著者

福田 美保 青野 辰雄 Zheng Jian 石丸 隆 神田 穣太

出版者

日本地球惑星科学連合

雑誌

日本地球惑星科学連合2018年大会

巻号頁・発行日

2018-03-14

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After the accident at the Fukushima Dai-ichi Nuclear Power Station (FDNPS) happened in March 2011, large amounts of radionuclides released from the FDNPS into the terrestrial and marine environments. The total amounts of 134Cs and 137Cs released from the accident were estimated as 18 PBq and 15 PBq, respectively. In contrast, those of 238Pu, 239Pu and 240Pu amounts were estimated as 0.0019 PBq, 0.0000032 PBq and 0.0000032 PBq and these amounts were not many compared to abundance before the accident (Report of Japan government to the IAEA Ministerial Conference on Nuclear safety, 2011). Based on the Pu atom ratio, it was estimated that the release of Pu from the accident was negligible in the marine environment (Bu et al., 2015). However, previous reports focused on the river (Evard et al., 2014) and offshore area (e.g. Zheng et al., 2012, Bu et al., 2013, 2015) and the lack of information on the distribution and behavior of plutonium in the estuarine area hampered the understanding of the process of radionuclide transport from river to ocean. In this study, the Niida River estuary was focused on, because the upstream portion of this river is located in Iidate Village, which was an area of high radiocaeasium deposition from the accident. We discussed temporal and vertical distributions of radiocaeasium and plutonium based on the results of the radiocesium (134Cs, 137Cs) and plutonium (239Pu, 240Pu, 241Pu) activity concentrations and plutonium atom ratios (240Pu/239Pu, 241Pu/239Pu) according to grain size in sediments.Sediment core samples at three monitoring stations (NR1: 37°39' N, 141°04' E, water depth: 25 m, NR2: 37°41' N, 141°09' E, water depth: 30 m, NR4: 37°38' N, 141°08' E, water depth: 35 m) were collected in mid-October 2013. Collected sediment cores were cut into 1 cm thick slices and dried. Then, the dried sediments were separated into four classes, based on grain sizes, using several mesh sizes: granules (grain size larger than 2 mm); very coarse to coarse sand particles (1-2 mm); coarse to very fine sand particles (0.063-1 mm); and silt to clay particles (smaller than 0.063 mm). Radiocesium (134Cs and 137Cs) activities were measured for each grain size class using high-purity gamma ray spectrometry and then corrected to the sampling date. Plutonium (239Pu, 240Pu and 241Pu) were extracted and concentrated based on Wang et al. (2017) and measured using SF-ICP-MS (Zheng et al., 2006; Zheng, 2015).Fractions for the classes of granules, very coarse sand, coarse to very fine sand, silt to clay particles were: 0.0-35%, 0.013-35%, 38-99%, and 0.0-29%, respectively. The fractions for coarse to very fine sand particles represented more than 70% of the total particle amount for each sediment layer and the highest fractions were obtained at NR1 and NR2, which are located northward from the river estuary. In contrast, fractions for granules and very coarse sand particle at NR4, which is located in an area of the same latitude as the river estuary, were relatively high and the total fraction for these particles ranged from 20-62 %. The 137Cs activities for very coarse sand, coarse to very fine sand, and silt to clay particles were in the ranges of 2.8-14 Bq/kg-dry, 4.1-751 Bq/kg-dry, and 731-837 Bq/kg-dry, respectively, and these activity concentrations tended to be higher with decreasing grain size. However, the profile patterns for the sand particles and silt to clay particles fraction were similar. In this presentation, we also report the results of grain-size distributions of Pu activity concentration and Pu atom ratio. This work was partially supported by Grants-in-Aid for Scientific Research on Innovative Areas, the Ministry of Education Culture, Sports, Science and Technology (MEXT), Japan (Nos. 24110004, 24110005), the JSPS KAKENHI (grant number JP17k00537) and Research and Development to Radiological Sciences in Fukushima Prefecture.

