Interannual changes in radiocesium concentrations in annually laminated tufa following the Fukushima Daiichi Nuclear Power Plant accident

Applied Geochemistry 102 (2019) 34–43

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Interannual changes in radiocesium concentrations in annually laminated tufa following the Fukushima Daiichi Nuclear Power Plant accident

T

Nagayoshi Katsutaa,∗, Yoshiki Miyatab,h, Takuma Murakamib,c, Yoshihisa Minod, Sayuri Naitoa, Koji Yasudaa, Shinya Ochiaib, Osamu Abee, Atsushi Yasudaf, Maki Morimotoa, Shin-ichi Kawakamia,g, Seiya Nagaob a

Faculty of Education, Gifu University, 1-1 Yanagido, Gifu, 501-1193, Japan Low Level Radioactivity Laboratory, Institute of Nature and Environmental Technology, Kanazawa University, Wake, Nomi, Ishikawa, 923-1224, Japan c Horonobe Research Institute for the Subsurface Environment, 5-3 Sakaemachi, Horonobe-cho, Teshio-gun, Hokkaido, 098-3221, Japan d Institute for Space-Earth Environmental Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi, 464-8601, Japan e Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi, 464-8601, Japan f Earthquake Research Institute, The University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032, Japan g Faculty of Education, Gifu Shotoku Gakuen University, 1-1 Takakuwanishi Yanaizu-cho, Gifu, 501-6194, Japan h Ventuer Business Laboratory, Organization of Frontier Science and Inovation, Kanazawa University, Kakuma-cho, Kanazawa, Ishikawa, 920-1192, Japan b

A R T I C LE I N FO

A B S T R A C T

Editorial handling by S. Stroes-Gascoyne.

Chemical and radiocesium concentrations in water and riverbed tufa of the Nigori River, flowing out of the southern foot of Mt. Asama volcano in central Japan, were investigated following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. This tufa is characterized by porous and dense laminae. Its annual lamination (varve) is formed by seasonal changes in dissolved CO2 concentration in water with temperature, and the tufa calcite is mostly precipitated during summer. We analyzed radiocesium concentrations in ∼4-mm-thick tufa samples that included four or five varves collected after the fall in 2011–2014. The activity ratio of 134Cs/137Cs in tufa collected in 2011 and 2014 was 0.96 ± 0.01 and 0.99 ± 0.01, respectively, which implies radiocesium generated as a result of the FDNPP. Sequential extractions indicate that 137Cs in tufa collected in 2011 is occupied in exchangeable and carbonate phases (55.2% and 44.8%, respectively). The 137Cs concentrations equivalent to tufa calcite that were precipitated after 2011 showed variations similar to the amount of summer rainfall during 2011–2014, which may be attributed to increases in dissolved 137Cs concentrations in river water associated with intensified rainfall and microbial action over the summer. In contrast, the 137Cs sorbed to tufa calcite was not retained in the riverbed, and its migration may be attributed to the porous texture of tufa and the high flow rate of the river. Therefore, tufa calcite is easier to dissolve even though it adsorbs compared with other minerals (mica, andosol etc.).

Keywords: Radiocesium Sequential extraction Tufa Varve Fukushima Daiichi nuclear power plant Summer rainfall

1. Introduction The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident was caused by damage as a result of a catastrophic earthquake (M = 9) and tsunami generated off the Pacific coast in Tohoku on March 11, 2011. Massive amounts of radionuclides were released into terrestrial eastern Japan, resulting in widespread soil contamination, especially by 134Cs (half-life [T1/2] = 2.06 years) and 137Cs (T1/2 = 30.17 years). The heavily contaminated soil spread into the area northwest of the FDNPP (MEXT, 2011; Yasunari et al., 2011). However, airborne radiation monitoring on November 1, 2011 showed that an area of over 10,000 km2, extending to the southwest of the FDNPP, was ∗

contaminated by more than 30 kBq/m2 of 134Cs and 137Cs (MEXT, 2013). The activity ratio of 134Cs and 137Cs was ∼1.0 (Komori et al., 2013; Tagami et al., 2011). Many studies have investigated the behavior and migration of deposited radiocesium in terrestrial environments covering forests, soil, sediments, rivers, and lakes (e.g., Evrard et al., 2015; Fukushima et al., 2018; Matsunaga et al., 2013; Nagao et al., 2015; Sakuma et al., 2018; Takahashi et al., 2018; Tsuji et al., 2016). Understanding the distribution of radiocesium in sediments is of importance because this includes information on the sedimentary processes such as transport of Csbearing materials and postdepositional migration. Deposited radiocesium in riverbeds is heavily adsorbed by the

Corresponding author. E-mail address: [email protected] (N. Katsuta).

https://doi.org/10.1016/j.apgeochem.2019.01.002 Received 24 September 2018; Received in revised form 28 December 2018; Accepted 4 January 2019 Available online 09 January 2019 0883-2927/ © 2019 Elsevier Ltd. All rights reserved.

