Abstract The largest concern on the cesium-137 (137Cs) deposition and its soil contamination due to the emission from the Fukushima Daiichi Nuclear Power Plant (NPP) showed up after a massive quake on March 11, 2011. Cesium-137 (137Cs) with a half-life of 30.1 y causes the largest concerns because of its deleterious effect on agriculture and stock farming, and, thus, human life for decades. Removal of 137Cs contaminated soils or land use limitations in areas where removal is not possible is, therefore, an urgent issue. A challenge lies in the fact that estimates of 137Cs emissions from the Fukushima NPP are extremely uncertain, therefore, the distribution of 137Cs in the environment is poorly constrained. Here, we estimate total 137Cs deposition by integrating daily observations of 137Cs deposition in each prefecture in Japan with relative deposition distribution patterns from a Lagrangian particle dispersion model, FLEXPART. We show that 137Cs strongly contaminated the soils in large areas of eastern and northeastern Japan, whereas western Japan was sheltered by mountain ranges. The soils around Fukushima NPP and neighboring prefectures have been extensively contaminated with depositions of more than 100,000 and 10,000 MBq km-2, respectively. Total 137Cs depositions over two domains: (i) the Japan Islands and the surrounding ocean (130–150 °E and 30–46 °N) and, (ii) the Japan Islands, were estimated to be more than 5.6 and 1.0 PBq, respectively. We hope our 137Cs deposition maps will help to coordinate decontamination efforts and plan regulatory measures in Japan.

A catastrophic earthquake and tsunami occurred on March 11, 2011, which caused destruction in northeastern Japan and severely damaged the Fukushima Daiichi Nuclear Power Plant (NPP). This event led to emissions of radioactive materials from the NPP (1), albeit at unknown and likely strongly varying release rates (1–3). Among these materials, with a half-life of 30.1 y (4), cesium-137 (137Cs) causes the largest concerns because of its deleterious effect on agriculture and stock farming, and, thus, human life for decades. Removal of 137Cs-contaminated soils or land use limitations in areas where removal is not possible is, therefore, an urgent issue. The Japanese government, general public, and scientists have been waiting for the information of the spatial distributions of 137Cs deposition and its soil contamination over all of Japan.

The aerosol-bound 137Cs can be removed from the atmosphere and brought to the surface by dry or wet deposition. Analysis of data collected after the Chernobyl accident has shown that 137Cs adsorbed in the top soil layer can remain there for many years (5, 6), restricting land use, e.g., for food production, of highly contaminated areas for a long time. To minimize the impacts on human health of soil contamination in Japan due to the Fukushima NPP accident, spatial maps of 137Cs deposition and concentrations in soil are urgently needed. Sporadic sampling of the soils in and around Fukushima prefecture has been carried out after the NPP accident under the instruction by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) (7) and others (Table S1). However, it is impossible to fully capture the distribution of 137Cs deposition across Japan from a limited number of in situ measurements alone. On the other hand, reliable estimates using dispersion models are also not available because of the largely unknown source term. Not only is the total release of 137Cs from the damaged NPP poorly known, but also its variation with time is even more uncertain. Although first attempts to estimate it have been made (2) and another study tried to estimate its deposition based on the previous study (2) over the limited areas around Fukushima prefecture with a regional chemical transport model (8), these estimates on the emission rate are highly uncertain and the discussion on deposition over all of Japan has not been made.

In this study we quantitatively estimate the spatial distribution of the 137Cs deposition and its soil contamination over all of Japan. We take relative deposition distribution patterns from a Lagrangian particle dispersion model, FLEXPART (Materials and Methods and SI Text) (9), using a constant source term [as assumed also in some simulations because of high uncertainty of the emission amount (10–16)]. We fuse daily varying observations of 137Cs deposition in each Japanese prefecture (17) into the modeled deposition fields to obtain quantitative deposition estimates.

