Relationship between 131I and 137Cs Deposition on Soil in the Territory of Belarus
after the Chernobyl Accident.
Vladimir. A. KNATKO, Igor I. IVACHKEVICH, Valentina D. ASIMOVA
Institute of Radiobiology, National Academy of Science of Belarus
Zhodinskaya Str., 2, 220141 Minsk, Republic of Belarus.
As a result of the Chernobyl accident a large part ofterritory of Belarus had been contaminated withradioactive fallout. One of important radionuclides was131
I which caused significant thyroid exposure in themost affected areas. Direct measurements of 131I
activity in thyroid and determination of 131I
concentration in foodstuffs and air were performed
only in limited areas regions of the contaminatedterritory. Therefore, reconstruction of thyroid doses
resulting from 131I intake is important problem solutionof which needs information of 131I deposition on soil
[1].
Izrael [2] noted expediency of using an isotope ratioof 131I to 137Cs content in soil for assessment of 131I
deposition in the areas where measured data on 131I areabsent or limited. The relationship between 131I and137
Cs deposition had been obtained by Mahonko et.al.
[3] on the basis of data on 131I and 137Cs activity
concentration in more than one hundred soil samples
taken in the area located between cities Kiev (Ukraine)and Tula (Russia) (north-eastern direction fromChernobyl). The regression function derived by
Mahonko et.al. [3] predicts an approximately linear
increase of 131I content in soil with a growth of 137Cs
deposition (131I ~ [137Cs]0.85).
The analysis of measured data performed by Orlov
et.al. [4] shows that the 131I/137Cs isotope deposition

ratio R on soil in the European part of the former USSRincreases in western regions. In particular the ratio R in
the western part of Belarus is larger by 4-5 times thanin the eastern part. The inhomogeneous deposition ofradioactive dispersed materials makes it necessary totake a regional approach to dose reconstruction. In thisconnection the relationship between 131I and 137Cs
content in deposition on soil in various contaminatedareas of Belarus is studied in the present paper.
The data for the analysis of 131I and 137Cs activity
concentration in soil have been taken from the reportby Slizov et.al. [5]. Their samples were collected in thesouthern and the eastern parts of the territory of Belarus
during May-June 1986. The location of the areas wheresamples were taken is shown in Fig. 1. Radionuclide
activity was determined by gamma-ray spectrometerwith semiconductor Ge(Li)-detector at the Institute of
Nuclear Energy of Belarus Academy of Sciences(Minsk). All results of the measurements were given as
values of activity on 10th May 1986. The numbers ofsoil samples taken in the southern and the eastern areas
were 139 and 213, respectively.
The southern area includes five administrative
districts of the Gomel region. The distance of thefarthest west point of this area is about 190 km fromChernobyl. The eastern area is located in the territoryof the Gomel and the Mogilev regions, including 6 andMINSKMOGILEV

Eestern area

GOMELSouthern area

CHERNOBYL

Fig. 1 Location (shaded) wher soil samples had been taken in the Belarus territory:
the southern area and the eastern area.
90

eastern areas are characterized by similar mean values.
The observed behavior of parameters means that theisotope ratio of 131I/137Cs in deposition on soil in thestudied areas can be characteristically different.
The empirical data presented in Figs. 2 and 3 wereused to derive the relationships between 131I and 137Cs
deposition on soil in the studied areas. It is noted that131
I and 137Cs activity concentration in soil are stronglyvariable. To minimize this variability and elucidate therelationship between 131I and 137Cs concentration, theempirical data were ln-transformed. As the 131I and137
Cs data follow the log-normal distribution, theln-transformed quantities will obey the normaldistribution which is more convenient for statisticaltreatment. Calculation of the relationship between 131I
and 137Cs activity concentration have been performed
without regard to the contribution of 137Cs global
fallout which is comparably small (~0.06 Ci/km2) andhas not remarkable influence on results. The
relationships between 131I and 137Cs deposition aregiven in Fig. 4. The corresponding regression functions
describing dependence of 131I concentration on 137Cs
contamination level in soil of the southern and theeastern areas are written as

5 administrative districts, respectively. The extent ofthe eastern area from the south end to the north end isabout 110 km. The level of 137Cs contamination in soilof the studied areas varies from tens to thousands ofkilo-becquerels per square meters and the distributionof 137Cs deposition is highly non-uniform. The wide
variability of deposition provides an opportunity toinvestigate the relationship between 131I and 137Cs
content in soil over a wide range of contamination.
Relative frequency distributions of 131I and 137Cs
activity concentration in soil of areas underconsideration are presented in Figs. 2 and 3. The resultsof performed calculations show that empirical data canbe described by log-normal distributionf ( x) =( 2πax) exp[− (ln x − b) 2a ] (1)
−1

2

2

The parameter a in (1) characterizes skewness ofthe distribution and is defined asa = [2(ln E − ln M)] ,12

where E is the mean value, and M=eb is the median ofthe distribution.
According to estimated parameters, the 137Cs meanvalue characterizing the distribution of data obtained
for the southern area (westward radioactive trace from
Chernobyl - WRT) is larger by two times than the meanvalue in the case of the eastern area (northernradioactive trace — NRT) (see Figs. 2 and 3). At thesame time the 131I data obtained in the southern and the(131I = 14.58 ⋅ 137 Cs)

