spatial movement and accumulation of 137Cs in a shallow lake in the initial phase after the Fukushima Daiichi nuclear power plant accident

Applied Radiation and Isotopes 147 (2019) 59–69

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Vertical/spatial movement and accumulation of 137Cs in a shallow lake in the initial phase after the Fukushima Daiichi nuclear power plant accident

T

Hideki Tsuji∗, Atsushi Tanaka, Kazuhiro Komatsu, Ayato Kohzu, Shin-ichiro S. Matsuzaki, Seiji Hayashi National Institute for Environmental Studies, Japan

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

movement in Lake Nishiura after • theCsFDNPP accident was investigated. mixing and accumulation of • Vertical Cs was active in sediment at 137

137

• • •

shallow depths. Spatial distribution of 137Cs was almost unchanged since March 2011. 137 Cs increased remarkably around the river mouth of the inflowing urban rivers. Suspended solids of urban rivers showed relatively high 137Cs levels and mobility.

A R T I C LE I N FO

A B S T R A C T

Keywords: 137 Cs Lake Kasumigaura Sediment Inventory Urban river Fukushima Daiichi nuclear power plant accident

Movement of 137Cs in the bottom sediment of a shallow lake was investigated by collecting columnar sediment cores from the entire area once a year, and also from three representative points every 3 months over 3 years from the 137Cs fallout in March 2011. The 137Cs inventory in the sediment increased remarkably near the mouths of urban rivers; however, the distribution of the 137Cs inventory did not change remarkably in most areas, and horizontally slow downstream movement of 137Cs was observed.

1. Introduction The Fukushima Daiichi nuclear power plant (FDNPP) accident in March of 2011 caused the deposition of substantial amounts of radioactive material (mainly 131I, 134Cs and 137Cs, which have half-lives of 8.0 days, 2.1 years and 30 years, respectively) in the surrounding area (Chino et al., 2011). Among these materials, 137Cs causes long-term environmental pollution of aquatic ecosystems (Rowan and Rasmussen, 1994). Some reports after the Chernobyl nuclear power plant accident estimated that the 137Cs concentration of freshwater fish will remain

∗

high for tens or hundreds of years (Smith et al., 2000; Sundbom et al., 2003). Moreover, contamination of fish is of particular concern in areas of stagnant water, such as lakes, because of the continuous accumulation of 137Cs inflow from upstream contaminated areas, and such cases have been reported in some lakes in Japan (Ochiai et al., 2013; Cao et al., 2017). Because 137Cs concentrations in fish are closely related to the 137Cs concentrations in sediment (Särkkä et al., 1996) and sedimentation rates (Saxén and Ilus, 2008), it is important to investigate the dynamics of 137Cs in lake sediment. Generally, 137Cs in lake sediment originates from two sources, direct

Corresponding author. 10-2, Fukasaku, Miharu, Tamura, Fukushima, 963-7700, Japan. E-mail address: [email protected] (H. Tsuji).

https://doi.org/10.1016/j.apradiso.2019.02.009 Received 25 April 2018; Received in revised form 5 February 2019; Accepted 5 February 2019 Available online 19 February 2019 0969-8043/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. (a) Major rivers in the Kasumigaura watershed and sampling points of river water. The 137Cs inventory distribution map was drawn using the modified values of the 137Cs deposition map of the fifth airborne survey conducted by the Ministry of Education, Culture, Sports, Science and Technology (2012) calculated by Kato et al. (b) Sediment core sampling points in Lake Nishiura. Vertical distribution of 137Cs in the sediment core was measured at St. 3, 7, and 9.

deposition by atmospheric fallout or flow into the lake via riverine transport. 137Cs deposited by fallout is quickly adsorbed onto suspended solids (SS) in the lake, after which it accumulates on the surface layer of the lake sediment, then slowly migrates downward (Kansanen et al., 1991; Kirikopoulos et al., 1994; Yoshimura et al., 2014). Following 137 Cs movement in the initial phase after the 137Cs fallout, the depth showing peak 137Cs concentration in the sediment gradually migrates downward as the sediment with lower 137Cs concentration accumulates on the lake bottom via riverine transport. Therefore, 137Cs has been widely used as a tracer to estimate the sedimentation rate of suspended solids to lake bottoms (Robbins and Edgington, 1975; Edgington et al., 1991; Erlinger et al., 2008). The planar sedimentation process of suspended particles flowing into the lake has also been estimated by measuring the temporary spatial distribution of 137Cs in lake sediment (Hongve et al., 1995; Abraham et al., 2000). However, few studies have focused on the overall movement of 137Cs in lakes based on temporal changes in vertical and spatial 137Cs distributions in sediment together with particulate 137Cs from inflowing rivers. Surveying 137Cs dynamics in the lake during the initial phase after the 137Cs fallout is particularly important for evaluating how the 137Cs contamination spreads in the lake. In deep lakes, horizontal movement of 137Cs can be ignored because the sediment is hardly disturbed, so most of the 137Cs behavior can be determined by the vertical deposition of particles. However, in flat and shallow lakes, particles repeatedly settle and become resuspended because of water movement caused by river inflow and wind waves (Hilton et al., 1986; Luettich et al., 1990); therefore, horizontal movement of 137Cs is likely to occur. Indeed, in Lake Teganuma in Japan, which is a shallow lake, 137Cs directly deposited at the inlet by the initial fallout reached the outlet 2 years after the FDNPP accident (Koibuchi et al., 2015). Moreover, 137Cs sedimentation rates in river mouth areas are expected to differ depending on land use characteristics in the watersheds of inflowing rivers (Andoh et al., 2015). Therefore, it is difficult to predict 137Cs distributions in sediment and changes in total 137Cs storage in lakes without comprehensive survey data. If there is a region in which 137Cs is likely to accumulate, it will strongly affect the radiological pollution of aquatic ecosystems. Lake Nishiura, which is located 150–170 km southwest of the FDNPP, is one of the largest lakes affected by radioactive contamination from the FDNPP accident (Fukushima and Arai, 2014). Indeed, the export of benthic fish such as eels (Anguilla japonica) and American

