Mass Interception Fractions and Weathering Half-lives of Iodine-131 and Radiocesium in Leafy Vegetables Observed after the Fukushima Daiichi Nuclear Power Plant Accident

Abstract

Background: This study was carried out to provide environmental transfer parameter values to estimate activity concentrations of these radionuclides in agricultural crops when direct contamination occurred.Materials and Methods: Mass interception fractions (<i>F_B</i>s) and weathering half-lives (<i>T_w</i>s) of 131I and radiocesium were calculated using openly available monitoring data obtained after the Fukushima Daiichi Nuclear Power Plant accident. <i>F_B</i> is the ratio between the initial radioactivity concentration of a radionuclide retained by the edible part of the plant (Bq‧ kg-1 fresh weight [FW]) and the amount of deposited radionuclide in that area (Bq‧m-2). <i>T_w</i> values can be calculated using activity concentrations of crops decreased with time after the initial contamination.Results and Discussion: Calculated <i>F_B</i> and <i>T_w</i> values for 131I and radiocesium were mostly obtained for leafy vegetables. The analytical results showed that there was no difference of <i>F_B</i>s between 131I and radiocesium by t-test; geometric mean values for leafy vegetables cultivated under outdoor conditions were 0.058 and 0.12 m2 ‧ kg-1 FW, respectively. Geometric mean <i>T_w</i> value of 131I in leafy vegetables grown under outdoor conditions was 8.6 days, and that of radiocesium was 6.6 days; there was no significant difference between <i>T_w</i> values of these radionuclides by Wilcoxon rank sum test.Conclusion: There was no difference between 131I and radiocesium for <i>F_B</i>s and <i>T_w</i>s. By using these factors, we would be able to carry out a rough estimation of the activity concentrations of 131I and radiocesium in the edible part of leafy crops when a nuclear accident occurred.