- Email: [email protected]
Effects of radiocesium inventory on 137Cs concentrations in river waters of Fukushima, Japan, under base-flow conditions
Journal of Environmental Radioactivity 144 (2015) 86e95
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Effects of radiocesium inventory on 137Cs concentrations in river waters of Fukushima, Japan, under base-flow conditions Shinya Ochiai a, *, Shinji Ueda a, Hidenao Hasegawa a, Hideki Kakiuchi a, Naofumi Akata a, b, Yoshihito Ohtsuka a, Shun'ichi Hisamatsu a a b
Department of Radioecology, Institute for Environmental Sciences, 1-7 Ienomae, Obuchi, Rokkasho, Kamikita, Aomori 039-3212, Japan National Institute for Fusion Science, 322-6 Oroshi-cho, Toki City, Gifu 509-5292, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 December 2014 Received in revised form 2 March 2015 Accepted 5 March 2015 Available online
To investigate the behavior of nuclear accident-derived 137Cs in river water under base-flow conditions, concentrations of dissolved and particulate 137Cs were measured at 16 sampling points in seven rivers of Fukushima Prefecture, Japan, in 2012 and 2013. The concentration of dissolved 137Cs was significantly correlated with the mean 137Cs inventory in the catchment area above each sampling point in both sampling years. These results suggest that the concentration of dissolved 137Cs under base-flow conditions is primarily determined by the 137Cs inventory of the catchment area above the sampling point. However, the concentration of particulate 137Cs did not show a clear relationship with either the mean 137 Cs inventory or the dissolved 137Cs concentration, thus indicating that particulate and dissolved forms do not effectively interact in rivers. To evaluate the contribution of the 137Cs inventory within catchment areas, we analyzed relations between the 137Cs concentration and the mean 137Cs inventory over the area within certain flow path lengths that were traced along the river and slope above the sampling point. Coefficients of determination for dissolved 137Cs concentrations were highest for the longest flow path, i.e., the whole catchment area, and lower for shorter flow paths. Coefficients of determination for particulate 137Cs concentrations were only moderately high for the shortest flow path in 2012, whereas the values were quite low for all flow paths in 2013. These results suggest that dissolved 137Cs can originate from a larger area of the catchment even under base-flow conditions; however, particulate 137Cs did not show such behavior. The results also show that under base-flow conditions, dissolved and particulate 137 Cs behave independently during their transport from river catchments to the ocean. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Fukushima Dai-ichi Nuclear Power Plant accident Radiocesium River water Base-flow Catchment inventory
1. Introduction After being damaged by the tsunami that followed the giant Tohoku earthquake of 11 March 2011, the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) of Tokyo Electric Power Company released a large amount of radiocesium (134Cs and 137Cs) into the surrounding environment (Nuclear Emergency Response Headquarters, 2011). The airborne radiocesium caused contamination over a broad area in northern Japan that was especially heavy in northeastern Fukushima Prefecture, where deposition of 137 Cs on the ground was estimated to exceed 3000 kBq m2 in the most contaminated areas (Ministry of Education, Culture, Sports,
* Corresponding author. Tel.: þ81 175 71 1426; fax: þ81 175 71 1492. E-mail address: [email protected] (S. Ochiai). http://dx.doi.org/10.1016/j.jenvrad.2015.03.005 0265-931X/© 2015 Elsevier Ltd. All rights reserved.
Science and Technology (MEXT), 2012, 2013; Mikami et al., 2015; Saito et al., 2015). Runoff carries 137Cs deposited on the ground into rivers; thus, the discharge rates of 137Cs from catchments to rivers and from rivers to downstream regions are important for evaluating the impacts of radiocesium on human health in riverine and coastal marine ecosystems. Long-term changes in the discharge rates of radiocesium from river catchments were studied for global fallout and the Chernobyl accident (e.g., Helton et al., 1985; Monte, 1995; Garcia-Sanchez, 2008). These studies have shown that the seasonal- and annual-scale discharge rate depends on the deposited amount and a transfer function, which expresses the temporal change in the discharge fraction of the deposited amount after deposition. Based on these models, long-term changes in discharge have been evaluated at the global scale (e.g., Smith et al., 2004).
S. Ochiai et al. / Journal of Environmental Radioactivity 144 (2015) 86e95
In Fukushima rivers, previous studies have investigated the relationship between observed fluvial discharge data and radiocesium deposition in the catchment to clarify the dominant factors controlling the radiocesium discharge from the catchment (Tsuji et al., 2014; Yoshimura et al., 2015). These studies have demonstrated that 137Cs concentrations are related to the 137Cs inventories in the catchments; hence, it is useful to estimate the initial discharge from the catchment. However, these relationships may be affected by hydrological fluctuations, especially those on shorter timescales, because the mobility of radiocesium depends on what forms (i.e., particulate and dissolved forms) are being transported and these forms are impacted differently by changes in discharge. For the annual timescale, Yamashiki et al. (2014) estimated that 84e92% of the total radiocesium discharge was transported in particulate form in the Abukuma River, which is the largest river in the Fukushima area. However, observations of shorter-term fluctuations show that particulate 137Cs largely fluctuates in accordance with the hydrological conditions. Ueda et al. (2013) determined that the percentage of particulate 137Cs fluctuated from 40% during base-flow conditions to over 90% during high-flow conditions in two small contaminated catchments. Similar observation results have been reported in southeastern Fukushima rivers, where the particulate percentages were 21eca.100% (Nagao et al., 2013), and in the Abukuma River and its tributaries, where the particulate percentages were 51e99% (Sakaguchi et al., 2015). Based on these results, the transport behaviors are expected to be different between particulate and dissolved 137Cs in the region. Therefore, it would be valuable to clarify the influences of such forms on the relationship between 137Cs concentration and catchment deposition for proper estimation of 137Cs discharge from the catchments. With this purpose in mind, we focused our efforts on fluvial transport of 137Cs under base-flow conditions, which is when the differences in behaviors of dissolved and particulate 137Cs are expected to be the most clear. Specifically, this study investigated the distribution of 137Cs in river water in eastern Fukushima during 2012e2013 and determined its relation to the catchment inventory based on a simple model and geographic analyses. From these data, we then attempted to clarify the transport processes of 137Cs under base-flow conditions. 2. Samples and methods This study targeted the main streams and tributaries of seven river systems (Uta, Mano, Niida, Ohta, Odaka, Ukedo, and Abukuma rivers) in eastern Fukushima Prefecture (Fig. 1), which received large amounts of radiocesium from the FDNPP accident. The catchment of the Abukuma River (5400 km2) is separated from the other six by the Abukuma Mountains. River water samples were collected at 16 sampling points during JuneeNovember 2012 and at 13 points during JuneeAugust 2013, when streamflows were at base level (Tables 1 and 2). Samples measuring 15e20 L were collected with a polyvinyl chloride bucket from 1 to 5 days after the latest rainfall (>0.5 mm/h). At the time of sample collection, water temperature, electrical conductivity, and pH were measured at the site with a water quality meter (DS4a, Hydrolab Co., USA). Water level data of Abukuma and Niida rivers during sampling intervals were provided by the Ministry of Land, Infrastructure, Transport and Tourism, Japan (MLIT) (2014a) and Fukushima Prefecture, respectively. Precipitation data observed in Fukushima City was provided by Japan Meteorological Agency. After the water samples were taken to the laboratory, subsamples of 1e4 L were filtered through membrane filters (0.45 mm pore size, Millipore) before the dissolved radiocesium measurement. Total radiocesium (dissolved and particulate) activities were measured in unfiltered samples. Concentrations of suspended
87
solids were obtained by weighing the residue on the filters after drying for more than 24 h at room temperature in a container containing desiccants (OZO-Z, OZO Kagakugiken Co., Japan). Each water sample, whether unfiltered or filtered, was passed through a column filled with a mixture of Powdex PAO and PCH resins (Ecodyn Co., USA) to separate both ionic and particulate radionuclides from the sample (Small et al., 1975; Kimura, 1980; Yamamoto et al., 1998). The homogenized resin was oven-dried at 80 C for more than 48 h and then packed into a plastic case (60 mm in diameter and 37 mm high). The activity concentrations of 134Cs (T1/2 ¼ 2.06 y) and 137Cs (T1/2 ¼ 30.17 y) were determined by a high-purity Ge detector (relative efficiency of 59%; Seiko EG&G, Japan). The particulate radiocesium concentration in water samples was estimated from the difference between the filtered and unfiltered samples. Geographic analyses of river catchments and radiocesium distributions were performed using QGIS 2.2.0 software and Fundamental Geospatial Data (Geospatial Information Authority of Japan, 2014) and National Land Numerical Information (MLIT, 2014b). Catchment areas upstream from each sampling point were delineated on the basis of digital elevation models (DEM) (250-m mesh for the Abukuma River and 10-m mesh for the other rivers), and their areas were determined from boundaries that were defined on the basis of inclinations between the map meshes. The 137Cs deposition data were based on airborne surveys by MEXT (2012, 2013) and Nuclear Regulation Authority (2013). Mean radiocesium inventories were calculated for each catchment from the catchment areas and the 137Cs deposition data. 3. Results and discussion 3.1. Concentration of
137
Cs in river water
Fig. 2 shows the changes in water levels of the Abukuma River (Kuroiwa observatory) and Niida River (Haramachi observatory) and hourly precipitation at Fukushima City for 10 days before each sampling date. The locations of the observatories are indicated in Fig. 1. Although high-flow events were observed during these intervals, especially in June 2012 just before the samples were collected, the samples used here were collected after the water level decreased to the base level. Therefore, hydrological conditions were similar among the sampling dates. Tables 3 and 4 list the 134Cs and 137Cs concentrations (dissolved and particulate) in river water in 2012 and 2013, respectively. The activity ratio 134Cs/137Cs of both dissolved and particulate forms, corrected to the value on the day of the FDNPP accident on 11 March 2011, was approximately 1.0, which indicates that most of the radiocesium in the river water was derived from the FDNPP accident rather than from global fallout. Therefore, we focus only on the 137Cs concentrations in the following discussion. Concentrations of dissolved 137Cs were 0.004e0.35 (arithmetic mean 0.16; median 0.14) Bq L1 in 2012 and 0.010e0.39 (mean 0.11; median 0.08) Bq L1 in 2013. Concentrations of particulate 137Cs were 0.010e0.40 (mean 0.19; median 0.12) Bq L1 in 2012 and 0.006e0.21 (mean 0.081; median 0.052) Bq L1 in 2013. Electrical conductivity was 47e200 mS cm1, which indicates that the influence of seawater was negligible at these sampling points. The geographic distributions of dissolved and particulate 137Cs concentrations for each sampling point are shown in Fig. 3 for 2012 and in Fig. 4 for 2013, together with the outlines of the catchments for each point and the 137Cs inventories based on airborne surveys. The 137Cs concentration in river water was relatively high at sampling points in the Niida, Ohta, Odaka, and Ukedo river systems, which roughly correspond to areas with high 137Cs inventories, and relatively low in the Uta, Mano, and Abukuma river systems, where 137 Cs inventories were likewise low.