catfish (Ictalurus punctatus) from the lake is still restricted because it contains radioactivity at levels exceeding the regulation value of 100 Bq/kg. Given these circumstances, the conditions and temporal changes in 137Cs concentrations of various aquatic organisms inhabiting the lake have been widely investigated (Matsuzaki et al., 2015; Satake et al., 2016; Wakimoto et al., 2016). Accordingly, it is necessary to identify areas in which 137Cs accumulates in high concentrations and to examine the possibility for such areas to form in the future by investigating the movement and accumulation of 137Cs in Lake Nishiura. Since high levels of 137Cs were deposited on the west (upstream) side of Lake Nishiura and its watershed, there is concern that 137Cs pollution in this area has been exacerbated by the influent 137Cs, and that 137Cs may be migrating to the east side of the lake (downstream) because of sediment movement. Temporal changes in the 137Cs vertical distribution within sediment of the central part of Lake Nishiura have already been reported (Arai et al., 2017; Fukushima et al., 2018), but there is currently no information regarding the spatial movement of 137Cs available. In the present study, we investigated the movement and accumulation of 137Cs in the sediment of Lake Nishiura from a relatively early period after the FDNPP accident and identified areas in which 137Cs pollution were exacerbated. To accomplish this, we measured the vertical and spatial distributions of 137Cs in sediment of Lake Nishiura for 3 years since the FDNPP accident, together with particulate 137Cs from inflowing rivers. We also clarified the dynamics of particulate 137Cs in urban rivers in which transportation and sedimentation of 137Cs to the lake occurs rapidly and examined the characteristics of dissolved 137Cs in the lake water, which are easily transferred to aquatic biota. 2. Materials and methods 2.1. Study site We investigated Lake Nishiura (172 km2 area), which is the largest lake of the series of Lake Kasumigaura (Fig. 1). Among the main rivers flowing into Lake Nishiura, five (the Sakura, Koise, Ono, Seimei, and Sonobe) were selected to monitor the flux of radiocesium 134Cs and 137 Cs. In addition, two rivers (the Tomoe and Hokota) flowing into Lake Kitaura (36 km2 area) were monitored to investigate the 137Cs behavior in the surrounding watershed with different land use. The watershed 60

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area of these rivers ranged from 25.6 to 352.7 km2. Moreover, the seven monitored rivers had variable land use characteristics (Supplemental Table A). Specifically, the ratio of the building area was relatively large in the Ono and Seimei watersheds, while forest area was most extensive in the Sakura and Koise watersheds, and agricultural land dominated the Sonobe, Tomoe, and Hokota watersheds (based on data from the Geospatial Information Authority of Japan). The 137Cs inventory in the Lake Nishiura and Kitaura watershed in July of 2011 was 12–77 kBq/ m2 (average, 23 kBq/m2) based on the 137Cs inventory map (Kato et al., under review). We also surveyed the vertical and spatial distribution of 137Cs in sediment of Lake Nishiura (Fig. 1). The average water depth was 4.0 m, and vertical mixing of the water and sediment is always promoted by wind waves (Tsushima et al., 2016). The average residence time of water in Lake Nishiura was ∼200 days. Inlets of the major rivers flowing into Nishiura are at the north and west sides of the lake, while the outlet is located in the southeast. The total 137Cs deposition by the fallout in Lake Nishiura was estimated to be 2.9 × 1012 Bq based on numerical simulation of the initial 137Cs depositions by Morino et al. (2013).

Fig. 2. Relationship between suspended solids concentration and concentration of 137Cs in suspended solids of water in rivers flowing into Kasumigaura.

found between the SS concentration and 137Cs concentration per weight of SS for each river, which was approximated by the following exponential function: 137

2.2. Sampling

Cs_SS = a C_SS

b

137

(1) 137

Cs_SS indicates the Cs concentration per unit weight of SS where [Bq/kg], a and b indicate fitting parameters, and C_SS indicates the SS concentration [kg/m3]. Two fitted parameters identified by the method of least squares with respect to log values of C_SS and 137Cs_SS are shown in Table 1. The 137Cs concentration in SS at each time was calculated from the continuous observed SS concentration with the identified equations for each river. Finally, the hourly SS and 137Cs flux values were summed to calculate the outflow flux of SS and 137Cs per year. To compare the hydrological and 137Cs flux characteristics between watersheds with different areas, the SS and 137Cs fluxes were normalized by dividing the values by the watershed area as specific discharge of SS and 137Cs. The 137Cs concentration in SS was normalized by dividing it by the average 137Cs inventory in each watershed (so-called entrainment coefficient of 137Cs [m2/kg]), and the characteristics between the 137Cs concentration in the SS of each river with the land use ratio were examined by regression analysis. The annual 137Cs flux was divided by the 137Cs inventory in the watershed to give the 137Cs runoff ratio.

