==== Front Sci Data Sci Data Scientific Data 2052-4463 Nature Publishing Group UK London 33319799 774 10.1038/s41597-020-00774-x Data Descriptor Dataset on the 6-year radiocesium transport in rivers near Fukushima Daiichi nuclear power plant http://orcid.org/0000-0002-3328-7808Taniguchi Keisuke taniguchi.keisuke.fu@u.tsukuba.ac.jp 12 http://orcid.org/0000-0001-8753-0378Onda Yuichi 1 http://orcid.org/0000-0003-0292-0284Smith Hugh G. 3 http://orcid.org/0000-0001-9447-1361Blake William 4 http://orcid.org/0000-0001-5256-2679Yoshimura Kazuya 5 Yamashiki Yosuke 6 http://orcid.org/0000-0002-3044-8127Kuramoto Takayuki 27 1 grid.20515.330000 0001 2369 4728Center for Research in Isotopes and Environmental Dynamics, University of Tsukuba, Tsukuba, Ibaraki Japan 2 Fukushima Prefectural Centre for Environmental Creation, Miharu, Fukushima Japan 3 grid.419186.30000 0001 0747 5306Landcare Research, Palmerston North, New Zealand 4 grid.11201.330000 0001 2219 0747School of Geography, Earth and Environmental Sciences, University of Plymouth, Plymouth, Devon UK 5 grid.20256.330000 0001 0372 1485Sector of Fukushima Research and Development, Japan Atomic Energy Agency, Minamisoma, Fukushima Japan 6 grid.258799.80000 0004 0372 2033Graduate School of Advanced Integrated Studies in Human Survivability, Kyoto University, Kyoto, Kyoto Japan 7 grid.265061.60000 0001 1516 6626School of Humanities and Culture, Tokai University, Hiratsuka, Kanagawa Japan 15 12 2020 15 12 2020 2020 7 43328 7 2020 13 11 2020 © The Author(s) 2020Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.The Creative Commons Public Domain Dedication waiver http://creativecommons.org/publicdomain/zero/1.0/ applies to the metadata files associated with this article.Radiocesium released from the Fukushima Daiichi nuclear power plant (FDNPP) and deposited in the terrestrial environment has been transported to the sea through rivers. To study the long-term effect of riverine transport on the remediation process near the FDNPP, a monitoring project was initiated by the University of Tsukuba. It was commissioned by the Ministry of Education, Culture, Sports, Science, and Technology, and the Nuclear Regulatory Commission in June 2011, and was taken over by the Fukushima Prefectural Centre for Environmental Creation from April 2015. The activity concentration and monthly flux of radiocesium in a suspended form were measured in the project. This provides valuable measurement data to evaluate the impact of the accidentally released radiocesium on residents and the marine environment. It can also be used as verification data in the development and testing of numerical models to predict future impacts. Measurement(s) Cesium-137 Gamma Radiation • Cesium-134 Gamma Radiation • Latitude • Longitude • grain size • radiocesium flux • radioactivity concentration Technology Type(s) gamma-ray spectroscopy • GPS navigation system • laser diffraction particle size analyzer • Calculation Factor Type(s) geographic location Sample Characteristic - Environment suspended sediment Sample Characteristic - Location Abukuma River • Fukushima Prefecture • Miyagi Prefecture Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.13055315 Subject terms Pollution remediationEnvironmental monitoringEnvironmental Radioactivity Research Network Centerhttps://doi.org/10.13039/501100001700Ministry of Education, Culture, Sports, Science and Technology (MEXT)Nuclear Regulation Authority, Japan Grant-in-Aid for ScientificResearch on Innovative Areas grant number 24110005 Environmental Radioactivity Research Network Centerissue-copyright-statement© The Author(s) 2020 ==== Body Background & Summary A 9.0 magnitude earthquake on March 11, 2011, caused the Tokyo Electric Power Company’s Fukushima Daiichi nuclear power plant (FDNPP) to be damaged by a tsunami, causing a large accident that spread radioactive materials into the environment1,2. This was the largest release of radioactivity into the environment since the Chernobyl nuclear power plant accident in 1986, and has been rated on the International Nuclear and Radiological Event Scale (INES) as a “Major Accident” by International Atomic Energy Agency (IAEA)3. Radiocesium (134Cs and 137Cs) is a very important nuclide for evaluating the impact of the accident on the radiation risk in the medium- to long-term because of the significant release amount (10 