88
S. Ochiai et al. / Journal of Environmental Radioactivity 144 (2015) 86e95
Fig. 1. River systems studied in the eastern Fukushima area, Japan. Inset shows the regional setting of the study area. Sampling points are indicated by open circles, and the location of the FDNPP is shown by a star. Open and closed squares indicate the locations of the water levels (Kuroiwa and Haramachi) and precipitation (Fukushima) observatories, respectively. Topography is based on Fundamental Geospatial Data (Geospatial Information Authority of Japan, 2014) and National Land Numerical Information (MLIT, 2014b).
3.2. Effects of
137
Cs inventory on the concentration in river water
To analyze the spatial variation of 137Cs concentrations in river water, we introduced a simple model of 137Cs transport in which the catchment above a sampling point was divided into small
segments. Water and 137Cs derived from these segments flow into a river and finally drain from the downstream end, corresponding to the sampling point. We assumed that the water discharge qi from the i-th segment was proportional to the area Ai of the segment:
Table 1 Sampling locations and water quality of river water in 2012. Sampling point
Sampling date Sampling location (year/month/day)
Water Conductivity pH temperature ( C) (mS cm1)
SS (mg L1) Catchment 137Cs inventory in the catchmenta area (km2) (kBq m2)
Latitude
Longitude
e 2012/9/26 2012/6/26 2012/6/26 2012/6/25 2012/6/25
e 37 290 53.000 37 330 50.600 37 350 44.600 37 430 07.600 37 470 41.200
e 141 000 24.800 141 000 17.3000 141 000 26.800 140 560 31.900 140 550 10.500
e e 20.5 18.2 16.4 16.0
e 78 93 55 80 87
e 7.2 6.7 6.8 6.8 6.8
e 4.6 7.3 22.1 5.2 4.3
Niida River Ganbe Dam Iitoi River Niida-U Hiso (Nagadoro) Mizunashi-U Mizunashi-D Niida-D Niida River Mouth
2012/6/26 2012/6/26 2012/6/26 2012/6/26 2012/6/26 2012/6/26 2012/6/26 2012/6/26
37 380 44.200 37 390 41.000 37 400 01.200 37 360 50.100 37 370 35.100 37 380 23.200 37 390 01.800 37 380 37.200
140 410 11.700 140 460 31.300 140 460 51.300 140 480 03.500 140 520 33.800 140 560 04.500 140 570 32.500 141 000 13.400
17.1 13.4 14.8 13.6 13.3 16.9 17.8 19.0
47 54 65 50 59 54 69 81
6.7 6.7 6.7 6.9 6.8 6.8 6.9 6.8
Abukuma River Abukuma River Hirose River Ishida River
2012/6/27 2012/11/28 2012/11/28
37 430 05.700 140 290 46.300 18.5 37 410 39.400 140 360 35.600 4.5 37 450 53.500 140 360 41.700 5.3
151 136 122
6.7 7.73 7.8
Ukedo River Ukedo-U Ukedo-D Odaka River Ohta River Mano River Uta River
e, Not measured. a Based on MEXT (2012).
Mean
Median
e 151.6 55.4 70.9 106.3 102.4
e 2138 555 1064 350 148
e 2000 370 990 325 140
3.2 4.5 2.8 2.1 2.3 13.8 3.3 8.0
3.3 47.4 52.9 29.4 8.6 18.1 205.5 258.6
715 839 638 1382 706 706 784 692
730 760 670 1500 855 700 710 680
8.1 1.6 1.4
2909.4 61.8 30.8
86 182 215
70 150 150
S. Ochiai et al. / Journal of Environmental Radioactivity 144 (2015) 86e95
89
Table 2 Sampling locations and water quality of river water in 2013. Sampling point
Sampling date Sampling location (year/month/day) Latitude
Longitude
Water Conductivity pH SS (mg L1) Catchment 137Cs inventory in the catchmenta temperature ( C) (mS cm1) area (km2) (kBq m2) Mean
Median
Ukedo River Ukedo-U Ukedo-D Odaka River Ohta River Mano River Uta River
2013/6/3 2013/6/4 2013/6/4 2013/6/4 2013/8/29 2013/8/29
37 330 30.700 37 290 53.000 37 330 50.600 37 350 44.600 37 440 08.300 37 470 35.500
140 450 12.000 141 000 24.800 141 000 17.3000 141 000 26.800 140 550 12.900 140 550 07.000
15.9 18.0 19.3 18.3 22.1 23.6
63 83 148 130 118 133
7.2 7.1 7.4 6.8 7.7 7.7
1.9 3.4 24.7 8.5 1.9 4.3
25.4 151.6 55.4 70.9 101.3 102.4
625 1743 445 883 300 134
361 1578 228 855 281 130
Niida River Ganbe Dam Iitoi River Niida-U Hiso (Nagadoro) Mizunashi-U Mizunashi-D Niida-D Niida River Mouth
2013/7/22 e 2013/8/30 2013/8/30 e 2013/8/29 2013/8/29 e
37 380 44.200 e 37 400 01.500 37 360 36.100 e 37 380 23.200 37 400 05.600 e
140 410 11.700 e 140 460 51.300 140 470 53.700 e 140 560 04.500 140 550 50.800 e
22.8 e 21.0 20.2 e 22.0 22.4 e
56 e 87 62 e 86 84 e
7.6 e 7.1 7.2 e 7.0 7.5 e
3.0 e 3.6 2.1 e 0.9 3.8 e
3.3 e 52.9 27.7 e 18.1 196.1 e
527 e 480 1080 e 574 648 e
522 e 491 1102 e 551 574 e
Abukuma River Abukuma River Hirose River Ishida River
2013/8/30 2013/8/30 e
37 430 05.600 140 290 46.400 37 410 52.200 140 360 33.000 e e
24.9 23.1 e
200 163 e
7.6 7.8 7.6 6.9 e e
2909.4 65.6 e
69b 149 e
57b 137 e
e, Not measured. a Based on Nuclear Regulation Authority (2013). b Based on MEXT (2013).
qi ¼ k1 Ai ;
(1)
where k1 is the proportionality constant. Removal flux of radionuclides from the catchment was assumed to be proportional to the inventory (e.g., Garcia-Sanchez, 2008). Based on this assumption, the flux of dissolved or particulate 137Cs fi from the i-th segment was expressed as follows:
fi ¼ k2 Ii Ai ;
(2)
where k2 and Ii are the proportionality constant and inventory of the segment, respectively. Assuming that the river water and 137Cs are conserved throughout the model, water discharge Q and 137Cs discharge at the sampling point CmQ are equal to the sum of components from the segments making up the catchment:
Q¼
X
Cm Q ¼
qi ¼
X
X
fi ¼
k1 Ai
X
k2 Ai Ii ;
(3) (4)
where Cm is the 137Cs concentration at the sampling point. Based on Eqs. (3) and (4), the dissolved or particulate 137Cs concentration at the sampling point Cm can be rewritten as
P Cm ¼
P k2 Ai Ii k AI ¼ P 2 i i: Q k1 Ai
(5)
If the constants k1 and k2 are uniform in a catchment, Eq. (5) is finally rewritten as
Cm ¼
P k2 Ai Ii P ; k1 Ai
(6)
where SAiIi/SAi is equivalent to the mean inventory across a catchment. This equation indicates that the 137Cs concentration is proportional to the catchment-mean inventory if water and 137Cs are conserved throughout the river (Eqs. (3) and (4)), i.e., dissolved
and particulate 137Cs are originating from every part of a catchment and do not interact each other during the transport. We derived the catchment-average 137Cs inventory (arithmetic mean and median) by using all data points from the airborne survey inside each catchment, which resulted in a wide range of mean values: 0.086e2.1 MBq m2 and 0.068e1.8 MBq m2 in 2012 and 2013, respectively (Tables 1 and 2). The arithmetic mean of the inventory was larger than the median for the Ukedo-U, Ukedo-D, and Odaka River sites, which is indicative of a skewed statistical distribution; this was due to the extremely high points in these catchments. Although the median is a suitable descriptor to avoid biased influence from a few extreme values, the concentrations in river water were related to the arithmetic mean of the inventory in the above model (Eq. (6)). Therefore, we also used the mean values in the following discussion. Fig. 5a shows that dissolved 137Cs concentration in river water is strongly correlated with catchment-mean 137Cs inventory, with r2 ¼ 0.90 (P < 0.001) and 0.92 (P < 0.001) for 2012 and 2013, respectively. We can conclude that the concentration of dissolved 137 Cs of river water under base-flow conditions was primarily determined by the catchment-mean 137Cs inventory. Those results are consistent with the conservation of dissolved 137Cs throughout the river. This correlation between dissolved concentration and inventory of 137Cs has been reported previously. Tsuji et al. (2014) related dissolved radiocesium concentrations to radiocesium inventories in the catchment of the Abukuma River and its tributaries in 2013. Yoshimura et al. (2015) documented the relationship between dissolved or particulate 137Cs concentrations and inventories in catchments of eastern Fukushima rivers in 2012. However, those papers reported data for only a single year. Our results show that this relationship prevailed in eastern Fukushima rivers during the two years after the accident. The slope of the regression line between concentrations and inventories, which reflects the mobility of dissolved 137Cs from the catchment, was 2.68 107 m2 L1 in 2012 and slightly less at 2.14 107 m2 L1 in 2013 (Fig. 5a). A parallelism test between the two regression lines indicated that parallelism could not be rejected (P > 0.05), i.e., differences
90
S. Ochiai et al. / Journal of Environmental Radioactivity 144 (2015) 86e95
Fig. 2. Temporal changes in water levels of the Abukuma River at the Kuroiwa Observatory (thin line) and the Niida River at the Haramachi Observatory (bold line). Hourly precipitation data at Fukushima City (bar graph) for 10 days before each sampling date are also shown. Open triangles indicate the sampling dates.
Table 3 Activity concentrations of Sampling point
134
Cs and
137
Cs of river water in 2012.