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At Mt. Asama volcano, located about 250 km to the west of the FDNPP, the 137Cs concentrations in soil were in part > 10 kBq/m2 (Fig. 1a) (MEXT, 2013). The Nigori River flows out of carbonic springs on the southern slope of Mt. Asama volcano, and then flows through the forested headwater. Upon flowing down, laminated carbonate (tufa) is deposited in the riverbed. To our knowledge, 137Cs activities in natural calcite have been not reported to date (Sengupta et al., 2017 and reference mentioned therein). By monitoring the water chemistry and the petrography of the riverbed tufa, we confirmed directly that the laminae represent annual depositions. Based on observations, 137Cs concentration and its speciation in the tufa were investigated in every 4 or 5 laminae deposited after the FDNPP accident, because it was difficult to measure 137Cs by dividing the layer (lamina) in tufa. We discuss interannual changes in tufa 137Cs concentrations and postdepositional mobilization of 137Cs in the tufa calcite.

frayed edge site in clay minerals (Bolsunovsky, 2010; Qin et al., 2012). Consequently, the riverine sediments preserve continuous records of the amount of radiocesium transported from the watershed to downstream areas. Tanaka et al. (2015a) investigated the size distribution of the FDNPP-derived radiocesium concentrations in riverbed sediments of the Abukuma River and its tributaries situated ∼40 km northwest of the FDNPP. Their results showed that the downstream radiocesium contained gaps between silt-sized and sand-sized grains, whereas the upstream sediments had continuous size distributions of radiocesium concentrations. It was suggested that selective transports of silt-sized materials in suspended loads resulted in radiocesium concentrations of riverbed sediments in the downstream area. Furthermore, from investigation of the upper 18 cm of sediments on a sandbar in the Abukuma River, Tanaka et al. (2015b) suggested that the radiocesium transport from the watershed had significantly reduced during the period 2011–2013. In regions of high fallout, downward migrations of deposited radiocesium have been observed over several years after the Fukushima and Chernobyl accidents. Of these, the vertical distributions of the FDNPP fallout in soils of croplands, grasslands, and forests were scarcely affected by intense rainfall events in the first year (Matsunaga et al., 2013). However, the subsequent monitoring of forested and cultivated soils and lake sediments revealed downward migrations with a rate of > 1.0 cm/year (Fukushima et al., 2018; Kang et al., 2017; Takahashi et al., 2018). Konoplev et al. (2016) calculated that the migration of radiocesium in undisturbed forest and grassland soils on territories generated from the FDNPP accident was faster than that in the soils of the 30 km zone of the Chernobyl NPP, which was attributed to the higher annual precipitation (∼2.5-fold) in Fukushima compared with Chernobyl.

2. Materials and methods 2.1. Study site The Nigori River is located on the southern slope of Mt. Asama volcano in central Japan and its headwaters run for ∼4.5 km from the springs down a slope of 12.4% (Fig. 1a and b). Because the river flows out of carbonic springs with high dissolved iron, the downstream water becomes brownish-red because of the iron precipitants. Water discharge from the main spring (NG 2 in Fig. 1b) is 0.03 m3/s, as measured by the float method. The riverbed deposits are clearly classified into three sections (Fig. 1b): (1) iron-rich deposits precipitating for ∼1 km from the springs; (2) tufa depositing for ∼3 km below section (1); and (3)

Fig. 1. (a) Maps showing the Nigori River at Mt. Asama volcano, central Japan. (b) Sampling locations on the Nigori River. (c) Monthly temperature and precipitation (1981–2010) at the Karuizawa Meteorological Station (indicated by the filled red star in (a)). In (b), the closed squares denote the sampling location at the main springs (NG 1 and 2), the closed circles the sampling locations on the river (NG 3–7), and the cross is the tufa sampling point. 35

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Table 1 Selected results of observation data from February to December 2011 at points NG 2, 3, and 6 on the Nigori River. Electrical conductivity (EC) is denoted in mS/cm, oxidation–reduction potential (ORP) in mV, and dissolved oxygen (DO) concentration in mg/L. Concentration of ion is given in mg/L, alkalinity (Alk) and ion balance in mmol/L, partial pressure of carbon dioxide (PCO2) in matm, and the PWP rate (PWP) in 10−10Mcm−2sec−1. SIc and SIf are the saturation index for calcite and ferrihydrite, respectively. Date & location (2011.2.11) NG2 NG3 NG6 (2011.4.16) NG2 NG3 NG6 (2011.6.19) NG2 NG3 NG6 (2011.8.28) NG2 NG3 NG6 (2011.10.13) NG2 NG3 NG6 (2011.12.9) NG2 NG3 NG6