2.2.1. River water 2.2.1.1. Sampling and processing method. In the downstream part of the seven rivers, water was collected from the center of the stream in 2012–2013 using a portable bucket made of polyethylene. We collected 4 L of river water continuously at intervals of 30 min to 1 h during rainfall events (two or three events in 2012, and three events in 2013). The total precipitation for 1 and 2 years after the FDNPP accident is shown in Supplemental Table B. Water samples collected at nearly the same time within the same rainfall events were composited to obtain a sufficient amount of SS for analysis of 137Cs. Following separation and extraction of the SS in water by centrifugation (15,760×g, 25 min), the solids were dried at 60 °C and put in a U-8 container (90 mL volume), after which the concentration of 137Cs per unit weight of SS [Bq/kg] was measured (a total of 12–21 samples of 137Cs in SS were measured for each river). In addition, 18 L of water was collected from the Seimei River using a portable bucket on 24 September 2012 and 19 August 2013, after which dissolved and particulate 137Cs in a unit volume of water [Bq/m3] was measured following preprocessing by filtering the water through a Whatman GF/C (GE Healthcare Life Sciences, Chicago, IL, USA; pore size 1.2 μm) and then concentrating the dissolved 137Cs in the filtrate with RAD disks (3M Japan, Ltd., Tokyo, Japan). For some samples, the particle size distribution of SS was also measured by laser diffraction analysis (SALD3100, Shimadzu Co., Ltd., Tokyo, Japan). The specific surface area [m2/kg] was obtained from discretization data of the obtained particle size distribution and compared with the 137Cs concentration in SS per watershed inventory of 137Cs as described later. To understand the regional and temporal characteristics of 137Cs distribution by different forms in river water, the apparent distribution coefficient (Kd), which was defined as the ratio of 137Cs in SS to dissolved 137Cs, was calculated.

2.2.2. Sediment and water in Lake Nishiura 2.2.2.1. Sampling and processing method. Water from Lake Nishiura was sampled with buckets at three locations on 11 September 2012 and 6 August 2013 (Fig. 1): St. 3 in the north (36.12167°N, 140.37750°E), St. 7 in the west (36.06558°N, 140.23164°E), and St. 9 in the center (36.03583°N, 140.40361°E). The station numbers corresponded to those used by Takamura and Nakagawa (2016). Thereafter, particulate and dissolved 137Cs were measured using the same method as that used for the filtration and concentration of dissolved 137 Cs in river water. Sediment cores were collected from St. 3, 7, and 9 (Fig. 1) in

2.2.1.2. Calculation of SS and 137Cs flux. Particulate 137Cs flux at monitoring points was calculated by the following procedure. First, the relationship between the SS concentration and turbidity for each river before the FDNPP accident (2007–2011) was determined based on measurements taken by the Kasumigaura River Office near the sampling points. The continuous SS concentration [kg/m3] after the accident was obtained by converting the continuous hourly turbidity data using the determined relationship equations. The hourly SS flux [kg/h] was then obtained by multiplying the flow rate [m3/h] by the SS concentration [kg/m3]. Finally, hourly particulate 137Cs flux [Bq/h] was obtained by multiplying the SS flux [kg/h] and 137Cs concentration per unit weight of SS [Bq/kg]. As shown in Fig. 2, a relationship was

Table 1 Parameters in equation (1) for each river.

Sakura Koise Ono Seimei Sonobe Tomoe Hokota

61

Coefficient a ( × 102)

Exponent b

estimate

p value

estimate

p value

1.6 6.8 12 29 8.8 4.2 1.1

0.74 0.36 < 0.01 < 0.01 0.03 0.55 0.44

−0.31 0.018 −0.31 −0.22 −0.080 −0.24 −0.026

< 0.01 0.80 < 0.01 0.13 0.03 0.42 0.70

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2.3. Evaluation of 137Cs storage in Lake Nishiura and the inflow amount of Cs

2012–2014 by a survey ship to evaluate temporal changes in the vertical distribution of 137Cs in the sediment. A vertical cylindrical sediment core with an inner diameter of 11 cm was collected using an HRtype gravity core sampler (RIGO Co., Ltd. Tokyo, Japan), so that the sediment core was not disturbed or compressed, every 3 months starting in August 2012. Sediment cores were cut at each 3 cm depth from the surface, and only the 3 cm of the surface layer was divided into two 1.5 cm layers. The core samples were then homogenized by hand in a plastic bag, after which raw core samples were put into U-8 containers and the 137Cs in wet sediment was measured. Next, samples were sufficiently lyophilized to calculate the 137Cs concentration per dry weight of the sediment. To eliminate the influence of the vertical distribution of bulk density variation between each sediment core sample, the vertically integrated value of sediment bulk density (kg/m2), or “mass depth,” was used as a vertical length indicator. The representative composition of the sediments was monitored from 1983 to 1990 and the volatile suspended solids at St. 3 and 9 were reported to be 13–19% (National Institute for Environmental Studies, 2018), with no large difference between the two points. Additionally, sediment cores collected from 68 points near the 1min mesh lattice point covering Lake Nishiura were collected to investigate the spatial distribution of the 137Cs inventory and to estimate the storage of 137Cs in Lake Nishiura on 18–20 December 2012, 20–27 September 2013, and 21–24 October 2014. Sediment cores with an inner diameter of 11 cm and length of about 30 cm were collected with a gravity core sampler (HR-type, RIGO Co., Ltd. Tokyo, Japan) at 52 of the 68 sampling points. At the other 16 points, which were mainly close to the lake shore, surface sediments were collected using an EkmanBerge type sampler (RIGO Co., Ltd. Tokyo, Japan) because the sediment was sandy and difficult to grasp with the weight-type core sampler (Fig. 1). The top 15 cm of the cylindrical sediment core sampled in 2012 was analyzed for 137Cs concentration. However, in some parts of the lake, 137Cs was found to have migrated deeper than 15 cm. Therefore, in 2013 and 2014, the sampling depth was extended to 25 cm and the sampled cores were divided into the top 15 cm and the underlying 10 cm. Each sediment sample was homogenized onboard with a steel spatula in a stainless-steel vat. Homogeneity of the sediment sample from the mixing operation was confirmed based on analysis of the water content of three aliquots of each sample, the difference of which was < 0.1%.