Activity concentrationa
134
1
Dissolved (Bq L 134
)
137
Cs
Cs
Particulate (Bq L
1
134
137
Cs
)
Particulate (Bq g 134
Cs
137
Cs
1
Cs
Cs/137Csb
Particulate to total
-SS) Dissolved
Particulate
Ukedo River Ukedo-U Ukedo-D Odaka River Ohta River Mano River Uta River
e 0.350 0.093 0.220 0.041 0.020
± ± ± ± ±
0.0066 0.0018 0.0064 0.0024 0.0012
e 0.590 0.140 0.340 0.062 0.030
± ± ± ± ±
0.0079 0.0020 0.0110 0.0023 0.0015
e 0.300 0.066 0.400 0.017 0.012
± ± ± ± ±
0.0140 0.0075 0.0110 0.0037 0.0024
e 0.480 0.100 0.600 0.022 0.017
± ± ± ± ±
0.0240 0.0091 0.0160 0.0039 0.0028
e e 3.1 110.0 ± 5.2 0.96 ± 1.0 14.2 ± 1.3 1.00 ± 0.5 27.1 ± 0.7 0.97 ± 0.7 4.1 ± 0.8 0.99 ± 0.6 3.8 ± 0.7 1.00 ±
0.02 0.02 0.04 0.07 0.08
e 1.01 0.99 1.00 1.16 1.06
± ± ± ± ±
0.07 0.14 0.04 0.33 0.27
e 44 40 64 27 36
Niida River Ganbe Dam Iitoi River Niida-U Hiso (Nagadoro) Mizunashi-U Mizunashi-D Niida-D Niida River Mouth
0.075 0.082 0.097 0.150 0.160 0.110 0.086 0.075
± ± ± ± ± ± ± ±
0.0022 0.0028 0.0048 0.0044 0.0061 0.0039 0.0034 0.0031
0.120 0.120 0.150 0.240 0.230 0.170 0.130 0.140
± ± ± ± ± ± ± ±
0.0037 0.0033 0.0056 0.0051 0.0069 0.0044 0.0043 0.0055
0.081 0.110 0.150 0.250 0.110 0.240 0.050 0.098
± ± ± ± ± ± ± ±
0.0053 0.0073 0.0060 0.0120 0.0110 0.0081 0.0056 0.0075
0.100 0.200 0.230 0.390 0.180 0.370 0.070 0.130
± ± ± ± ± ± ± ±
0.0068 25.6 ± 1.7 32.1 ± 2.1 0.94 ± 0.0086 24.7 ± 1.6 44.9 ± 1.9 1.03 ± 0.0071 52.5 ± 2.1 80.4 ± 2.5 0.97 ± 0.0150 121.5 ± 5.8 186.9 ± 7.2 0.94 ± 0.0130 50.0 ± 5.0 79.1 ± 5.6 1.04 ± 0.0097 17.6 ± 0.6 27.0 ± 0.7 0.97 ± 0.0069 15.3 ± 1.7 21.5 ± 2.1 0.99 ± 0.0100 12.2 ± 0.9 16.5 ± 1.2 0.80 ±
0.04 0.04 0.06 0.03 0.05 0.04 0.05 0.05
1.22 0.83 0.98 0.96 0.92 0.97 1.07 1.13
± ± ± ± ± ± ± ±
0.11 0.07 0.05 0.06 0.11 0.04 0.16 0.12
45 63 61 63 44 69 35 48
Abukuma River Abukuma River Hirose River Ishida River
0.015 ± 0.0009 0.025 ± 0.0010 0.029 ± 0.0025 0.046 ± 0.0030 0.004 ± 0.0005 0.008 ± 0.0006 0.010 ± 0.0019 0.014 ± 0.0023 0.004 ± 0.0006 0.008 ± 0.0008 0.013 ± 0.0014 0.022 ± 0.0022
e, Not measured. a Corrected to the value at sampling date. b Corrected to the value on 11 March 2011.
e 65.0 9.1 18.0 3.2 2.7
± ± ± ± ±
3.6 ± 0.3 6.6 ± 1.2 9.2 ± 1.1
5.7 ± 0.4 0.90 ± 0.06 0.95 ± 0.10 65 8.6 ± 1.5 0.84 ± 0.12 1.22 ± 0.31 64 16.0 ± 1.6 0.99 ± 0.16 1.01 ± 0.15 73
137
Cs (%)
S. Ochiai et al. / Journal of Environmental Radioactivity 144 (2015) 86e95 Table 4 Activity concentrations of Sampling point
134
Cs and
137
Cs of river water in 2013.
Activity concentrationa
134
Dissolved (Bq L1) 134
91
137
Cs
Cs
Particulate (Bq L1)
Particulate (Bq g1-SS)
134
134
137
Cs
Cs
Cs
Ukedo River Ukedo-U Ukedo-D Odaka River Ohta River Mano River Uta River
0.030 0.190 0.035 0.086 0.025 0.007
± ± ± ± ± ±
Niida River Ganbe Dam Iitoi River Niida-U Hiso (Nagadoro) Mizunashi-U Mizunashi-D Niida-D Niida River Mouth
0.056 e 0.036 0.076 e 0.055 0.049 e
± 0.0017 0.130 e ± 0.0008 0.081 ± 0.0017 0.160 e ± 0.0015 0.130 ± 0.0014 0.110 e
Abukuma River Abukuma River Hirose River Ishida River
0.006 ± 0.0005 0.015 ± 0.0008 0.016 ± 0.0022 0.028 ± 0.0035 6.9 ± 0.9 0.005 ± 0.0005 0.010 ± 0.0007 0.021 ± 0.0021 0.052 ± 0.0039 10.0 ± 1.0 e e e e e
0.0016 0.0010 0.0012 0.0008 0.0013 0.0007
0.063 0.390 0.071 0.180 0.053 0.016
± ± ± ± ± ±
0.0026 0.0020 0.0022 0.0016 0.0022 0.0008
0.023 0.099 0.068 0.039 0.021 0.004
± 0.0033 0.017 e ± 0.0016 0.037 ± 0.0032 0.054 e ± 0.0028 0.007 ± 0.0026 0.090 e
± ± ± ± ± ±
0.0023 0.0019 0.0038 0.0023 0.0024 0.0010
0.047 0.210 0.150 0.083 0.040 0.006
± 0.0031 0.030 e ± 0.0036 0.080 ± 0.0052 0.110 e ± 0.0025 0.013 ± 0.0049 0.200 e
± ± ± ± ± ±
0.0040 0.0038 0.0073 0.0045 0.0038 0.0014
12.0 ± 1.2 29.0 ± 0.6 2.8 ± 0.2 4.6 ± 0.3 29.0 ± 3.3 2.2 ± 0.6
± 0.0062 5.7 ± 1.0 e ± 0.0069 28.0 ± 2.7 ± 0.0098 67.0 ± 6.5 e ± 0.0049 34.0 ± 11.0 ± 0.0094 63.0 ± 3.4 e
137
Cs
Cs/137Csb
Dissolved
Particulate to total
Cs (%)
Particulate
24.0 ± 2.0 63.0 ± 1.1 6.1 ± 0.3 9.7 ± 0.5 55.0 ± 5.3 3.7 ± 0.9
0.96 0.98 0.99 0.96 1.02 1.00
± ± ± ± ± ±
9.9 ± 2.0 e 59.0 ± 5.1 130.0 ± 12.0 e 59.0 ± 23.0 140.0 ± 6.6 e
0.90 e 0.96 1.03 e 0.92 0.97 e
± 0.04 1.19 e ± 0.03 1.00 ± 0.03 1.07 e ± 0.03 1.22 ± 0.04 0.98 e
12.0 ± 1.5 25.0 ± 1.9 e
0.87 ± 0.08 1.24 ± 0.23 65 1.05 ± 0.12 0.88 ± 0.11 84 e e e
0.06 0.01 0.05 0.01 0.07 0.10
137
0.98 0.95 0.91 0.95 1.14 1.30
± ± ± ± ± ±
0.13 0.03 0.07 0.08 0.17 0.47
43 35 68 32 43 27
± 0.33 19 e ± 0.13 50 ± 0.14 41 e ± 0.62 9 ± 0.07 65 e
e, Not measured. a Corrected to the value at sampling date. b Corrected to the value on March 11, 2011.
Fig. 3. Distribution of the 137Cs inventory on 28 June 2012 (colors; from MEXT, 2012) and dissolved and particulate measured in 2012. Open circles indicate sampling points. Black lines outline the catchments for the sampling points.
137
Cs concentrations (white and black bars, respectively)
92
S. Ochiai et al. / Journal of Environmental Radioactivity 144 (2015) 86e95
Fig. 4. Distribution of the 137Cs inventory on 28 September 2013 (colors; from MEXT, 2013; Nuclear Regulation Authority, 2013) and dissolved and particulate 137Cs concentrations (white and black bars, respectively) measured in 2013. Open circles indicate sampling points. Black lines outline the catchments for the sampling points.
between the two slopes were not significant. However, longer observations will be required to confirm whether this tendency changes in Fukushima rivers in the future. The geology, land use, and vegetation cover in the catchment may affect the permeability of rock to groundwater, which may be related to the constants k1 and k2 here. The geology of the Abukuma Mountains (Fig. 1) mainly consists of granite, and sedimentary rocks are distributed in the plain along the coast, indicating that these conditions may be different within a given catchment. Although the significant relationship between the dissolved 137Cs concentration and mean 137Cs inventory implies that influences of these conditions on dissolved 137Cs transportation are relatively small, detailed investigations will be needed in the future to clarify this issue. Concentrations of particulate 137Cs in river water and in suspended solids (SS) were found to have weak correlations with the catchment-mean 137Cs inventory in 2012, and no significant correlations in 2013 (Fig. 5b and c). Those poor correlations may be attributed to the fact that our samples were collected during baseflow conditions, when SS mobility was low. In periods of low mobility, base flow does not transport particulate 137Cs from distant parts of a catchment; thus, the assumption in Eq. (4) that particulate 137Cs is equally originated from the every part of a catchment is not valid. Therefore, 137Cs concentrations in SS were not correlated with the catchment-mean 137Cs inventory (Fig. 5c). The low
mobility of particulate 137Cs was reflected in its relatively small contribution to the total 137Cs concentration, which averaged 49% with a range of 9e84% (Tables 3 and 4). In previous studies of Fukushima rivers, in which data during high-flow conditions were included, the mean percentage of particulate 137Cs in 2011 was reported as 74% in the Hiso River and 82% in the Wariki River, both of which are tributaries of the Niida River (Ueda et al., 2013). The relationship between 137Cs concentrations dissolved in river water and in SS (Fig. 5d) reflects the degree of interaction (sorption/ desorption) between dissolved and particulate 137Cs in river water. As no clear relationship was found (r2 ¼ 0.35 and 0.10 for 2012 and 2013, respectively), it appears that dissolved and particulate 137Cs in river water interact quite weakly under base-flow conditions. 3.3. Contribution of
137
Cs inventory in portions of catchments
Our previous discussion suggests that dissolved 137Cs can be equally contributed from every part of a catchment, while particulate 137Cs cannot originate from every part under base-flow conditions. To test these results statistically, we tried to evaluate the contributions of different portions of catchments to 137Cs concentrations. Because a certain point in the catchment contributes to the sampling point via water flow, the distance between them should be measured along the flow path instead of by the linear distance. Therefore, we introduced flow path lengths into the analysis, and
S. Ochiai et al. / Journal of Environmental Radioactivity 144 (2015) 86e95
93
Fig. 5. Relationships between the catchment-mean 137Cs inventory and (a) dissolved 137Cs concentration, (b) particulate 137Cs concentration, and (c) 137Cs concentration in SS. Open and closed circles indicate the values in 2012 and 2013, respectively. (d) Relationship between the dissolved 137Cs concentration and 137Cs concentration in SS. Regression lines are dashed for 2012 and solid for 2013.
