Evaluation of particulate 137Cs discharge from a mountainous forested catchment using reservoir sediments and sinking particles

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Evaluation of particulate 137Cs discharge from a mountainous forested catchment using reservoir sediments and sinking particles Hironori Funakia,∗, Kazuya Yoshimuraa, Kazuyuki Sakumaa, Shatei Irib, Yoshihiro Odaa a b

Fukushima Environmental Safety Center, Japan Atomic Energy Agency, 10-2, Fukasaku, Miharu-machi, Fukushima, 963-7700, Japan West Japan Engineering Consultants, Inc., 1-1, 1-chome, Watanabe-dori, Chuo-ku, Fukuoka-shi, Fukuoka, 810-0004, Japan

A B S T R A C T

The time and size dependencies of particulate 137Cs concentrations in a reservoir were investigated to evaluate the dynamics of 137Cs pollution from a mountainous forested catchment. Sediment and sinking particle samples were collected using a vibracorer and a sediment trap at the Ogaki Dam Reservoir in Fukushima, which is located in the heavily contaminated area that formed as a result of the Fukushima Dai-ichi Nuclear Power Plant accident of 2011. The inventory of 137Cs discharged into the reservoir during the post-accident period (965 days) was estimated to be approximately 3.0 × 1012–3.9 × 1012 Bq, which is equivalent to 1.1%–1.4% of the initial estimated catchment inventory. The particulate 137Cs concentration showed a decline with time, but the exponent value between the specific surface area and the 137Cs concentration for the fine-sized (< 63 μm) particle fraction remained almost constant from the immediate aftermath of the accident. These quantitative findings obtained by reconstructing the contamination history of particulate 137Cs in reservoir sediments and sinking particles have important implications for the evaluation of 137Cs dynamics in mountainous forested catchments.

1. Introduction Large quantities of volatile radionuclides, particularly radioiodine and radiocaesium, were released into the atmosphere and the Pacific Ocean from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) as a result of the damage caused by the massive earthquake and the associated tsunami in March 2011 (Chino et al., 2011; MEXT, 2011a). Caesium-137 contamination is considered to be the most serious risk from the standpoint of long-term human health and ecosystems because of its relatively long half-life (30.2 years) (Saito et al., 2015). When 137 Cs is deposited on the ground, it is strongly adsorbed on the surfaces of fine soil particles such as clay, which have large specific surface areas, providing more adsorption sites for radiocaesium (Cremers et al., 1988; He and Walling, 1996). In the Fukushima case, particle size has been shown to affect the concentrations of 137Cs in soils, suspended solids, and sediments (Kato et al., 2012; Lepage et al., 2014; Yoshimura et al., 2014; Sakaguchi et al., 2015; Tanaka et al., 2015). Therefore, understanding the dynamics of particulate 137Cs by considering the particle size distributions resulting from processes such as soil erosion, and the subsequent particle-transport processes in water systems is important for understanding the effects of radioactive contamination on terrestrial and aquatic ecosystems as well as on human health. The direct monitoring of particulates and dissolved 137Cs within river systems provides the quantitative data required to evaluate 137Cs

∗

transport (Nagao et al., 2013; Ueda et al., 2013; Yoshikawa et al., 2014; Yamashiki et al., 2014; Tsuji et al., 2014, 2016; Sakaguchi et al., 2015; Yoshimura et al., 2015a). Many studies have predicted future 137Cs distributions in water systems using modelling approaches (Kitamura et al., 2014; Kurikami et al., 2014, 2016; Mouri et al., 2014; Iwasaki et al., 2015; Kinouchi et al., 2015; Yamada et al., 2015; Sakuma et al., 2017; Wei et al., 2017). In particular, field-monitoring data from the aftermath of an accident are important to estimate total 137Cs discharge from the catchment after the accident because the 137Cs concentrations decreased relatively steeply in the first years after the Chernobyl and FDNPP accidents (Smith et al., 1999, 2004; NRA, 2015; Yoshimura et al., 2016; Iwagami et al., 2017a). Anthropogenic alterations to riverine morphologies (e.g., dams and reservoirs) have played important roles in delaying the migration, storage, and release of particulate 137Cs (Kurikami et al., 2014; Mouri et al., 2014; Lepage et al., 2014; Evrard et al., 2015). In previous studies, 137Cs concentrations in sediments derived from atmospheric nuclear weapons testing and the Chernobyl accident were used as time markers to calculate sediment accumulation rates in lakes and reservoirs and to evaluate the dynamics of 137Cs from catchments (Richie and McHenry, 1990; Whicker et al., 1994; He et al., 1996; Albrecht et al., 1998; Saxén and Ilus, 2008). In this study, we investigated a reservoir in the Fukushima Prefecture in which the catchment has a high 137Cs inventory. We