著者

乙坂 重嘉 福田 美保 青野 辰雄

出版者

日本地球惑星科学連合

雑誌

日本地球惑星科学連合2018年大会

巻号頁・発行日

2018-03-14

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In the coastal region of Fukushima, 137Cs concentration which is higher than before the accident is detected from the seabed even though the concentration in seawater has declined sufficiently. From this fact, it is pointed out that seabed sediment can be a source of radiocesium to coastal areas. In this study, behavior of dissolved radiocesium near the seafloor is discussed from the distributions of 137Cs in seawater, seabed sediment and pore water collected from the area around Fukushima. Between October 2015 and September 2017, seawater and surface (0~10 cm) sediments were collected at 17 stations at 1.5~105 km away from the Fukushima Daiichi Nuclear Power Plant. Seawater was collected at the surface layer (0~3 m depth), intermediate layer (5 m above the seabed), and the layer immediately on the seabed (overlying layer with 0.3 m in thickness). At four stations, pore water in sediment was also collected. The 137Cs concentration in seawater and sediment was measured by gamma-ray spectrometry. The 137Cs concentration in the overlying water ranged from 5 to 283 mBq L-1, and was 2~3 times higher than that in the intermediate layer water. The 137Cs concentration in the pore water was 33~1166 mBq L-1, which was 10~40 times higher than that in the overlying water. The 137Cs concentration in the overlying water did not show clear differences regardless of the pore size (0.45 μm, 0.2 μm and 1 kDa) of the filter used for filtration. From these results, it was confirmed that radiocesium in the seabed sediment was "dissolved" in pore water and diffused to the benthic layer. The 137Cs abundance in the pore water in the surface sediment corresponded to 0.1~0.6% of the 137Cs existing in the solid phase of sediment. At most stations, the 137Cs concentrations in the overlying water and the pore water were approximately proportional to those in the sediment. The apparent distribution coefficient between pore water and sediment was [0.9-4.2]×102 L kg-1, with no difference depending on the year of sampling. These results indicated that equilibrium of 137Cs between pore water and sediment has established in a relatively short period. From the above-mentioned results and kinetic parameters such as 137Cs desorption rate from sediment obtained from laboratory experiments, we estimated the mass balance of 137Cs in the sediments and the overlying water along the coast of Fukushima. The results showed that the 137Cs in the sediment was reduced by about 4~9% per year by desorption/diffusion of 137Cs from the seabed. This rate was lower than the reduction rate of 137Cs in sediments (~29%) observed in this region, and it was estimated that this process was not the main factor of decreasing the 137Cs inventory in sediments. In addition, as of 2017, since the 137Cs concentration presumed to migrate to benthos via the pore water will not exceed the regulatory limit of fishery products, the impact of supply to the benthic environment of 137Cs is considered to be limited.

著者

福田 美保 山崎 慎之介 青野 辰雄 石丸 隆 神田 穣太

出版者

日本地球惑星科学連合

雑誌

JpGU-AGU Joint Meeting 2017

巻号頁・発行日

2017-03-10

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After the accident at the Fukushima Dai-ichi Nuclear Power Station (FDNPS) happened in March 2011, large amounts of radionuclides including radiocaesium also released from the FDNPS into the terrestrial and marine environments. In marine environment, parts of particulate radiocaesium have transported in seawater and accumulated to seafloor. Then, radiocaesium in sediment have partly re-suspended as particulate form and re-eluted as dissolved form due to several factors such as bottom current and deformation. The characters of seafloor topography are more different in the area off the coast of northern and southern part of Fukushima Prefecture, dividing areas at the Onahama port (Mogi and Iwabuchi, 1961). Because the wave bases in fine and stormy weather are about 20 and 80 m, respectively (Saito et al., 1989), it seems that the area of shallower than 100 m is also affected by erosion and re-sedimentation near seafloor with ocean wave degree. Thus, it is necessary to elucidate interaction for radiocaesium between sediment and seawater close to seafloor with more stations in order to guess radiocaesium activity variation at long times. For example, in the case of collected bottom-layer water with the Conductivity-Temperature-Depth (CTD) system, it is very difficult to collect seawater close to sediment because it is careful not to touch CTD system seafloor. This study was aimed at elucidating the relationship for radioacesium activity concentration between sediment and trapped water on sediment collected using Multiple Corer, which is considered as overlying water.Sediment samples were collected using a Multiple Corer during UM14-04 cruise in May 2014 at three stations: I01 (37°14’N, 141°07’E, water depth:60 m), I02 (37°14’N, 141°13’E, water depth:120 m) and C (36°55’N, 141°20’E, water depth:190 m).Overlying waters were collected using tube for 2 hours later from collected sediment. In laboratory, collected sediment sample are dried and overlying water samples were filtered through a 0.2-μm pore size filter and was concentrated by the ammonium phosphomolybdate (AMP) method (Aoyama and Hirose, 2008). The radiocaesium activity concentrations in each sediment and overlying water samples were measured by gamma-ray spectrometry using a high-purity Ge-detector and corrected to sampling date.In overlying water, the dissolved 137Cs activity concentrations (mBq/l) were 3.1-16 and the activity at I01, I02 and C in order from the higher. In the surface-layer sediments (core depth 0-3cm), the activity concentrations (Bq/kg-dry) were 8.4-286 and the high activities at I01 and I02 have characters of relatively high percentage for silt to clay particle compared to those at C. At I02 and C, the activity in overlying water were same value compared those in bottom-layer of seawater, which collected above water depth 10 m from seafloor. On the other hand, the activity in overlying water at I01 was five time higher than those in bottom water. The calculated Kd’ (L/kg) of apparent distribution coefficient using 137Cs activity concentrations in surface-layer sediment and overlying water were 8.8×102-1.5×104 and within rages of recommended Kd value of 2.0×103 for caesium by IAEA TRS422.This work was partially supported by Grants-in-Aid for Scientific Research on Innovative Areas, the Ministry of Education Culture, Sports, Science and Technology (MEXT), Japan (nos. 24110004, 24110005) and Research and Development to Radiological Sciences in Fukushima Prefecture.