these lengths were traced along not only the river course but also the course on the slope (Fig. 6). As mentioned earlier, the catchments were divided into meshes to determine their catchment boundaries. The flow direction (slope aspect) at each mesh was calculated using a DEM. The length of the flow path from each of these meshes to the sampling point was then calculated based on sov the flow direction in the catchment (Mita a and Hofierka, 1993), and these values were normalized against the maximum flow path length (the flow path to the most upstream end) in the catchment. Fig. 6 shows the normalized flow path lengths for some points and contour lines (white line). We derived mean 137Cs inventories over the area within the certain normalized flow path lengths. The correlations between the 137 Cs inventory and dissolved 137Cs concentration (Fig. 7a) had lower coefficients of determination (r2) when the flow path was shorter, suggesting that dissolved 137Cs concentration was not come from only catchment area near sampling point. The value of r2 increases with the flow path length and has the maximum value at 1.0 of flow path length: i.e., the whole catchment area of the sampling point. Thus, the mean inventory for the whole catchment showed the best correlation with the dissolved 137Cs concentration. On the other hand, the analogous exercise for 137Cs concentration in SS (Fig. 7b) resulted in higher r2 values for shorter flow path lengths; r2 decreased with path length in 2012 and showed no clear relationship with flow path length in 2013. Thus, particulate 137Cs
was more closely related to inventory near the sampling point than to inventory in the whole catchment in 2012, although this trend disappeared in 2013. 3.4. Transport processes of
137
Cs under base-flow conditions
Based on the above results, transport processes of dissolved and particulate 137Cs under base-flow conditions can be estimated as follows. During high-flow events, particulate 137Cs supplied from slope erosion is transported downstream for a short time as well as dissolved 137Cs. During such conditions, the concentrations are expected to be correlated to the catchment inventory. Actually, Yoshimura et al. (2015) reported that particulate 137Cs concentrations collected using a time-integrated SS sampler were correlated with the inventories of Fukushima river catchments; the sampler collects combined material during base- and high-flow conditions (Phillips et al., 2000). After water discharge decreases, a part of the suspended sediment is deposited and temporally stored in the riverbed. Under base-flow conditions, fine particles in the riverbed are the main sources of suspended sediment (Grimshaw and Lewin, 1980), and sediment supply from slope erosion to the channel is negligible. Because of the low transport force of river flow during such conditions, particulate 137Cs concentrations originate from areas within close sections of the river (as shown in Fig. 7b). At such
94
S. Ochiai et al. / Journal of Environmental Radioactivity 144 (2015) 86e95
Fig. 6. Conceptual illustration of the flow path length for an example catchment. The map indicates the catchment area above the sampling point. The flow paths are traced along the slope (dashed line) and river (solid line) from each point (open circle) to the sampling point. Small numbers beside the each point indicate the normalized flow path lengths for each point. Flow path lengths for the sampling point and the most upstream end are 0.0 and 1.0, respectively. White lines indicate contours of the normalized flow path length.
times, the correlation with the catchment inventory became weak. As time passes after the last discharge event, stored sediment sources decrease from the riverbed. Accordingly, the weaker correlations of particulate 137Cs in 2013 may have resulted from the
longer base-flow before sampling during this year (Fig. 2b). Dissolved 137Cs can originate from a larger area of the catchment (as shown in Fig. 7a), and hence, the correlation with the catchment inventory was high even under base-flow conditions. These data show that dissolved and particulate 137Cs behaved independently under base-flow conditions. Our results regarding the transport processes of 137Cs under base-flow conditions are based on observations and a simple model explaining the relationship between the 137Cs concentration and catchment inventory. In the future, investigations of the spatial distribution of 137Cs concentrations and their relation with riverbed sediments, soil, and geological features along the river may provide more clear evidence of the transport processes taking place in this region.
4. Conclusions
Fig. 7. Coefficients of determination r2 between 137Cs inventory averaged over the area within segments of the normalized flow path above the sampling point and (a) dissolved 137Cs concentration and (b) 137Cs concentration in SS for 2012 and 2013.
We investigated the distribution of radiocesium in river water in eastern Fukushima and the factors controlling concentrations under base-flow conditions. Observations of dissolved and particulate 137 Cs concentrations at 16 sampling points in seven river systems during 2012 and 2013 lead to the following conclusions. Dissolved 137Cs concentrations had a significant correlation with the mean 137Cs inventory in the catchment above each sampling point during 2012 and 2013. On the other hand, the particulate 137 Cs concentrations were not correlated with either the 137Cs inventory or the dissolved 137Cs concentration. Those results show that 1) the concentration of dissolved 137Cs under base-flow conditions is dominated by the 137Cs inventory of the catchment, which is conserved during transport by the river, and 2) there is little or no interaction between particulate and dissolved forms of radiocesium. We examined relations between the 137Cs concentration in river water and the mean 137Cs inventory over the area within the certain flow path lengths above the sampling point to evaluate the contributions of different portions of catchments to 137Cs
S. Ochiai et al. / Journal of Environmental Radioactivity 144 (2015) 86e95
concentrations. Coefficients of determination for dissolved 137Cs concentrations were highest for the longest flow path, i.e., the whole catchment area, and weaker with shorter flow paths. Coefficients of determination for particulate 137Cs concentrations were only moderately high for the shortest flow path in 2012 and quite low throughout the flow path in 2013. These results suggest that particulate 137Cs under base-flow conditions is mainly derived from riverbed sediments temporally stored near the sampling point after the last high-flow event. Dissolved 137Cs can originate from a larger area of the catchment even under base-flow conditions. Additionally, the results show that dissolved and particulate 137Cs concentrations under base-flow conditions behave independently during transport in catchments. Acknowledgments We are grateful to H. Kakimoto and S. Takaya (Zax Co. Ltd.) for their technical help in sample pretreatment. This work was performed under a contract with the government of Aomori Prefecture, Japan. References Garcia-Sanchez, L., 2008. Watershed wash-off of atmospherically deposited radionuclides: review of the fluxes and their evolution with time. J. Environ. Radioact. 99, 563e573. Geospatial Information Authority of Japan, 2014. Fundamental Geospatial Data. Available at: http://www.gsi.go.jp/kiban/index.html (in Japanese). Grimshaw, D.L., Lewin, J., 1980. Source identification for suspended sediments. J. Hydrol. 47, 151e162. Helton, J., Muller, A., Bayer, A., 1985. Contamination of surface-water bodies after reactor accidents by the erosion of atmospherically deposited radionuclides. Health Phys. 48, 757e771. Kimura, T., 1980. A simple and rapid method of collecting radionuclides from rain water. Anal. Chim. Acta 120, 419e422. Mikami, S., Maeyama, T., Hoshide, Y., Sakamoto, R., Sato, S., Okuda, N., Demongeot, S., Gurriaran, R., Uwamino, Y., Kato, H., Fujiwara, M., Sato, T., Takemiya, H., Saito, K., 2015. Spatial distributions of radionuclides deposited onto ground soil around the Fukushima Dai-ichi Nuclear Power Plant and their temporal change until December 2012. J. Environ. Radioact. 139, 320e343. Ministry of Education, Culture, Sports, Science and Technology (MEXT), 2012. Results of the (i) Fifth Airborne Monitoring Survey and (ii) Airborne Monitoring Survey Outside 80km from the Fukushima Dai-ichi NPP. Available at: http:// radioactivity.nsr.go.jp/en/contents/6000/5790/24/203_0928_14e.pdf. Ministry of Education, Culture, Sports, Science and Technology (MEXT), 2013. (i) Results of the Sixth Airborne Monitoring and (ii) Airborne Monitoring out of the 80km Zone of Fukushima Dai-ichi NPP. Available at: http://radioactivity.nsr.go. jp/en/contents/7000/6099/24/203_e_0301_18.pdf. Ministry of Land, Infrastructure, Transport and Tourism, Japan (MLIT), 2014a. Water Information System. Available at: http://www1.river.go.jp/ (in Japanese).
95
Ministry of Land, Infrastructure, Transport and Tourism, Japan (MLIT), 2014b. National Land Numerical Information. Available at: http://nlftp.mlit.go.jp/ksj-e/ index.html. , H., Hofierka, J., 1993. Interpolation by regularized spline with tension: II. Mit asova Application to terrain modeling and surface geometry analysis. Math. Geol. 25, 657e669. Monte, L., 1995. Evaluation of radionuclide transfer functions from drainage basins of fresh water systems. J. Environ. Radioact. 26, 71e82. Nagao, S., Kanamori, M., Ochiai, S., Tomihara, S., Fukushi, K., Yamamoto, M., 2013. Export of 134Cs and 137Cs in the Fukushima river systems at heavy rains by Typhoon Roke in September 2011. Biogeosci. Dis. 10, 2767e2790. Nuclear Emergency Response Headquarters, Government of Japan, 2011. Report of the Japanese Government to the IAEA Ministerial Conference on Nuclear Safety - The Accident at TEPCO's Fukushima Nuclear Power Stations. Available at: http://japan.kantei.go.jp/kan/topics/201106/iaea_houkokusho_e.html. Nuclear Regulation Authority, Japan, 2013. Monitoring air dose rates from a series of aircraft surveys 30 months after the Fukushima Daiichi NPS accident. Available at: http://www.nsr.go.jp/english/library/data/special-report_0114.pdf. Phillips, J.M., Russell, M.A., Walling, D.E., 2000. Time-integrated sampling of fluvial suspended sediment: a simple methodology for small catchments. Hydrol. Process 14, 2589e2602. Saito, K., Tanihata, I., Fujiwara, M., Saito, T., Shimoura, S., Otsuka, T., Onda, Y., Hoshi, M., Ikeuchi, Y., Takahashi, F., Kinouchi, N., Saegusa, J., Seki, A., Takemiya, H., Shibata, T., 2015. Detailed deposition density maps constructed by large-scale soil sampling for gamma-ray emitting radioactive nuclides from the Fukushima Dai-ichi Nuclear Power Plant accident. J. Environ. Radioact. 139, 308e319. Sakaguchi, A., Tanaka, K., Iwatani, H., Chiga, H., Fan, Q., Onda, Y., Takahashi, Y., 2015. Size distribution studies of 137Cs in river water in the Abukuma Riverine system following the Fukushma Dai-ichi Nuclear Power Plant accident. J. Environ. Radioact. 139, 379e389. Small, H., Stevens, T.S., Bauman, W.C., 1975. Novel ion exchange chromatographic method using conductometric detection. Anal. Chem. 47, 1801e1809. n, R., Smith, J.T., Wright, S.M., Cross, M.A., Monte, L., Kudelsky, A.V., Saxe Vakulovsky, S.M., Timms, D.N., 2004. Global analysis of the riverine transport of 90 137 Sr and Cs. Environ. Sci. Technol. 38, 850e857. Tsuji, H., Yasutaka, T., Kawabe, Y., Onishi, T., Komai, T., 2014. Distribution of dissolved and particulate radiocesium concentrations along rivers and the relations between radiocesium concentration and deposition after the nuclear power plant accident in Fukushima. Water Res. 60, 15e27. Ueda, S., Hasegawa, H., Kakiuchi, H., Akata, N., Ohtsuka, Y., Hisamatsu, S., 2013. Fluvial discharges of radiocaesium from watersheds contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident, Japan. J. Environ. Radioact. 118, 96e104. Yamamoto, M., Kofuji, H., Shiraishi, K., Igarashi, Y., 1998. An attempt to evaluate dry deposition velocity of airborne 210Pb in a forest ecosystem. J. Radioanal. Nucl. Chem. 227, 81e87. Yamashiki, Y., Onda, Y., Smith, H.G., Blake, W.H., Wakahara, T., Igarashi, Y., Matsuura, Y., Yoshimura, K., 2014. Initial flux of sediment-associated radiocesium to the ocean from the largest river impacted by Fukushima Daiichi Nuclear Power Plant. Sci. Rep. 4, 3714,. http://dx.doi.org/10.1038/srep03714. Yoshimura, K., Onda, Y., Sakaguchi, A., Yamamoto, M., Matsuura, Y., 2015. An extensive study of the concentrations of particulate/dissolved radiocaesium derived from the Fukushima Dai-ichi Nuclear Power Plant accident in various river systems and their relationship with catchment inventory. J. Environ. Radioact. 139, 370e378.