DOI of original article: https://doi.org/10.1016/j.jenvrad.2018.03.004 Corresponding author. E-mail address: [email protected] (H. Funaki).

https://doi.org/10.1016/j.jenvrad.2018.09.012 Received 28 September 2017; Received in revised form 24 February 2018; Accepted 13 March 2018 0265-931X/ © 2018 Elsevier Ltd. All rights reserved.

Please cite this article as: No Author, Journal of Environmental Radioactivity, https://doi.org/10.1016/j.jenvrad.2018.09.012

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Fig. 1. Location of the Ogaki Dam Reservoir and sampling stations. The deposition density of 137Cs was obtained from the Third Airborne Monitoring Survey (MEXT, 2011a).

samples were collected from the Ogaki Dam Reservoir, which is located in the Ukedo River (Fig. 1). The catchment of the Ogaki Dam Reservoir has the highest deposition density of 137Cs among the catchments of the Fukushima Prefecture and is located in the heavily contaminated evacuated area; the area downstream of the reservoir is being remediated in advance of repopulation and recommencement of crop cultivation. Therefore, residents are concerned about the long-term migration of 137Cs from the reservoir catchment into the paddy fields via irrigation water. The deposition densities of 137Cs in the reservoir catchment range from 0.2 to 7.2 MBq m−2 with an average of 2.5 MBq m−2, and the total inventory of 137Cs in the reservoir catchment is approximately 2.8 × 1014 Bq (Fig. 1) (MEXT, 2011a). The site characteristics of the Ogaki Dam Reservoir and its catchments are provided in Tables 1 and 2. The Ogaki Dam is a rockfill structure constructed in 1985 by the Tohoku Regional Agricultural Administration Office (TRAAO). The water level of the reservoir is regulated by outlet gates at multiple depths. The elevation within the

collected reservoir sediments using a vibracorer to evaluate particulate 137 Cs discharge from a mountainous forested catchment after the FDNPP accident. Additionally, we sampled sinking particles using a sediment trap after obtaining sediment cores and measured 137Cs concentrations for each particle size fraction. Finally, we discussed the time and size dependencies of particulate 137Cs dynamics from the mountainous forested catchment. 2. Material and methods 2.1. Site description Dam reservoirs to store irrigation water have been constructed on the middle stretches of many rivers in the eastern part of Fukushima Prefecture because these rivers are generally short in length. These reservoir catchments have similar land uses, soil types, landforms, and climatic conditions. In this study, sediment, sinking particle, and water 2

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outer parts were used sediment analyses. After obtaining sediment cores, we deployed a sediment trap (Nichiyu Giken Kogyo, SMD13S6000) to collect sinking particles at Station F, which is the closest to the reservoir outlet and was located approximately 1 m from the reservoir bottom from December 2013 to January 2017.

Table 1 Site characteristics of the Ogaki Dam Reservoir catchments. Characteristics Area (km2) Ukedo River Kodeya River Other Stream Land use (%) Forest Cultivated land Residential land and Other land use

88 15 7

2.3. Radionuclide analysis

80 15 5

The core sections were dried at 105 °C for 24 h to determine the dry weight for the calculation of sediment bulk density (g cm−3) and dry mass depth (g cm−2). After homogenization, the samples were transferred into plastic containers for gamma-ray measurement. Specific gamma rays from 134Cs (605 keV) and 137Cs (662 keV) in our samples were measured using an n-type high-purity germanium detector (GMX40P4-76, Seiko EG&G ORTEC, Tokyo, Japan) with a relative efficiency of 40%. All the activities of sediment cores discussed herein were corrected for radioactive decay up to 11 March 2011. For pulseheight analysis, a mulch channel analyzer (MCA7600, Seiko EG&G ORTEC) was used in line with spectrum analysis software (Gamma Studio, Seiko EG&G ORTEC). Efficiency calibration was conducted with a multiple-gamma-ray emitting standard source (including 10 nuclides) packed in the same type of vessel (Eckert & Ziegler Isotope Products, California, USA). The detection limits corresponding to a measurement time of 600 s for 137Cs in sediments and sinking particles were on the order of 10−1 kBq kg−1.