137

The 137Cs storage in the sediment of Lake Nishiura based on the observed data was compared with the amount of 137Cs inflow from the upstream watershed in 2012–2014. The 137Cs storage in the lake immediately before the FDNPP accident was evaluated based on the assumption that the 137Cs inventory in the lake just before the 137Cs fallout in 2011 was homogeneous and equal to that at the center of the lake in 2007 measured by Fukushima et al. (2010). According to the Japan Radioactivity Database of the Analytical Center, the 137Cs concentration in the lake water did not increase significantly from 1967 to 2011. Therefore, 137Cs in the lake in 2011 just before the 137Cs fallout was thought to have originated only from the global fallout in 1963. The total 137Cs storage in the lake immediately after the initial 137Cs fallout from the FDNPP was evaluated based on an estimated amount of 137 Cs fallout simulated by Morino et al. (2013). In 2012, the amount of 137 Cs in the top 15 cm layer in the sediment was considered equal to the total 137Cs storage in the lake sediment, including underestimation of 137 Cs stored at depths greater than 15 cm, or with uncertainty associated with considering that the 137Cs in grab-sampled sediment corresponded to that at 5 cm. Similarly, the total 137Cs storage in 2013 and 2014 was considered to be equal to the total 137Cs in the top 25 cm. Particulate and dissolved 137Cs in the lake water was calculated by multiplying the water volume and the average 137Cs concentrations measured at St. 3, 7, and 9. The inflow of 137Cs by riverine transport was calculated by multiplying the 137Cs runoff ratio, which was the average value of the runoff ratio in a year for the five inflowing rivers, by the total 137Cs amount in the Lake Nishiura watershed (3.6 × 1013 Bq). Because no remarkable additional atmospheric 137Cs fallout has been reported since the FDNPP accident, the increase in 137Cs storage in the lake was completely derived from the surrounding watershed, mainly via river transport.

2.4. Analysis of radioactivity The 134Cs and 137Cs concentrations in the SS, RAD disks and lake sediments were measured based on a dataset of coaxial high-purity germanium detectors of GC2518 and GCW7023 (Mirion Technologies Canberra Japan Co. Ltd., Tokyo, Japan) using the Spectrum Explorer analysis software (Canberra Japan Co. Ltd., Tokyo, Japan), and a set of detectors (GWL-450-15-S, LOAX-70550/30, GEM65P4-83 and GMX45P4-76; Seiko EG&G Co. Ltd., Tokyo, Japan) using the Gamma Studio software (Seiko EG&G Co. Ltd., Tokyo, Japan). Sediment samples were input to the detector in a wet state, and the 137Cs concentration per unit weight of sediment was calculated using a gamma ray absorption correction assuming homogeneous mixing of water and soil, after which the detected 137Cs concentration per weight was divided by the water content. The instruments were calibrated against MX033U8PP (The Japan Radioisotope Association, Tokyo, Japan) as the standard volume radioactivity sources. The radioactivity of each sample was then decay-corrected to the date of the FDNPP accident on March 11, 2011. The uncertainty between measuring instruments was ∼10%, which was confirmed by proficiency testing conducted by the International Atomic Energy Agency (IAEA-TEL-2012-03). The reproducibility of the radioactivity measurements was ∼0.5% and the total uncertainty associated with the radioactivity analysis was ∼10%. The 134Cs and 137Cs derived from the FDNPP accident (March 2011) was identified when the ratio of 134Cs–137Cs corrected to the date of the accident was near 1.0 because almost equal amounts of 134Cs and 137Cs were generated in the nuclear fission reaction (Nishihara et al., 2012).

2.2.2.2. Evaluation of the spatial distribution of 137Cs inventory. The amount of 137Cs radioactivity in the sediment per unit area was calculated as an inventory (Bq/m2). For the cylindrical sediment collected at 52 points, the 137Cs inventory for depths of 0–15 cm, 15–25 cm, and 0–25 cm was calculated. For the grab-sampled sediment collected at 16 points, the 137Cs inventory was calculated assuming that the sample was collected from the corresponding top 5 cm layer. Using these datapoints, a plane distribution map with a 250 m grid of 137Cs inventory was drawn for each year from 20122014. The 137Cs inventory map, which had higher resolution than the actual sampling points, was drawn by quadratic spline interpolation between sampling points or extrapolation from the points to the edge of the lake using the geographic information system software ArcGIS ver. 10.0 (Esri Co., Ltd., Redlands, CA, USA). In addition, we calculated the total amount of 137 Cs in the top 25 cm of Lake Nishiura sediment by integrating the 137 Cs inventory of the 250 m mesh resolution. Similarly, we created plane distribution maps of the 137Cs inventory with depths of 0–15 cm and 15–25 cm, then calculated the total amount of 137Cs in each layer calculated. The 137Cs inventory at points sampled using the EkmanBerge sampler was included in the 0–15 cm layer, and that in the 15–25 cm layer was regarded as 0 Bq/m2.

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0.23 0.75 2.5 4.8 1.1 0.87 1.2