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Effects of radiocesium inventory on 137Cs concentrations in river waters of Fukushima, Japan, under base-flow conditions Shinya Ochiai a, *, Shinji Ueda a, Hidenao Hasegawa a, Hideki Kakiuchi a, Naofumi Akata a, b, Yoshihito Ohtsuka a, Shun'ichi Hisamatsu a a b
Department of Radioecology, Institute for Environmental Sciences, 1-7 Ienomae, Obuchi, Rokkasho, Kamikita, Aomori 039-3212, Japan National Institute for Fusion Science, 322-6 Oroshi-cho, Toki City, Gifu 509-5292, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 December 2014 Received in revised form 2 March 2015 Accepted 5 March 2015 Available online
To investigate the behavior of nuclear accident-derived 137Cs in river water under base-flow conditions, concentrations of dissolved and particulate 137Cs were measured at 16 sampling points in seven rivers of Fukushima Prefecture, Japan, in 2012 and 2013. The concentration of dissolved 137Cs was significantly correlated with the mean 137Cs inventory in the catchment area above each sampling point in both sampling years. These results suggest that the concentration of dissolved 137Cs under base-flow conditions is primarily determined by the 137Cs inventory of the catchment area above the sampling point. However, the concentration of particulate 137Cs did not show a clear relationship with either the mean 137 Cs inventory or the dissolved 137Cs concentration, thus indicating that particulate and dissolved forms do not effectively interact in rivers. To evaluate the contribution of the 137Cs inventory within catchment areas, we analyzed relations between the 137Cs concentration and the mean 137Cs inventory over the area within certain flow path lengths that were traced along the river and slope above the sampling point. Coefficients of determination for dissolved 137Cs concentrations were highest for the longest flow path, i.e., the whole catchment area, and lower for shorter flow paths. Coefficients of determination for particulate 137Cs concentrations were only moderately high for the shortest flow path in 2012, whereas the values were quite low for all flow paths in 2013. These results suggest that dissolved 137Cs can originate from a larger area of the catchment even under base-flow conditions; however, particulate 137Cs did not show such behavior. The results also show that under base-flow conditions, dissolved and particulate 137 Cs behave independently during their transport from river catchments to the ocean. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Fukushima Dai-ichi Nuclear Power Plant accident Radiocesium River water Base-flow Catchment inventory
1. Introduction After being damaged by the tsunami that followed the giant Tohoku earthquake of 11 March 2011, the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) of Tokyo Electric Power Company released a large amount of radiocesium (134Cs and 137Cs) into the surrounding environment (Nuclear Emergency Response Headquarters, 2011). The airborne radiocesium caused contamination over a broad area in northern Japan that was especially heavy in northeastern Fukushima Prefecture, where deposition of 137 Cs on the ground was estimated to exceed 3000 kBq m2 in the most contaminated areas (Ministry of Education, Culture, Sports,
* Corresponding author. Tel.: þ81 175 71 1426; fax: þ81 175 71 1492. E-mail address: [email protected] (S. Ochiai). http://dx.doi.org/10.1016/j.jenvrad.2015.03.005 0265-931X/© 2015 Elsevier Ltd. All rights reserved.
Science and Technology (MEXT), 2012, 2013; Mikami et al., 2015; Saito et al., 2015). Runoff carries 137Cs deposited on the ground into rivers; thus, the discharge rates of 137Cs from catchments to rivers and from rivers to downstream regions are important for evaluating the impacts of radiocesium on human health in riverine and coastal marine ecosystems. Long-term changes in the discharge rates of radiocesium from river catchments were studied for global fallout and the Chernobyl accident (e.g., Helton et al., 1985; Monte, 1995; Garcia-Sanchez, 2008). These studies have shown that the seasonal- and annual-scale discharge rate depends on the deposited amount and a transfer function, which expresses the temporal change in the discharge fraction of the deposited amount after deposition. Based on these models, long-term changes in discharge have been evaluated at the global scale (e.g., Smith et al., 2004).
S. Ochiai et al. / Journal of Environmental Radioactivity 144 (2015) 86e95
In Fukushima rivers, previous studies have investigated the relationship between observed fluvial discharge data and radiocesium deposition in the catchment to clarify the dominant factors controlling the radiocesium discharge from the catchment (Tsuji et al., 2014; Yoshimura et al., 2015). These studies have demonstrated that 137Cs concentrations are related to the 137Cs inventories in the catchments; hence, it is useful to estimate the initial discharge from the catchment. However, these relationships may be affected by hydrological fluctuations, especially those on shorter timescales, because the mobility of radiocesium depends on what forms (i.e., particulate and dissolved forms) are being transported and these forms are impacted differently by changes in discharge. For the annual timescale, Yamashiki et al. (2014) estimated that 84e92% of the total radiocesium discharge was transported in particulate form in the Abukuma River, which is the largest river in the Fukushima area. However, observations of shorter-term fluctuations show that particulate 137Cs largely fluctuates in accordance with the hydrological conditions. Ueda et al. (2013) determined that the percentage of particulate 137Cs fluctuated from 40% during base-flow conditions to over 90% during high-flow conditions in two small contaminated catchments. Similar observation results have been reported in southeastern Fukushima rivers, where the particulate percentages were 21eca.100% (Nagao et al., 2013), and in the Abukuma River and its tributaries, where the particulate percentages were 51e99% (Sakaguchi et al., 2015). Based on these results, the transport behaviors are expected to be different between particulate and dissolved 137Cs in the region. Therefore, it would be valuable to clarify the influences of such forms on the relationship between 137Cs concentration and catchment deposition for proper estimation of 137Cs discharge from the catchments. With this purpose in mind, we focused our efforts on fluvial transport of 137Cs under base-flow conditions, which is when the differences in behaviors of dissolved and particulate 137Cs are expected to be the most clear. Specifically, this study investigated the distribution of 137Cs in river water in eastern Fukushima during 2012e2013 and determined its relation to the catchment inventory based on a simple model and geographic analyses. From these data, we then attempted to clarify the transport processes of 137Cs under base-flow conditions. 2. Samples and methods This study targeted the main streams and tributaries of seven river systems (Uta, Mano, Niida, Ohta, Odaka, Ukedo, and Abukuma rivers) in eastern Fukushima Prefecture (Fig. 1), which received large amounts of radiocesium from the FDNPP accident. The catchment of the Abukuma River (5400 km2) is separated from the other six by the Abukuma Mountains. River water samples were collected at 16 sampling points during JuneeNovember 2012 and at 13 points during JuneeAugust 2013, when streamflows were at base level (Tables 1 and 2). Samples measuring 15e20 L were collected with a polyvinyl chloride bucket from 1 to 5 days after the latest rainfall (>0.5 mm/h). At the time of sample collection, water temperature, electrical conductivity, and pH were measured at the site with a water quality meter (DS4a, Hydrolab Co., USA). Water level data of Abukuma and Niida rivers during sampling intervals were provided by the Ministry of Land, Infrastructure, Transport and Tourism, Japan (MLIT) (2014a) and Fukushima Prefecture, respectively. Precipitation data observed in Fukushima City was provided by Japan Meteorological Agency. After the water samples were taken to the laboratory, subsamples of 1e4 L were filtered through membrane filters (0.45 mm pore size, Millipore) before the dissolved radiocesium measurement. Total radiocesium (dissolved and particulate) activities were measured in unfiltered samples. Concentrations of suspended
87
solids were obtained by weighing the residue on the filters after drying for more than 24 h at room temperature in a container containing desiccants (OZO-Z, OZO Kagakugiken Co., Japan). Each water sample, whether unfiltered or filtered, was passed through a column filled with a mixture of Powdex PAO and PCH resins (Ecodyn Co., USA) to separate both ionic and particulate radionuclides from the sample (Small et al., 1975; Kimura, 1980; Yamamoto et al., 1998). The homogenized resin was oven-dried at 80 C for more than 48 h and then packed into a plastic case (60 mm in diameter and 37 mm high). The activity concentrations of 134Cs (T1/2 ¼ 2.06 y) and 137Cs (T1/2 ¼ 30.17 y) were determined by a high-purity Ge detector (relative efficiency of 59%; Seiko EG&G, Japan). The particulate radiocesium concentration in water samples was estimated from the difference between the filtered and unfiltered samples. Geographic analyses of river catchments and radiocesium distributions were performed using QGIS 2.2.0 software and Fundamental Geospatial Data (Geospatial Information Authority of Japan, 2014) and National Land Numerical Information (MLIT, 2014b). Catchment areas upstream from each sampling point were delineated on the basis of digital elevation models (DEM) (250-m mesh for the Abukuma River and 10-m mesh for the other rivers), and their areas were determined from boundaries that were defined on the basis of inclinations between the map meshes. The 137Cs deposition data were based on airborne surveys by MEXT (2012, 2013) and Nuclear Regulation Authority (2013). Mean radiocesium inventories were calculated for each catchment from the catchment areas and the 137Cs deposition data. 3. Results and discussion 3.1. Concentration of
137
Cs in river water
Fig. 2 shows the changes in water levels of the Abukuma River (Kuroiwa observatory) and Niida River (Haramachi observatory) and hourly precipitation at Fukushima City for 10 days before each sampling date. The locations of the observatories are indicated in Fig. 1. Although high-flow events were observed during these intervals, especially in June 2012 just before the samples were collected, the samples used here were collected after the water level decreased to the base level. Therefore, hydrological conditions were similar among the sampling dates. Tables 3 and 4 list the 134Cs and 137Cs concentrations (dissolved and particulate) in river water in 2012 and 2013, respectively. The activity ratio 134Cs/137Cs of both dissolved and particulate forms, corrected to the value on the day of the FDNPP accident on 11 March 2011, was approximately 1.0, which indicates that most of the radiocesium in the river water was derived from the FDNPP accident rather than from global fallout. Therefore, we focus only on the 137Cs concentrations in the following discussion. Concentrations of dissolved 137Cs were 0.004e0.35 (arithmetic mean 0.16; median 0.14) Bq L1 in 2012 and 0.010e0.39 (mean 0.11; median 0.08) Bq L1 in 2013. Concentrations of particulate 137Cs were 0.010e0.40 (mean 0.19; median 0.12) Bq L1 in 2012 and 0.006e0.21 (mean 0.081; median 0.052) Bq L1 in 2013. Electrical conductivity was 47e200 mS cm1, which indicates that the influence of seawater was negligible at these sampling points. The geographic distributions of dissolved and particulate 137Cs concentrations for each sampling point are shown in Fig. 3 for 2012 and in Fig. 4 for 2013, together with the outlines of the catchments for each point and the 137Cs inventories based on airborne surveys. The 137Cs concentration in river water was relatively high at sampling points in the Niida, Ohta, Odaka, and Ukedo river systems, which roughly correspond to areas with high 137Cs inventories, and relatively low in the Uta, Mano, and Abukuma river systems, where 137 Cs inventories were likewise low.
88
S. Ochiai et al. / Journal of Environmental Radioactivity 144 (2015) 86e95
Fig. 1. River systems studied in the eastern Fukushima area, Japan. Inset shows the regional setting of the study area. Sampling points are indicated by open circles, and the location of the FDNPP is shown by a star. Open and closed squares indicate the locations of the water levels (Kuroiwa and Haramachi) and precipitation (Fukushima) observatories, respectively. Topography is based on Fundamental Geospatial Data (Geospatial Information Authority of Japan, 2014) and National Land Numerical Information (MLIT, 2014b).
