Article pubs.acs.org/est
Isotopic Composition and Distribution of Plutonium in Northern South China Sea Sediments Revealed Continuous Release and Transport of Pu from the Marshall Islands Junwen Wu,†,‡ Jian Zheng,*,‡ Minhan Dai,*,† Chih-An Huh,§ Weifang Chen,† Keiko Tagami,‡ and Shigeo Uchida‡ †
State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, Fujian 361005, P. R. China Research Center of Radiation Protection, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage, Chiba 263-8555, Japan § Institute of Earth Sciences, Academia Sinica, P.O. Box 1-55, Nankang, Taipei 11529, Taiwan ‡
S Supporting Information *
ABSTRACT: The 239+240Pu activities and 240Pu/239Pu atom ratios in sediments of the northern South China Sea and its adjacent Pearl River Estuary were determined to examine the spatial and temporal variations of Pu inputs. We clarified that Pu in the study area is sourced from a combination of global fallout and close-in fallout from the Pacific Proving Grounds in the Marshall Islands where above-ground nuclear weapons testing was carried out during the period of 1952−1958. The latter source dominated the Pu input in the 1950s, as evidenced by elevated 240Pu/239Pu atom ratios (>0.30) in a dated sediment core. Even after the 1950s, the Pacific Proving Grounds was still a dominant Pu source due to continuous transport of remobilized Pu from the Marshall Islands, about 4500 km away, along the North Equatorial Current followed by the transport of the Kuroshio current and its extension into the South China Sea through the Luzon Strait. Using a simple two endmember mixing model, we have quantified the contributions of Pu from the Pacific Proving Grounds to the northern South China Sea shelf and the Pearl River Estuary are 68% ± 1% and 30% ± 5%, respectively. This study also confirmed that there were no clear signals of Pu from the Fukushima Daiichi Nuclear Power Plant accident impacting the South China Sea. 240
Pu/239Pu atom ratios ranging from 0.2 to 1.0 depending on the fuel burn-up, while weapons-grade Pu has a lower 240 Pu/ 239 Pu atom ratio (0.01−0.07). 11,12 The average 240 Pu/239Pu atom ratio of global fallout is 0.176 ± 0.014.13,14 In the past few decades, these anthropogenic radionuclides have been extensively investigated in order to elucidate their source terms and to assess their environmental behavior and impacts.8,15 The Marshall Islands are located in the northern Pacific Ocean, about 4500 km from the Luzon Strait, the entrance of the Kuroshio current to the South China Sea (SCS). The 240 Pu/239Pu atom ratio of close-in fallout from the Pacific Proving Grounds (PPG) in the Marshall Islands is characteristically higher (0.30−0.36) than that of global fallout.16−18 It has been demonstrated that Pu from the PPG can be transported long distances in the western Pacific Ocean and into its adjacent marginal seas by the North Equatorial Current
1. INTRODUCTION Plutonium (Pu) is an artificial element that has been introduced into the environment since 1945 mainly through above ground nuclear weapons testing1,2 and through various accidental releases (e.g., Chernobyl and Fukushima)3,4 and reprocessing plants of nuclear materials.5−7 The oceans covering ∼71% of the earth’s surface are major recipients of Pu released to the environment by atmospheric and aquatic pathways. It was estimated that ∼11 PBq (1 PBq = 1015 Bq) 239+240Pu has been deposited in the world’s oceans.8 Due to their high toxicity, internal radiation threat, and long half-lives, the fate of plutonium isotopes is of great environmental concern. When deposited in the oceans, Pu participates in a variety of oceanic processes and can be transported rather far from the source point. Such environmental concerns can thus be beyond a regional scale because of this transport. For example, Pu isotopes in Arctic kelp indicated such transport from nuclear fuel-reprocessing wastes in the Atlantic Ocean.9 The relative abundances of Pu isotopes, typically 239Pu and 240 Pu, can be used to trace the specific Pu source because Pu isotopic ratios vary with reactor type, nuclear fuel burn-up time, neutron flux and energy, and for fallout from nuclear detonations, weapon type, and yield.10 Reactor-grade Pu has © 2014 American Chemical Society
Received: Revised: Accepted: Published: 3136
December 1, 2013 February 15, 2014 February 24, 2014 February 24, 2014 dx.doi.org/10.1021/es405363q | Environ. Sci. Technol. 2014, 48, 3136−3144
Environmental Science & Technology
Article
Figure 1. Maps of the South China Sea and the western North Pacific Ocean showing (a) a schematic chart of the North Equatorial CurrentKuroshio current system and the location of the Pacific Proving Grounds (PPG) and (b) sampling sites in the northern South China Sea. The PPG (red triangle) is located in the Marshall Islands. The range of both the North Equatorial Current and Kuroshio (blue solid arrows) in the Pacific Ocean has also been roughly sketched. The North Equatorial Current flows westward to the Philippine Sea and bifurcates into the northwardflowing Kuroshio current with its extension into the northern South China Sea. The sampling stations (red squares) indicate those visited in 2009; the sediment cores were taken at stations A8 and S101. Blue circles represent stations where surface sediment samples were collected during the 2012 cruise. Green dashed lines are isobaths, and the numbers on them indicate water depth in meter.