Table 2 Site characteristics of the Ogaki Dam Reservoir. The shoreline (October 2013) in Fig. 1 corresponds to a reservoir water height of approximately 140 m above sea level. The maximum water height is approximately 170 m above sea level. Characteristics

Reservoir water level (m)

Surface area (105 m2) Water volume (106 m3) Average water depth (m) Mean annual discharge rate (108 m3 y−1) Water residence time (d)

170

140

10.4 18.6 17.8 1.1 62

2.8 2.9 9.1 1.1 10

reservoir catchment ranges from approximately 170 m near the reservoir to approximately 1050 m on the western rim of the catchment; the slope ranges from 0° to 35°, with an average of 10°. The dominant surface geology is granite covered by combisols and andosols. Data from the Automated Meteorological Data Acquisition System (AMeDAS) of the Japan Meteorological Agency for Tsushima and Namie Stations (Fig. 1) indicate mean annual precipitations of 1340 and 1510 mm, respectively, from 1981 to 2010. Since the FDNPP accident, the catchment area of the reservoir has been evacuated, and no crops have been grown in the area. Therefore, the outlet gates of the Ogaki Dam have been continuously left open by the operators, and the water was at a low level (140 m above sea level) during this investigation.

2.4. Sediment analysis We analyzed the particle size distribution of the nine cores in each section to evaluate the sedimentary facies using a laser diffraction particle size analyzer (SALD-3100, Shimadzu Co., Ltd., Kyoto, Japan). Particle size distribution measurements were made on the < 2-mm sediment fraction. The particle size fractions were defined as (i) clay < 4 μm; (ii) silt 4–63 μm; (iii) sand 63–2000 μm according to Wentworth (1922). Additionally, to measure the radioactivity of radiocaesium in each size fraction, the sediment and sinking particle samples collected at Stations F (Fig. 1), which located away from the reservoir inflows considered to be under the less influence of sediment erosion and resuspension, were divided into five size fractions: < 2, 2–4, 4–16, 16–63, and > 63 μm by a wet-sieving method (> 63 μm) and sedimentation according to Stokes' law (< 63 μm). The specific surface area of each sample was calculated based on the assumption that an average particle density was 2.6 g mL−1 (kaolinite) and a particle shape was spherical. The radioactivity of radiocaesium was measured using the same method as described above.

2.2. Field sampling Using a vibracoring technique, sediment cores were collected by divers at nine locations within the Ogaki Dam Reservoir in 2013 (Fig. 1, Table 3). The core sampler comprised a polycarbonate tube with an inner diameter of 100 mm. The cores were vertically extruded and divided into 1-cm-thick sections from the top to a depth of 50 cm and then into 2-cm-thick sections for the remaining length of the cores using scrapers and a polyvinyl chloride tube (75 mm in diameter). The inner parts of the core samples were used for radionuclide analyses, while the

Table 3 Sampling date, location, water depth, contamination density, and sediment accumulation rate at each sampling station. Sampling station

Sampling

Latitude (°N)

Longitude (°E)

Water Depth (m)

Core length (cm)

137

Cs deposition density (MBq m−2)

Sediment accumulation rate (g cm−2 y−1)

37° 37° 37° 37° 37° 37° 37° 37° 37°

140° 140° 140° 140° 140° 140° 140° 140° 140°

18 21 24 21 25 21 13 12 10

68 31 48 43 11 10 8 9 8

28.5 15.4 28.6 21.4 4.8 7.0 5.0 6.9 5.9

12.9 4.2 7.8 6.1 0.4 0.7 0.4 0.7 1.1

Date (year/month/ day) A B C D E F G H I

2013/11/1 2013/11/5 2013/11/1 2013/11/5 2013/10/31 2013/10/28 2013/10/25 2013/10/24 2013/10/29

31′ 31′ 31′ 31′ 31′ 31′ 31′ 30′ 30′

16.1″ 12.3″ 11.6″ 11.4″ 08.0″ 04.8″ 03.5″ 59.6″ 53.0″

53′ 53′ 53′ 53′ 53′ 53′ 52′ 52′ 52′

19.9″ 17.9″ 10.4″ 04.3″ 03.3″ 04.0″ 56.3″ 51.6″ 58.8″

3

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Fig. 2. Vertical profiles of

137

Cs and particle size in sediments at each sampling station of the Ogaki Dam Reservoir.