PBq each) and the long-half-lives (2.07 and 30.1 years, respectively)2. Approximately 2.7 PBq of the total 137Cs release of 10 PBq was deposited on land in eastern Japan4. In terrestrial environments, deposited radiocesium exists in particulate and dissolved forms. The former are those adsorbed on soil particles and the latter are dissolved in water as ions5. Although a part of the dissolved form existed as a colloidal form, the colloidal form ratio in the dissolved form was low6. During rainfall events, the surface soil that has absorbed radiocesium is eroded and transported into rivers, and finally reaches the ocean. Due to the high Kd values (i.e., the ratio between suspended and dissolved 137Cs concentrations)7–12, high levels of precipitation13–15, and steep topography16, >90% radiocesium was transported in the particulate form in Japanese rivers7,8,10,17. This ratio is much higher than in the rivers of Europe following the Chernobyl nuclear power plant (CNPP) accident5,13. In response to the FDNPP accident, the Japanese government designated an evacuation zone based on the air dose rate (Transition of evacuation designated zones. Fukushima Prefecture https://www.pref.fukushima.lg.jp/site/portal-english/en03-08.html) and to conducted decontamination activities around the residential areas (Off-site Environmental Remediation in Affected Areas in Japan. Ministry of the Environment http://josen.env.go.jp/en/decontamination/). This resulted in regional differences in land-use conditions, with all human activities halted in the areas that received evacuation orders while agriculture in rice paddies and fields continued outside of these areas. The evacuation orders were lifted in stages as the decontamination process progressed. Continuous environmental monitoring is essential for safe habitation in these areas. A long-term monitoring campaign began in June 2011, approximately three months after the accident, in order to comprehensively understand the movement of radiocesium in the terrestrial environment from the source to the ocean18. A considerable amount of data has been acquired and valuable knowledge has been provided, e.g., mapping of the air dose rate and initial deposition19–21, runoff from paddy fields22 and other land uses23,24, and riverine transport10,17. The riverine monitoring in the campaign was taken over by the Fukushima Prefectural Centre for Environmental Creation in April 2015 and is currently ongoing. Data on radiocesium concentrations and fluxes collected in rivers within 80 km of the FDNPP from June 2011 (three months after the accident) to March 2017 (6 years after the accident) is available on the web pages10,25–29. The data is expected to be widely used for validation of the radiocesium transport model, comparison between the FDNPP and CNPP accidents, influence on the health of residents, and evaluation of the effect of the environmental remediation measures in Fukushima30. Additional data for Cs concentrations in size-fractionated sediments and information on size fractionation as a function of flow rate could be useful for further improving numerical models to predict future impacts. Methods Monitoring Sites A total of 30 river observation sites were established within 80 km of the FDNPP (Fig. 1 and Table 1). Seventeen of these sites are located in the watershed of the Abukuma River which is the largest river system in the region. The remaining 13 sites are located in relatively small river systems on the coastal area of Fukushima Prefecture. The long-term monitoring sites (sites 1–6) and the other sites (sites 7–30) were respectively established in June 2011 and from October to December 2012 as a part of the monitoring campaigns by the Ministry of Education, Culture, Sports, Science and Technology (MEXT)18. These sites were selected because they were located in high 137Cs deposition areas and where data on water level and discharge could be obtained. The wide-spread monitoring network aimed to study temporal and regional variations in radiocesium transport through river networks to the ocean.Fig. 1 Monitoring site map. The numbers correspond to the site numbers in Table 1. The red shaded area is the area where the evacuation order was issued in the past. The solid blue line indicates the river channel where the observation points are located, and the area surrounded by black