Temp

pH

EC

ORP

DO

Na+

Mg2+

K+

Ca2+

Fe2+

Mn2+

Cl−

SO42–

Alk

IB

PCO2

SIc

SIf

PWP

12.5 12.8 1.30

5.74 6.95 8.58

0.802 0.823 0.683

– – –

– – –

19.03 30.33 30.50

17.88 27.22 26.94

3.68 5.93 5.83

54.91 74.13 64.44

68.04 6.74 0.01

1.09 0.96 0.23

11.37 20.25 20.91

46.06 97.19 101.66

6.18 5.07 4.15

2.19 0.06 −1.25

538.4 29.2 0.50

−1.64 −0.38 1.04

−3.06 −4.83 1.28

−6.37 −0.28 1.89

13.9 15.6 11.5

5.78 6.98 8.65

0.703 0.752 0.628

10 −150 207

0.87 5.76 9.12

21.99 29.85 31.12

18.58 23.75 24.54

4.46 5.97 6.10

55.93 67.47 52.18

58.83 0.00 0.00

1.02 0.79 0.09

12.84 19.46 20.45

54.47 91.88 97.43

6.13 4.75 3.59

−0.69 −2.98 −0.61

487.8 25.5 0.40

−1.59 −0.37 1.14

−3.99 −4.07 1.67

−5.50 −0.73 4.44

13.0 15.3 17.3

5.95 7.20 8.49

0.673 0.751 0.592

69 −148 151

0.77 6.75 8.60

15.42 29.87 30.27

17.39 24.22 24.80

4.15 6.08 5.91

53.93 66.01 51.87

61.05 1.35 0.00

0.77 0.63 0.00

11.51 18.74 19.13

47.18 87.75 91.28

5.18 4.52 3.46

−0.13 −0.55 0.01

311.8 14.6 0.60

−1.45 −0.18 0.91

−2.42 −4.07 1.29

−3.80 −0.43 2.20

13.2 15.5 19.7

5.75 6.92 8.52

0.644 0.763 0.530

87 −143 135

0.94 6.42 6.72

21.12 30.38 30.01

19.25 26.07 25.72

4.20 6.28 5.86

59.57 78.23 51.52

65.33 3.49 0.00

0.85 0.72 0.00

11.40 18.40 18.59

54.29 93.29 96.22

6.18 5.16 3.46

1.79 0.05 1.08

523.4 30.6 0.50

−1.59 −0.36 0.94

−2.68 −4.41 1.12

−5.82 −0.73 2.59

12.7 14.7 12.9

5.74 7.16 8.55

0.684 0.774 0.625

– – –

1.50 8.62 8.12

20.30 29.90 31.14

18.94 26.43 26.61

4.12 6.12 6.11

59.03 75.27 61.01

68.33 1.38 0.00

0.92 0.76 0.01

11.11 18.41 19.35

53.51 95.32 97.09

6.08 4.52 4.01

2.32 2.86 1.35

523.4 15.9 0.50

−1.62 −0.18 1.01

−4.24 −1.61 0.20

−5.79 −0.40 2.39

12.9 13.1 1.50

5.74 7.06 8.54

0.705 0.790 0.647

68 −67 223

1.46 6.48 11.96

21.70 30.36 31.03

18.19 25.27 25.53

5.76 7.64 8.14

56.14 70.45 62.64

62.85 4.18 0.01

1.06 0.93 0.19

11.74 20.36 21.24

48.72 97.41 102.36

5.99 4.65 4.15

2.17 0.14 −0.85

519.6 20.2 0.50

−1.64 −0.32 0.85

−3.07 −2.65 2.39

−5.80 −0.63 1.06

1993) (Fig. 1a). The depth of river terrace of NG 7 was ∼4.0 m. The paleo-tufa sample used in this study was obtained from a depth of ∼30 cm from the top surface of the pyroclastic deposits. Its formation is thus thought to have been ∼800 years ago.

downstream nondeposition (riverbeds of pebbles and gravels thinly coated by the iron and calcite precipitants and moss/algae). From 1981 to 2010, the annual mean temperature was 8.2 °C and the annual rainfall was 1241.7 mm at the Karuizawa Meteorological Station (N36°20.5′, E138°32.8′), located at an altitude of 999 m a.s.l. (http://weather.time-j.net/Climate/Chart/Karuizawa) (Fig. 1a and c). Vegetation in the watershed around the study site is dominated by forests of Castanea crenata and Quercus mongolica, Pinus densiflora, and Larix kaempferi (Environment Agency, 1983).

2.3. Analyses Cation and anion concentrations in river water were determined using inductively coupled plasma atomic emission spectroscopy (Horiba Jobin-Yvon Ultima 2) and ion chromatography (Thermo Scientific Dionex ICS-1100), respectively. Dissolved inorganic nitrogen was measured using an autoanalyzer (TRACCS 2000, BL Tech). Analytical precision was < 5% for Na+ and K+, 0.6% for NO3−, 2.5% for NO2−, and < 3% for the other ions. The saturation index for calcite (SIc) and ferrihydrite (SIf) and the partial pressure of CO2 (PCO2) were calculated using the chemical speciation program WATEQ4F (Ball and Nordstrom, 2001). The inorganic precipitation rate for calcite (PWP rate) followed the equation of Plummer et al. (1978) and its four constants were based on Plummer (1978) and Kaufmann and Dreybordt (2007). When SIc ≥ 0 and SIf ≥ 0, the water is saturated with respect to calcite (CaCO3) and ferrihydrite (FeOOH), respectively. Negative values indicate undersaturation. The iron-rich deposit and tufa samples for analyses were dried in a desiccator, and were then powdered and homogenized in an agate mortar. Thin sections of dry tufa samples were embedded in low viscosity epoxy resin (E205, Nichika Inc.) in a desiccator under low pressure. Cs speciation in the tufa samples was performed using the Tessier extraction technique for the following five fractions (Tessier et al., 1979): exchangeable, bound to carbonates, bound to Fe-eMn hydroxides, bound to organic matter, and residual (silicates). Samples for radiocesium analyses prior to milling were adjusted to give four or five laminae from the outer surface of the tufa (Tables 2 and 3), using microblasting and air scribes for fossil cleaning. Then, the sample corners were cut with a band saw to align the lengths of each lamina. Minerals and amorphous materials were identified using a MAC Science MXP3V X-ray diffractometer (XRD) with CuKα radiation (35 kV, 20 mA). Amorphous materials were also investigated using Fe

