Burial Experiment of Soil Contaminated By Radiocaesium At a Paddy Field in Iidate Village, Fukushima Prefecture.

Most of radioactive cesium released from Fukushima Daiichi nuclear power plant has been accumulated in the topsoil within 5 cm. For decontamination of the top soil, Japanese government (Ministry of Agriculture, Forestry and Fisheries) has authorized three methods: topsoil stripping method, puddling method, and plowing method to replace surface soil with subsoil. Among three methods, the topsoil stripping method is being carried out and a lot of flexible container bags containing contaminated topsoil are piled up in the paddy field. We have not yet found the final disposal site of the contaminated soil. For agricultural regeneration and early return village, it is urgent and important to find a feasible decontamination method that farmers can conduct by themselves. Therefore, we are challenging a field test that buried the contaminated soil in the ground by a combination of the topsoil stripping method and the plowing method. We named this method "Madei-method" that means we treat contaminated soil very carefully. Currently, we are monitoring the radiation dose from the buried contaminated soil by using a newly developed soil radiation sensor. At the moment, leakage of radioactive cesium has not been confirmed from the buried contaminated soil despite rapid changes in ground water due to rainfall and irrigation to the paddy. This can be explained in terms of the filtering effect of the clay particles even if water penetrates the contaminated soil and colloidal clay particles move downward.