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 2015, we studied:
1) the downward migration rate of radioiodine in core soil samples (3–20 m), and 2) factors controlling thedownward migration rate of radioiodine in surface soil.
A core sample of 3–20 m was collected from a pasture field in Rokkasho to study the downward
migration rate of I. The distribution coefficient (Kd) values of 125I, as well as 85Sr and 137Cs, in the soil
samples fractionated from different depths of the soil core sample were measured by the batch sorption
method. Downward migration rate of the nuclides was estimated up to the depth of 11.7 m at which a watertable was found. The downward migration rate was estimated by using a retardation factor, which wasobtained by the measured Kd value, under the assumption that half of the precipitation in Rokkashopermeates in the soil. The downward migration rate of the 125I from the 3 m depth and deeper was higher
than that in the upper 3 m and almost constant up to the water table, while the rates of 137Cs and 85Sr
gradually decreased with depth in the soil profile.
To study the controlling factor of the migration rate of radioiodine, surface soil samples were collected
at four points around the nuclear fuel reprocessing plant from different depths up to 50 cm. Soil samples
before and after the pot cultivation of pasture grasses were also collected. The Kd of 125I and soil chemical
properties were determined for those samples and the relationships between the Kd and soil properties wereexamined. The Kd value of 125I in the soil samples was positively correlated with Kd value of organic carbon,
which was calculated from the concentrations of soil total carbon and dissolved organic carbon. The results
suggest that radioiodine is transformed from inorganic to organic form in surface soil rapidly after deposition,and migration in soil is controlled by the partitioning of dissolved organic substances.

Downward migration rate (mm y-1)

Surface soils
Pot cultivation soils
Pasture soils
Forest soils

Depth (m)y = 0.99 x ‒ 1.1
r = 0.82

Fig. 1 Downward migration rate 125I-, 85Sr and 137Cs
in soil samples from different depths in a
pasture field. Results were obtained in
FY2012, FY2014 and FY2015.

Fig. 2 Kd of 125I- and organic carbon in soils.
Kd (125I-), Kd of 125I- evaluated from batch sorption
experiment of 125I-. Kd (OC), Kd of organic carbon
evaluated from concentration of soil total carbon anddissolved organic carbon. Surface soils represents
soil samples from different depths (0-50 cm)
collected from four different sites around the nuclear
fuel reprocessing plant.
Pot cultivation soils represents soil samples before
and after the pot cultivation experiments. Pasturesoils represents soil samples from different depths(0-3 m) in a pasture field. Forest soils represents soil
samples from different depths (0-3 m) in a forestarea.