2.2. Observation and sampling River water was observed and sampled bimonthly from February to December 2011 at seven fixed stations (Fig. 1b): (NG 1 and 2) springs; (NG 3) iron-precipitating site; (NG 4–6) tufa-depositing sites; and (NG 7) nondeposition site (Table 1). The water temperature, pH, electrical conductivity (EC), oxidation–reduction potential (ORP), and dissolved oxygen (DO) concentration were measured using a Horiba portable meter (D-55S). Alkalinity (Alk) was determined by colorimetric titration with 0.05 N H2SO4 placed in a burette. Water samples for ion and radiocesium analyses were collected after passing through 0.45-μm acetate cellulose filters, and then the cation samples were acidified to pH ∼0.5 with nitric acid. In addition, subsamples for dissolved inorganic nitrogen (nitrate and nitrite) from February to August 2016 were collected bimonthly after passing through 0.20-μm filters and were frozen until measurement. Iron-rich deposit samples were taken in the riverbed at NG 3 using a hand scoop (Fig. 1b). Similarly, tufa samples were carefully collected from the riverbed at the fixed station located between NG 5 and 6, and a paleo-tufa sample was excavated from the river terrace at NG 7 (see Fig. 1b). In addition, tufa samplings on the riverbed were conducted at the same location every time. Hereafter, the riverbed modern tufa and terrace-buried tufa samples are called ‘tufa’ and ‘paleo-tufa’ samples, respectively. The five tufa samples collected during 2010–2014 and the paleo-tufa sample were used for radiocesium analyses (Tables 2 and 3). The river terrace of the study site is composed of Oiwake pyroclastic flow deposits that erupted in 1108 AD from Mt. Maekake (Aramaki, 36

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Table 2 134 Cs and fraction.

137

Cs concentrations in water and tufa in the Nigori River. Values in parentheses of the sequential extraction indicate the relative percentage of each

Sample

Points

(For river water) Water Water (For riverine tufa) Paleo-tufa Tufa Tufa Tufa Tufa Tufa (For tufa at 2011.12) Exchangeable Carbonate Fe-eMn hydroxide Organic matter Residual

137

134

(Bq/kg)

(Bq/kg)

2011.4.16 2011.4.16

0.045 ± 0.017 n.d.

n.d. n.d.

After 1108AD 2010.08 2011.12 2012.10 2013.10 2014.12

n.d. n.d. 1.229 0.364 0.332 0.884

n.d. n.d. 1.183 ± 0.317 n.d. n.d. 0.878 ± 0.284

2011.12 2011.12 2011.12 2011.12 2011.12

0.578 ± 0.178 (55.2) 0.470 ± 0.170 (44.8) n.d. n.d. n.d.

Date

NG3 NG6

Cs (2011.3.11)

± ± ± ±

134

Cs (2011.3.11)

0.120 0.106 0.086 0.114

Cs/137Cs

Varve numbers

Amount of sample

90 mL 90 mL 11 7 5 5 6 5

0.96 ± 0.01

0.99 ± 0.02

10.852 g 7.957 g 5.767 g 3.124 g 4.340 g 5.210 g

n.d. n.d. n.d. n.d. n.d.

and Fig. 2. The water temperature showed a seasonal variation that reflected air temperature (Fig. 2a). The differences between August and February were 0.7 °C at NG 2 (spring), 2.7 °C at NG 3 (Fe-rich deposit), and 18.4 °C at NG 6 (tufa deposit). The pH values at NG 2, 3, and 6 were respectively about 5.78 ± 0.08, 7.05 ± 0.12, and 8.56 ± 0.06 throughout the year (Fig. 2b). The Ca2+ concentrations and DO at NG 6 decreased during summer and increased during winter (Fig. 2c and g), as did EC and Alk values at the three stations (Table 1). The ORP was positive except at NG 3 (Fig. 2h). SIc, SIf, and the PWP rate at NG 6 showed positive values from February to December 2011 (Fig. 2f and Table 1) and the river water was saturated with respect to calcite and ferrihydrite. PCO2 abruptly declined from ∼500 to 0.5 matm toward downstream throughout the year (Fig. 2e). NO3 concentrations increased toward downstream in both February and August and the temporal variations show a higher increment in summer compared with that in winter (Fig. 2i). Similarly, NO2 concentrations in summer increased toward downstream but were relatively constant in winter (Fig. 2j).

K-edge X-ray adsorption near-edge structure (XANES) spectra captured using a synchrotron radiation beam. The XANES measurements were conducted at BL12C, the Photon Factory, KEK, Tsukuba (Murakami et al., 2012). Reference samples including FeO, FeCO3, Fe2O3, Fe3O4, and γ-FeOOH were used as the Fe standards, which were prepared by mixing the powder sample with a BN powder to form a disk. Similarly, the Fe-rich deposit and tufa samples were prepared for the XANES analyses. The radioactivity of 134Cs and 137Cs in water and tufa was measured using gamma-ray spectrometry with low background Ge detectors (EGMP60-30-R, EGM3800-30-R: CANBERRA, France) equipped with a multichannel analyzer (Multiport II: CANBERRA, France) under ultralow background conditions at the Ogoya Underground Laboratory in the Low Level Radioactivity Laboratory of Kanazawa University over 6–18 days (Hamajima and Komura, 2004). Water and tufa samples were packed in a plastic case without pretreatment. The detector was calibrated with a certified reference material (JSAC0471) including 134Cs and 137Cs from the Japan Society for Analytical Chemistry. Gamma emission peaks were used for calculating activity at 605 and 795 keV for 134Cs and 661 keV for 137Cs. The detection limits of 134C and 137Cs are 0.01–0.03 and 0.01–0.04 Bq/kg for water samples, respectively, and 0.05–0.31 and 0.06–0.23 Bq/kg for tufa samples, respectively.