3.2. Effects of
137
Cs inventory on the concentration in river water
To analyze the spatial variation of 137Cs concentrations in river water, we introduced a simple model of 137Cs transport in which the catchment above a sampling point was divided into small
segments. Water and 137Cs derived from these segments flow into a river and finally drain from the downstream end, corresponding to the sampling point. We assumed that the water discharge qi from the i-th segment was proportional to the area Ai of the segment:
Table 1 Sampling locations and water quality of river water in 2012. Sampling point
Sampling date Sampling location (year/month/day)
Water Conductivity pH temperature ( C) (mS cm1)
SS (mg L1) Catchment 137Cs inventory in the catchmenta area (km2) (kBq m2)
Latitude
Longitude
e 2012/9/26 2012/6/26 2012/6/26 2012/6/25 2012/6/25
e 37 290 53.000 37 330 50.600 37 350 44.600 37 430 07.600 37 470 41.200
e 141 000 24.800 141 000 17.3000 141 000 26.800 140 560 31.900 140 550 10.500
e e 20.5 18.2 16.4 16.0
e 78 93 55 80 87
e 7.2 6.7 6.8 6.8 6.8
e 4.6 7.3 22.1 5.2 4.3
Niida River Ganbe Dam Iitoi River Niida-U Hiso (Nagadoro) Mizunashi-U Mizunashi-D Niida-D Niida River Mouth
2012/6/26 2012/6/26 2012/6/26 2012/6/26 2012/6/26 2012/6/26 2012/6/26 2012/6/26
37 380 44.200 37 390 41.000 37 400 01.200 37 360 50.100 37 370 35.100 37 380 23.200 37 390 01.800 37 380 37.200
140 410 11.700 140 460 31.300 140 460 51.300 140 480 03.500 140 520 33.800 140 560 04.500 140 570 32.500 141 000 13.400
17.1 13.4 14.8 13.6 13.3 16.9 17.8 19.0
47 54 65 50 59 54 69 81
6.7 6.7 6.7 6.9 6.8 6.8 6.9 6.8
Abukuma River Abukuma River Hirose River Ishida River
2012/6/27 2012/11/28 2012/11/28
37 430 05.700 140 290 46.300 18.5 37 410 39.400 140 360 35.600 4.5 37 450 53.500 140 360 41.700 5.3
151 136 122
6.7 7.73 7.8
Ukedo River Ukedo-U Ukedo-D Odaka River Ohta River Mano River Uta River
e, Not measured. a Based on MEXT (2012).
Mean
Median
e 151.6 55.4 70.9 106.3 102.4
e 2138 555 1064 350 148
e 2000 370 990 325 140
3.2 4.5 2.8 2.1 2.3 13.8 3.3 8.0
3.3 47.4 52.9 29.4 8.6 18.1 205.5 258.6
715 839 638 1382 706 706 784 692
730 760 670 1500 855 700 710 680
8.1 1.6 1.4
2909.4 61.8 30.8
86 182 215
70 150 150
S. Ochiai et al. / Journal of Environmental Radioactivity 144 (2015) 86e95
89
Table 2 Sampling locations and water quality of river water in 2013. Sampling point
Sampling date Sampling location (year/month/day) Latitude
Longitude
Water Conductivity pH SS (mg L1) Catchment 137Cs inventory in the catchmenta temperature ( C) (mS cm1) area (km2) (kBq m2) Mean
Median
Ukedo River Ukedo-U Ukedo-D Odaka River Ohta River Mano River Uta River
2013/6/3 2013/6/4 2013/6/4 2013/6/4 2013/8/29 2013/8/29
37 330 30.700 37 290 53.000 37 330 50.600 37 350 44.600 37 440 08.300 37 470 35.500
140 450 12.000 141 000 24.800 141 000 17.3000 141 000 26.800 140 550 12.900 140 550 07.000
15.9 18.0 19.3 18.3 22.1 23.6
63 83 148 130 118 133
7.2 7.1 7.4 6.8 7.7 7.7
1.9 3.4 24.7 8.5 1.9 4.3
25.4 151.6 55.4 70.9 101.3 102.4
625 1743 445 883 300 134
361 1578 228 855 281 130
Niida River Ganbe Dam Iitoi River Niida-U Hiso (Nagadoro) Mizunashi-U Mizunashi-D Niida-D Niida River Mouth
2013/7/22 e 2013/8/30 2013/8/30 e 2013/8/29 2013/8/29 e
37 380 44.200 e 37 400 01.500 37 360 36.100 e 37 380 23.200 37 400 05.600 e
140 410 11.700 e 140 460 51.300 140 470 53.700 e 140 560 04.500 140 550 50.800 e
22.8 e 21.0 20.2 e 22.0 22.4 e
56 e 87 62 e 86 84 e
7.6 e 7.1 7.2 e 7.0 7.5 e
3.0 e 3.6 2.1 e 0.9 3.8 e
3.3 e 52.9 27.7 e 18.1 196.1 e
527 e 480 1080 e 574 648 e
522 e 491 1102 e 551 574 e
Abukuma River Abukuma River Hirose River Ishida River
2013/8/30 2013/8/30 e
37 430 05.600 140 290 46.400 37 410 52.200 140 360 33.000 e e
24.9 23.1 e
200 163 e
7.6 7.8 7.6 6.9 e e
2909.4 65.6 e
69b 149 e
57b 137 e
e, Not measured. a Based on Nuclear Regulation Authority (2013). b Based on MEXT (2013).
qi ¼ k1 Ai ;
(1)
where k1 is the proportionality constant. Removal flux of radionuclides from the catchment was assumed to be proportional to the inventory (e.g., Garcia-Sanchez, 2008). Based on this assumption, the flux of dissolved or particulate 137Cs fi from the i-th segment was expressed as follows:
fi ¼ k2 Ii Ai ;
(2)
where k2 and Ii are the proportionality constant and inventory of the segment, respectively. Assuming that the river water and 137Cs are conserved throughout the model, water discharge Q and 137Cs discharge at the sampling point CmQ are equal to the sum of components from the segments making up the catchment:
Q¼
X
Cm Q ¼
qi ¼
X
X
fi ¼
k1 Ai
X
k2 Ai Ii ;
(3) (4)
where Cm is the 137Cs concentration at the sampling point. Based on Eqs. (3) and (4), the dissolved or particulate 137Cs concentration at the sampling point Cm can be rewritten as
P Cm ¼
P k2 Ai Ii k AI ¼ P 2 i i: Q k1 Ai
(5)
If the constants k1 and k2 are uniform in a catchment, Eq. (5) is finally rewritten as
Cm ¼
P k2 Ai Ii P ; k1 Ai
(6)
where SAiIi/SAi is equivalent to the mean inventory across a catchment. This equation indicates that the 137Cs concentration is proportional to the catchment-mean inventory if water and 137Cs are conserved throughout the river (Eqs. (3) and (4)), i.e., dissolved
and particulate 137Cs are originating from every part of a catchment and do not interact each other during the transport. We derived the catchment-average 137Cs inventory (arithmetic mean and median) by using all data points from the airborne survey inside each catchment, which resulted in a wide range of mean values: 0.086e2.1 MBq m2 and 0.068e1.8 MBq m2 in 2012 and 2013, respectively (Tables 1 and 2). The arithmetic mean of the inventory was larger than the median for the Ukedo-U, Ukedo-D, and Odaka River sites, which is indicative of a skewed statistical distribution; this was due to the extremely high points in these catchments. Although the median is a suitable descriptor to avoid biased influence from a few extreme values, the concentrations in river water were related to the arithmetic mean of the inventory in the above model (Eq. (6)). Therefore, we also used the mean values in the following discussion. Fig. 5a shows that dissolved 137Cs concentration in river water is strongly correlated with catchment-mean 137Cs inventory, with r2 ¼ 0.90 (P < 0.001) and 0.92 (P < 0.001) for 2012 and 2013, respectively. We can conclude that the concentration of dissolved 137 Cs of river water under base-flow conditions was primarily determined by the catchment-mean 137Cs inventory. Those results are consistent with the conservation of dissolved 137Cs throughout the river. This correlation between dissolved concentration and inventory of 137Cs has been reported previously. Tsuji et al. (2014) related dissolved radiocesium concentrations to radiocesium inventories in the catchment of the Abukuma River and its tributaries in 2013. Yoshimura et al. (2015) documented the relationship between dissolved or particulate 137Cs concentrations and inventories in catchments of eastern Fukushima rivers in 2012. However, those papers reported data for only a single year. Our results show that this relationship prevailed in eastern Fukushima rivers during the two years after the accident. The slope of the regression line between concentrations and inventories, which reflects the mobility of dissolved 137Cs from the catchment, was 2.68 107 m2 L1 in 2012 and slightly less at 2.14 107 m2 L1 in 2013 (Fig. 5a). A parallelism test between the two regression lines indicated that parallelism could not be rejected (P > 0.05), i.e., differences
90
S. Ochiai et al. / Journal of Environmental Radioactivity 144 (2015) 86e95
Fig. 2. Temporal changes in water levels of the Abukuma River at the Kuroiwa Observatory (thin line) and the Niida River at the Haramachi Observatory (bold line). Hourly precipitation data at Fukushima City (bar graph) for 10 days before each sampling date are also shown. Open triangles indicate the sampling dates.
Table 3 Activity concentrations of Sampling point
134
Cs and
137
Cs of river water in 2012.