Discussion There are many important agricultural regions in Japan. In Japan, the limit for the sum of 134Cs and 137Cs concentrations (as total cesium) in soil is 5,000 Bq kg-1 under the Food Sanitation Law (24). Considering that about half (2,500 Bq kg-1) of the total radioactive cesium deposition is due to 137Cs, the east Fukushima prefecture exceeded this limit and some neighboring prefectures such as Miyagi, Tochigi, and Ibaraki are partially close to the limit under our upper bound estimate (Movie S4) and, therefore, local-scale exceedance is likely given the strong spatial variability of 137Cs deposition. For those three prefectures, detailed soil sampling is recommended in the near future. Estimated and observed contaminations in the western parts of Japan were not as serious, even though some prefectures were likely effected to some extent (Fig. 3, Movie S4, and Table S4). Concentrations in these areas are below 25 Bq kg-1, which is far below the threshold for farming. However, we strongly recommend each prefecture to quickly carry out some supplementary soil samplings at city levels to validate our estimates even if the concentrations are low. The relatively low contamination levels over western Japan can be well explained by the Japanese topography. The eastern and northeastern parts of Japan are surrounded by mountain ranges such as the Kanto, Echigo, and Ohwu mountain ranges (Fig. S6) (25), which, to a large extent, sheltered the northwestern and western parts of Japan from the dispersion of radioactive material. It is worth noting, however, that relatively higher contamination levels can be seen over the Hida, Chugoku, and Shikoku mountain ranges (Fig. 3, Fig. S6, and Movie S4), probably due to orographic enhancement of precipitation and, thus, wet deposition of 137Cs. In Hokkaido, to the north of Japan’s main island, both lower altitude and higher altitudes such as the Yubari and Hidaka mountain ranges are effected by 137Cs deposition, partially due to direct transport from the Fukushima NPP via the Pacific Ocean as shown in Movies S1 and S2 and also as simulated by another atmospheric transport model (12). We estimate that a total of more than 5.6 and 1.0 PBq 137Cs were deposited over Japan and the surrounding ocean (130–150 °E and 30–46 °N), and the Japan Islands in this domain only, respectively (Fig. 2A). Although the estimate for the larger domain is quite uncertain because it is constrained only by measurements in Japan, these numbers are consistent with a suspected total release of about 12 PBq 137Cs (2). Most of the deposition occurred over the Pacific Ocean, yet soil concentrations of 137Cs are above 100 Bq kg-1 over large areas of eastern Japan (Fig. 3). According to our results, food production in eastern Fukushima prefecture is likely severely impaired by the 137Cs loads of more than 2,500 Bq kg-1 (upper limit of farming) and also partially impacted in neighboring provinces such as Iwate, Miyagi, Yamagata, Niigata, Tochigi, Ibaraki, and Chiba, where values of more than 250 Bq kg-1 cannot be excluded (Fig. 3 and Movie S4). Notice also that our estimates are based on a transport model driven with meteorological analysis data from a global model. Such a model cannot fully capture all complexities of the regional wind field over Japan and, in particular, does not resolve the high spatiotemporal variability of precipitation. Therefore, we expect the true soil contamination across Japan to be considerably more variable than in our estimate. Even in regions where we find relatively low soil contamination levels, hot spots with high concentrations (e.g., due to convective rain fall, orographic enhancement of rainfall, or fine-grain soil flow by rainwater on the ground) may be possible. In contrast, relatively clean patches may also be present in areas with high overall contamination levels. Despite these shortcomings, we expect our results to be useful for regulatory measures and for guiding monitoring activities toward areas with expected high 137Cs burdens. We hope this study will contribute to understanding the contamination issue in Japan.

