投稿日：2018年 3月 3日(土)21時36分24秒 KD182250243012.au-net.ne.jp
Spatiotemporal distribution and fluctuation of radiocesium in Tokyo Bay in the five years following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident
Hideo Yamazaki, Masanobu Ishida , Ryoichi Hinokio, Yosuke Alexandre Yamashiki, Ryokei Azuma
Published: March 1, 2018 https://doi.org/10.1371/journal.pone.0193414
A monitoring survey was conducted from August 2011 to July 2016 of the spatiotemporal distribution in the 400 km2 area of the northern part of Tokyo Bay and in rivers flowing into it of radiocesium released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. The average inventory in the river mouth (10 km2) was 131 kBq?m-2 and 0.73 kBq?m-2 in the central bay (330 km2) as the decay corrected value on March 16, 2011. Most of the radiocesium that flowed into Tokyo Bay originated in the northeastern section of the Tokyo metropolitan area, where the highest precipitation zone of 137Cs in soil was almost the same level as that in Fukushima City, then flowed into and was deposited in the Old-Edogawa River estuary, deep in Tokyo Bay. The highest precipitation of radiocesium measured in the high contaminated zone was 460 kBq?m-2. The inventory in sediment off the estuary of Old-Edogawa was 20.1 kBq?m-2 in August 2011 immediately after the accident, but it increased to 104 kBq?m-2 in July 2016. However, the radiocesium diffused minimally in sediments in the central area of Tokyo Bay in the five years following the FDNPP accident. The flux of radiocesium off the estuary decreased slightly immediately after the accident and conformed almost exactly to the values predicted based on its radioactive decay. Contrarily, the inventory of radiocesium in the sediment has increased. It was estimated that of the 8.33 TBq precipitated from the atmosphere in the catchment regions of the rivers Edogawa and Old-Edogawa, 1.31 TBq migrated through rivers and was deposited in the sediments of the Old-Edogawa estuary by July 2016. Currently, 0.25 TBq?yr-1 of radiocesium continues to flow into the deep parts of Tokyo Bay.
Tokyo Bay is a closed bay that extends 70 km from north to south and 20 km from east to west, covers a total area of 1,380 km2, is an average of 15 m deep, and is connected to the Pacific Ocean by a 7 km wide strait at its south end. The average retention time of seawater varies seasonally but is reported to be approximately 31 days . Central Tokyo is located on the west side of the bay, which is surrounded by a zone of large cities that forms the heart of Japan, and has a total population of 38 million. The catchment basins of rivers flowing into Tokyo Bay from the greater Tokyo region occupy a land area of 9,100 km2, and the total quantity of inflowing river water fluctuates greatly, but averages approximately 1.4 × 107 m3?day-1. The major rivers are the Edogawa, Old-Edogawa, Arakawa, Tamagawa, Sumidagawa, and Tsurumigawa. Even though Tokyo Bay is closed, its seawater flow is complex. In addition to tidal currents, permanent currents flow throughout the bay, and the surface water movement is dominated by circular drifts: clockwise in the winter and counterclockwise in the summer. The bottom water moves in the opposite direction to the flow of the surface water. The pelagic water from the Pacific Ocean flows north on the bottom inside the bay until it reaches the Bay’s deepest section .
Aircraft monitoring of the 134+137Cs precipitation was conducted by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT) , and the results were published by the Geospatial Information Authority of Japan (GSI) , showing that the catchment basin of Edogawa River was contaminated from 30 to 100 kBq?m-2 by radiocesium discharged from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, but the radioactive contamination levels in the catchment basins of the Bay’s other rivers were lower than those in the catchment basin of Edogawa. Radioactive materials precipitated on the ground surface in the greater Tokyo region are, as in the case of artificially discharged environmental contaminants, presumably carried by these rivers until they finally flow into Tokyo Bay.