Activity concentrationa
134
1
Dissolved (Bq L 134
)
137
Cs
Cs
Particulate (Bq L
1
134
137
Cs
)
Particulate (Bq g 134
Cs
137
Cs
1
Cs
Cs/137Csb
Particulate to total
-SS) Dissolved
Particulate
Ukedo River Ukedo-U Ukedo-D Odaka River Ohta River Mano River Uta River
e 0.350 0.093 0.220 0.041 0.020
± ± ± ± ±
0.0066 0.0018 0.0064 0.0024 0.0012
e 0.590 0.140 0.340 0.062 0.030
± ± ± ± ±
0.0079 0.0020 0.0110 0.0023 0.0015
e 0.300 0.066 0.400 0.017 0.012
± ± ± ± ±
0.0140 0.0075 0.0110 0.0037 0.0024
e 0.480 0.100 0.600 0.022 0.017
± ± ± ± ±
0.0240 0.0091 0.0160 0.0039 0.0028
e e 3.1 110.0 ± 5.2 0.96 ± 1.0 14.2 ± 1.3 1.00 ± 0.5 27.1 ± 0.7 0.97 ± 0.7 4.1 ± 0.8 0.99 ± 0.6 3.8 ± 0.7 1.00 ±
0.02 0.02 0.04 0.07 0.08
e 1.01 0.99 1.00 1.16 1.06
± ± ± ± ±
0.07 0.14 0.04 0.33 0.27
e 44 40 64 27 36
Niida River Ganbe Dam Iitoi River Niida-U Hiso (Nagadoro) Mizunashi-U Mizunashi-D Niida-D Niida River Mouth
0.075 0.082 0.097 0.150 0.160 0.110 0.086 0.075
± ± ± ± ± ± ± ±
0.0022 0.0028 0.0048 0.0044 0.0061 0.0039 0.0034 0.0031
0.120 0.120 0.150 0.240 0.230 0.170 0.130 0.140
± ± ± ± ± ± ± ±
0.0037 0.0033 0.0056 0.0051 0.0069 0.0044 0.0043 0.0055
0.081 0.110 0.150 0.250 0.110 0.240 0.050 0.098
± ± ± ± ± ± ± ±
0.0053 0.0073 0.0060 0.0120 0.0110 0.0081 0.0056 0.0075
0.100 0.200 0.230 0.390 0.180 0.370 0.070 0.130
± ± ± ± ± ± ± ±
0.0068 25.6 ± 1.7 32.1 ± 2.1 0.94 ± 0.0086 24.7 ± 1.6 44.9 ± 1.9 1.03 ± 0.0071 52.5 ± 2.1 80.4 ± 2.5 0.97 ± 0.0150 121.5 ± 5.8 186.9 ± 7.2 0.94 ± 0.0130 50.0 ± 5.0 79.1 ± 5.6 1.04 ± 0.0097 17.6 ± 0.6 27.0 ± 0.7 0.97 ± 0.0069 15.3 ± 1.7 21.5 ± 2.1 0.99 ± 0.0100 12.2 ± 0.9 16.5 ± 1.2 0.80 ±
0.04 0.04 0.06 0.03 0.05 0.04 0.05 0.05
1.22 0.83 0.98 0.96 0.92 0.97 1.07 1.13
± ± ± ± ± ± ± ±
0.11 0.07 0.05 0.06 0.11 0.04 0.16 0.12
45 63 61 63 44 69 35 48
Abukuma River Abukuma River Hirose River Ishida River
0.015 ± 0.0009 0.025 ± 0.0010 0.029 ± 0.0025 0.046 ± 0.0030 0.004 ± 0.0005 0.008 ± 0.0006 0.010 ± 0.0019 0.014 ± 0.0023 0.004 ± 0.0006 0.008 ± 0.0008 0.013 ± 0.0014 0.022 ± 0.0022
e, Not measured. a Corrected to the value at sampling date. b Corrected to the value on 11 March 2011.
e 65.0 9.1 18.0 3.2 2.7
± ± ± ± ±
3.6 ± 0.3 6.6 ± 1.2 9.2 ± 1.1
5.7 ± 0.4 0.90 ± 0.06 0.95 ± 0.10 65 8.6 ± 1.5 0.84 ± 0.12 1.22 ± 0.31 64 16.0 ± 1.6 0.99 ± 0.16 1.01 ± 0.15 73
137
Cs (%)
S. Ochiai et al. / Journal of Environmental Radioactivity 144 (2015) 86e95 Table 4 Activity concentrations of Sampling point
134
Cs and
137
Cs of river water in 2013.
Activity concentrationa
134
Dissolved (Bq L1) 134
91
137
Cs
Cs
Particulate (Bq L1)
Particulate (Bq g1-SS)
134
134
137
Cs
Cs
Cs
Ukedo River Ukedo-U Ukedo-D Odaka River Ohta River Mano River Uta River
0.030 0.190 0.035 0.086 0.025 0.007
± ± ± ± ± ±
Niida River Ganbe Dam Iitoi River Niida-U Hiso (Nagadoro) Mizunashi-U Mizunashi-D Niida-D Niida River Mouth
0.056 e 0.036 0.076 e 0.055 0.049 e
± 0.0017 0.130 e ± 0.0008 0.081 ± 0.0017 0.160 e ± 0.0015 0.130 ± 0.0014 0.110 e
Abukuma River Abukuma River Hirose River Ishida River
0.006 ± 0.0005 0.015 ± 0.0008 0.016 ± 0.0022 0.028 ± 0.0035 6.9 ± 0.9 0.005 ± 0.0005 0.010 ± 0.0007 0.021 ± 0.0021 0.052 ± 0.0039 10.0 ± 1.0 e e e e e
0.0016 0.0010 0.0012 0.0008 0.0013 0.0007
0.063 0.390 0.071 0.180 0.053 0.016
± ± ± ± ± ±
0.0026 0.0020 0.0022 0.0016 0.0022 0.0008
0.023 0.099 0.068 0.039 0.021 0.004
± 0.0033 0.017 e ± 0.0016 0.037 ± 0.0032 0.054 e ± 0.0028 0.007 ± 0.0026 0.090 e
± ± ± ± ± ±
0.0023 0.0019 0.0038 0.0023 0.0024 0.0010
0.047 0.210 0.150 0.083 0.040 0.006
± 0.0031 0.030 e ± 0.0036 0.080 ± 0.0052 0.110 e ± 0.0025 0.013 ± 0.0049 0.200 e
± ± ± ± ± ±
0.0040 0.0038 0.0073 0.0045 0.0038 0.0014
12.0 ± 1.2 29.0 ± 0.6 2.8 ± 0.2 4.6 ± 0.3 29.0 ± 3.3 2.2 ± 0.6
± 0.0062 5.7 ± 1.0 e ± 0.0069 28.0 ± 2.7 ± 0.0098 67.0 ± 6.5 e ± 0.0049 34.0 ± 11.0 ± 0.0094 63.0 ± 3.4 e
137
Cs
Cs/137Csb
Dissolved
Particulate to total
Cs (%)
Particulate
24.0 ± 2.0 63.0 ± 1.1 6.1 ± 0.3 9.7 ± 0.5 55.0 ± 5.3 3.7 ± 0.9
0.96 0.98 0.99 0.96 1.02 1.00
± ± ± ± ± ±
9.9 ± 2.0 e 59.0 ± 5.1 130.0 ± 12.0 e 59.0 ± 23.0 140.0 ± 6.6 e
0.90 e 0.96 1.03 e 0.92 0.97 e
± 0.04 1.19 e ± 0.03 1.00 ± 0.03 1.07 e ± 0.03 1.22 ± 0.04 0.98 e
12.0 ± 1.5 25.0 ± 1.9 e
0.87 ± 0.08 1.24 ± 0.23 65 1.05 ± 0.12 0.88 ± 0.11 84 e e e
0.06 0.01 0.05 0.01 0.07 0.10
137
0.98 0.95 0.91 0.95 1.14 1.30
± ± ± ± ± ±
0.13 0.03 0.07 0.08 0.17 0.47
43 35 68 32 43 27
± 0.33 19 e ± 0.13 50 ± 0.14 41 e ± 0.62 9 ± 0.07 65 e
e, Not measured. a Corrected to the value at sampling date. b Corrected to the value on March 11, 2011.
Fig. 3. Distribution of the 137Cs inventory on 28 June 2012 (colors; from MEXT, 2012) and dissolved and particulate measured in 2012. Open circles indicate sampling points. Black lines outline the catchments for the sampling points.
137
Cs concentrations (white and black bars, respectively)
92
S. Ochiai et al. / Journal of Environmental Radioactivity 144 (2015) 86e95
Fig. 4. Distribution of the 137Cs inventory on 28 September 2013 (colors; from MEXT, 2013; Nuclear Regulation Authority, 2013) and dissolved and particulate 137Cs concentrations (white and black bars, respectively) measured in 2013. Open circles indicate sampling points. Black lines outline the catchments for the sampling points.
between the two slopes were not significant. However, longer observations will be required to confirm whether this tendency changes in Fukushima rivers in the future. The geology, land use, and vegetation cover in the catchment may affect the permeability of rock to groundwater, which may be related to the constants k1 and k2 here. The geology of the Abukuma Mountains (Fig. 1) mainly consists of granite, and sedimentary rocks are distributed in the plain along the coast, indicating that these conditions may be different within a given catchment. Although the significant relationship between the dissolved 137Cs concentration and mean 137Cs inventory implies that influences of these conditions on dissolved 137Cs transportation are relatively small, detailed investigations will be needed in the future to clarify this issue. Concentrations of particulate 137Cs in river water and in suspended solids (SS) were found to have weak correlations with the catchment-mean 137Cs inventory in 2012, and no significant correlations in 2013 (Fig. 5b and c). Those poor correlations may be attributed to the fact that our samples were collected during baseflow conditions, when SS mobility was low. In periods of low mobility, base flow does not transport particulate 137Cs from distant parts of a catchment; thus, the assumption in Eq. (4) that particulate 137Cs is equally originated from the every part of a catchment is not valid. Therefore, 137Cs concentrations in SS were not correlated with the catchment-mean 137Cs inventory (Fig. 5c). The low
mobility of particulate 137Cs was reflected in its relatively small contribution to the total 137Cs concentration, which averaged 49% with a range of 9e84% (Tables 3 and 4). In previous studies of Fukushima rivers, in which data during high-flow conditions were included, the mean percentage of particulate 137Cs in 2011 was reported as 74% in the Hiso River and 82% in the Wariki River, both of which are tributaries of the Niida River (Ueda et al., 2013). The relationship between 137Cs concentrations dissolved in river water and in SS (Fig. 5d) reflects the degree of interaction (sorption/ desorption) between dissolved and particulate 137Cs in river water. As no clear relationship was found (r2 ¼ 0.35 and 0.10 for 2012 and 2013, respectively), it appears that dissolved and particulate 137Cs in river water interact quite weakly under base-flow conditions. 3.3. Contribution of
137
Cs inventory in portions of catchments
Our previous discussion suggests that dissolved 137Cs can be equally contributed from every part of a catchment, while particulate 137Cs cannot originate from every part under base-flow conditions. To test these results statistically, we tried to evaluate the contributions of different portions of catchments to 137Cs concentrations. Because a certain point in the catchment contributes to the sampling point via water flow, the distance between them should be measured along the flow path instead of by the linear distance. Therefore, we introduced flow path lengths into the analysis, and
S. Ochiai et al. / Journal of Environmental Radioactivity 144 (2015) 86e95
93
Fig. 5. Relationships between the catchment-mean 137Cs inventory and (a) dissolved 137Cs concentration, (b) particulate 137Cs concentration, and (c) 137Cs concentration in SS. Open and closed circles indicate the values in 2012 and 2013, respectively. (d) Relationship between the dissolved 137Cs concentration and 137Cs concentration in SS. Regression lines are dashed for 2012 and solid for 2013.