dotted lines indicates the catchment area of each observation point. The colour contours in the background of the map show the 137Cs deposition o as of July 20114. Table 1 Location of the observation points and period of data acquisition. No. Site name (abbreviation) River name River system Latitude longitude S D Period of data availability [km2] [kBq/m2] Radiocesium concentrations Radiocesium flux 1 Mizusakai (Miz) Kuchibuto Abukuma 140°41′32″ 37°35′08″ 7.5 745.2 Jun.2011–Mar.2017 Jun.2011–Mar. 2017 2 Kuchibuto_Upper (KU) Kuchibuto Abukuma 140°41′18″ 37°35′27″ 21.4 477.4 Jun.2011–Mar.2017 Jun.2011–Mar. 2017 3 Kuchibuto_Middle (KM) Kuchibuto Abukuma 140°36′50″ 37°31′55″ 62.8 357.2 Jun.2011–Mar.2017 Jun.2011–Mar. 2017 4 Kuchibuto_Down (KD) Kuchibuto Abukuma 140°32′31″ 37°34′52″ 135.2 269.1 Jun.2011–Mar.2017 Jun.2011–Mar. 2017 5 Fushiguro (Fus) Abukuma Abukuma 140°31′12″ 37°49′43″ 3644.5 95.9 Jul.2011–Mar.2017 Jun.2011–Mar. 2017 6 Iwanuma (Iwa) Abukuma Abukuma 140°52′19″ 38°05′41″ 5313.2 88.4 Jul.2011–Feb.2017 Jun.2011–Mar. 2017 7 Mano (Man) Mano Mano 140°50′22″ 37°42′57″ 75.6 498.7 Aug.2011–Feb.2017 Oct.2012–Mar. 2017 8 Ojimadazeki (Oji) Mano Mano 140°57′19″ 37°42′42″ 110.8 405.5 Aug.2011–Feb.2017 Oct.2012–Mar. 2017 9 Matsubara (Mat) Same Same 140°46′43″ 36°55′03″ 570.9 40.0 Sep.2011–Feb.2017 Oct.2012–Mar. 2017 10 Onahama (Ona) Fujiwara Fujiwara 140°52′35″ 36°57′03″ 70.1 38.8 Sep.2011–Feb.2017 Oct.2012–Mar. 2017 11 Tsukidate (Tsuk) Hirose Abukuma 140°36′22″ 37°43′59″ 83.6 222.8 Aug.2011–Aug.2016 Oct.2012–Mar. 2017 12 Nihonmatsu (Nih)* Abukuma Abukuma 140°27′40″ 37°35′40″ 2380.3 81.8 Aug.2011–Mar.2017 Oct.2014*–Mar. 2017 13 Miyota (Miy) Abukuma Abukuma 140°22′25″ 37°20′47″ 1286.6 74.1 Sep.2011–Aug.2016 Oct.2012–Mar. 2017 14 Nishikawa (Nis) Shakado Abukuma 140°21′16″ 37°17′19″ 289.4 132.0 Sep.2011–Feb.2017 Oct.2012–Mar. 2017 15 Kitamachi (Kit) Mizunashi Niida 140°57“26″ 37°38′40″ 35.8 564.5 Aug.2011–Mar.2017 Oct.2012–Mar. 2017 16 Kawamata (Kaw) Hirose Abukuma 140°36′54″ 37°40′46″ 56.6 229.1 Aug.2011–Mar.2017 Oct.2012–Mar. 2017 17 Marumori (Mar)*2 Abukuma Abukuma 140°45′44″ 37°55′18″ 4123.9 105.1 Dec.2012–Feb.2017 — 18 Funaoka-ohashi (Fun)*3 Shiroishi Abukuma 140°46′14″ 38°03′43″ 775.2 20.2 — — 19 Senoue (Sen) Surigami Abukuma 140°29′45″ 37°48′40″ 313.3 41.9 Dec.2012–Mar.2017 Oct.2012–Mar. 2017 20 Yagita (Yag)*4 Ara Abukuma 140°27′10″ 37°45′04″ 184.6 52.7 Dec.2012–Apr.2016*4 Oct.2012–Mar. 2016*4 21 Kuroiwa (Kur) Abukuma Abukuma 140°28′23″ 37°43′38″ 2921.4 103.4 Dec.2012–Mar.2017 Oct.2012–Mar. 2017 22 Tomita (Tom) Ouse Abukuma 140°21′41″ 37°24′54″ 72.6 98.5 Dec.2012–Feb.2017 Oct.2012–Mar. 2017 23 Ota (Ota) Ota Ota 140°57′18″ 37°36′12″ 49.9 1767.5 Dec.2012–Mar.2017 Oct.2012–Mar. 2017 24 Odaka (Oda) Odaka Odaka 140°59′1″ 37°33′58″ 50.3 724.2 Dec.2012–Mar.2017 Oct.2012–Mar. 2017 25 Asami (Asa) Asami Asami 140°59′33″ 37°12′20″ 25.8 193.8 Dec.2012–Feb.2017 Oct.2012–Mar. 2017 26 Tsushima (Tsus)*5 Ukedo Ukedo 140°45′12″ 37°33′36″ 25.4 951.5 Dec.2012–Mar.2017 Oct.2012–Aug. 2015*5 27 Ukedo (Uke) Ukedo Ukedo 141° 0′38″ 37°29′41″ 152.6 2565.9 Dec.2012–Mar.2017 Oct.2012–Mar. 2017 28 Takase (Tak) Takase Ukedo 140°59′53″ 37°29′7″ 263.7 726.0 Dec.2012–Mar.2017 Oct.2012–Mar. 2017 29 Haramachi (Har) Niida Niida 140°57′29″ 37°39′4″ 200.3 963.7 Dec.2012–Mar.2017 Oct.2012–Mar. 2017 30 Akanuma (Aka) Otakine Abukuma 140°25′44″ 37°22′13″ 242.6 52.6 Dec.2012–Mar.2017 Oct.2012–Mar. 2017 The geodetic datum for all locations is WGS1985 (ERSG = 4326). *At point No. 12, fluxes could not be calculated due to the poor quality of turbidity data prior to April 2014 *2 At point No. 17, the flux could not be calculated due to the poor quality of turbidity data. *3 There is no data on suspended radiocesium at point No. 18 because only water sampling was carried out at that point. *4 Observations at No. 20 were suspended at the end of FY 2016 due to a major change in the shape of the flow channel caused by a typhoon. *5 At point No. 26, due to the impact of the typhoon in September 2015, data from that point has not been released yet. Following the FDNPP incident, evacuation orders have been issued for areas of concern for the health of residents (Transition of evacuation designated zones. Fukushima Prefecture https://www.pref.fukushima.lg.jp/site/portal-english/en03-08.html). The area covered a maximum of approximately 1600 km2 in 11 municipalities (Fig. 1). As of March 2020, the designated evacuation area has been reduced to approximately 330 km2 in seven municipalities (∼Fukushima Today∼Steps for Reconstruction and Revitalization in Fukushima Prefecture (2020.3.24). Fukushima Prefecture https://www.pref.fukushima.lg.jp/site/portal-english/ayumi-en-15.html). Eleven of the monitoring sites (nos. 1, 2, 7, 15, 23, 24, 25, 26, 27, 28, and 29) were located in the previously evacuated area. One of the monitoring sites (no. 26) remains located in the evacuated area. Sample collection for measurement of suspended radiocesium concentration and calculation of suspended radiocesium flux Activity concentrations of suspended radiocesium C Time-integrate SS samplers31 were set at 29 monitoring sites (i.e., all except for site no. 18). A bulk sample of SS was collected every few days to months. The samples were dried at 105‒110 °C for 24 hours. Then, the radiocesium concentration [C (Bq kg−1)] was measured using germanium semiconductor detectors (EGC25-195-R, GC3018, and GC4020 by Canberra-Eurisys, GEM20-70, GEM40P4-76, and GMX30-70-HJ by Ortec). All the data were corrected to the sample collection date. Grain size distributions were also measured using laser diffraction/scattering particle size distribution analyzers (SALD-3100 by Shimazu, LA-960A by HORIBA) to obtain the mean diameter and grain size distribution of the sample. Since the laser-diffraction (LD) method can measure a wide range of particle size distributions, from sand to clay, the LD method has been widely used for particle size correction of Fukushima sediments containing a large amount of silt and clay. The specific surface area SSA [m2 g−1] was estimated using the grain size distribution and the spherical approximation of particles in each size fractions as per the following equation9. 1 SSA=∑i6ρ−1di−1pi, where pi and di [m] are the ratio and diameter of the grain size fraction, ρ is the particle density (i.e., 2.6 × 103 kg m−3 for silica-sand). The radiocesium concentrations for the period from August 2011 to January 2015 were previously published on the JAEA’s database site25. However, for the 29 samples collected between August 2011 and March 2012 at 10 sites (nos. 7–16), the data were measured without drying the samples. Also, all data were rounded to two significant digits. We obtained 12 samples of the 29 samples, dried them as well as the other samples for 24 hours at 105 °C, and then re-measured the radiocesium concentrations. The re-measured data, along with other initial data, were published again on the CRiED Database site26. The time series of 137Cs radioactivity concentrations between June 2011 and March 2017 is shown in Fig. 2.Fig. 2 Time series of the radioactivity concentrations of suspended 137Cs. There are two types of points related to site no.6 in the left plot. The green point indicates the suspended 137Cs concentration collected on the left bank side and the dark blue point indicates the concentration collected on the right bank side. The points at the right bank were added in 2014 to confirm the representativeness of the data. Radiocesium flux L The suspended radiocesium (134Cs and 137Cs) fluxes (L [Bq month−1]) were obtained by the accumulation of the product of the activity concentrations of suspended radiocesium C, water discharge Q and suspended solid concentration SSC for one month as following equation. 2 L=∑(Q×SSC×C) Figure 3 schematically illustrates the process of data collection and calculation.Fig. 3 Schematic image of calculation of riverine radiocesium flux. The monthly fluxes of 134Cs and 137Cs were calculated by accumulating the water discharge (Q), SS concentration (SSC), and activity concentration (C) by month. The water discharge (Q) was converted from the water level (h) obtained from water level gauges or from the webpages of Ministry of Land, Infrastructure, Transport, and Tourism (MLIT) and Fukushima Prefecture using h-Q equations. The SS concentration was calculated from the turbidity using the conversion formula prepared using a standard solution. The activity concentrations of suspended 134Cs and 137Cs were obtained by measuring SS samples collected by SS samplers, dried at 105‒110 °C, and then measured with a hp Germanium semiconductor detector. The water discharge (Q [m3 s−1]) was calculated from the water level (h [m]). The water level