Spatial prediction of radioactive Cs concentration in agricultural soil in eastern Japan

As a result of the accident at the Fukushima Dai-ichi Nuclear Power Station (FDNPS) operated by the Tokyo Electric Power Company, radioactive cesium (Cs) was released into the surrounding environment. To determine the extent of decontamination required in agriculture fields and to consider management options, we surveyed and measured soil Cs concentrations in 3461 agricultural fields, and used these data to construct a distribution map of radioactive Cs concentration in agricultural soil in eastern Japan. Soil Cs concentration was positively correlated with radiation dose (r(2) = 0.89, n = 2199). This linear correlation was affected to some extent by soil surface condition, soil group and land use type. Linear regression analysis was conducted by land surface condition, soil type and land use type. We constructed a soil Cs concentration map by regression and by a regression-kriging (RK) method that combines regression equations with ordinary kriging of the regression residuals. Prediction accuracy of the RK was higher than that of the regression; we therefore adopted the RK. Total radioactive Cs concentration in soil was highest in the 20-km evacuation zone surrounding FDNPS, and tended to be higher to the northwest of FDNPS than in other directions. About 5900 ha of paddy fields and 3000 ha of upland fields in Fukushima Prefecture were above contamination level 2 (> 5000 Bq kg(-1)), and were mostly distributed in the evacuation zone.