Cs concentration in SS and

137

Cs flux of

12 20 39 140 26 21 18

0.15% 0.077% 0.11% 0.24% 0.13% 0.099% 0.064%

The decay-corrected 134Cs/137Cs ratio was close to 1.0, which confirmed that most 137Cs in the SS of the river water was derived from the FDNPP accident (Supplemental table C). The specific discharge of SS from the seven watersheds in the first and second year from March 2011 was 1.6–5.4 × 10−2 kg/m2 (Table 2), while the specific discharge of 137Cs was 4.5–140 Bq/m2. The discharge was particularly large in the Seimei River, where 137Cs was deposited in high amounts. However, the runoff ratio of 137Cs in the first and second year from March 2011 was only 0.053–0.28% for all of the rivers, and the average 137Cs runoff ratio of the five rivers flowing into Lake Nishiura was 0.15% in the first and second year. These values were similar to those reported in another watershed; specifically, 0.02% per year in 10 years after the Chernobyl accident (Strebl et al., 1999), 1.13% per 274 days within the period of 2011–2012 (Yamashiki et al., 2014), and 0.072% per year during 2014–2015 (Tsuji et al., 2016). The 137Cs concentration in SS was < 8 kBq/kg, and values exceeding 2 kBq/kg were only observed in the Seimei and Ono rivers, where 137Cs deposition was relatively high (Fig. 2). The 137Cs concentration of SS in some rivers decreased remarkably as the SS concentration increased. The exponential value in the approximate expression of 137Cs concentration in the SS to SS concentration was negative in all rivers, while significant values (p value < 0.05) appeared in the Ono, Sakura, and Sonobe rivers (Table 1). Based on the average value of the 137Cs concentration in the SS of each river, a strong correlation was found between the 137Cs concentration in the SS and the average amount of 137Cs inventory in the watershed (Fig. 3). A similar significant positive relationship was reported in the nearby watershed of Lake Nishiura between the average amount of 134+137Cs deposited in the upstream watershed and 134+137Cs in riverbed sediments (Tabayashi and Yamamuro, 2013). When the 137Cs concentration in SS was normalized by dividing it by the average 137Cs inventory in the watershed (entrainment coefficient of 137Cs [m2/kg]), a significant positive correlation was found for the ratio of the building area, with a correlation coefficient (r) of 0.86 (Fig. 4). Correlation coefficients for other major land uses were r = −0.17 for paddy fields, −0.11 for agricultural land, and −0.51 for forests. The specific surface area of SS [m2/kg] was negatively correlated with the entrainment coefficient of 137 Cs [m2/kg] (r = −0.41, p = 0.04, Supplemental Figure A). This result contradicted the knowledge that 137Cs concentrations in SS [m2/ kg] are high in fine particles because they have a large specific surface area to capture 137Cs (He and Walling, 1996).

4.5 4.0 6.7 3.1 2.1 2.5 0.98 19.2 5.9 2.5 0.77 2.0 3.2 0.85 310 98 65 12 55 100 42 ##### 0.11% 0.12% 0.26% 0.18% 0.10% ##### 5.1 24 46 130 35 18 23 19 36 16 28 33 23 20

Specific discharge of SS( × 10−3 kg/m2)

Fig. 3. Relationship between average 137Cs deposition in river basins after the FDNPP accident and concentration of 137Cs in suspended solids in water of rivers flowing into Kasumigaura.

6.9 8.1 2.6 711 0.71 3.0 1.1 140 160 77 16 70 110 45 Sakura Koise Ono Seimei Sonobe Tomoe Hokota

Discharge of SS ( × 106 kg) Flow volume ( × 106 m3)

Discharge of 137 Cs ( × 109 Bq)

137

1.8 5.5 7.3 3.3 2.8 2.4 1.3

Discharge of 137 Cs ( × 109 Bq) Discharge of SS ( × 106 kg) Flow volume ( × 106 m3)

Mar. 12, 2012–Mar. 11, 2013

Discharge ratio of 137 Cs Specific discharge of 137 Cs (Bq/ m2)

Cs from river basins.

Mar. 12, 2011–Mar. 11, 2012 River

137

Table 2 Discharge ratio of

3.1. Characteristics of the inflowing rivers

54 26 16 30 25 24 16

Specific discharge of SS( × 10−3 kg/m2)

Specific discharge of 137 Cs (Bq/ m2)

Discharge ratio of 137 Cs

Averaged 137Cs concentration in SS (kBq/kg)

3. Results

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concentrations were larger than the 137Cs concentrations of 130–560 Bq/kg in the SS of the Sakura River, which has a mouth near St. 7 (Fig. 1). The 137Cs inventory increased from September 2012 to December 2012, after which it decreased. At St. 9, the 137Cs concentration in the top 11 kg/m2 in July 2012 showed a peak value of 530 Bq/kg. A very similar 137Cs profile was reported for sediment cores collected from the same point during the same period by Arai et al. (2017). A remarkable increase in the peak 137Cs concentration and 137Cs inventory was observed from July 2012 to October 2012, after which the 137 Cs vertical profile was almost completely unchanged, and most 137Cs was stored in the top 15 cm. The maximum 137Cs concentration of the sediment in 2007 of ∼20 Bq/kg at 60 kg/m2 (Fukushima et al., 2010) did not appear in the 137Cs profile in July of 2012. For the 134Cs/137Cs profile decay-corrected to March 2011 in the sediment (Fig. 5b), values of the ratio at depths that contained > 20 Bq/kg of 137Cs were close to 1.0. Assuming that 137Cs concentrations in the sediment before the FDNPP accident were ∼20 Bq/kg and those in the sediment deposited by the 137Cs fallout in 2011 were 800 Bq/kg, a 1:1 mixture of the two types of sediments gives a 134Cs/137Cs value of 0.98. When estimating the ratio of 137Cs derived from the FDNPP accident by the 134Cs/137Cs profile (Supplemental Figure C), mixing of the two types of sediment was more active in the order of St. 3, 7 and 9. In particular, the two types of sediment were mixed almost at 1:1 at St. 3 after November 2012. These results indicate that movement of sediment by vertical mixing was very active at up to 60 kg/m2 when compared with new sedimentation.

Fig. 4. Relationship of ratio of building area in river basins and normalized 137 Cs concentration in suspended solids.

3.2.

137

Cs concentration in inflowing rivers

The particulate and dissolved 137Cs concentrations in the Seimei River were 34 and 56 Bq/m3 on 24 September 2012, while they were 14 and 69 Bq/m3 on 19 August 2013 (Table 3). The apparent distribution coefficient value (Kd) was 2.0–3.2 × 104 L/kg. This value was nearly the same as that of the outlet river of Lake Nishiura of 1.0–6.1 × 104 L/ kg that was measured in 1985–1987 (Hirose and Aoyama, 1990). It was also one order of magnitude less than 105 L/kg reported in the river near the FDNPP (Ueda et al., 2013; Ochiai et al., 2015; Yoshimura et al., 2015; Eyrolle-Boyer et al., 2016). 3.3. Vertical distribution of

137

3.4. Spatial distribution of

137

Cs concentration in river and lake water; ± indicates counting error. Sampled date