data were obtained every 10 minutes from the web pages for 24 monitoring sites located near water level monitoring stations operated by Fukushima Prefecture (http://kaseninf.pref.fukushima.jp/gis/) and the Ministry of Land, Infrastructure, Transport, and Tourism (MLIT) (http://www1.river.go.jp/). At other sites (nos. 1, 2, 3, 7, 15, 26), we measured the water level every 10 minutes using a pressure type water level gauge (Rugged Troll 100 by In situ inc.). That water level data was converted to a flow rate (Q) using the h-Q equation. Equations for each of the monitoring sites were obtained from flow measurement. For 16 (nos. 5, 6, 9, 10, 12, 13, 14, 17, 19, 20, 21, 22, 24, 27, 29, 30) sites, we use the equations obtained by Fukushima Prefecture and MLIT, while equations for the other 13 sites (nos. 1, 2, 3, 4, 7, 8, 11, 15, 16, 23, 25, 26, 28) were made by the authors based on the data obtained by Fukushima Prefectural Centre for Environmental Creation. When long-term data loss occurred due to water level gauge loss, the flow rate estimated from the ratio with another monitoring station on the same or adjacent river was used. The suspended solid concentration SSC was calculated from the turbidity data obtained every 10 minutes for 29 sites (i.e., all except for site 18). Initially, turbidity meters (NEP9350 by McVAN inc.) were installed at all sites. The turbidity meters were replaced with three different models after the 2015 fiscal year (SE-TV30MS by Senecom, Sensor DAKUDO by FieldPro, and ACLW2-USB by JFE Advantech) depending on the size of the river. All turbidity meters were calibrated using the same standard (bottom sediments in the Horai Dam reservoir on the Abukuma River). Then, conversion equations from turbidity to SSC were developed for all turbidity meters. Turbidity data may remain unusable for long periods of time due to sensor failure, burial, or obstruction. In such cases, the data were complemented in the following two methods. The power equation (Eq. 3) was used to estimate the monthly Qss (i.e. the products of Q and SSC) from the monthly flow rate Q for the data up to August 201510: 3 (MonthlyQss)=a×monthlyQb, where a and b are constants calculated from the monthly Qss and Q. In this case, the L is calculated as the product of C and monthly Qss. For data since September 2015, SSCs were estimated from Q every 10 minutes using the linear rating curve shown below28: 4 SSC=c×Q+d, where c and d are constants calculated from the relationship between the Q and the SSC at the same point that was used to supplement the SSC value. Figure 4 shows the monthly flux of suspended 137Cs accumulated from the start of the observation.Fig. 4 Cumulative flux of suspended 137Cs in the rivers in Fukushima area. The graph on the left shows the total fluxes since June 2011 for the six sites (nos. 1‒6) where turbidity has been observed since 2011. The graphs in the center and on the right show the accumulated fluxes since October 2012 for the other sites (nos. 7–11, 13–16, 19–30). Data Records Data on the 134Cs and 137Cs concentrations and grain size distribution of SS between June 2011 and March 2017 and fluxes of suspended radiocesium from September 2015 to March 2017 is available on Environmental Radioactivity Datasets website at CRiED, Univ. of Tsukuba (http://www.ied.tsukuba.ac.jp/database/)26–29 Monthly 137Cs flux data from June 2011 to August 2015 were published in the supporting information of Taniguchi et al.10 (10.1021/acs.est.9b02890). Table 2 summarized the number of data and differences in data processing methods between the two data sources.Table 2 Data sources. No. 1 2 Source Supporting Information of Taniguchi et al.10 (https://pubs.acs.org/doi/10.1021/acs.est.9b02890) Environmental Radioactivity Datasets website at CRiED, Univ. of Tsukuba (http://www.ied.tsukuba.ac.jp/database/) Target Flux of suspended 137Cs Activity concentrations of suspended 134Cs and 137Cs Grain size distribution of the SS samples Flux of suspended 134Cs and 137Cs Data number 27 Sites Total 1041 data 29 Sites Total 1917 data Temporal range Jun.2011 - Aug. 2015 Jun. 2011 - Mar. 2017 for radiocesium concentration Sep. 2015 – Mar. 2017 for the fiux of suspended 134Cs and 137Cs Differences in calculation methods for flux A power function was used to fill in the missing values for SSC. A linear rating curve is used to fill