3. Results and discussion

vertical distribution patterns (Table S1 in supporting data). The Cs/137Cs activity ratios corrected to those of March 11, 2011 ranged from 0.875 to 1.10 with an average of 1.00. Therefore, the contributions of global fallout 137Cs to the deposition density and the concentration in the cores were negligible in this reservoir. In the following 134

3.1. Accumulation of

137

134

137

The

Cs and

Cs in reservoir sediments

Cs measurements indicated nearly identical 4

5

River

River River Reservoir outflow River River

River

Reservoir

Ueda et al. (2013)

Yamashiki et al. (2014) Tsuji et al. (2016) Hayashi et al. (2016)

Muto et al. (2017)

This study

b

a

Ogaki

Ibaraki

Kawamata Uda Sakashita (KE–plot) Sakashita (KW–plot) Ogi (KA–plot) Hiso (Niida) Wariki (Niida) Abukuma Ota Matsugabo (Uda) Uda Koutaishi Iboishi Ishidaira

P = Paticulate, D = Dissolved and S = Sediments. Percentage of initial catchment inventory.

Iwagami et al. (2017a)

Forest Forest Forest

Yoshimura et al. (2015b) Hayashi et al. (2016) Niizato et al. (2016)

Location

4.5 3.2 5172 21 25.4 95.6 0.54 0.17 0.075 0.6

Cs Cs

134&137

134&137

& & & &

D D D D

S

P&D

P P P P P

Cs Cs

137

137

137

Cs Cs 137 Cs 137 Cs

137

110

0.05

Cs

137

P P&D

0.06

Cs

137

137

P

80

85 70 62 99 80 79 94 71 92 100

(%)

(km2) 0.11 0.34 0.07

Forested area (%)

Catchment

Cs Cs 137 Cs

137

Radionuclide

P P&D P

Measured materiala

Cs discharge derived from the FDNPP accident.

Topography

137

Author

Table 4 Summary of monitoring results for

274 365 365 365 390

365 365 965

1 Jan. – 31 Dec., 2013 1 Jan. – 31 Dec., 2014 11 Mar., 2011–31 Oct., 2013

489 365 234 195 143 195 160 179 290

(d)

Period

10 Aug., 2011–11 May, 2012 29 May 2014–29 May 2015 2014 2014 23 Aug., 2012–18 Sep., 2013

17 Jul., 2011–18 Nov., 2012 2014 29 Mar. – 19 Nov., 2013 7 Apr. – 20 Oct., 2014 28 Jun. – 19 Nov., 2013 7 Apr. – 20 Oct., 2014 10 Jun. – 18 Nov., 2013 11 Apr. – 8 Oct., 2014 15 Mar. – 31 Dec., 2011

Date

2.8E+14

9.0E+12 6.4E+12 8.9E+14 4.0E+13 6.6E+09 2.0E+10 4.9E+08 9.2E+07 2.2E+07 1.8E+07

2.5E+07

3.0E+07

4.9E+07 5.8E+07 3.3E+07

(Bq)

Catchment inventory

3.4E+04 4.8E+04 1.6E+04 2.0E+04 2.9E+04 3.6E+04 2.6E+04 9.0E+03 4.5E+10 2.0E+10 1.0E+13 2.9E+10 4.6E+05 1.1E+07 1.2E+05 2.7E+05 1.3E+04 1.5E+07 2.2E+07 3.0E+12 – 3.90E+12

(Bq)

0.07 0.08 0.05 0.06 0.10 0.12 0.11 0.04 0.5 0.3 1.1 0.07 0.01 0.06 0.02 0.3 0.1 0.08 0.11 1.1 – 1.4

(%b)

1.4E-04 2.2E-04 2.1E-04 3.1E-04 7.0E-04 6.2E-04 6.9E-04 2.2E-04 1.7E-03 1.0E-03 4.0E-03 2.0E-04 2.7E-05 1.6E-04 5.1E-05 7.7E-04 2.6E-04 2.2E-04 3.0E-04 1.1E-03 – 1.5E-03

(% d−1)

Radiocaesium Discharge

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Table 5 137 Cs concentrations for different size fractions of sediments and scaling factor reflecting the initial 137Cs concentration of suspended particles discharged from the catchment at Station F. Depth (cm)

Cs concentration (kBq kg−1)