Seimei river Lake Nishiura

St.3 St.7 St.9

Cs in Lake Nishiura sediment

The spatial distribution of 137Cs in the top 15 cm in 2012 and the top 25 cm in 2013–2014 was almost the same as that observed for the initial deposition of 137Cs in the surrounding area (Fig. 6a). The 137Cs storage in the top 15 cm and 25 cm of sediment in Lake Nishiura was calculated to be 3.1–3.5 × 1012 Bq in 2012–2014, which corresponds to an average 137Cs inventory of 18–20 kBq/m2. When calculating the variation of 137Cs inventory in the top 25 cm from 2013 to 2014 (Fig. 6b), the 137Cs inventory decreased mainly in the western and northern areas near the river mouth of the Sakura, Koise, and Sonobe rivers. In contrast, the 137Cs inventory increased remarkably near the river mouth of the Seimei and Ono rivers. In the region farthest from the mouth of the inflowing rivers, the area in which the 137Cs inventory increased was located on the downstream side; therefore, the sediment gradually moved from the north and west to the east with the flow direction of water in the lake, but at a much slower rate than lake water, with an average residence time of ∼200 days. The average 137Cs inventory of 0–15 cm decreased from 16 to 14 kBq/m2 in 2013–2014, while that of 15–25 cm increased from 4.2 to 5.9 kBq/m2 (Fig. 7). Comparison of these values indicated that 137Cs migrated downward or newly accumulated over the entire area of Lake Nishiura.

Cs in Lake Nishiura sediment

The mass corresponding to a depth of 15 cm was 29 ± 5 kg/m2 at St. 3, 58 ± 5 kg/m2 at St. 7, and 24 ± 4 kg/m2 at St. 9 (Fig. 5a). The mass corresponding to a depth of 25 cm was 52 ± 8 kg/m2 at St. 3, 107 ± 4 kg/m2 at St. 7, and 45 ± 6 kg/m2 at St. 9. Based on these findings, the lake sediment was more densely accumulated around St. 7 than the other two stations, which was confirmed by the sediment bulk density at St. 7 increasing with depth (Supplemental Figure B); however, the reason for such variations in density were unclear. At St. 3, the surface peak 137Cs concentration of 780 Bq/kg in August 2012 decreased to 380 Bq/kg and diffused up to 60 kg/m2 in November 2012, after which the vertical 137Cs profile became nearly stable. These peak 137Cs concentrations were lower than the 137Cs concentration in the SS of the Sonobe River (770–1,300 Bq/kg), which formed a river mouth near St. 3 (Fig. 1), while they were at the same level or lower than those of the Koise River (350–1,860 Bq/kg), which formed a river mouth upstream of St. 3. The 137Cs inventory increased from 8.8 kBq/m2 in August 2012 to 22 kBq/m2 in November 2012, after which it did not increase remarkably. At St. 7, the peak 137Cs concentration of 900 Bq/kg in September 2012 gradually decreased to < 500 Bq/kg in September of 2013. Moreover, the peak 137Cs Table 3 Particulate and dissolved

137

Sep. 24, 2012 Aug. 19, 2013 Sep. 11, 2012 Aug. 6, 2013 Sep. 11, 2012 Aug. 6, 2013 Sep. 11, 2012 Aug. 6, 2013

137

Cs concentration (Bq/m3)

Particulate

Dissolved

36 ± 2 15 ± 1 17 ± 2 13 ± 1 26 ± 4 17 ± 1 13 ± 2 16 ± 1

58 ± 4 73 ± 4 36 ± 4 20 ± 2 48 ± 3 33 ± 1 55 ± 4 19 ± 1

64

137

Cs in SS (kBq/kg)

1900 1500 740 350 920 680 860 660

Apparent Kd (m3/kg)

33 21 20 18 19 20 16 34

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Fig. 5. (a) Vertical distribution of 137Cs concentration in sediment. Horizontal dashed line indicates average mass depth corresponding to a depth of 15 cm, and dashdot line indicates a depth of 25 cm. Total 137Cs inventory is shown in parentheses. (b) Vertical distribution of ratio of 134Cs–137Cs in sediment. The ratio was corrected as of March 11 in 2011.

3.5.

137

2.9 × 1012 Bq by Morino et al. (2013). In 2012, the total amount of 137 Cs in the sediment was estimated to be 3.1 × 1012 Bq, while in 2013 and 2014 it was estimated to be 3.5 × 1012 Bq (Fig. 6a). The dissolved and particulate form of 137Cs in the lake water was calculated to be 1.0 × 1010 Bq and 3.2 × 1010 Bq in 2012, while it was found to be 1.1 × 1010 Bq and 1.7 × 1010 Bq in 2013. These results were calculated by multiplying the average dissolved and particulate 137 Cs concentration of St. 3, 7, and 9 by the average water volume in Lake Nishiura (4.0 m mean water depth and 172 km2 area). The inflow of 137Cs by riverine transport was calculated to be 4.8 × 1010 Bq in 2012 and 5.0 × 1010 Bq in 2013 by multiplying the average value of the runoff ratio in a year for the five inflowing rivers of 0.15% by the total 137 Cs amount in the Lake Nishiura watershed (3.6 × 1013 Bq). The estimated values of 137Cs inflow from rivers were two orders of magnitude less than the 137Cs storage in the sediment. Accordingly, 137Cs directly deposited in the lake was still dominant relative to 137Cs flowing from rivers at 2–3 years after the initial fallout in 2011. We

Cs concentration in water of Lake Nishiura

The dissolved 137Cs concentration in lake water was higher than that in particulate form, and the dissolved 137Cs concentrations decreased at all three stations from 2012 to 2013 (Table 3). The apparent Kd value was 2.0–3.2 × 104 L/kg, which was nearly the same as in the Seimei River and one-order of magnitude smaller than that reported in the lake near the FDNPP (Tsukada et al., 2017; Wakiyama et al., 2017). 3.6.