missing SSC values Files es9b02890_si_001.pdf DOI00014_data.csv DOI00015_data.csv DOI00020_data.csv DOI00021_data.csv This dataset is available on two websites. Differences in data processing methods are also summarized. Technical Validation Uncertainty of radiocesium concentrations The SS sampler, which was introduced by Phillips et al.31, has been reported as being problematic because it does not reproduce the particle size distribution of the suspended sand sized particles correctly due to the poor collection efficiency of fine particles32. As a result of water flume experiments, it was found that the collection efficiency improved as the flow velocity increased and the collection efficiency of particles with a diameter of <4 microns decreased33. This means that the SS sampler is able to collect SSs more efficiently during floods due to the higher flow velocity and larger mean grain size. The adsorption capacity of cesium (including 134Cs and 137Cs) on SSs is dependent on the particle size34,35. Since the adsorption of cesium occurs at the surface, the radiocesium concentration of each size fractions can be represented by the power function of SSA36. In the case of rivers in Fukushima, the following equations can be used to correct for the effect of particle size distribution9,37: 5 C′=C/P, and 6 P=Sr/Ss0.65, where C [Bq kg−1] and C’ [Bq kg−1] are the measured and corrected values of radiocesium concentration, respectively; P is a particle-size correction factor; Sr and Ss are the SSA of the standard sample and measured sample. The clay fraction by the LD method can be underestimated relative to that by the sieving/settling and pipette method in soil and coastal sediment cases38,39. Errors in the SSA caused by the difference in the measurement methods could not be quantitatively evaluated. A solution would be to avoid mixing data from different measurement methods in the cases of clay-rich sediments. In the measurement of the radioactivity concentration by the Germanium semiconductor detector, only the counting errors were described. If the amount of sampled SS is small, the error is large. Although many types of detectors were used, the accuracy was checked by measuring standard samples to ensure that there were no differences in measurement results between detectors. Uncertainty of radiocesium fluxes Errors in the 134Cs and 137Cs flux were assessed for data after September 201528. The errors included in the three quantities (the activity concentration of radiocesium [C], water discharge [Q], and SS concentration [SSC]) used to calculate the flux are treated as follows:The error in the radiocesium concentration C was considered only for the coefficient error of the measurement with the germanium semiconductor detector (eC). Errors included in the water discharge Q were not evaluated. This is because the h-Q formula provided by the Ministry of Land, Infrastructure, Transport and Tourism (MLIT), and the Public Works Department of Fukushima Prefecture does not include information on the errors. However, these public agencies conduct surveys and data analysis in accordance with Technical Guidelines for River Erosion Control to maintain the quality of the h-Q formula (http://www.mlit.go.jp/river/shishin_guideline/index.html). The error in the conversion from turbidity to SSC was not evaluated because it was confirmed to be highly linear (R2 > 0.99). When the SSC was complemented by the rating curve, the 95% prediction interval was used as the error essc. From the law of propagation of error, the error el contained in the flux every 10 minutes is calculated as follows. 7 l±el=Q×SSC±eSSC×C±ec=Q×SSC×C±C×eSSC2+(SSC×ec)2 The error eL included in the monthly flux L is expressed as follows. 