137

Clay 1a Station F 5–6 4–5 3–4 2–3 1–2 0–1 a b c d

883.2 420.1 320.2 265.0 248.4 200.3

± ± ± ± ± ±

Clay 2b

5.3 4.8 3.9 3.6 3.8 3.2

797.5 348.6 277.8 208.6 208.9 178.8

± ± ± ± ± ±

15.2 4.8 4.2 4.1 3.3 3.2

Silt 1c

Silt 2d

521.6 ± 5.5 231.6 ± 4.3 139.5 ± 1.9 137.7 ± 2.0 117.9 ± 1.8 99.0 ± 1.6

386.6 ± 3.9 213.9 ± 2.4 115.8 ± 3.2 123.3 ± 2.4 92.2 ± 2.6 83.6 ± 2.7

Scaling factor μn

Exponent value γn

695 324 228 197 179 150

0.20 0.17 0.26 0.19 0.25 0.22

The particle size and the specific surface area were below 2 μm and 5.16 m2 g−1, respectively. The particle size and the specific surface area were 2–4 μm and 0.82 m2 g−1, respectively. The particle size and the specific surface area were 4–16 μm and 0.29 m2 g−1, respectively. The particle size and the specific surface area were 16–63 μm and 0.07 m2 g−1, respectively.

Table 6 137 Cs concentrations for different size fractions of sinking particles and scaling factor reflecting the initial 137Cs concentration of suspended particles discharged from the catchment at Station F. Datea

Period after FDNPP accident (y)

137

Cs concentration (kBq kg-1) a

21 Dec., 2013–26 Mar., 2014 27 Mar., 2014–23 May, 2014 24 May, 2014–10 Jul., 2014 11 Jul., 2014–5 Aug., 2014 6 Aug., 2014–9 Sep., 2014 10 Sep., 2014–4 Oct., 2014 5 Oct., 2014–27 Oct., 2014 24 May, 2014–10 Jul., 2014 18 Jan., 2015–27 May, 2015 28 May, 2015–28 Jul., 2015 30 Jul., 2015–7 Oct., 2015 8 Oct., 2015–13 Jan., 2016 14 Jan., 2016–20 Apr., 2016 21 Apr., 2016–27 Jul., 2016 28 Jul., 2016–18 Oct., 2016 19 Oct., 2016–18 Jan., 2017 a b c d

2.78–3.04 3.04–3.20 3.20–3.33 3.33–3.40 3.41–3.50 3.50–3.57 3.57–3.63 3.75–3.86 3.86–4.21 4.21–4.38 4.39–4.57 4.58–4.84 4.85–5.11 5.11–5.38 5.38–5.61 5.61–5.86

b

Clay 1

Clay 2

99.8 ± 0.9 101.1 ± 0.9 119.1 ± 0.5 172.4 ± 1.0 143.0 ± 0.6 178.6 ± 1.3 128.3 ± 0.8 143.2 ± 0.8 108.6 ± 0.8 112.2 ± 0.8 79.6 ± 0.7 83.6 ± 0.6 67.1 ± 0.7 66.9 ± 0.8 46.3 ± 0.4 53.1 ± 0.5

99.9 ± 0.7 89.0 ± 0.7 92.5 ± 0.4 134.8 ± 1.0 116.3 ± 0.7 134.0 ± 1.3 97.6 ± 0.7 105.7 ± 0.7 73.2 ± 0.6 84.6 ± 0.6 70.7 ± 0.6 66.1 ± 0.5 63.8 ± 0.8 66.9 ± 0.6 43.1 ± 0.5 51.8 ± 0.7

c

Silt 1 74.7 67.2 81.0 83.2 97.3 96.9 83.9 81.1 62.2 66.3 47.1 50.5 48.5 61.3 22.7 34.0

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

Scaling Factor μn

Exponent Value γn

86 80 95 120 117 130 99 105 76 83 60 61 55 58 34 46

0.16 0.20 0.15 0.28 0.13 0.21 0.18 0.20 0.25 0.22 0.27 0.28 0.22 0.21 0.26 0.11

d

Silt 2 0.4 0.3 0.3 0.5 0.4 0.7 0.4 0.4 0.3 0.3 0.3 0.3 0.5 0.5 0.2 0.5

50.3 42.0 61.7 54.6 83.2 74.8 57.5 61.7 35.6 43.6 25.1 24.6 25.0 25.1 15.7 35.5

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.7 0.7 0.7 0.7 0.8 1.7 1.0 0.9 0.8 1.0 0.8 0.9 1.2 0.3 0.8 1.8