and subsequently via the Kuroshio current, thus impacting the northwestern Pacific Ocean,19−23 the East China Sea,24,25 and the Japan Sea.26,27 Note that the transit time of surface seawater masses from the PPG to the western Pacific margin was in the range of 290−380 days.28,29 The SCS, located in the western margin of the North Pacific, is the largest marginal sea of the Pacific Ocean. The northern SCS, considered in the present study, is adjacent to one of the world’s most economically dynamic regions, notably the Pearl River Delta including metropolises such as Hong Kong, Shenzhen, and Guangzhou. There are two nuclear power plants, i.e., Daya Bay and Ling Ao, located in Shenzhen that have been operating since 1994 and 2003, respectively, with another four under construction in Guangdong and Hainan Provinces adjacent to the northern SCS. In addition, there are plans to build five more nuclear power plants in Guangdong Province. Potential impact of Pu and other anthropogenic radionuclides to the surrounding environment is therefore an issue of great concern to the public. To date, however, there are
only four Pu data points reported for the SCS basin; they led to a hypothesis that Pu in this region is sourced from the PPG.23,30 This study thus sought to further test the hypothesis by using an expanded data set with good spatial coverage of the northern SCS shelf and the Pearl River Estuary (PRE). Such spatial coverage aids in examining the Kuroshio transport of Pu into the SCS with spatial variations which would in turn help in elucidating the source terms. Such source identification is further aided by the measurements of the relative abundance of Pu isotopes. For this study we also aimed at investigating temporal evolution of this Pu input in a dated sediment core collected on the northern SCS shelf. Moreover, with the growing concerns aroused by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, this study sought to assess its potential impact to the China Seas by examining the levels of Pu isotopes in northern SCS sediments before and after the FDNPP accident. Finally, we contend that information on Pu activities and isotopic composition would greatly help in 3137
dx.doi.org/10.1021/es405363q | Environ. Sci. Technol. 2014, 48, 3136−3144
Environmental Science & Technology
Article
2.2. Sample Collection. Sampling was conducted onboard the R/V Dongfanghong II during her 2009 and 2012 summer cruises to the northern SCS organized by the CHOICE-C project (Carbon cycling in the China Seas-budget, controls and ocean acidification) and the R/V Tianlong during the 2012 summer and winter cruises to the PRE. Detailed sampling locations are marked in Figure 1. Longitudes, latitudes, sampling date, and water depths at the stations are presented in Table S1 (Supporting Information). Sediment samples were taken by the recovery of box cores at the stations and processed by extruding vertically with a hydraulic jack. Sediment cores A8 and S101 (collected in 2009) were sectioned at 2 cm intervals. Then, they were frozen and kept in that state until returned to the shore-based laboratory where they were dried at 60 °C for 24 h and pulverized using agate mortar and pestle sets in preparation for 210Pb and 137Cs analyses, followed by Pu isotope analysis. 2.3. Determination of 210Pb and 137Cs. As described in our previous work,11,19 the dried and pulverized samples were weighed and transferred to plastic counting jars for nondestructive analysis of 210 Pb and 137 Cs using gamma spectrometry. For this study, we used an HPGe detector (ORTEC GMX-100-230) interfaced to a digital spectrometer (DSPec Plus). Activities of 210Pb and 137Cs were determined from the count rate of gamma-ray energies at 46.52 and 661.62 keV, respectively. In addition, 214Pb (351.99 keV) was also measured as an index of supported 210Pb, which should be subtracted from the measured 210Pb so as to obtain excess 210Pb as a sediment chronometer. 