Many reports outlining the FDNPP accident have already been released to the public [4?7]. However, many of these are analyses of the accident process, whereas few address the environmental radioactive contamination that was caused [2, 8?10]. In particular, the movement in the greater Tokyo region of radioactive contamination produced by the FDNPP accident has been insufficiently analyzed. Nevertheless, after the accident, high concentration radioactive plumes arrived in the greater Tokyo region, radionuclides washout with precipitation (rainfall) on March 16 and 22 in 2011 . Clarifying the movement of environmental radioactive contaminants in the heavily populated greater Tokyo region is an important issue related to the problem of low dose exposure to large populations. Evaluation of the migration process of radiocesium from the Tokyo metropolitan area also is important from the viewpoint of reduction and decontamination of radioactive contamination in these areas. In our previous paper, the behavior of radioactive contaminants of the soil in the Tokyo metropolitan area was discussed . It is estimated that 10 to 22% of the radiocesium precipitated in the surface soil and migrated to Tokyo Bay via rivers in the five years after the FDNPP accident.
This study was a continuous time-series analysis of the distribution and fluctuation of radiocesium in sediments and waters in Tokyo Bay and in the rivers flowing into Tokyo Bay starting in August 2011, immediately after the FDNPP accident. Based on the results, the process of the movement from the land and deposition in Tokyo Bay of radiocesium that was precipitated in the greater Tokyo region via the FDNPP accident were evaluated. However, before the FDNPP accident the Chernobyl accident and the Three Mile Island (TMI) accident affected many affecting people. TMI was located about 150 km west of Washington DC but because it avoided the destruction of the pressure vessel, the emission of radioactive nuclides to the environment is only rare gas nuclides, and the release amount of 131I is estimated to be about 0.5 TBq [13,14]. In the case of the Chernobyl accident, Kiev City was located 130 km south, and 4 million residents lived in that metropolitan area. It was reported at the Chernobyl Forum by the IAEA in 2006  that radioactive plume flew to Kyiv City by the north wind on May 1, 1986 immediately after the accident. However, regulations on information disclosure were made by the Soviet government at the time, and the actual state and dynamics of radioactive contamination in Kiev City are still hardly understood even now. Of course, do not forget the radioactive contamination by the atomic bombs of Hiroshima and Nagasaki. The results of our investigation on the environmental dynamics of 60 years after radiation exposure in Nagasaki has already been reported [16?18]. From such a viewpoint, we think that the FDNPP accident was the first time that an urban region as densely populated as Tokyo was contaminated by radioactive material over a wide area.
Studies on the behavior of radiocesium in an urban environment have been performed through simulations , but fluctuation in this radionuclide’s spatiotemporal distribution has not been monitored nor analyzed over wide areas for long periods. Furthermore, the behavior of cesium as an alkaline element is often complicated and unknown in the estuary where seawater and river water mix [20,21]. In this study, the important roles that Tokyo Bay and rivers flowing into it play in the movement of radiocesium contaminants and the transport and accumulation mechanisms of such in the greater Tokyo region have been clarified.
The Nuclear Regulation Authority of the Japanese Government (NRA) has monitored the radioactive contamination derived from the FDNPP accident in the surface sediment of Tokyo Bay since June 2013 . The Japan Coast Guard (JCG) has also been measuring the radioactive contamination of surface sediments in Tokyo Bay since 1981 . On the other hand, survey results of radiocesium contamination in the Tokyo Bay area immediately after the accident have been published . However, since their monitoring is limited spatiotemporally, it is insufficient to evaluate the dynamics of radioactive contamination throughout the environments of Tokyo Bay.
Sampling and analytical methods
Material and methods
Sediment and water were sampled in Tokyo Bay and in the rivers in its catchment basin. The locations are shown in Fig 1. Sampling was performed at the same points one to seven times during the study period, which ran from August 20, 2011, until July 12, 2016. Sediment samples were collected at 77 points in Tokyo Bay, 10 points in Edogawa River, and 6 points in Sakagawa River. Of the sediment samples collected, 68 were core sediments and 142 surface sediments. Sediment cores were sampled at Point S1 (Fig 1), where Sakagawa flows into Edogawa, to evaluate the role of Sakagawa in the process of transporting radiocesium. To compare with sediment, soil samples were also collected from the 14 points shown in Fig 1B.