8 L±eL=∑(l±el)=∑l±∑el2 Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Acknowledgements We received meaningful advice from experts invited by Fukushima Prefecture and the IAEA cooperation project. This work was partially supported by the Ministry of Education, Culture, Sports and Science (MEXT), and Nuclear Regulation Authority (NRA), Japan, Grant-in-Aid for Scientific Research on Innovative Areas grant number 24110005, and Environmental Radioactivity Research Network Center (F-19-10, F-19-47). Author contributions K.T., T.K. and K.Y. collected SS samples and measured the activity concentrations of radiocesium. Y.Y., H.S., W.B. and K.T. calculated the radiocesium flux. Y.O. planned the monitoring flamework and managed the network. Competing interests The authors declare no competing interests. ==== Refs References 1. Katata G Detailed source term estimation of the atmospheric release for the Fukushima Daiichi Nuclear Power Station accident by coupling simulations of an atmospheric dispersion model with an improved deposition scheme and oceanic dispersion model Atmos. Chem. Phys. 2015 15 1029 1070 10.5194/acp-15-1029-2015 2. Terada H Refinement of source term and atmospheric dispersion simulations of radionuclides during the Fukushima Daiichi Nuclear Power Station accident J. Environ. Radioact. 2020 213 106104 10.1016/j.jenvrad.2019.106104 31983441 3. Steinhauser G Brandl A Johnson TE Comparison of the Chernobyl and Fukushima nuclear accidents: A review of the environmental impacts Sci. Total Environ. 2014 470–471 800 817 10.1016/j.scitotenv.2013.10.029 4. Kato H Onda Y Gao X Sanada Y Saito K Reconstruction of a Fukushima accident-derived radiocesium fallout map for environmental transfer studies J. Environ. Radioact. 2019 210 105996 10.1016/j.jenvrad.2019.105996 31262609 5. Smith, J. T., Voitsekhovitch, O. V., Konoplev, A. V. & Kudelsky, A. V. Radioactivity in aquatic systems. In Chelnobyl - Catastrophe and Consequences (eds. Smith, J. T. & Beresford, N. A.) 139–190 (Praxis Pablishing, 2005). 6. Eyrolle F Charmasson S Importance of colloids in the transport within the dissolved phase (<450 nm) of artificial radionuclides from the Rhône river towards the Gulf of Lions (Mediterranean Sea) J. Environ. Radioact. 2004 72 273 286 10.1016/S0265-931X(03)00178-4 14972410 7. Nagao S Export of 134 Cs and 137 Cs in the Fukushima river systems at heavy rains by typhoon roke in September 2011 Biogeosciences 2013 10 6215 6223 10.5194/bg-10-6215-2013 8. Ueda S Fluvial discharges of radiocaesium from watersheds contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident, Japan J. Environ. Radioact. 2013 118 96 104 10.1016/j.jenvrad.2012.11.009 23274616 9. Yoshimura K Onda Y Sakaguchi A Yamamoto M Matsuura Y 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. 2015 139 370 378 10.1016/j.jenvrad.2014.08.021 25242014 10. Taniguchi K Transport and Redistribution of Radiocesium in Fukushima Fallout through Rivers Environ. Sci. Technol. 2019 53 12339 12347 10.1021/acs.est.9b02890 31490064 11. Tsuji H Factors controlling dissolved 137 Cs concentrations in east Japanese Rivers Sci. Total Environ. 2019 697 134093 10.1016/j.scitotenv.2019.134093 31487585 12. Nakanishi T Sakuma K Trend of 137 Cs concentration in river water in the medium term and future following the Fukushima nuclear accident Chemosphere 2019 215 272 279 10.1016/j.chemosphere.2018.10.017 30317098 13. Konoplev A Behavior of accidentally released radiocesium in soil–water environment: Looking at Fukushima from a Chernobyl perspective J. Environ. Radioact. 2016 151 568 578 10.1016/j.jenvrad.2015.06.019 26143175 14. Laceby JP Rainfall erosivity in catchments contaminated with fallout from the Fukushima Daiichi nuclear power plant accident Hydrol. Earth Syst. Sci. 2016 20 2467 2482 10.5194/hess-20-2467-2016 15. Chartin C The impact of typhoons on sediment connectivity: lessons learnt from contaminated coastal catchments of the Fukushima Prefecture (Japan) Earth Surf. Process. Landforms 2017 42 306 317 10.1002/esp.4056 16. Evrard O Radiocesium transfer from hillslopes to the Pacific Ocean after the Fukushima Nuclear Power Plant accident: A review J. Environ. Radioact. 2015 148 92 110 10.1016/j.jenvrad.2015.06.018 26142817 17. Yamashiki Y Initial flux of sediment-associated radiocesium to the ocean from the largest river impacted by Fukushima Daiichi Nuclear Power Plant Sci. Rep. 2014 4 1 7 18. Saito K Onda Y Outline of the national mapping projects implemented after the Fukushima accident J. Environ. Radioact. 2015 139 240 249 10.1016/j.jenvrad.2014.10.009 25457373 19. Saito K 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. 2015 139 308 319 10.1016/j.jenvrad.2014.02.014 24703526 20. Mikami S The air dose rate around the Fukushima Dai-ichi Nuclear Power Plant: Its spatial characteristics and temporal changes until December 2012 J. Environ. Radioact. 2015 139 250 259 10.1016/j.jenvrad.2014.08.020 25246092 21. Andoh M 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. 