2.4. Pu Isotope Analysis. The separation of Pu using twostage ion-exchange chromatography has been described elsewhere.41 Briefly, sediment samples (∼2.0 g) were weighed out and spiked with 0.57 pg 242Pu (CRM130, New Brunswick Laboratory, USA). The spiked samples were digested by heating on a hot-plate at 180−200 °C for ∼4 h using 20 mL of concentrated (conc.) HNO3 in a sealed Teflon digestion tube. Pu in the sample solution was subsequently purified by the twostage anion-exchange columns using AG 1-X8 and AG MP-1 M (Bio-Rad). A small drop of the final sample solution was dissolved in 4% ultrapure HNO3 (0.9 mL) and filtered for SFICP-MS analysis. The chemical yield for Pu resulting from this procedure was 63.9% ± 7.3% (n = 65). For Pu measurements by SF-ICP-MS, the most significant interferences are usually caused by formation of isobaric uranium hydrides (238UH+) and peak tailing from the 238U+ peak, which would result in overestimation of the 239Pu signal. The analytical procedure we employed in this work is capable of completely eliminating the U interferences by achieving an extremely high U decontamination factor of 2.0 × 106. The determination of Pu isotopes was conducted using the SF-ICP-MS (Thermo Element 2, Bremen, Germany) in a low resolution mode in order to obtain the maximal sensitivity. Meanwhile, an APEX-Q high efficiency sample introduction system (Elemental Scientific Inc., Omaha, NE, USA) with membrane desolvation unit and a conical concentric nebulizer were used. Detailed optimization of APEX-Q-SF-ICP-MS for the determination of Pu isotopes was described in Zheng and Yamada.42 The mixed Pu isotope standard solution (NBS-947) (240Pu/239Pu = 0.240 ± 0.009 (n = 4), verified value: 0.234 ± 0.011) was applied to mass bias correction. Meanwhile, the analytical method was validated by analyzing reference materials IAEA-368 (ocean sediment), IAEA-Soil-6 (soil)
establishing a baseline for future environmental risk assessment related to the nuclear power plant operations.
2. MATERIALS AND METHODS 2.1. Study Area. The SCS has a unique circulation pattern with dynamic exchanges with the North Pacific Ocean via the Luzon Strait with a sill depth of 2000 m. The circulation pattern is largely dominated by the East Asian monsoon and shows a cyclonic gyre in winter and an anticyclonic gyre in summer.31 The entire SCS can be viewed as a semienclosed basin with continental shelves influenced by terrestrial inputs and a central gyre characterized by oligotrophic features similar to that of most ocean basins.32 What is relevant to the present study is the Kuroshio, the most important western ocean boundary current. The origin of the Kuroshio current is from the northward bifurcation of the North Equatorial Current which flows westward to the Philippine Sea.33,34 The Kuroshio extension pathway to the SCS through the Luzon Strait is temporally variable and significantly modulates the surface seawater chemistry of the northern SCS.35 In addition to the Kuroshio extension, there exists the South China Sea Warm Current that flows toward the northeast intermittently within the SCS but year-round along the shelf break of the northern SCS, spanning from the southeast of Hainan Island to the southwest of Taiwan, affecting material transport in the northern SCS, the focus area of this study.36 The Pearl River Plume, discharging into the northern SCS and dispersing northeastward in summer but southwestward in winter, also significantly modulates the material distribution on the northern SCS shelf.37 The surface sediments across the northern SCS continental shelf show grain size gradations, from gravel inshore to silt offshore.38 The grading analysis of sediment samples at transect A in the northern SCS shelf show that particles less than 63 μm in diameter account for ∼61−95% of the total weight.39 The sediments consist of both terrigenous and biogenous detritus as well as small amounts of authigenic minerals.38 The Pearl River system empties into the SCS through three subestuaries, Lingdingyang, Modaomen, and Huangmaohai, among which Lingdingyang (hereafter referred to as the PRE) is a funnel-shaped estuary with a surface area of 1180 km2. The depth of the PRE is
