Parameters of Iodine Migration in Soil
Mutsumi YAMAGAMI, Akira TAKEDA, Yusuke UNNO
Yuichi TAKAKU, Shun’ichi HISAMATSU
Department of Radioecology
Abstract
7Iodine-129 (half-life, 1.6 × 10 y) is one of the important radionuclides discharged from the firstcommercial nuclear fuel reprocessing plant in Rokkasho, Japan for the assessment of radiation dose to thepublic. A part of the 129I discharged to the atmosphere from the plant is deposited on the land surface andretained in surface soil. Downward migration of 129I in soil is important for the prediction of its concentrationin both ground water and surface soil during and after long-term operation of the reprocessing 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 2014, we studied:
1) the downward migration rate of 125I in core samples of subsurface soil (0.5–3 m); 2) chemical form of
stable I in surface soil in grassland and forest; and 3) the effect of pasture grassed and wild plants on thechemical form of stable I in soil solution by an experiment with cultivation pots.
A core sample in subsurface soil (0.5–3 m) was collected from grassland in Rokkasho to study thedownward migration rate of I. The distribution coefficient (Kd) values of 125I, as well as 85Sr and 137Cs, in thesoil samples fractionated from different depths of the soil core sample 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, under the assumption that half of the precipitation in Rokkasho
permeates in the soil. The downward migration rate of 125I, 85Sr and 137Cs ranged from 0.04 to 41 mm y−1,0.02–0.16 mm y−1 and 0.005–0.022 mm y−1, respectively. The downward migration rate of 125I significantly
increased in soil under 2 m depth, while that of 137Cs and 85Sr in 0.5-3.0 m depth showed gradually
increasing and decreasing trends with soil depth, respectively.
To study the chemical forms of stable I in soil, surface soil samples were collected at four points ineach grassland area and forest around the nuclear fuel reprocessing plant at 5 cm depth intervals up to 20 cm.
The organic layer including the root mat and O-horizon were also collected at each point. Soil solution of thesurface soil sample was obtained by a centrifugation method and analyzed for I-, IO3- and total I separately.
The concentration of the total I in the soil solution ranged from 2.8 to 46 μg-I L-1. The major form of I in thesoil solutions was I− (61%), whereas IO3- was not detected in any of the soil solution samples. Organic-I,
which is assumed to be the difference between the total I and the sum of I- and IO3-, was also a significant
form of I in the soil solution. Water-soluble I, which was extracted at 1:10 (w/v) ratio with deionized water,
occupied 0.03-1.2% of the total I in the organic layer and subjacent soil (0–5 cm) samples. The concentrationof the water-soluble I positively correlated with that of water-soluble organic carbon, suggesting that
migration of I would be affected by behavior of the organic carbon in the surface soils.
The chemical forms of stable I in the soil solution in cultivation pots of pasture grasses and wild plants
were examined to study the effect of plant cultivation on I in soil. Concentrations of I-, IO3- and organic-I in
the soil solution were analyzed 1 d after adding I- or IO3- solutions to the cultivated soil surface.

Concentrations of both I- and organic-I in the soil solution samples were increased by the plant cultivation.
The increasing ratio of cultivated soil to non-cultivated control depended on the cultivated plants. No IO3−
was detected in any soil solution samples.

Fig. 1

Vertical profiles of downward migration rate.
Open circles, 125I-; Open squares, 85Sr; Smallopen rhombuses, 137Cs. Migration rates of 0 to 0.5
m were adapted from the previous report in FY