The particle size and the specific surface area were below 2 μm and 5.16 m2 g−1, respectively. The particle size and the specific surface area were 2–4 μm and 0.82 m2 g−1, respectively. The particle size and the specific surface area were 4–16 μm and 0.29 m2 g−1, respectively. The particle size and the specific surface area were 16–63 μm and 0.07 m2 g−1, respectively.

discussion, we consider the 137Cs concentrations within the extracted cores from each of the reservoir sampling stations. The concentration of 137Cs and the variation in the particle size distribution with depth in the sediment cores from the Ogaki Dam Reservoir are illustrated in Fig. 2. The majority of the vertical profiles of 137 Cs show a distinct peak in the lower part of the contaminated sediments, and a sharp decrease in 137Cs activity can be observed from below each peak to the base of the core. These peaks represent a major time marker because they were formed by the direct adsorption of 137Cs onto the bottom of reservoir and/or the deposition of suspended particles immediately after the FDNPP accident fallout. The deposition densities and sediment accumulation rates of each core are listed in Table 3. The deposition densities of 137Cs in the sediments were 4.8–28.6 MBq m−2, higher than the values for the surrounding soil (3.2 MBq m−2) collected in the vicinity of the reservoir (MEXT, 2011b). The values at the upstream sites (Stations A, B, C, and D; Fig. 1) were between two and six times greater than those at the downstream sites (Stations E, F, G, H, and I; Fig. 1). Similarly, the sediment accumulation rates for the post-accident period at the upstream sites were 4.2–12.9 g cm−2 y−1 (Table 3), considerably higher than the rates for the downstream sites (0.4–1.1 g cm−2 y−1). The sediments were predominantly composed of silt-sized fractions with clay-sized fractions (Fig. 2). The proportions of the clay- and silt-

sized fractions in the sediments reached 2.9%–51.1% (average = 21.2%) and 33.2%–87.7% (average = 70.6%), respectively. The majority of the sand-sized fractions were deposited at the upstream sites. Focusing on the upstream sampling stations, the layers at mass depths of 19.8–24.8 g cm−2 at Station A contained more of sand- and gravel-sized fractions than above and below these layers (Fig. 2). The 137 Cs concentration above these layers was approximately half of that below these layers. In cores B, C, and D at mass depths of 5.7–6.2, 10.8–11.4, and 11.5–12.7 g cm−2 contained coarse-grained fractions and were showed a similar vertical distribution of 137Cs concentration in core A (Fig. 2). These coarse-sized fraction layers in the upstream sites were likely formed by a same unusual mode of sediment transport (e.g., a typhoon flood event). In contrast, the sediments at Stations E and F near the reservoir outlet were mainly composed of clay- and siltsized fractions; sand- and gravel-sized fractions were absent, and the sediment accumulation rates were very low (Table 3). These results indicate that the bed load and coarse fraction of the suspended load were directly deposited at the sites upstream of the reservoir, whereas fine suspended particles with lower settling velocities were transported downstream via a tributary that enters the impounded reach as expected, resulting in decreased flow velocity. TRAAO, the administrator of the dam, has monitored the river discharge rates and concentrations of suspended solids in the water at 6