3.2. Constituent materials of riverbed deposits The XRD pattern of the Fe-rich deposit sample shows two broad peaks at 35° and 65° in 2-theta (Fig. 3a), which corresponds to poorly ordered ferrihydrite (Fe2O3∙0.5H2O) (Henmi et al., 1980). The K-edge XANES spectrum of the Fe-rich deposit also shows much similarity with γ-FeOOH, as well as with the tufa samples (Fig. 3b). The tufa laminations are characterized by repeated layers of alternating porous and dense laminae, composed of micritic and sparitic calcite crystals, respectively (Figs. 3a and 4a). Petrographic observation shows that the

3. Results 3.1. Water chemistry The physicochemical parameters of water at three selected stations (NG 2, 3, and 6) at the Nigori River in 2011 are presented in Table 1

Table 3 Cumulative 137Cs concentrations (Ccum) in the tufa samples after 2011 and rainfall in 2011–2014 at the Karuizawa Meteorological Station. Ccum (predicted) is the cumulative 137Cs concentration of tufa calcite after 2011, in the case where the 137Cs was adsorbed/sorbed to tufa calcite during 2011 only; Ccum (predicted) therefore decreases with the number of varves accumulated after 2011. Ccum (measured) is the cumulative 137Cs concentration of tufa calcite after 2011, calculated using the bulk 137Cs concentrations (Cbulk) in the tufa samples in 2011–2014. N is the number of varves in the tufa samples and Na is the number of varves after 2011. Date

Tufa sample thickness

N

(mm) 2011.12 2012.10 2013.10 2014.12 a b

4.08 4.19 4.81 4.46

Varve thickness

Na

(mm) 5 5 6 5

0.816 0.838 0.802 0.892

1 2 3 4

Cbulk

Ccum (predicted)

Ccum (measured)

Ccum (measured)

Rainfall (from 2011 to 2014)

(Bq/kg)

(Bq/kg)

(Bq/kg)a

(Bq/kg)b

Jul–Sept (mm)

Annual (mm)

498.0 436.0 368.5 490.5

1121.5 1127.5 964.5 1343.5

1.229 0.346 0.332 0.884

± ± ± ±

0.120 0.106 0.086 0.114

2.753 1.376 0.918 0.688

Xcalclite = 1.000. Xcalclite = 0.448. 37

± ± ± ±

0.269 0.134 0.090 0.067

2.753 0.865 0.664 1.105

± ± ± ±

0.269 0.265 0.172 0.143

2.753 0.388 0.297 0.495

± ± ± ±

0.269 0.119 0.077 0.064

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Fig. 2. Spatial variations in physicochemical parameters from the springs to downstream. (a) water temperature; (b) pH; (c) Ca; (d) Fe; (e) log10(PCO2); (f) SIc; (g) DO; (h) ORP; (i) NO3; and (j) NO2. (a)–(h) were observed in 2011; (i) and (j) were observed in 2016. The numbers on the upper axis indicate the seven fixed stations (NG 1–7). NO2 concentrations of NG 3 and 4 in (h) are < 0.1 (mg/L).

38

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Fig. 3. (a) XRD pattern and (b) Fe K-edge XANES spectra showing the Fe-rich deposit at NG 3 and tufa at the sampling point between NG 5 and 6 on the Nigori River (Fig. 1b). H and C in (a) denote ferrihydrite and calcite, respectively.

4. Discussion

outer surface of the tufa samples presents sparitic calcite (Fig. 3b). This outer surface corresponds to late-August 2013 when the sample was collected.

4.1. Formation process of riverbed deposits 4.1.1. Iron hydroxide Carbonic springs of the Nigori River flow out with high dissolved Fe (Fig. 2d) and low DO concentrations (Table 1). During the subsequent downflow, the ferrous iron in river water changes to ferric iron around NG 3 and then precipitation of the ferrihydrite (Fe2O3∙0.5H2O or Fe (OH)3) in downstream water (Fig. 3a and b; SIf > 0 in Table 1) as follows (Langmuir, 1997):

3.3. Radiocesium and its chemical speciation Radiocesium concentrations in tufa and water of the Nigori River are presented in Table 2. The dissolved 137Cs concentration in river water was 0.045 ± 0.017 Bq/kg at NG 3 in April 2011 and it was below the limit of detection at NG 6 on the same date. The paleo-tufa sample and tufa samples collected before 2011 were also below the limit of detection. The 137Cs concentrations were 1.229 ± 0.120 Bq/kg for the tufa sample collected in December 2011, 0.364 ± 0.106 Bq/kg in October 2012, 0.332 ± 0.086 Bq/kg in October 2013, and 0.884 ± 0.114 Bq/kg in December 2014 (Table 2). Of these, the activity ratio of 134Cs/137Cs in December 2011 and December 2014 was 0.96 ± 0.01 and 0.99 ± 0.01 (corrected to March 11, 2011) for the samples, respectively. The results of sequential extraction experiments for the tufa samples in December 2011 showed that the fraction of 137Cs bound to the exchangeable phase was 55.2% and that to the carbonate phase was 44.8% (Table 2).