these lengths were traced along not only the river course but also the course on the slope (Fig. 6). As mentioned earlier, the catchments were divided into meshes to determine their catchment boundaries. The flow direction (slope aspect) at each mesh was calculated using a DEM. The length of the flow path from each of these meshes to the sampling point was then calculated based on sov the flow direction in the catchment (Mita a and Hofierka, 1993), and these values were normalized against the maximum flow path length (the flow path to the most upstream end) in the catchment. Fig. 6 shows the normalized flow path lengths for some points and contour lines (white line). We derived mean 137Cs inventories over the area within the certain normalized flow path lengths. The correlations between the 137 Cs inventory and dissolved 137Cs concentration (Fig. 7a) had lower coefficients of determination (r2) when the flow path was shorter, suggesting that dissolved 137Cs concentration was not come from only catchment area near sampling point. The value of r2 increases with the flow path length and has the maximum value at 1.0 of flow path length: i.e., the whole catchment area of the sampling point. Thus, the mean inventory for the whole catchment showed the best correlation with the dissolved 137Cs concentration. On the other hand, the analogous exercise for 137Cs concentration in SS (Fig. 7b) resulted in higher r2 values for shorter flow path lengths; r2 decreased with path length in 2012 and showed no clear relationship with flow path length in 2013. Thus, particulate 137Cs
was more closely related to inventory near the sampling point than to inventory in the whole catchment in 2012, although this trend disappeared in 2013. 3.4. Transport processes of
137
Cs under base-flow conditions
Based on the above results, transport processes of dissolved and particulate 137Cs under base-flow conditions can be estimated as follows. During high-flow events, particulate 137Cs supplied from slope erosion is transported downstream for a short time as well as dissolved 137Cs. During such conditions, the concentrations are expected to be correlated to the catchment inventory. Actually, Yoshimura et al. (2015) reported that particulate 137Cs concentrations collected using a time-integrated SS sampler were correlated with the inventories of Fukushima river catchments; the sampler collects combined material during base- and high-flow conditions (Phillips et al., 2000). After water discharge decreases, a part of the suspended sediment is deposited and temporally stored in the riverbed. Under base-flow conditions, fine particles in the riverbed are the main sources of suspended sediment (Grimshaw and Lewin, 1980), and sediment supply from slope erosion to the channel is negligible. Because of the low transport force of river flow during such conditions, particulate 137Cs concentrations originate from areas within close sections of the river (as shown in Fig. 7b). At such
94
S. Ochiai et al. / Journal of Environmental Radioactivity 144 (2015) 86e95
Fig. 6. Conceptual illustration of the flow path length for an example catchment. The map indicates the catchment area above the sampling point. The flow paths are traced along the slope (dashed line) and river (solid line) from each point (open circle) to the sampling point. Small numbers beside the each point indicate the normalized flow path lengths for each point. Flow path lengths for the sampling point and the most upstream end are 0.0 and 1.0, respectively. White lines indicate contours of the normalized flow path length.
times, the correlation with the catchment inventory became weak. As time passes after the last discharge event, stored sediment sources decrease from the riverbed. Accordingly, the weaker correlations of particulate 137Cs in 2013 may have resulted from the
longer base-flow before sampling during this year (Fig. 2b). Dissolved 137Cs can originate from a larger area of the catchment (as shown in Fig. 7a), and hence, the correlation with the catchment inventory was high even under base-flow conditions. These data show that dissolved and particulate 137Cs behaved independently under base-flow conditions. Our results regarding the transport processes of 137Cs under base-flow conditions are based on observations and a simple model explaining the relationship between the 137Cs concentration and catchment inventory. In the future, investigations of the spatial distribution of 137Cs concentrations and their relation with riverbed sediments, soil, and geological features along the river may provide more clear evidence of the transport processes taking place in this region.
4. Conclusions
Fig. 7. Coefficients of determination r2 between 137Cs inventory averaged over the area within segments of the normalized flow path above the sampling point and (a) dissolved 137Cs concentration and (b) 137Cs concentration in SS for 2012 and 2013.
We investigated the distribution of radiocesium in river water in eastern Fukushima and the factors controlling concentrations under base-flow conditions. Observations of dissolved and particulate 137 Cs concentrations at 16 sampling points in seven river systems during 2012 and 2013 lead to the following conclusions. Dissolved 137Cs concentrations had a significant correlation with the mean 137Cs inventory in the catchment above each sampling point during 2012 and 2013. On the other hand, the particulate 137 Cs concentrations were not correlated with either the 137Cs inventory or the dissolved 137Cs concentration. Those results show that 1) the concentration of dissolved 137Cs under base-flow conditions is dominated by the 137Cs inventory of the catchment, which is conserved during transport by the river, and 2) there is little or no interaction between particulate and dissolved forms of radiocesium. We examined relations between the 137Cs concentration in river water and the mean 137Cs inventory over the area within the certain flow path lengths above the sampling point to evaluate the contributions of different portions of catchments to 137Cs
S. Ochiai et al. / Journal of Environmental Radioactivity 144 (2015) 86e95
concentrations. Coefficients of determination for dissolved 137Cs concentrations were highest for the longest flow path, i.e., the whole catchment area, and weaker with shorter flow paths. Coefficients of determination for particulate 137Cs concentrations were only moderately high for the shortest flow path in 2012 and quite low throughout the flow path in 2013. These results suggest that particulate 137Cs under base-flow conditions is mainly derived from riverbed sediments temporally stored near the sampling point after the last high-flow event. Dissolved 137Cs can originate from a larger area of the catchment even under base-flow conditions. Additionally, the results show that dissolved and particulate 137Cs concentrations under base-flow conditions behave independently during transport in catchments. Acknowledgments We are grateful to H. Kakimoto and S. Takaya (Zax Co. Ltd.) for their technical help in sample pretreatment. This work was performed under a contract with the government of Aomori Prefecture, Japan. References Garcia-Sanchez, L., 2008. Watershed wash-off of atmospherically deposited radionuclides: review of the fluxes and their evolution with time. J. Environ. Radioact. 99, 563e573. Geospatial Information Authority of Japan, 2014. Fundamental Geospatial Data. Available at: http://www.gsi.go.jp/kiban/index.html (in Japanese). Grimshaw, D.L., Lewin, J., 1980. Source identification for suspended sediments. J. Hydrol. 47, 151e162. Helton, J., Muller, A., Bayer, A., 1985. Contamination of surface-water bodies after reactor accidents by the erosion of atmospherically deposited radionuclides. Health Phys. 48, 757e771. Kimura, T., 1980. A simple and rapid method of collecting radionuclides from rain water. Anal. Chim. Acta 120, 419e422. Mikami, S., Maeyama, T., Hoshide, Y., Sakamoto, R., Sato, S., Okuda, N., Demongeot, S., Gurriaran, R., Uwamino, Y., Kato, H., Fujiwara, M., Sato, T., Takemiya, H., Saito, K., 2015. Spatial distributions of radionuclides deposited onto ground soil around the Fukushima Dai-ichi Nuclear Power Plant and their temporal change until December 2012. J. Environ. Radioact. 139, 320e343. Ministry of Education, Culture, Sports, Science and Technology (MEXT), 2012. Results of the (i) Fifth Airborne Monitoring Survey and (ii) Airborne Monitoring Survey Outside 80km from the Fukushima Dai-ichi NPP. Available at: http:// radioactivity.nsr.go.jp/en/contents/6000/5790/24/203_0928_14e.pdf. Ministry of Education, Culture, Sports, Science and Technology (MEXT), 2013. (i) Results of the Sixth Airborne Monitoring and (ii) Airborne Monitoring out of the 80km Zone of Fukushima Dai-ichi NPP. Available at: http://radioactivity.nsr.go. jp/en/contents/7000/6099/24/203_e_0301_18.pdf. Ministry of Land, Infrastructure, Transport and Tourism, Japan (MLIT), 2014a. Water Information System. Available at: http://www1.river.go.jp/ (in Japanese).
95
Ministry of Land, Infrastructure, Transport and Tourism, Japan (MLIT), 2014b. National Land Numerical Information. Available at: http://nlftp.mlit.go.jp/ksj-e/ index.html. , H., Hofierka, J., 1993. Interpolation by regularized spline with tension: II. Mit asova Application to terrain modeling and surface geometry analysis. Math. Geol. 25, 657e669. Monte, L., 1995. Evaluation of radionuclide transfer functions from drainage basins of fresh water systems. J. Environ. Radioact. 26, 71e82. Nagao, S., Kanamori, M., Ochiai, S., Tomihara, S., Fukushi, K., Yamamoto, M., 2013. Export of 134Cs and 137Cs in the Fukushima river systems at heavy rains by Typhoon Roke in September 2011. Biogeosci. Dis. 10, 2767e2790. Nuclear Emergency Response Headquarters, Government of Japan, 2011. Report of the Japanese Government to the IAEA Ministerial Conference on Nuclear Safety - The Accident at TEPCO's Fukushima Nuclear Power Stations. Available at: http://japan.kantei.go.jp/kan/topics/201106/iaea_houkokusho_e.html. Nuclear Regulation Authority, Japan, 2013. Monitoring air dose rates from a series of aircraft surveys 30 months after the Fukushima Daiichi NPS accident. Available at: http://www.nsr.go.jp/english/library/data/special-report_0114.pdf. Phillips, J.M., Russell, M.A., Walling, D.E., 2000. Time-integrated sampling of fluvial suspended sediment: a simple methodology for small catchments. Hydrol. Process 14, 2589e2602. Saito, K., Tanihata, I., Fujiwara, M., Saito, T., Shimoura, S., Otsuka, T., Onda, Y., Hoshi, M., Ikeuchi, Y., Takahashi, F., Kinouchi, N., Saegusa, J., Seki, A., Takemiya, H., Shibata, T., 2015. Detailed deposition density maps constructed by large-scale soil sampling for gamma-ray emitting radioactive nuclides from the Fukushima Dai-ichi Nuclear Power Plant accident. J. Environ. Radioact. 139, 308e319. Sakaguchi, A., Tanaka, K., Iwatani, H., Chiga, H., Fan, Q., Onda, Y., Takahashi, Y., 2015. Size distribution studies of 137Cs in river water in the Abukuma Riverine system following the Fukushma Dai-ichi Nuclear Power Plant accident. J. Environ. Radioact. 139, 379e389. Small, H., Stevens, T.S., Bauman, W.C., 1975. Novel ion exchange chromatographic method using conductometric detection. Anal. Chem. 47, 1801e1809. n, R., Smith, J.T., Wright, S.M., Cross, M.A., Monte, L., Kudelsky, A.V., Saxe Vakulovsky, S.M., Timms, D.N., 2004. Global analysis of the riverine transport of 90 137 Sr and Cs. Environ. Sci. Technol. 38, 850e857. Tsuji, H., Yasutaka, T., Kawabe, Y., Onishi, T., Komai, T., 2014. Distribution of dissolved and particulate radiocesium concentrations along rivers and the relations between radiocesium concentration and deposition after the nuclear power plant accident in Fukushima. Water Res. 60, 15e27. Ueda, S., Hasegawa, H., Kakiuchi, H., Akata, N., Ohtsuka, Y., Hisamatsu, S., 2013. Fluvial discharges of radiocaesium from watersheds contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident, Japan. J. Environ. Radioact. 118, 96e104. Yamamoto, M., Kofuji, H., Shiraishi, K., Igarashi, Y., 1998. An attempt to evaluate dry deposition velocity of airborne 210Pb in a forest ecosystem. J. Radioanal. Nucl. Chem. 227, 81e87. Yamashiki, Y., Onda, Y., Smith, H.G., Blake, W.H., Wakahara, T., Igarashi, Y., Matsuura, Y., Yoshimura, K., 2014. Initial flux of sediment-associated radiocesium to the ocean from the largest river impacted by Fukushima Daiichi Nuclear Power Plant. Sci. Rep. 4, 3714,. http://dx.doi.org/10.1038/srep03714. Yoshimura, K., Onda, Y., Sakaguchi, A., Yamamoto, M., Matsuura, Y., 2015. An extensive study of the concentrations of particulate/dissolved radiocaesium derived from the Fukushima Dai-ichi Nuclear Power Plant accident in various river systems and their relationship with catchment inventory. J. Environ. Radioact. 139, 370e378.