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2016), was estimated to be 3.0 × 1012–3.9 × 1012 Bq (Table 4). The lowest value was calculated assuming that almost all the fallout 137Cs on the reservoir surface estimated from surrounding ground contamination (3.2 MBq m−2; MEXT, 2011b) were deposited directly on reservoir bottom. On the other hand, the highest value was calculated assuming that the fallout 137Cs on reservoir surface were released downstream and most of the particulate 137Cs were supplied from the upstream catchment of the reservoir. We considered that the 137Cs discharge from the catchment falls inside this range. The inventory of 137 Cs was equivalent to 1.1%–1.4% of the initial catchment inventory (Table 4). Table 4 shows the 137Cs discharge from catchments affected by the FDNPP accident estimated by direct monitoring within forests and rivers in previous studies. Based on the comprehensive interpretation of these results, the following conclusions regarding 137Cs discharge can be drawn. The radiocaesium wash-off from the forests and rivers was noticeably low (Yoshimura et al., 2015b; Hayashi et al., 2016; Niizato et al., 2016; Tsuji et al., 2016; Iwagami et al., 2017a), and the radiocaesium levels tended to be preserved in the forest ecosystem through present day. On the other hand, the radiocaesium discharge rate estimated by Ueda et al. (2013), Yamashiki et al. (2014), were above 1.0 × 10−3% d−1, were higher than other monitoring data (Table 4). These data were included the radiocaesium discharge from the catchments within a year of the accident. In addition, the discharge rate by Ueda et al. (2013) was also included the dissolved radiocaesium. In previous studies, the dissolved and particulate radiocaesium concentration in river and irrigation water in the Fukushima affected area declined steeply through time in the first year after the accident (NRA, 2015; Yoshimura et al., 2016). Therefore, the initial radiocaesium discharge is considered to contribute significantly to the evaluation of the total radiocaesium discharge from the catchment. In this study, the 137Cs discharge rate estimated by only particulate form was 1.5 × 10−3–1.5 × 10−3% d−1, relatively higher than other monitoring data. This indicates that the reservoir sediments captured the initial discharge records from the catchment in the early phase after the accident. Therefore, to evaluate 137 Cs accumulation in reservoir sediments is useful for determining total particulate 137Cs discharges, particularly initial discharges from catchments.

Fig. 3. Time dependency of the scaling factor reflecting the initial 137Cs concentration of suspended particles discharged from the Ogaki Dam Reservoir catchment. Solid line indicates fitted curve to the scaling factor. Dotted lines indicate 95% confidential interval.

the inlet and outlet of the reservoir since September 2012. TRAAO reported that approximately 90% of 137Cs entering the Ogaki Dam Reservoir is trapped in the reservoir, and almost all the sand-sized fractions are deposited onto the reservoir bed, with only a part of the clay-sized fraction being discharged downstream (Kurikami et al., 2014; TRAAO, 2016). Therefore, it is possible to estimate the 137Cs discharge from the mountainous forested catchment by understanding the inventory of 137Cs in the sediments at the Ogaki Dam Reservoir. 3.2. Caesium-137 discharge from the mountainous forested catchment We derived the particulate 137Cs discharge from the mountainous forested catchment to the reservoir using deposition densities and the reservoir surface area divided into nine sectors according to the perpendicular bisector lines between two adjacent sampling stations. We didn't collect the sediment cores in a transverse direction of the reservoir. Therefore, in our estimation approach, large uncertainties remain about the details of sediment yield and 137Cs inventory discharged from the catchment. In this study, we roughly estimated the 137Cs discharge from catchments using sediment cores and made a comparative review of focus on catchment scale 137Cs transfers based on field-monitoring data. The range of 137Cs inventory discharged from the catchment in the post-accident period (965 days), including the flow loss of 10% of particulate 137Cs to the downstream site (Kurikami et al., 2014; TRAAO, Table 7 Decrease rate constants of particulate

137

3.3. Effects of time and particle size on particulate

137

Cs dynamics

As discussed above, reservoir sediments are important indicators for estimating total and historical 137Cs discharge from catchments. After obtaining the sediment cores, we collected sinking particles from December 2013 to January 2017 using a sediment trap to track the time and size dependencies of particulate 137Cs concentration. Tables 5 and 6 show the concentration of 137Cs for clay- and silt-sized fractions in the sediment core and in the sinking particles obtained from Station F near the reservoir outlet (Fig. 1). Sand-sized fractions were virtually absent in these samples (Table S1) and were not measured the radioactivity. In the following discussion, we consider the 137Cs dynamics of fine-sized fractions (< 63 μm). The highest 137Cs concentration among all samples was obtained for

Cs concentration derived from the FDNPP accident.

Author

Location

Date

Rate constant k

NRA (2015)

Abukuma River

2012–Jan. 2015 2012–Jan. 2015 2012–Jan. 2015 2012–Jan. 2015 Aug. 2012–Sep. 2013 Sep. 2011–Sep. 2013 Dec. 2013–Jan. 2017

0.694 0.410 0.441 0.160 0.75 0.48 0.36

Iwagami et al. (2017a) Yoshimura et al. (2016) This Study

Main stream Tributary of eastrn part Tributary of westrn part Rivers of eastern part of Fukushima Prefecture Outflow of Ishidaira watershed (Tributary of eastrn part of Abukuma River) Paddy field derived Suspended solids in Fukushima Prefecture Ogaki Dam Reservoir

