Behavior of radiocesium in decontaminated paddy fields in Fukushima Prefecture, Japan

Abstract

The 2011 Great East Japan earthquake and tsunami led to dispersal of radionuclides into the surrounding area from the Tokyo Electric Power Company’s Fukushima Daiichi Nuclear Power Plant. Before agricultural activities can safely resume after this accident, it is necessary to evaluate the dynamics of radiocesium in irrigation water in the area. We measured the concentrations of total and dissolved radiocesium in irrigation water and analyzed radiocesium concentrations in rice and soil samples, the soil to brown rice transfer factor (TF) in two decontaminated paddy fields located within the zone where paddy rice culture was prohibited, and radiocesium/water balance in a paddy field in 2014. Our key findings were as follows: (1) about 85% of the radiocesium in the irrigation water did not drain away but accumulated in the paddy field; (2) in comparison with the amount of the radiocesium deposited in the fields, the amount of radiocesium that accumulated during irrigation was approximately 0.076%; (3) the outflow of total radiocesium from paddy field was 13.0% of the inflow to a paddy field (irrigation water and fallout); (4) the soil to brown rice TF was 0.0015–0.0068 in the decontaminated paddy fields where soil improvement was performed to increase the content of exchangeable potassium to 200 (mg K)/kg soil before the conventional application; and (5) the concentration of radiocesium in brown rice was about 2% of the standard limit in Japan (100 Bq/kg), and the impact on brown rice from radiocesium in the irrigation water was limited.