Time Dependence of the 137Cs Concentration in Particles Discharged from Rice Paddies to Freshwater Bodies after the Fukushima Daiichi NPP Accident

The concentration of particulate Cs-137 in paddy fields, which can be a major source of Cs-137 entering the water system, was studied following the Fukushima Daiichi Nuclear Power Plant accident. To parametrize the concentration and to estimate the time dependence, paddy fields covering various levels of Cs-137 deposition were investigated over the period 2011-2013 (n = 121). The particulate Cs-137 concentration (kBq kg(_ss)(-1)) showed a significant correlation with the initial surface deposition density (kBq m(-2)). This suggests that the entrainment coefficient (m(2) kg(-ss)(-1)), defined as the ratio between the particulate Cs-137 concentration and the initial surface deposition density, is an important parameter when modeling Cs-137 wash-off from paddy fields. The entrainment coefficient decreased with time following a double exponential function. The decrease rate constant of the entrainment coefficient was clearly higher than that reported for other land uses for river water. The difference in the decrease rates of the entrainment coefficient suggests that paddy fields play a major role in radiocesium migration through the water system. An understanding of the decrease rate of the entrainment coefficient of paddy fields is therefore crucial to understand the migration of radiocesium in the water system.