2015 139 266 280 10.1016/j.jenvrad.2014.05.014 24951121 22. Wakahara T Onda Y Kato H Sakaguchi A Yoshimura K Radiocesium discharge from paddy fields with different initial scrapings for decontamination after the Fukushima Dai-ichi Nuclear Power Plant accident Environ. Sci. Process. Impacts 2014 16 2580 2591 10.1039/C4EM00262H 25247992 23. Yoshimura K Onda Y Kato H Evaluation of radiocaesium wash-off by soil erosion from various land uses using USLE plots J. Environ. Radioact. 2015 139 362 369 10.1016/j.jenvrad.2014.07.019 25113169 24. Wakiyama Y Onda Y Yoshimura K Igarashi Y Kato H Land use types control solid wash-off rate and entrainment coefficient of Fukushima-derived 137 Cs, and their time dependence J. Environ. Radioact. 2019 210 105990 10.1016/j.jenvrad.2019.105990 31255315 25. MEXT & NRA. Nuclide Analysis of Suspended Sediment Sampled from Rivers in Fukushima Prefecture in the Distribution Survey of Radioactive Substances. Japan Atomic Energy Agencyhttps://emdb.jaea.go.jp/emdb/en/portals/b1030114/ (2017). 26. Taniguchi K Yoshimura K Onda Y 2020 Initial data on suspended 134 Cs and 137 Cs concentrations in the rivers within the 80km radius from Fukushima Daiichi Nuclear Power Plant Center for Research in Isotopes and Environmental Dynamics, University of Tsukuba 10.34355/CRiED.U.Tsukuba.00020 27. Taniguchi K Yoshimura K Onda Y 2020 Suspended 134 Cs and 137 Cs concentrations in the rivers within the 80km radius from TEPCO’s Fukushima Daiichi Nuclear Power Plant Center for Research in Isotopes and Environmental Dynamics, University of Tsukuba 10.34355/Fukushima.Pref.CEC.00014 28. Taniguchi K Yoshimura K Onda Y 2020 Monthly fluxes of suspended 134 Cs and 137 Cs in the rivers within the 80km radius from TEPCO’s Fukushima Daiichi Nuclear Power Plant Center for Research in Isotopes and Environmental Dynamics, University of Tsukuba 10.34355/Fukushima.Pref.CEC.00015 29. Taniguchi K Yoshimura K Onda Y 2020 Grain size distribution of the suspended solids in the rivers within the 80km radius from TEPCO’s Fukushima Daiichi Nuclear Power Plant Center for Research in Isotopes and Environmental Dynamics, University of Tsukuba 10.34355/Fukushima.Pref.CEC.00021 30. Onda Y Radionuclides from the Fukushima Daiichi nuclear power plant in terrestrial systems Nat. Rev. Earth Environ. 2020 1 644 660 10.1038/s43017-020-0099-x 31. Phillips JM Russell MA Walling DE Time-integrated sampling of fluvial suspended sediment: A simple methodology for small catchments Hydrol. Process. 2000 14 2589 2602 10.1002/1099-1085(20001015)14:14<2589::AID-HYP94>3.0.CO;2-D 32. Smith TB Owens PN Flume- and field-based evaluation of a time-integrated suspended sediment sampler for the analysis of sediment properties Earth Surf. Process. Landforms 2014 39 1197 1207 10.1002/esp.3528 33. Osanai, N., Mizuno, H., Okinaka, T. & Haramaki, T. Experimental study in sampling efficiency of suspended load with time-integrated sampler. Report no. Technical note of NILIM no.266 (National Institute for Land and Infrastructure Management, 2005). (in Japanese with English abstract). 34. Tanaka K Iwatani H Sakaguchi A Fan Q Takahashi Y Size-dependent distribution of radiocesium in riverbed sediments and its relevance to the migration of radiocesium in river systems after the Fukushima Daiichi Nuclear Power Plant accident J. Environ. Radioact. 2015 139 390 397 10.1016/j.jenvrad.2014.05.002 24874435 35. Sakaguchi A Size distribution studies of 137 Cs in river water in the Abukuma Riverine system following the Fukushima Dai-ichi Nuclear Power Plant accident J. Environ. Radioact. 2015 139 379 389 10.1016/j.jenvrad.2014.05.011 24929977 36. He Q Walling DE Interpreting particle size effects in the adsorption of 137 Cs and unsupported 210 Pb by mineral soils and sediments J. Environ. Radioact. 1996 30 117 137 10.1016/0265-931X(96)89275-7 37. Ministry of the Environment. Sampling and analytical method of radionuclide in the aquatic environment. Ministry of the Environmenthttps://www.env.go.jp/jishin/monitoring/report_method-w_1303.pdf (2012). 38. Yang X Determination of soil texture by laser diffraction method Soil Sci. Soc. Am. J. 2015 79 1556 1566 10.2136/sssaj2015.04.0164 39. Ramaswamy V Rao PS Grain size analysis of sediments from the northern Andaman Sea: Comparison of laser diffraction and sieve-pipette techniques J. Coast. Res. 2006 22 1000 1009 10.2112/04-0162.1