Materials and Methods Observations of Cesium-137 Deposition and Concentration in Soil in each Prefecture. From March 18, MEXT has been observing daily radioactivity levels in deposition in most of the prefecture (17). The exact coordinates of the sampling locations were individually accessible through our contacts to MEXT (Table S2). The deposition data between March 18 and 19 were not used in our estimate because of no depositions at observatories from the modeled DR maps as mentioned in the main text. In some prefectures, data were missing or unavailable [Miyagi, March 18–April 19 (completely no observations); Yamagata, March 29–April 3; Fukushima, March 18–March 26 and April 4; Gifu, March 24, 25, 27, 28, and 30; Nara, March 18–21 and April 15–18; Oita, March 22–26). FLEXPART and Estimated 137Cs Deposition. FLEXPART (9) is a Lagrangian particle dispersion model simulating transport, diffusion, dry and wet deposition, and radioactive decay of radioactive materials such as 131I, 137Cs, and 133Xe (See http://transport.nilu.no for further details on FLEXPART). In this study, continuous emission from the Fukushima Daiichi NPP was assumed after 1800 hours coordinated universal time (UTC) on March 11, 2011. The simulation ended at 0000 hours UTC on April 20. FLEXPART was forced with the European Center for Medium-Range Weather Forecasts (ECMWF) operational analysis data with a global resolution of 1° × 1° and 0.18° × 0.18° for 120–168 °E and 25–50 °N. The output had a resolution of 0.2° × 0.2° and was recorded every 3 h (SI Text). For each day, we first normalized the modeled daily accumulated deposition in each grid cell with the maximum accumulated deposition value for the model domain, hereafter called daily deposition ratio (DDR) maps: [1]where FPD (x,y)i is the three-hourly modeled deposition in grid cell (x,y) and FPD max is the maximum daily deposition value found in the entire model domain. T is the number of model output timesteps per day (T = 8). Daily gridded deposition values of 137Cs were estimated by scaling the DDR map with available daily observed 137Cs depositions in each prefecture by MEXT (17) by the following equation: [2]where Depo (x,y) is the estimated daily total 137Cs deposition in grid cell (x,y), Depo i(Obs.) is the observed 137Cs deposition at location i (Table S2), N ≤ 47 is the number of available counts on a certain day in Japan’s 47 prefectures, DDR i(Obs.Loc.) is the DDR (x,y) in the grid point where 137Cs deposition was observed, and DDR (x,y) is the DDR in grid cell (x,y). Only the cases with both the observed deposition and the DDR i(Obs.Loc.) not equal to zero at each observatory location were used for counting N on each day. Because the Depo i(Obs.) to DDR i(Obs.Loc.) scaling factor in Eq. 2 becomes infinite when the simulated DDR value is close to zero but deposition is actually observed, a minimum positive DDR value, DRT, needs to be used to derive the scaling. Several DRTs of 0.001, 0.005, 0.007, 0.01, 0.05, and 0.1 for DDR i(Obs.Loc.) within the simulation domain on each day were used to avoid abnormally high Depo (x,y) values due to dividing by small values (SI Text). If DDR i(Obs.Loc.) at a certain grid point was less than a DRT value, DDR i(Obs.Loc.) was set to the DRT value. For computing total 137Cs deposition between March 20 and April 19, we corrected all values to April 19 using a half-life of 137Cs of 30.1 y (4). The sum of all the daily observed or estimated 137Cs depositions is the total 137Cs deposition (Fig. 2 and Fig. S4A). Observations on 137Cs Concentrations in Soil and Grass. For comparison with our estimates, measurements of 137Cs concentrations in soil or grass were used (SI Text and Table S1). Mean transfer factor of soil-to-grass of 0.13, which was obtained from the observations in Japanese soil and grass, was used to convert grass contamination to soil equivalent contamination (grass contamination divided by the transfer factor) (23). The times and locations of those samplings varied. To cover the time period of our study (March 20–April 19), we also used some soil samples from later dates, but we did not use any data after May 19. Notice also that the soil samples were also effected by 137Cs deposition before March 20 (SI Text). Some observatories measured total cesium concentration including both 137Cs and 134Cs. In that case, we assumed that half of the total Cs was 137Cs. To convert the 137Cs deposition into soil concentration, soil depth and density information are needed. However, it is currently difficult to obtain this information across all of Japan. There is an empirical relationship on the ratio between 137Cs concentration and deposition from 0 to 5 cm soil, paddy soil, and field soil samples (22) (Fig. S5). We considered the mean value of the ratio as CC of 53 ± 15 kg m-2 reflecting the 5-cm depth soil information and its density. Our estimated CC value is close to the CC value of 65 kg m-2 assumed by MEXT (26) with 5-cm soil and a soil density of 1,300 kg m-3. Dividing our estimated deposition (MBq km-2 = Bq m-2) by the CCs, we empirically obtained the mean 137Cs concentration in soil (Bq kg-1).

Acknowledgments Useful comments were obtained from K.-M. Kim Morgan State University (MSU)/Goddard Earth Sciences Technology and Research (GESTAR) and Q. Tan Universities Space Research Association (USRA)/GESTAR. Daily deposition of radioactive materials, atmospheric radiations, and concentrations in soil used in this study were observed by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), each prefecture in Japan, the Ministry of Agriculture, Forestry and Fisheries (MAFF), and Prof. Yamazaki et al. at Kinki University. We also appreciate all the people working on these measurements. The tropical rainfall measuring mission (TRMM, 3B42 V6 product) data used in this study were acquired using the Goddard Earth Sciences (GES)-Data and Information Services Center (DISC) as part of the National Aeronautics and Space Administration’s GES-DISC. The Grid Analysis and Display System (GrADS) was use for plotting. This paper was partially supported by Universities Space Research Association.

Footnotes Author contributions: T.J.Y. designed research; T.J.Y., A.S., J.F.B., and S.E. performed research; T.J.Y. and R.S.H. analyzed data; and T.J.Y., A.S., J.F.B., and T.Y. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

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