Fig 1. Study areas and sampling points.
Geographical distribution of the radiocesium precipitation is indicated by the values for eight months after the accident, adapted from “Extension Site of Distribution Map of Radiation Dose, etc.” . (a) Study area. (b) Sampling points in the Edogawa river system. (c) Sampling points in the Tokyo Bay area. V: Tamagawa estuary, W: Sumidagawa estuary, X: Old-Edogawa estuary, Y: Off the Old-Edogawa estuary, Z: Center of Tokyo Bay, Aqua Line: Cross road of Tokyo Bay. River water in Old-Edogawa flows in the direction of the blue arrow in Fig 1C.
Sediment core sampling was done using an acrylic pipe with a diameter of 10 cm and length of 100 cm. The core samples were collected by a diver pushing the pipe into the seabed by hand. Core samples of 20 to 80 cm length were obtained. Surface sediment specimens were sampled from a boat using an Ekman-Birge bottom sampler. Then, on the boat, after the sediments were collected, the material was inserted into an acrylic pipe with a diameter of 5 cm and length of 10 cm to obtain samples a top 5 cm sediment. Most of soil and sediment samples consisted of silt and sand with a particle size of 2 mm or less. However, more pebbles, plant pieces, shell fragments, etc. were removed with tweezers. Grain size sorting by sieve was not done. The sediments were pushed out of the pipes, cut into 1 or 2 cm thick slices in the depth direction, and then thermally dried to a constant weight in a 60°C oven to remove the water from the sediments. The dried samples were pulverized in an agate mortar, and then the radioactivity of the samples was measured. Water samples were obtained from the surface of the water by lowering buckets from boats. Divers obtained bottom water from about 1 m above the seabed. Without filtering suspended materials out of the water, the radiocesium in 20 L of sample water was concentrated using an ammonium phosphomolybdate (AMP) method . After standing overnight, the AMP precipitate was filtrated and collected on a membrane filter (pore size 0.8 μm), then the radioactivity of the dried AMP precipitate was measured. In this way, it was confirmed in a preliminary experiment that the ionic and suspended radiocesium in the sample water can be recovered quantitatively.
Measurement of radioactivity
Radionuclides in the samples were quantified by connecting a 4096-multichannel pulse height analyzer (Lab Equipment, MCA600) to a low energy HPGe detector (ORTEC, LO-AX/30P) shielded in lead 10 cm thick, sealing the specimens inside a plastic container with a diameter of 5.5 cm and depth of 2.0 cm, then measuring them via γ-ray spectrometry. The Ge detector calculated the geometric efficiency relative to the sample weight using the American NIST (National Institute of Standards and Technology) Environmental Radioactivity Standards, SRM 4350B (River Sediment) and SRM 4354 (Freshwater Lake Sediment), and the efficiency was corrected to within a range of 2 to 30 g of the sample weight . The measurement time was set so that the counting error would be less than 5% according to the radioactive intensity of the samples. 134Cs (605 keV) and 137Cs (662 keV) were quantified in this study. A 134Cs solution with known concentration was used to correct the sum peak effect for 134Cs counting. The detection limits of 134Cs and 137Cs under appropriate conditions were 0.6 Bq?kg-1 in sediment samples and 0.3 mBq?L-1 in water samples. Radiocesium activity was indicated by the values per sampling day, but was corrected for radioactive decay to the value on March 16, 2011, as necessary. In that case, it is denoted as “corrected activity.”
