Article pubs.acs.org/est
Radiostrontium in the Western North Pacific: Characteristics, Behavior, and the Fukushima Impact Pavel P. Povinec,*,† Katsumi Hirose,‡,§ and Michio Aoyama∥ †
Department of Nuclear Physics and Biophysics, Comenius University, Bratislava, Slovakia Department of Materials and Life Sciences, Sophia University, Tokyo, Japan § Geosphere Research Institute, Saitama University, Saitama, Japan ∥ Geochemical Research Department, Meteorological Research Institute, Tsukuba, Japan Downloaded via UNIV OF WINNIPEG on July 10, 2018 at 10:41:41 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
ABSTRACT: The impact of the Fukushima-derived radiostrontium (90Sr and 89Sr) on the western North Pacific Ocean has not been well established, although 90Sr concentrations recorded in surface seawater offshore of the damaged Fukushima Daiichi nuclear power plant were in some areas comparable to or even higher than (as those in December 2011 with 400 kBq m−3 90Sr) the 137Cs levels. The total amount of 90Sr released to the marine environment in the form of highly radioactive wastewater could reach about 1 PBq. Long-term series (1960−2010) of 90Sr concentration measurements in subtropical surface waters of the western North Pacific indicated that its concentration has been decreasing gradually with a half-life of 14 y. The pre-Fukushima 90Sr levels in surface waters, including coastal waters near Fukushima, were estimated to be 1 Bq m−3. To better assess the impact of about 4−5 orders of magnitude increased radiostrontium levels on the marine environment, more detail measurements in seawater and biota of the western North Pacific are required.
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INTRODUCTION The Great East Japan Earthquake and resulting gigantic tsunami on March 11, 2011, caused the Fukushima Dai-ichi nuclear power plant (NPP) accident. As a result, large amounts of radionuclides were released into the atmosphere due to venting of gases from the damaged nuclear reactors, hydrogen explosions, and the fire in the spent nuclear fuel pond.1,2 Major radionuclides released to the atmosphere were 131I (153 PBq) and radiocesium (134Cs and 137Cs, each about 13 PBq), which were then distributed globally3 and due to wet and dry deposition contaminated the terrestrial and marine environments. Radiostrontium (89Sr and 90Sr) belongs to the group of refractory elements, and due to its lower volatility at the reactor temperatures than that of Cs, about 3 orders of magnitude lower amounts were released to the atmosphere than in the case of 137Cs.1,2 Large amounts of radionuclides were directly released into the ocean as liquid wastes via channels in the Fukushima Daiichi NPP, which widely contaminated seawater in areas offshore of the Fukushima NPP.4,5 The total amounts of 137Cs directly released into the sea have been estimated to be from 1 to 27 PBq;6−8 however, the partial release rates of radiostrontium in the form of liquid radioactive wastes are unknown. As the cooling water directly interacted with ruptured nuclear fuel rods, it is expected that large amounts of radiostrontium have been released to the ocean as well. 137 Cs and 90Sr have been recognized radioecologically as the most important long-lived radionuclides9 (half-lives of 30.17 and 28.78 y, respectively), which are released from nuclear fission10 and accumulate in the marine environment.11 Their © 2012 American Chemical Society
most dominant source in the western North Pacific is global fallout originating from atmospheric nuclear-weapons testing.12,13 The major input from global fallout into the ocean occurred in the 1960s due to wet and dry deposition processes after large-scale atmospheric nuclear weapons tests carried out at Novaya Zemlya in the Kara Sea.11,12 The 90Sr/137Cs activity ratio in global fallout is 0.63, which is usually found in terrestrial and marine environments.12 However, a fluvial input of 90Sr deposited on a land surface may be an important pathway for oceanic 90Sr because of its higher mobility and release from the soil than in the case of 137 Cs.14 Other potential sources15,16 are accidental releases of 90Sr from radioisotope thermoelectric generators (RTGs), which have been used as power generators in lighthouses in the Former USSR. The first accidental lost of a RTG containing about 25 PBq of 90Sr occurred near the eastern coast of Sakhalin Island in the Sea of Okhotsk on Aug 20, 1987.15 The second lost of a RTG of 1.3 PBq of 90Sr occurred on Aug 8, 1997. However, there were no direct observations of elevated 90 Sr levels from the lost RTGs in the Sea of Okhotsk.17 Authorized releases of 90Sr from reprocessing nuclear facilities18 at Sellafield (U.K.) (about 6 PBq) and from La Hague (France) (about 2 PBq) were much smaller when compared with 137Cs releases from the Sellafield plant (about Received: Revised: Accepted: Published: 10356
May 21, 2012 August 6, 2012 August 8, 2012 August 8, 2012 dx.doi.org/10.1021/es301997c | Environ. Sci. Technol. 2012, 46, 10356−10363
Environmental Science & Technology
Article
Table 1. Source Terms of Anthropogenic Radionuclides in the Atmosphere and the Ocean (PBq) global fallout radionuclide
half-life
atm12
ocean18
Chernobyl atm12
131
I
8.02 d
137
Cs
30.17 y
950
600
85
28.78 y
600
380
1
90
Sr
ocean18
1760 16
Fukushima atm1,2 160 153 15 13 0.14 0.01d
ocean
reprocessing facilities18
total ocean inventory in 2010
3.57 46 278 0.1a 2.2b
42c
195
7c
105
a Based on liquid 137Cs discharges of 3.5−4 PBq.6,7 bBased on a liquid 137Cs discharge of 27 PBq.8 cOnly discharges into the sea from the Sellafield (U.K.) and La Hague (France) nuclear reprocessing facilities. dEstimates from the 90Sr/137Cs ratio (0.001) in environmental samples.