4H+ + O2(g) + 4e− → 2H2O, E° = +1.23 V Fe

2+

+ 3H2O → Fe(OH)3 + 3H

+

−

+ e , E° = − 0.975 V

(1) (2)

When the reactions are added so that the electrons cancel, the net reaction is 4Fe2+ + O2(g) + 10H2O → 4Fe(OH)3 + 8H+, E° = − 2.67 V

(3)

As a result, the negative ORP water occurred at station NG 3 because atmospheric O2 gas is dissolved in water and then the reaction

Fig. 4. Photomicrographs of tufa sections. (a) Laminated textures in tufa: crossed polarized light. (b) Outer surface of tufa sample, collected on August 27, 2013: plane-polarized light. The laminated structure is composed of dense (D) and porous (P) laminae. Scale = 500 μm. 39

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exchangeable and carbonate phase (55.2% and 44.8%, respectively; Table 2). Based on this observation, we consider that the carbonate phase originates from the 137Cs sorbed to (1) and (2) and the exchangeable phase from the 137Cs adsorbed to (3). Hereafter, for convenience, we refer to (1) and (2) as ‘sorption’ and to (3) as ‘adsorption’.

progresses to the right hand of Eq. (3). 4.1.2. Tufa and its lamination Similar to the dissolved Fe, the dissolved CO2 concentration in river water abruptly decreases from upstream to downstream (Fig. 2e). This CO2 degassing brings about an increase in pH of river water (Table 1) and a precipitation of calcite (Figs. 2f and 3a) as follows: H

+

+

HCO3−

→ H2O + CO2

Ca2+ + 2HCO3− → CaCO3 + H2O + CO2

4.3. Assessment of

(4)

137

Cs concentrations in tufa calcite

137 Cs concentrations in the tufa samples decreased in 2011–2013, followed by an increase in 2013–2014 (Table 2). As described in Section 3.3, the four tufa samples were collected in October or December 2011–2014. However, the varve thickness of the four samples was approximately the same at 0.8–0.9 mm (Table 3). Such a coincidence implies that the annual depositional rate is almost identical among the four samples and is mainly controlled by the formation of dense laminae in the summer (Fig. 3). Therefore, 137Cs concentrations in the tufa samples collected in October or December can be approximated as an annual amount of 137Cs sorbed/incorporated to the tufa calcite lamination. Transport of the radioactive plume from the FDNPP to the Nigori River had considerably decreased by the fall of 2011. Monthly observations at monitoring stations within 250 km of the FDNPP show that 137Cs deposition peaked in March 2011, and then quickly decreased with half-lives of 11–14 days until the fall of 2011 (Hirose, 2013). Similarly, monthly observations at Nagano City located ∼40 km northwest of Mt. Asama show that the annual 137Cs deposition was significantly reduced from 1257 MBq/km2 in 2011 to less than 9.0 MBq/km2 in 2012–2014 (Nagano Prefecture, 2018). The same conclusion was drawn from the observation of 137Cs atmospheric concentration at the Meteorological Research Institute, Tsukuba located ∼170 km west of the FDNPP (Igarashi et al., 2015). Furthermore, although the emission rate of radioactive materials into the atmosphere at the FDNPP continued until at least the fall of 2011, its emission rate at that time decreased by more than three orders of magnitude compared with the amounts immediately after the accident (TEPCO, 2012). These atmospheric monitoring results suggest that the radiocesium deposition at the Nigori River and the associated sorption of 137Cs on the tufa calcite mostly occurred during 2011. Therefore, we calculated whether the cumulative 137Cs concentration in tufa calcite (Ccum) accumulated after 2011 would be approximately reduced in proportion to the number of varves (Fig. 5a) as follows:

(5)

Tufa calcite precipitation of the Nigori River occurs throughout the year because SIf > 0 and the PWP rate > 0 in both February and August (Fig. 2f and Table 1). The lamination of the tufa is composed of dense and porous laminae (Fig. 4a). The dense laminae are formed during the summer (Fig. 4b), which may be attributed to the increase in water temperature. The associated decrease in CO2 solubility in water leads to the promotion of calcite precipitation (Eq. (5)). This interpretation was supported by lower Ca concentration and Alk in river water during the summer compared with the winter (Fig. 3c and Table 1). Therefore, the dense and porous lamina couplets can be regarded as an annual cycle deposition (known as a varve). A similar laminated texture in tufa associated with seasonal climate changes has been reported in streambeds in limestone areas in Southwest Japan (Kano et al., 2004; Matsuoka et al., 2001). 4.2. Speciation of radiocesium in tufa and its origin As shown in Fig. 3a, clay mineral and mica, which have strong adsorption affinity, were not detected in the riverbed sediments of the Nigori River (e.g., Qin et al., 2012). The tufa samples are mainly composed of calcite, which also contains ferrihydrite as shown in the XANES spectra (Fig. 3b). However, sequential extraction results show that radiocesium in the tufa samples was adsorbed to exchangeable and carbonate phases only and was not included in the Fe-eMn hydroxide phase (Table 2). Radiocesium adsorption on the ferrihydrite in tufa is therefore negligible. Experimental studies and geological observations were consistent with our results. Sequential extraction on radionuclidecontaminated sediments of Yenisei River suggests that the amount of uptake of 137Cs by calcite is comparable with the concentration of 137Cs extracted from exchangeable ions (Bolsunovsky, 2010). Batch sorption experiments indicate that hematite (Fe2O3) as well as calcite are poor sorbents of Cs over the range of pH 7–9 (Cornell, 1993). The activity ratio of 134Cs/137Cs in tufa samples from the Nigori River was 0.96–0.99 (Table 1); these values are within the range of 0.9–1.05, which represent FDNPP-derived radiocesium (Komori et al., 2013; Tagami et al., 2011). The 134Cs/137Cs ratio reported for the Chernobyl accident was 0.50–0.52 (International Atomic Energy Agency, 2001) and no radiocesium following the Three Mile Island accident was released into the environment (Miller, 1994). The numerical simulation with a Lagrangian particle dispersion model estimated that the 137Cs deposition of our study site was 10–25 Bq/kg (see Fig. 3 in Yasunari et al., 2011). The tufa 137Cs concentrations in samples in 2011–2014 were therefore comparable with 5–12% of the 137Cs deposition amounts (Table 2). As shown in Fig. 2f, river water at NG 3 in April 2011 was undersaturated in calcite (SIc = −0.37; Table 1) and the dissolved 137Cs concentration was 0.045 ± 0.017 (Bq/kg). On the other hand, river water at NG 6 on the same date was supersaturated in calcite (SIc = 1.14), whereas it was below the limit of detection for dissolved 137 Cs (i.e., below 0.01 Bq/kg). This implies that the tufa calcite of Nigori River was precipitated while it incorporated with the FDNPP-derived 137Cs+ in the water. Therefore, we considered the possibility that the 137Cs in the calcite occurs (1) inside the grains, (2) at the grain boundaries, and (3) on the porous grain surfaces. Sequential extraction results indicate that 137Cs in tufa collected in 2011 is in the

