy metal As had a certain correlation with the 239+240Pu activity. Sediment components, particle sizes, and contents of heavy metals (except As) had no connection with the 239+240Pu activity.

Samples and Methods

Sample Collection and Sample Preparation

A total of 53 surface sediments in this study were collected from river basins in southern China in 2016, located 105°–119°E, 18°–26°N (Figure ). Sediment samples were collected by box core recovery and treated by vertical extrusion using a hydraulic jack from river bed, with top 3 cm sediments. These samples included 30 samples from the Pearl River Basin (Beijing, Dongjiang, Hanjiang, Xijiang), 11 samples from WGC, eight samples from Jinjiang basin, one sample from Minjiang basin, and three samples from some rivers in Hainan Province. The regions of these samples were within inner river basins and estuaries, and the samples were packed into clean polypropylene sample bags and then kept in a 4 °C refrigerator until analysis. The detailed site information and sample properties are shown in Table S1.

Clay Content and TOC Content Analysis

Separated sample aliquot (2 g) from each sample was tested to determine the grain size using a Master-size 2000 (Malvern Instruments, U.K.). Carbonate-free samples (decalcified using 2 M HCl) were used for total organic carbon (TOC) measurements. The TOC was determined using an elemental analyzer (Heraeus, Germany) and a set of tyrosine standards were used to calibrate the TOC concentrations. The measurements of particle size and TOC were completed in the laboratory of Nanjing University, and the relative errors of the test were within ±3%.

Pu Isotope Measurement

After the analysis of particle sizes and TOC, these surface sediments were dried in an 80° oven and crushed in an agate mortar to prepare for Pu isotopes analyses. The detailed experiments regarding the separation of Pu isotopes and its analytical methods have been previously described.[25] First, we weighed 2.5 g of each sample in a Teflon cup and added 1.14 pg 242Pu as a chemical yield tracer. The Pu was extracted from samples by heating sample solutions on a hot plate at 200 °C for 4 h using 8 M HNO3. Subsequently, the sample solutions were filtered with AG 1-x8 and AG MP-1 M anion-exchange resins to purify Pu isotopes. The final purified liquids were evaporated to almost dryness and were volume to 1 mL with 4% HNO3. Then, an APEX-Q sample introduction system with a membrane desolvation unit (ACM) and a conical concentric nebulizer were together used as sample introduction systems to improve the sensitivity of sector field inductively coupled plasma mass spectrometry (SF-ICP-MS). The total recovery of Pu in these sample solutions was 80–95% and the average recovery was 87 ± 3%. A certified Pu isotope standard solution (NBS-947) was used for mass deviation correction.[25]

The Pu isotope measurement was completed in the Key Laboratory of Radiation Medicine and Protection at Soochow University, Suzhou, China. Relative standard deviations of the replicate analyses were all within ±5%.