water, environmental half-life, processes in the water column) and assess the impact of radiostrontium released from the damaged Fukushima NPP on the marine environment. The impact of the Fukushima-derived radiocesium on the marine environment has been discussed in several papers;27,28 however, an assessment of radiostrontium in the marine environment has been missing, although the total radioactivity released to the marine environment and 90Sr levels observed offshore of Fukushima at some areas have been comparable to those of 137 Cs.
41 PBq), but comparable for the La Hague plant (about 1 PB). These releases have been responsible for increased levels of the radionuclides in the European seas,19 however, with no impact on the North Pacific Ocean. The Chernobyl accident is the biggest single release of 137Cs into the environment (85 PBq), about 16 PBq of which was deposited in the ocean.18 The 90Sr release was much lower, about 1 PBq only.20 Estimated radionuclide releases and their inventories in the world ocean (Table 1) indicate that the most significant source of anthropogenic radionuclides in the ocean is still global fallout. The estimated 137Cs and 90Sr inventories in the world ocean for the year 2010 are about 195 and 105 PBq, respectively. Almost 50% of this inventory is in the Pacific Ocean. The 90Sr concentrations in seawater have been determined since the mid-1950s.21 However, the number of data on the 90 Sr concentration in seawater is much smaller than that of 137 Cs because of more complicated and time-consuming analytical procedures used for determinations of the 90Sr concentration in seawater. The background 90Sr levels in the world ocean (including the Pacific), which are mostly due to global fallout, were summarized in the Worldwide Marine Radioactivity Studies (WOMARS) project.18,22 Oceanic behavior of 90Sr has been considered to be similar to that of 137Cs due to the finding that both radionuclides predominantly exist in an ionic form in seawater and act as conservative tracers. If we consider the Kd values of 90Sr and 137 Cs in coastal seawater environments (8 and 4000, respectively),23 one would expect that 137Cs should be easier to remove from seawater (also supported by the observed higher deposition of 137Cs in sediments).24 Radiostrontium is, however, more weakly adsorbed to particles in comparison to radiocesium because the elemental concentration of inactive Sr is significantly higher in seawater than that of Cs. Sr therefore acts more strongly as a carrier and is much better stabilized in seawater. A high concentration of Ca in seawater also supports this effect. However, knowledge of the detailed oceanic behavior of 90Sr is still poor because of rather complicated interaction processes with celeslite (SrSO4) particles25 (in contrast to 137Cs), with preferential removal of 90Sr in surface water layers and its dissolution in deeper layers. As both radionuclides are dissolved in seawater with a weak interaction with sediment,24 they have been used as conservative traces of water movement in the open ocean.26 In the present paper we evaluate historical 90Sr levels in Pacific waters with the aim to elucidate its characteristics and behavior in the water column (e.g., present levels in surface
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DATA SOURCES Recently, global marine radioactivity databases have been developed at the Meteorological Research Institute in Tsukuba (the HAM database)21 and at the International Atomic Energy Agency’s Environment Laboratories in Monaco (the GLOMARD/MARIS29,30 database). We shall use data gathered until 2010 in the HAM database, which focuses on 137Cs, 90Sr, and 239,240 Pu concentrations in seawater of the Pacific Ocean and its marginal seas. The data on 90Sr in coastal seawater near Fukushima were taken from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) database.31 After releases of a large amount of radioactivity from the Fukushima Dai-ichi NPP, the Tokyo Electric Power Co. (TEPCO) measured radionuclide concentrations in the coastal seawater near the Fukushima Dai-ichi NPP (reported on the TEPCO Web site32). Figure 1 shows the location of the Fukushima Dai-ichi NPP and the main current system (Kuroshio current) affecting the transport of water masses in the coastal regions. The data on activity concentrations of Sr and Cs isotopes in surface waters offshore of the Fukushima NPP, extracted from the MEXT31 and TEPCO32 Web sites, are presented in Table 2.