137

Cs storage in Lake Nishiura and inflow amount of

137

Cs

The 137Cs storage in the lake immediately before the FDNPP accident was estimated to be 2.1 × 1011 Bq (Table 4) by multiplying the lake area of 172 km2 and the 137Cs inventory at the center of the lake in 2007 of 1.2 kBq/m2 (Fukushima et al., 2010). The total 137Cs storage in the lake immediately after the initial 137Cs fallout from the FDNPP was ∼3.1 × 1012 Bq based on an estimated amount of 137Cs fallout of 65

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Fig. 6. (a) 137Cs distribution and total storage in Lake Nishiura. (b) Increase in 137Cs inventory in sediment of Lake Nishiura from 2013 to 2014. Arrows indicate inlets of five major inflowing rivers and the outlet.

dust to topsoil of cultivated land as flow rate in the river increased. In addition, Yamashita et al. (2015) showed that 137Cs concentration of road dust was larger than that of river sediment in an urban river, and that road dust remained the major source of SS even 2 years after the FDNPP accident. Such components of SS are unique to urban rivers and known to have high mobility. Moreover, a more rapid decline in the air dose rate of 137Cs in building areas than forested watersheds was observed in Fukushima Prefecture (Andoh et al., 2015). These reports suggest that the main SS component in the Ono and Seimei river was road dust with relatively high levels of 137Cs, and the contribution of eroded topsoil with relatively low levels of 137Cs from cultivated and forested areas increased when the SS concentration became high. Moreover, water samples with low SS concentrations were mainly collected in 2012; therefore, the observed results might have been influenced by time decay of 137Cs (Supplemental Figure D). However, this tendency was not clear because of the limited number of samples.

could not evaluate the amount of 137Cs outflow because outflow water was not sampled, but considering the very slow movement of 137Cs in the sediment (Fig. 6b), 137Cs outflow would be much smaller than 137Cs storage in the lake.

4. Discussion 4.1. Characteristics of

137

Cs in suspended solids of urban river water

We found a significant decrease in the 137Cs concentration of SS in the Ono and Seimei rivers against the SS-concentrations (Fig. 2), a high 137 Cs runoff ratio of SS from the Seimei River (Table 2), and a strong correlation between the entrainment factor of 137Cs in SS and the ratio of the building area (Fig. 4). These results would exhibit the characteristics of SS in urban rivers. Murakami et al. (2016) reported a decreasing trend of apparent Kd value vs. SS concentration in an urban river and suggested that such a trend was caused by an increase in the proportion of SS with large particle size with increasing SS concentration because the specific surface area of SS has a positive correlation with the entrainment coefficient of 137Cs in SS [m2/kg] if the composition of SS is homogeneous (He and Walling, 1996; Yoshimura et al., 2015). However, in the present study, the median particle size measured by laser diffraction analysis showed the negative correlation with the entrainment coefficient of 137Cs in SS. Accordingly, the composition of SS may have been different for each sample. Carter et al. (2003) showed that the main source of SS in urban rivers shifted from road

4.2. Distribution coefficient value (Kd) of

137

Cs in river and lake water

An apparent Kd value of 104 L/kg was observed in river and lake water, which was smaller than the value of ∼105 L/kg reported in freshwater near the FDNPP. One of the main factors regulating the Kd value was reported to be the geology of bedrock (Yoshimura et al., 2015). The geology around Lake Nishiura is primarily volcanic ash soils (National Institute of Advanced Science and Technology), and the substrate of that soil is composed of allophane, which adsorbs 137Cs at 66

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more likely to occur in Lake Nishiura than in similarly contaminated water environments in Fukushima Prefecture. 4.3. Movement of

Table 4 Changes in 137Cs storage and 137Cs inflow amount in Lake Nishiura. All amounts are corrected to March 2011. Unit: × 1012 Bq 137 Cs inflow from the watershed

Cs storage

Dec. 2012 Sep. 2013 Oct. 2014

Sediment

Dissolved form

Particulate form

3.1 3.5 3.5

0.032 0.017

0.010 0.011

Cs in sediment

4.3.1. Sediment movement at the three representative points At the northern inlet of the lake, the maximum 137Cs concentration decreased from August through November 2012 at St. 3 (Fig. 5a), and the layer that included 137Cs derived from the FDNPP accident reached a depth of more than 25 cm (Fig. 5b). Together with the increase of 137 Cs inventory in this period, simultaneous vertical mixing and accumulation of sediment would be active in this area. The newly accumulated 137Cs in this period was probably mainly transported from the Sonobe River or movement of sediments in the upstream northern bay; however, turbidity recorded in the main inflowing rivers did not reveal any remarkable runoff events in this period that brought disturbance of the sediment. The vertical 137Cs profile and 137Cs inventory were almost unchanged after November 2012, indicating that sediment was difficult to resuspend at this point. At the western inlet, the peak 137Cs concentration gradually decreased together with the 137Cs inventory from December 2012 at St. 7. Because the peak 137Cs concentration in the sediment was higher than that of SS in the Sakura River, the remarkable decrease in 137Cs inventory around St. 7 (Fig. 6b) indicates that, although the direct 137Cs deposition of the fallout was prominent immediately after the accident at this station, the sediments then flowed downstream and were replaced by sediments transported from the Sakura River. The main factors influencing vertical mixing of sediments at St. 7 were probably wind-wave disturbances because of the shallow water depth (Seki et al., 2006; Tsushima et al., 2016), and bioturbation by benthic worms or fish because of the nutrition-rich and benthos-abundant environment (Tarasiuk et al., 2009; Zhang et al., 2010). At this area, 137Cs that accumulated after the FDNPP fallout was dominant below the 15 cm layer according to the 134Cs/137Cs ratio (Fig. 5b), despite the low concentration of 137Cs as before the FDNPP accident. Although the cause of this result was not clear, one probable factor was dredging work conducted on the west side of the lake as a eutrophication countermeasure during 1975–2010, which removed most of the 137Cs at St. 7 immediately before the 137Cs fallout. In addition, the 137Cs concentration at St. 7 decreased vertically in the downward direction, which differed from the profile at St. 3. This was probably because the particle size in the deep layer was larger because the bulk density profile in the sediment increased in the downward direction (Supplemental Figure A). Such a slope of the bulk density profile likely indicates that large particles with less 137Cs in SS precipitated faster than small particles during the process of resuspension according to Stokes' law. In the central part of the lake, vertical mixing of sediment was not active in from October 2012 to October 2013 (Fig. 5b) at St. 9. One of the main reasons for this stability was that this area was the deepest portion of the lake (7.3 m at St. 9), so disturbance by surface wind waves on the lake bottom was weak. Biological disturbances were also weak because the euphotic zone would not reach the lake bottom. However, downward migration or new accumulation of 137Cs was observed from September 2013 to October 2014 (Fig. 7). Fukushima et al. (2018) also reported that the vertical profile of 137Cs in sediment at St. 9 was unchanged before November 2013, but afterward the depth including 137Cs at 500–600 Bq/kg reached more than 15 cm as the 137Cs inventory increased. Therefore, accumulation of resuspended sediment derived from the upstream area can sometimes change the vertical 137 Cs profile in this area.

