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

The largest concern on the cesium-137 (Cs-137) 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 (Cs-137) 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 Cs-137 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 Cs-137 emissions from the Fukushima NPP are extremely uncertain, therefore, the distribution of Cs-137 in the environment is poorly constrained. Here, we estimate total Cs-137 deposition by integrating daily observations of Cs-137 deposition in each prefecture in Japan with relative deposition distribution patterns from a Lagrangian particle dispersion model, FLEXPART. We show that Cs-137 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 Cs-137 depositions over two domains: (i) the Japan Islands and the surrounding ocean (130-150 degrees E and 30-46 degrees N) and, (ii) the Japan Islands, were estimated to be more than 5.6 and 1.0 PBq, respectively. We hope our Cs-137 deposition maps will help to coordinate decontamination efforts and plan regulatory measures in Japan.