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RESULTS AND DISCUSSION Long-Term Variability of Surface 90Sr in the Western North Pacific. We examined the long-term changes of surface 90 Sr concentrations in the subtropical waters (25−36° N), including Kuroshio current water, and in the mixed region waters (36− 45° N), corresponding to the east area of the Fukushima NPP (A and B areas, respectively, Figure 1). The 90 Sr concentrations in surface waters of both areas decreased gradually (Figure 2), although a peak in the surface 90Sr concentration appeared in the early 1990s. Assuming that the surface 90Sr concentrations decreased exponentially during the period 1970−2010, apparent half-residence times of 90Sr in surface waters of sea areas A and B were calculated to be 14.9 ± 0.4 and 14.5 ± 0.4 y, respectively. These values are in good 10357
dx.doi.org/10.1021/es301997c | Environ. Sci. Technol. 2012, 46, 10356−10363
Environmental Science & Technology
Article
agreement with previously examined temporal changes in surface 90Sr concentrations in the western North Pacific (25− 40° N) in which an effective half-life of 14.4 ± 0.7 y was obtained22 and the average concentration of 90Sr in 2000 was estimated to be 1.1 ± 0.1 Bq m−3. The surface 90Sr concentrations in coastal waters adjacent to the Fukushima and Tokai NPPs are also plotted in Figure 2. The surface 90Sr concentrations in coastal waters were in the range of variability of surface 90Sr levels in the corresponding area B, although there was no obvious 90Sr peak in the early 1990s. Higher 90Sr concentrations in surface water offshore of Tokai occurred in the 1980s, when the nuclear fuel reprocessing plant operated and discharged liquid radioactive wastes during the period from 1977 to 1997.33 These results suggest that the surface 90Sr concentration in 2011 during the pre-Fukushima time, including coastal waters near Fukushima, was 1.0 ± 0.1 Bq m−3. To elucidate the oceanic behavior of 90Sr in surface waters, it is important to compare 90Sr and 137Cs temporal variations in their concentrations in surface waters of the corresponding sea areas, because these radionuclides are major components of anthropogenic marine radioactivity. The temporal variations of the surface 137Cs have been studied in the world ocean, and an apparent half-residence time was determined for each sea area.22,29,34,35 We examined in detailed temporal variations of the surface 137Cs concentrations in the A and B areas during the period 1970−2010, which exponentially decreased with apparent half-residence times of 16.2 ± 0.7 and 15.4 ± 0.7 y, respectively, as shown in Figure 3. There is no significant difference in the apparent half-residence time of surface 137Cs when compared with previous estimates in the western North Pacific midlatitude region (15 ± 1 y).18,22 The half-residence
Figure 1. Sampling locations (indicated as black dots): site 1, 37°25′52″ N, 141°12′4″ E; site 2, 37°24′55″ N, 141°02′2″ E. The loop shows a coastal monitoring area off the Tokai nuclear fuel reprocessing plant.
Table 2. Concentrations of Radiostrontium and Radiocesium in Surface Water Offshore of the Fukushima NPP Extracted from the MEXT31 and TEPCO32 Web Sites (kBq m−3) site 1 (37°25′52″ N, 141°12′4″ E) date
89
Apr 18 May 9 June 13 July 11 Aug 15 Sept 12 Oct 14 Nov 14 Dec 5 Dec. 10 Jan 16 Feb 13
62 2.4 13 7.4 2.7 1.6 1.3 1.3
date Apr 18 May 9 June 13 July 14 Aug 27 Sept 12 Oct 14 Nov 15 Dec 10 Jan 16 Feb 13
89
Sr
1.2 0.13