7

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confidential interval of 0.23–0.49 y−1). The decrease rate is affected by land use (Yoshimura et al., 2015b), human activities such as decontamination (NRA, 2015), and the time period of calculation (Smith et al., 2000b). Therefore, it is difficult to directly compare the decrease rate between reported values. Further long-term monitoring considering the above is necessary in order to clarify the predominant factor of the decrease rate.

the finest size fraction (< 2 μm), and the 137Cs concentration clearly decreased with increasing particle size. Radiocaesium is known to strongly adsorb onto fine soil particles, particularly clay minerals (Cremers et al., 1988; Spezzano, 2005). In addition, smaller particles have larger specific surface areas, providing more adsorption sites for radiocaesium. He and Walling (1996) demonstrated that the relationship between specific surface area (or particle size) and 137Cs concentration can be well fitted using a power function for various types of soil and sediment samples. The particle size dependency of the particulate 137Cs concentration was normalized as follows:

Cn (Ssp) = μn Sspvn ,

4. Conclusions We quantitatively estimated particulate 137Cs discharge from a mountainous forested catchment to a reservoir using reservoir sediments and sinking particles obtained from the Ogaki Dam Reservoir, which is located in the heavily contaminated area that formed as a result of the FDNPP accident. The inventory of 137Cs discharged into the reservoir during the post-accident period (965 days) was estimated to be approximately 3.0 × 1012–3.9 × 1012 Bq, equivalent to 1.1%–1.4% of the initial estimated catchment inventory. This indicates that the reservoir sediments captured the initial discharge records from the catchment in the early phase after the accident. Furthermore, the particulate 137Cs concentration decreased with time, but the exponent value between the specific surface area and the 137Cs concentration of the fine-sized fraction (< 63 μm) remained almost constant. The results obtained from reconstructing the contamination history of particulate 137 Cs in the reservoir provide important quantitative information for evaluating the environmental recovery rate of the mountainous forested catchment.

(2)

where Cn is the concentration of 137Cs (kBq kg−1), Ssp is the specific surface area (m2 g−1) estimated assuming spherical particles, μn (kBq kg−1) is the scaling factor reflecting the initial 137Cs concentration of suspended particles discharged from the catchment, and νn is the exponent value. To compare the obtained νn values between sediments and sinking particles (Tables 5 and 6), a t-test (for two datasets) was carried out, and no statistically significant difference was found (average = 0.21; P > 0.05). These findings suggest that the particulate 137 Cs concentration decreased with time but the exponent value between specific surface area (particle size) and 137Cs concentration for the fine-sized fractions (< 63 μm) has remained constant since the immediate aftermath of the accident. Meanwhile, the scaling factor ( μn ) of sinking particles decreased with time (Fig. 3). In previous studies, temporal decreases in dissolved and particulate 137Cs concentrations in river and irrigation water could be classified into two phases. The first phase is a steep decline, and the second is a slow decline described by a double-exponential function (Smith et al., 2000a; Yoshimura et al., 2016; Iwagami et al., 2017a; b). In the Fukushima case, the first decreasing phase of dissolved and particulate 137Cs concentrations in river and irrigation water had a characteristic time scale of six months to a year (NRA, 2015; Yoshimura et al., 2016; Iwagami et al., 2017a, b). In this study, the sampling of sinking particles was initiated in December 2013. Therefore, the declining trend of 137Cs concentration constituting only the second phase was demonstrated with a single-component exponential line, which is represented in following equation:

μn (t ) = α × e−kt ,

Acknowledgments We would like to express our gratitude to the Tohoku Regional Agricultural Administration Office for the permission to undertake our fieldwork. The authors are grateful to Dr. K. Iijima, Dr. T. Tsuruta, Dr. T. Nakanishi at the Fukushima Environmental Safety Center of the Japan Atomic Energy Agency for their valuable comments and suggestions in our research and assistance with fieldwork and measurements. Appendix A. Supplementary data

(3) Supplementary data related to this article can be found at https:// doi.org/10.1016/j.jenvrad.2018.03.004.

where α (kBq kg−1) and k (y−1) are empirically determined constants, and t (y) is time after the accident. The parameters of the Ogaki Dam Reservoir were calculated as α = 337 kBq kg−1 and k = 0.36 y−1. Finally, the time and size dependences of the 137Cs concentrations of particulates discharged from the catchment to the reservoir can be described by the following equation: 0.21 Cn (t , Ssp) = 337 × e−0.36t × Ssp

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