Behavior of Trace Elements on Leaf Surfaces of Crop Plants
Hitoshi KAWABATA, Hidenao HASEGAWA, Hirofumi TSUKADA,
Yuichi TAKAKU, Shun'ichi HISAMATSU
Department of Radioecology
Abstract
Radionuclides released into the atmosphere are deposited on the leaf surfaces of crop plants, taken up
by the plants, and translocated to other parts from the leaves. Some amount of the radionuclides deposited
onto the leaves is removed from the surface by the environmental process called weathering, i.e. removal byrain, wind, etc. Although weathering and translocation are important processes involved in the radiation dose
assessment of radionuclides from plants, site-specific parameters to describe those processes have not yet
been elucidated. This work aims to establish site-specific parameters for those processes for Cs, Sr, and I
using stable elements in a climate chamber in which meteorological conditions are controlled. The effect ofrainfall or fog on the behaviors of I (I-, IO3-) on leaf surfaces was studied in FY 2010. Compartment models
were constructed for describing the behaviors of Cs, Sr and I by using results from the studies in FY
2007-2010, and the weathering half-lives by rainfall for the elements were obtained with the models.
After applying solid aerosols of NaCl containing NaI or NaIO3 onto the leaf surfaces of radish,Raphanus sativus, the plants were treated at different rainfall intensities and rainfall durations using a rain
simulator or at different atmospheric liquid water contents and fog durations using a fog simulator. After aplant sample was collected, its leaf surface was washed with a solution containing detergent. The resulting
washing solution and the plant sample were analyzed for I. The proportion of I remaining on the leaf surface
decreased as the rainfall intensity and duration increased, and in an experiment with a fixed rainfall intensity,
the proportion of I remaining decreased according to a function with two exponential terms for rainfall
duration.

Exposure to fog with the atmospheric liquid water content of 3.5 mg m-3, that is the geometricmean value for fog in Rokkasho, had no effect on removing I from the leaf surface. When the atmosphericliquid water content exceeded 3.5 mg m-3, I was removed from the leaf surface depending on the liquidwater content and duration of the fog. The proportion of I remaining on the leaf surface in an experimentwith a fixed liquid water content exponentially decreased with the duration.
Compartment models of the target elements were constructed for describing foliar uptake,translocation and weathering by rainfall. Typical half-lives of the target elements were estimated by using
the models and actual weather conditions. A unit amount of radionuclide was assumed to deposit onto theleaf surface at noon of a given date, and then the behavior of the radionuclide was simulated with the modeland actual weather condition during 20 d after the date. A half-life was obtained from the remaining
proportion in the plant and on the leaf surface on the assumption of exponential decrease. The half-lives
were calculated for each day from May to October in 2005 - 2010. The median values of 820 half-lives

obtained for Cs, Sr, I- and IO3- were 10, 7.6, 25 and 6.0 d, respectively. Only the half-life for I- in this study
was longer than the weathering half-life (14 d) used in the safety review of the large-scale commercial spent
nuclear fuel reprocessing plant located in Rokkasho, Aomori Prefecture, Japan.

Weathring

Table 1 Statistics of half-lives calculated for Cs, Sr, Iand IO3- using the compartment model (Fig. 1).

Parameter of no rain condition

Equation (1)Statistic
Deposition

Leafsurface 1(LS1)

UptakeTranslocation

Arithmetic mean
Leaf(L)Leaf
surface 2(LS2)Weathring

Half life (d)Volatilization*

Roottuber(R)Volatilization*

Fig. 1 Compartment model describing foliar
uptake, translocation, volatilization andweathering by rainfall of Cs, Sr and I inplant.
* Only for I.

Cs
9.7

Sr-

IO3

I

7.3

23-

5.7

Standard deviation

3.7

2.6

5.9

0.9

Geometric mean

8.7

6.7

21

5.6

Median

10.3

7.6

25

6.0

Minimum

0.32

0.32

0.67

0.41

Maximum

15.1

11.7

33

8.6

n=

820

820

820

820