Fig. 7. Distribution of 137Cs inventory in sediment of 0–15 cm and 15–25 cm in Lake Nishiura in 2013 and 2014.

137

137

0.048 0.050

one order of magnitude less than other minerals such as kaolinite and montmorillonite based on the of Kd value (Tamura et al., 2013). Therefore, the dissolved 137Cs concentration in natural water of this area was one order of magnitude higher than that of rivers or lakes in Fukushima prefecture at the same period in which 137Cs was deposited at the same level (Sato et al., 2013; Tsuji et al., 2014; Ochiai et al., 2015). As another considerable factor, the suspended solids in the Lake Nishiura watershed may contain less spherical water-insoluble Csbearing particles than watersheds near the FDNPP. These particles are believed to have been emitted during a relatively early stage after the FDNPP accident and fallen to the ground mainly by dry deposition (Adachi et al., 2013). Therefore, few of these particles likely reached our survey area, which was 150–170 km from the FDNPP. The low Kd value of 137Cs indicates a higher proportion of bioavailable 137Cs; therefore, we have to consider the risk that bioconcentration of 137Cs is

4.3.2. Sediment movement throughout the lake Evaluation of the entire area of Lake Nishiura revealed that slow movement of sediment including 137Cs downstream was observed (Fig. 6b). The active resuspension and mixing of sediments in the western and northern shallow area (Fig. 5a and b) decreased the 137Cs 67

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inventory, while in the relatively deep area around St. 9, newly accumulated suspended solids in the lake water increased the 137Cs inventory. However, the increase or decrease of 137Cs inventory from 2012 to 2014 was sufficiently smaller than the 137Cs inventory in 2012 in most of the area. Therefore, when compared with the horizontal movement of sediment, the initial 137Cs deposition in March 2011 had a much greater effect on the spatial 137Cs inventory distribution in the lake, even 3 years after the FDNPP accident. A remarkable increase in 137Cs inventory was only observed near the mouths of the urban rivers (Seimei and Ono), which suggests the accumulation of sediments generated in urban areas with high concentrations of 137Cs and high mobility. 137Cs accumulation in this area would continue for tens of years because of the small 137Cs runoff ratio in the watershed of inflowing rivers (Table 2); therefore, it is necessary to pay attention to whether the organisms inhabiting this area take up 137 Cs. As a future research topic, it is important to investigate whether benthic fish inhabiting the river mouths of urban rivers accumulate 137 Cs faster than those living in other areas in the lake. Furthermore, since we only evaluated the distribution of 137Cs across the lake at 1 year and 9 months after the initial 137Cs fallout in 2011, the 137Cs distribution in the lake immediately after the accident should be evaluated by numerical simulation using a method such as the migration–accumulation model of suspended solids. Such studies will facilitate assessment of the risk posed to lakes in the early phase of future nuclear disasters.

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5. Conclusions By investigating the spatial and vertical distribution of 137Cs in the sediment of Lake Nishiura, the following was determined. In most of the lake, 137Cs in the sediment migrated vertically downward via mixing and new accumulation, but the 137Cs inventory was almost unchanged since the fallout of 137Cs following the FDNPP accident in 2011. In the horizontal direction, gradual movement of sediment downstream was observed, but at a much slower rate than the movement of water. Moreover, the 137Cs inventory was only likely to increase near the river mouths of urban rivers because of the high 137Cs concentrations and high mobility of suspended solids in these areas. The results presented herein will facilitate removal of the restrictions on exports of fish from Lake Nishiura. Acknowledgements We appreciate the Kasumigaura River Office for providing hydrologic data. We thank Drs. Nohara, Ueno and Satake and Ms. Nakagawa for help with sampling and analysis. We also thank Steven Hunter, M.S. and J. Kamen M.Sc. from Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apradiso.2019.02.009. References Abraham, J.P., Whicker, F.W., Hinton, T.G., Rowan, D.J., 2000. Inventory and spatial pattern of 137Cs in a pond: a comparison of two survey methods. J. Environ. Radioact. 51, 157–171. Adachi, K., Kajino, M., Zaizen, Y., Igarashi, Y., 2013. Emission of spherical cesiumbearing particles from an early stage of the Fukushima nuclear accident. Sci. Rep. 3, 2554. Andoh, M., Nakahara, Y., Tsuda, S., Yoshida, T., Matsuda, N., Takahashi, F., Mikami, S., Kinouchi, N., Sato, T., Tanigaki, M., Takamiya, K., Sato, N., Okumura, R., Uchihori, Y., Saito, K., 2015. Measurement of air dose rates over a wide area around the Fukushima Dai-ichi Nuclear Power Plant through a series of car-borne surveys. J. Environ. Radioact. 139, 266–280.

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