Cesium-137 deposition and contamination of Japanese soils due to the Fukushima nuclear accident

Abstract

The largest concern on the cesium-137 ((137)Cs) 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 ((137)Cs) 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 (137)Cs 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 (137)Cs emissions from the Fukushima NPP are extremely uncertain, therefore, the distribution of (137)Cs in the environment is poorly constrained. Here, we estimate total (137)Cs deposition by integrating daily observations of (137)Cs deposition in each prefecture in Japan with relative deposition distribution patterns from a Lagrangian particle dispersion model, FLEXPART. We show that (137)Cs 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 (137)Cs 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 approximately 6.7 and 1.3 PBq, [corrected] respectively.We hope our (137)Cs deposition maps will help to coordinate decontamination efforts and plan regulatory measures in Japan.

Fig. 1.

Cesium-137 deposition maps. (A) Relative deposition contributions between March 11 and 19, showing the areas potentially effected by ¹³⁷Cs before the start of measurements. The sums of the depositions during the period were divided by the maximum deposition in the accumulated field. (B) The same as in A, but for March 20–April 19. (C) An example of estimated daily deposition of ¹³⁷Cs on March 21. Squares in black denote the observation locations in each prefecture (Table S2). (D) Daily accumulated rainfall on March 21 by TRMM.

Fig. 2.

Total deposition of ¹³⁷Cs. (A) Gridded total ¹³⁷Cs deposition values for the period March 20–April 19 using our reference DRT value of 0.005. Outputs with 0.2° × 0.2° were interpolated to finer grid using cubic interpolation. Squares in black denote the observation locations in each prefecture (Table S2). (B) Comparisons between total observed depositions for the period March 20–April 19 and estimates at the grid point of each observatory location (Table S2) in the selected prefectures, using different DRT values to derive the scaling factor for the model output. Orange, gray, and black boxes denote no observation (Miyagi) and missing observations (Yamagata, between March 29 and April 3; Fukushima, before March 27 and April 4), respectively.

Fig. 3.

The ¹³⁷Cs concentration in soil. We used DRT of 0.001 and CC of 53 kg m^-2. Outputs with 0.2° × 0.2° were interpolated to finer resolution using cubic interpolation. The Merged IBCAO/ETOPO5 Global Topographic Data Product (25) was used to mask out ocean area below 0 m above sea level (a.s.l.).

Fig. 4.

Atmospheric, soil, and grass observation-based Cs-137 concentrations, and estimates based on the scaled model output and for different DRT values used for the scaling. (A) Comparisons in northern prefectures. Aomori and Miyagi prefectures had no ¹³⁷Cs detections on the daily deposition data and no measurements, respectively. The minimum value in Yamagata prefecture is no detection and no lower error bar is shown. (B) The same as in A, but around Kanto area. Lower and upper error bars denote minimum and maximum concentrations using CC of 68 and 38 kg m^-2 based on Fig. S5, respectively. Orange, gray, and black boxes denote no observation (Miyagi) and missing observations (Yamagata, between March 29 and April 3; Fukushima, before March 27 and April 4), respectively. A soil-to-grass transfer factor of 0.13 (23) was used to convert grass to soil contamination. For Fukushima prefecture, only the soil observations in Fukushima City were used, excluding other observations close to the Fukushima NPP. The data source for the comparisons are summarized in Table S1.