Measurements of heavy metals and particle size distributions in the sediments
The heavy metals in the sediments were measured via an XRF method (Rigaku, ZSX-Primus Ⅱ) using the NIST SRM 1646 (Estuarine Sediment) as the standard sample. Sample measured were made from cellulose powder pressed into 4 cm diameter aluminum ring 0.4 ton?cm-2, and then 1.2 g of powdered sample was placed on the disk and repressed at 1.6 ton?cm-2. The correction of matrix effect was achieved by X-ray intensity ratio of peak to back ground for each element . Mercury in the sediments was measured via a heating-vaporization atomic absorption spectrometry (Hiranuma, HG-300). The particle size distribution of the wet sediment samples was measured using a laser diffractometer (Shimadzu, SALD-3000) with a measurement range of particle size 0.05 to 3000 μm. Dispersion of sedimentary particles was carried out via ultrasonic irradiation (Shimadzu, SUS-200, 42 kHz) using sodium hexametaphosphate as a dispersant. In this paper, the particle size obtained is presented as the volume-based average particle diameter.
Spatiotemporal distribution of radiocesium in Tokyo Bay sediment
All measured values obtained in this study are shown in S1?S4 Tables of the supporting information file. Geographic coordinates of sampling points are also shown in S5 Table. Sampling was done on different days; hence, the radiocesium activities are shown after the radioactive decay correction based on the value of March 16, 2011. A plurality of measured values collected at different times were subjected to statistical processing. Fig 2 (S6 Table) shows the spatial distribution of the 134+137Cs activity (total value of 134Cs and 137Cs) in the surface layer of the sediment, from 0 to 5 cm depth. When multiple measurements were done at the same point, the 134+137Cs activity was evaluated based on the value in a weighted average with the counting error. The deviation of the weighted average approximated according to the law of uncertainty propagation.
Fig 2. Activities of 134+137Cs in the surface sediments throughout the Tokyo Bay water system.
Sediment samples were collected from August 20, 2011, to July 12, 2016. The activity of 134+137Cs was radioactive decay corrected based on the value of March 16, 2011. The value of activity is shown as an average of the values from the surface to 5 cm depth. When there are multiple data at the same point, the activity is expressed as a weighted average value for the counting error.
The highest 134+137Cs activity among all measured values in Tokyo Bay was 1340 ± 13 Bq?kg-1, found in the surface sediments sampled at Point 01 in the Old-Edogawa estuary on November 1, 2012. As shown in Fig 2, the 134+137Cs activity in surface sediment in Tokyo Bay was ranked from high to low contamination level as the Old-Edogawa estuary (X), off the Old-Edogawa estuary (Y), center of Tokyo Bay (Z), Tamagawa estuary (V), and Sumidagawa estuary (W). Throughout the survey period, the 134+137Cs activity of the surface sediments was highest in the Area X, and fell remarkably towards the Area Z. The 134+137Cs activity indicated by the weighted average value was 424 ± 1 Bq?kg-1 (78?1340 Bq?kg-1, n = 55) for X, 131 ± 1 Bq?kg-1 (40?371 Bq?kg-1, n = 40) for Y, and 17 ± 0.3 Bq?kg-1 (1?162 Bq?kg-1, n = 50) for Z. In the Tamagawa estuary (V), which flows through farmland in western metropolitan Tokyo, the weighted average activity was 57 ± 1 Bq?kg-1 (5?234 Bq?kg-1, n = 7). In the Sumidagawa mouth (W), which flows through central Tokyo, it was 103 ± 2 Bq?kg-1 (32?374 Bq?kg-1, n = 16). Both are lower activities than that of the Old-Edogawa estuary (X).
We inferred the inventory and flux of radiocesium accumulation in Tokyo Bay sediment from the catchment basin owing to the FDNPP accident for the five years studied. Table 1 shows the inventory, flux, and 134Cs/137Cs activity ratio of radiocesium in the sediments collected from the Edogawa water system and Tokyo Bay. The 134Cs/137Cs activity ratio in 117 sediment samples (Table 1, S6 Table), with counting error of the radioactivity measurements within 5%, was 1.006 ± 0.003 (weighted average value), which conforms to the 134Cs/137Cs ratio of radiocesium discharged by the FDNPP accident [28?30].