N Ccum = Cbulk × Xcalcite × ⎛ ⎞, ⎝ Na ⎠ ⎜

⎟

(6)

where Cbulk is bulk 137Cs concentrations in the tufa sample, Xcalcite is the relative percentage of 137Cs in the carbonate fraction of the tufa sample (Table 2), N is the number of varves including the tufa samples, and Na is the number of varves after 2011 (Table 3). Here, we assumed that Xcalcite during 2011–2014 equaled 44.8% (Table 2). The sequential extractions of the tufa samples from 2012 to 2014 should be performed to determine Xcalcite. However, it is difficult to measure those samples because of the very low 137Cs concentrations. Therefore, our assumption underestimates Xcalcite. The exchangeable phase in 2012–2014 would be reduced compared with that in 2011, considering temporal variations in dissolved 137Cs concentrations in river waters observed in the vicinity of the FDNPP (Iwagami et al., 2017; Nagao et al., 2015). As shown in Fig. 5b and Table 3, cumulative 137Cs concentrations in the tufa calcite Ccum (measured) rapidly decreased in 2011–2012, then decreased more gradually in 2012–2013, and increased in 2013–2014. To investigate whether the 137Cs in the tufa calcite is retained or not after deposition, we calculated the Ccum (predicted) in the case where the 137Cs was adsorbed/sorbed to the tufa calcite during 2011 only; the Ccum (predicted) therefore decreases with the number of varves accumulated after 2011. The trends of Ccum (measured) are significantly 40

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Fig. 5. (a) Schematic sketches of time evolution of varve formation and 137Cs sorption in tufa calcite of the Nigori River. (b) Comparison of the tufa calcite 137Cs concentrations with annual and summer rainfall at the Karuizawa Meteorological Station. In (a), N is the number of varves in a sample and Na is the number of varves formed after 2011. In (b), Xcalcite is the relative percentage of 137Cs in the carbonate fraction of the tufa sample.

discrepancy between simulations and observations during summer might be attributed to not considering the effects on microbial action and leaching from organic matter in forest litters. Indeed, the nutrients (nitrate and nitrite) in the Nigori River increased in the downstream tufa deposited during summer (Fig. 2i and j), which suggests intensified microbial decomposition in the forested headwater (Section 2.1). In addition to meteorological factors, the similarity of variation between tufa calcite 137Cs and rainfall records (Fig. 5b) suggests that the sorbed 137Cs did not accumulate in the dense laminae, which migrated until the following summer. This migration rate is estimated to be ∼4 mm/year based on the tufa sample thickness (Table 3) (i.e., the 137 Cs residence time in tufa is possibly less than half a year). We believe that the migration of 137Cs adsorbed/sorbed to tufa calcite may be attributed to the physical and chemical properties of calcite and the river flow conditions. An experimental study showed that the adsorption of Cs to calcite was nearly zero in any of the electrolyte solutions (Fujiwara et al., 1997). A similar experiment revealed that 137Cs adsorption on a stalagmite sample increased with pH (from pH 1 to pH 6), whereas its adsorption was only 1% in a solution of pH 6 (Sengupta et al., 2017). This lower sorption on calcium carbonate was thought to result from the ionic charge and size mismatches. In contrast, as shown in the tufa sample thin sections (Fig. 4), the Nigori River tufa occupies many pore spaces, especially in porous laminae, which corresponded to deposition during winter. The porous tufa textures are formed under the condition of high river flow down a slope that averages 12% (Fig. 1b and Section 2). As reported in shallow lake sediments (Fukushima et al., 2018), the high porosity and flow stress exerted on tufa calcite may lead to the promotion of 137Cs penetration. Fallout 137Cs activity peaks in undisturbed sediments have been used as time markers, some of which correspond to 1954 (onset of the global nuclear fallout), 1963 (maximum fallout deposition), and 1986 (Chernobyl fallout) (Appleby, 2001; Pennington et al., 1973). The combined use of 137Cs and excess 210Pb techniques reduces uncertainty in dating recent sediment archived proxies (0–150 years ago), which then permits the assessment of climatic and human impacts on lake and watershed evolutions in the Japanese archipelago with high reliability (e.g., Hyodo et al., 2017; Itono et al., 2015; Ochiai et al., 2015; Mizugaki et al., 2006). However, our investigation of the radiocesium

