Parameters of Iodine Migration in Soil
Hirofumi TSUKADA, Mutsumi YAMAGAMI, Akira TAKEDA,
Yuichi TAKAKU, Shun’ichi HISAMATSU
Department of Radioecology
Abstract
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Iodine-129 (half-life, 1.6 × 10 y) is one of the important radionuclides discharged from the firstcommercial nuclear fuel reprocessing plant in Rokkasho, Japan that must be considered for the assessment ofradiation dose to the public. A part of the 129I discharged to the atmosphere from the plant is deposited on theland surface and retained in surface soil. Downward migration of 129I in soil is important for the prediction of
its concentration in both ground water and surface soil during and after long-term operation of thereprocessing plant. The aims of this study are to evaluate the rate of downward migration of 129I in soil
around the reprocessing plant and to clarify physico-chemical and biological factors affecting the migration
rate. In FY 2013, we studied 1) the downward migration rate of 125I− and 125IO3− in core samples of surface
soil, 2) the effect of soil water condition on the chemical form of stable I in soil solution, and 3) the effect ofrice plant root activity on the chemical form of stable I in cultivated soil solution.
Undisturbed soil core samples were collected at a forest in Rokkasho to study the downward migration
rate of I. The distribution coefficient (Kd) values of 125I− and 125IO3−, as well as 85Sr and 137Cs, in the soil
samples fractionated from different depths (0–50 cm) of the soil core samples were measured by the batch
sorption method. Downward migration rate of the nuclides in a soil was estimated by using a retardation
factor which was obtained by the measured Kd value. The downward migration rate of 125I in the two forms
examined ranged from 0.1 to 69 mm y−1, where higher values were observed within 3 cm in depth. The
downward migration rates of 85Sr and 137Cs were 0.9–7.5 mm y−1 and 0.03–2.2 mm y−1, respectively.
Undistributed soil columns were prepared from the core samples, and the solution of 125I was added on thesoil surface of each column. Vertical distribution of the nuclides was investigated after passing simulated
rainwater through it. The vertical distribution was not different between 125I– and 125IO3– in the 0–20 cm soilcolumn while the distribution in a soil column deeper than 20 cm suggested that the downward migration
rate of 125IO3- was higher than that of 125I-.
Surface soil samples were incubated during 28 d under different water conditions: 30, 60, 100, 200 and300% of the maximum water holding capacity. Concentrations of I-, IO3- and total I in the soil solution
collected at pre-determined periods after starting the incubation were analyzed. The concentration of the total
I in the soil solution was remarkably high under the soil reducing condition at higher water content, in whichI- was the main form of iodine in the solution. No IO3− was detected in any soil solution samples.
The chemical forms of stable I in the cultivation solution of different rice cultivars with different root
activities were examined to study the effect of plants on I. Concentrations of I-, IO3- and organic-I in thesoil solution were analyzed 4 d after adding I- or IO3- solutions to the cultivated soil surface.
Concentrations of I− in all soil solution samples were not different from that in the soil solution without
plants, so independent of the rice cultivars, while organic-I concentrations were increased for the plantcultivation, showing dependence on the rice cultivars. No IO3− was detected in any soil solution samples.

To investigate the effect of plant root activity on iodine speciation, rice plant cultivars were cultivatedby sand-hydroponics using the cultivation solution with added I- and IO3-. The concentrations of both Iand IO3- in the solution decreased in the cultivation period. The I- concentration depended on the rice
plant cultivars, while the IO3- concentration did not.