0.605

(2)

and

0,50
0,40
Model:LogNorm al
a = 0.827 (0.042)
b = 4.24 (0.037)
Mean = 97.71
Med = 69.41

0,400,30

Model:LogNorm al
a = 0.912 (0.038)
b = 4.10 (0.027)
Mean = 91.46
Med = 60.34

0,30

0,200,20131

0,10131

0,10II

Ci/km 2

Ci/km 20,000,00
0100

200

300

400

0500

100200300

400500

600

0,50

0,50
0,40
0,30

Model:LogNorm al
a = 1.11 (0.046)
b = 3.09 (0.027)
Mean = 40.69
Med = 21.98

0,40

Model:LogNorm al
a = 1.23 (0.050)
b = 2.39 (0.039)
Mean = 23.25
Med = 10.91

0,30

0,200,20

137

Ci/km

137

Cs

0,10Cs0,10Ci/km 2

20,00

0,00
0

20

4060
131

80

0100

137

50

100

150
131

200137

Fig. 3 Distribution of
I andCs activity
concentration in soil of the eastern area ofBelarus.

Fig. 2 Distribution ofI andCs activity
concentration in soil of the southern area ofBelarus.
91

131

I = 8.85 ⋅( Cs)
137

0.627

(3)

respectively, where levels of contamination are
expressed with activity (Ci/km2) on 10th May 1986.
The similar function was derived from analysis of
the joined set of data including measurements for bothstudied areas (see Fig. 5). In this case the obtained
regression equation is131

I = 1147. ⋅( Cs)137

0.591.

(4)

Using the regression equations (2), (3) and (4), theisotopic ratio R can be taken to be(R = C ⋅ 137 Cs)−0.40,(5)where the coefficient C is equal 14.15, 8.85 and 11.47
for the southern, the eastern and the joined(southern + eastern) areas, respectively. According tocoefficient C values, the isotope ratio R in deposition
on soil in the southern area is larger by 1.6 times thanin the eastern one.
The relationship (5) predicts that R values increaseby a factor of about 4.8 with decreasing 137Cs contentof soil from 50 to 1 Ci/km2. The observed behavior isin agreement with the data of Orlov et al. [4] whoshow that R values obtained in the western regions ofBelarus where the level of soil contamination with137
Cs is low (1-5 Ci/km2) are higher by 3-5 times thanin the strongly contaminated areas of the south-easternand the eastern parts of Belarus.
Thus the results of the present study performed fora wide range of soil contamination with 137Cs predicts
non-linear behavior of 131I/137Cs isotope ratio in thestudied areas of Belarus. Use of the obtained
non-linear relationship between 131I and 137Cs content
in soil may provide a more realistic regional estimationof 131I deposition and help to improve radiologicalassessments.
REFERENCES
1. Likhatev I.A., Gulko G.M., Sobolev B.G. et al. Thyroid
dose assessment for the Chernobyl region (Ukraine):estimation based on 131I thyroid measurements andextrapolation of the results to districts withoutmonitoring. Radiat. Environ. Biophys., 33 (1994)
149-166.
2. Chernobyl: Radioactive Contamination of Environments.
Ed. by Y.A. Izrael. Lenigrad, 1990 (In Russian).
3. Mahonko K.P., Kozlova E.G., Silantiev A.N. et al.
Contamination of Regions with 131I after ACCIDENT AT
THE Chernobyl NPP and Assessments of Upper Dosesfrom 131I. Atomic Energy, 72 (1992) 377-382 (InRussian).
4. Orlov M.Y., Snikov V.P., Hvalenski Y.A., Volokitin
A.A. Contamination of the Soil in European Part ofTerritory of USSR after Accident at the Chernobyl NPP.
Atomic energy, 80 (1996) 466-471 (In Russian).

Ln 131I7A
Linear Regression
Y=A+B*X--------------------------------ParamValuesd--------------------------------A
2,680,13
B0,6050,05--------------------------------R = 0,71
SD = 0,75, N = 1396543210

Ln 137Cs-1-2Ln131-101I

72345B
Linear Regression
Y=A+B*X--------------------------------ParamValuesd--------------------------------A
2,180,09B
0,630,03--------------------------------R = 0,82
SD = 0,62, N = 213

6543210

Ln 137Cs-1-3-2-10

123456Fig. 4 Relationship between 131I and 137Cs activity
concentration (Ci/km2) in soil of the southern (A)and the eastern (B) areas of Belarus.
A+BLn 131I
7Linear Regression
Y=A+B*X--------------------------------Param Value sd--------------------------------A
2,440,08B
0,590,03--------------------------------R = 0,75
SD = 0,71, N = 352

6543210Ln 137Cs-1-3-2-10123456Fig. 5 Relationship between 131I and 137Cs activity
concentration (Ci/km2) in soil of the Belarus
territory including both the southern (A) and theeastern (B) areas.
5. Slizov V.P., Ananich P.I., Shekin Y.K. et al. Bank ofData on Isotope Analysis of Soil Samples Taken at
Territory of BSSR and Processing Programs. Report ofInstitute of Nuclear Energy, Academy of Sciences ofBelarus, № 4616, Minsk, 1987 (In Russian).

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