different from the predicted rate of reduction with varve accumulation (Fig. 5b), even though Xcalcite after 2012 was assumed to be 1.0. 4.4. Radiocesium behavior in the tufa-depositing system and its relation to climate Cumulative 137Cs concentrations in the tufa calcite (Ccum (measured)) show a similar pattern to the amount of annual rainfall, particularly during the summer in 2011–2014 (Fig. 5b and Table 3). As described in Sections 3.1 and 3.2, the tufa calcite of the Nigori River is mostly precipitated during the summer (Figs. 3 and 4), which implies that its 137Cs concentration reflects the amount of 137Cs sorption in that season. The discharge rate of river water (runoff rate) seems be approximately linearly proportional to the average rainfall intensity as given by rational method equation (Ascough et al., 1997). Therefore, we believe that annual changes in 137Cs concentrations in the tufa calcite may be attributed to variations in dissolved 137Cs concentrations in the river during the summer. Indirect evidence supporting this hypothesis has been reported on a high-dose-rate area located 35 km northwest of the FDNPP (Iwagami et al., 2017; Ueda et al., 2013). Ueda et al. (2013) investigated total 137 Cs concentrations (dissolved and particulate forms) in two small river waters in 2011 and reported their strong dependence on the rate of water discharge, which increased during rainfall or flood events in July and September. A similar conclusion was drawn from studies of dissolved 137Cs in stream water from June 2011 to July 2013 (Iwagami et al., 2017). Furthermore, Iwagami et al. (2017) pointed out that the abrupt reductions in dissolved 137Cs in the water in 2011–2013 were similar to those of loss of canopy Cs by throughfall. Such an abruptly decreasing trend was observed in cumulative 137Cs concentrations of tufa calcite in the Nigori River in 2011–2012 (Fig. 5b). Tsuji et al. (2016) observed an increase in dissolved 137Cs concentrations in the river water of a forested watershed during summer and suggested that increased temperatures might cause leaching from organic matter by microbial activity and that storm runoff enhanced the transport of the dissolved 137Cs into river water. A similar suggestion was made by Sakuma et al. (2018), who simulated seasonal changes in dissolved 137Cs in the watershed and noted that the significant 41

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behavior of the riverine tufa-depositing system demonstrates that the Fukushima-derived 137Cs was not retained in the annual lamination as a time marker corresponding to 2011.

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5. Conclusions The radiocesium behavior of the tufa-depositing system in the Nigori River, located at the southern foot of Mt. Asama volcano, central Japan, was investigated following the FDNPP accident using chemical analysis of riverbed and water samples. Our results lead to the following conclusions. (1) The lamination of tufa comprises annually deposited structures that formed in the riverbed because of seasonal change in CO2 degassing in river water. The tufa calcite is mostly precipitated during the summer. (2) The activity ratio of 134Cs and 137Cs in tufa collected after 2011 was 0.96 ± 0.01 and 0.99 ± 0.01, which implies radiocesium emitted from the FDNPP. (3) The sequential extraction experiments indicate that 137Cs in the tufa sample is present in exchangeable and carbonate phases (55.2% and 44.8%, respectively). (4) The similarity of variation between tufa calcite 137Cs and annual/ summer rainfall records in 2011–2014 may explain the dissolved 137 Cs concentrations in the river, in association with intensified rainfall and microbial action during summer. (5) The 137Cs sorbed to tufa calcite migrates at a rate of ∼4 mm/year. Acknowledgements Part of the present study was supported by a Grant-in-Aid for Young Scientists (B) (No. 24700947) from the Ministry of Education, Culture, Sports, Sciences, and Technology, Japan. This study was also performed under the cooperative research program of Institute of Nature and Environmental Technology, Kanazawa University (Accept No. 7, 2015; No. 30, 2016) and was supported by the Earthquake Research Institute, The University of Tokyo (ERI JURP 2014-G-17). Two anonymous reviewers provided constructive reviews that improved this manuscript. We deeply thank S. Nishide, T. Nagaya M. Kanbara and K. Sakai for analyses and samplings. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apgeochem.2019.01.002. References Aramaki, S., 1993. Geological Map of Asama Volcano 1:50,000. Geological Survey of Japan (in Japanese with English abstract). Appleby, P.G., 2001. Chronostratigraphic techniques in recent sediments. In: Last, W.M., Smol, J.P. (Eds.), Tracing Environmental Change Using Lake Sediments Volume 1: Basin Analysis, Coring, and Chronological Techniques. Kluwer Academic, pp. 171–203. Ascough II, J.C., Baffaut, C., Nearing, M.A., Liu, B.Y., 1997. The WEPP watershed model: I. Hydrology and erosion. Trans. ASAE (Am. Soc. Agric. Eng.) 40, 921–933. Ball, J.W., Nordstrom, D.K., 2001. User's manual for WATEQ4F, with revised thermodynamic data base and test cases for calculating speciation of major, trace, and redox elements in natural waters. U. S. Geological Survey, Open File Report 91–183. Bolsunovsky, A., 2010. Artificial radionuclides in sediment of the Yenisei River. Chem. Ecol. 26, 401–409. Cornell, R.M., 1993. Adsorption of cesium on minerals: a review. J. Radioanal. Nucl. Chem. 171, 483–500. Environment Agency, 1983. Actual Vegetation Map, Karuizawa. The 3rd National Survey on the Natural Environment (Vegetation). Evrard, O., Laceby, J.P., Lepage, H., Onda, Y., Cerdan, O., Ayrault, S., 2015. Radiocesium transfer from hillslopes to the pacific ocean after the Fukushima nuclear power plant accident: a review. J. Environ. Radioact. 146, 92–110. Fujikawa, Y., Fukui, M., 1997. Radionuclide sorption to rocks and minerals: effects of pH and inorganic anions. Part 1. Sorption of cesium, cobalt, strontium and manganese. Radiochim. Acta 76, 153–162.

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