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J.RADIOANAL.NUCL.CHEM.,LETTERS 186 (2) 143-155 (1994) DISTRIBUTION OF 137Cs AND 228Ra IN THE SEDIMENTS OF ASWAN HIGH DAM LAKE S.S. Ismail, E. Unfried, F. Grass Atominstitut der Osterreichischen Universit~ten Wien, A-1020 SchOttelstraBe 115, Vienna, Austria Received 31 May 1993 Accepted 16 June 1993 Sediment samples of the High Dam lake were investigated for their 137Cs, 226Ra, 228Th, and 40K content, using low-level y-spectro- scopy. The results show that at the begin- ning of the lake (500 km from the High Dam), where sediments consist mostly of sand, the level of 137Cs is very low (0.1 Bq kg-1). The maximum value (22.3 Bq kg -I) was found 40 km from the wall of the High Dam, where the composition of the.sediments is nearly 50% clays. The distribution of the natural nuclides 226Ra, 228Th , and 40K shows a different trend. INTRODUCTION Since the beginning of 1950 a series of atomic weap- ons tests were carried out in different parts of the world. As a result, a large amount of radioactive mate- rial, viz. 137Cs, 90Sr and 239pu was widely distributed in the environment. The maximum release of these iso- topes was in 1965-1966. Kampe I estimated that in 1986 the 137Cs concentration in soil was in the range of ] 43 Elsevier Sequoia ,9. A, Louaa.ne Akad&mial Kiadd,Budapest

Distribution of137Cs and228Ra in the sediments of Aswan High Dam lake

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J.RADIOANAL.NUCL.CHEM.,LETTERS 186 (2) 143-155 (1994)

DISTRIBUTION OF 137Cs AND 228Ra IN THE SEDIMENTS OF ASWAN HIGH DAM LAKE

S.S. Ismail, E. Unfried, F. Grass

Atominstitut der Osterreichischen Universit~ten Wien, A-1020 SchOttelstraBe 115, Vienna, Austria

Received 31 May 1993 Accepted 16 June 1993

Sediment samples of the High Dam lake were investigated for their 137Cs, 226Ra, 228Th, and 40K content, using low-level y-spectro- scopy. The results show that at the begin- ning of the lake (500 km from the High Dam), where sediments consist mostly of sand, the level of 137Cs is very low (0.1 Bq kg-1). The maximum value (22.3 Bq kg -I) was found 40 km from the wall of the High Dam, where the composition of the.sediments is nearly 50% clays. The distribution of the natural nuclides 226Ra, 228Th , and 40K shows a different trend.

INTRODUCTION

Since the beginning of 1950 a series of atomic weap-

ons tests were carried out in different parts of the

world. As a result, a large amount of radioactive mate-

rial, viz. 137Cs, 90Sr and 239pu was widely distributed

in the environment. The maximum release of these iso-

topes was in 1965-1966. Kampe I estimated that in 1986

the 137Cs concentration in soil was in the range of

] 43 Elsevier Sequoia ,9. A, Louaa.ne Akad&mial Kiadd, Budapest

ISMAIL et al.: 137Cs AND 228Ra IN SEDIMENTS

4.8 Bq kg -I soil, which is about 370 Bq m -2 for a soil

depth of 5 cm. However, the significant amount of radio-

active material released into the atmosphere due to the

Chernobyl accident in April 1986 caused an additional

contamination to our environment. 2

To investigate the level of 137Cs in the Egyptian

environment, we analyzed the High Dam lake sediments.

The lake was created after building the High Dam in

1964. The reservoir of this lake is about 500 km long,

300 of which is in Egypt, and the rest in Sudan. Mil-

lions of tons of sediments are transported annually

with the Nile water in the reservoir.

EXPERIMENTAL

45 sediment samples from the bottom of the lake were

collected along 480 km (Fig. I). The samples were dried

for 10 h at 105 ~ powdered and then divided using the

quartering method to obtain representative samples.

About 50 g of each was prepared for measurement by low-

level y-spectroscopy, using a 23% n'type HP-Ge detector.

The FWHM at 1332 keV of 60Co is 1.9 keY. The samples

were counted up to 150,000 s and evaluated after testing

the accuracy of the system with different reference ma-

terials, i.e IAEA/RGK-I (K2SO 4) , IAEA/RGTH-I (Th ore) and

IAEA/RGU-I (U ore). 40K with a half-life (HL) of 1.26xi09

y was determined using its y-line at 1460.8 keY (10.67%),

while for 137Cs (HL=30.25 y), the y-line ~t 661.6 keY

(85.2%) was used. 226Ra (HL=1600 y) was determined using

y-lines from its decay products 214pb and 214Bi in equi-

librium with 222Rn (HL=3.8 d) and 226Ra. Storage of the

gas-tight encapsulated samples for 3 weeks prior to

measurement allows for ingrowth of 222Rn to equilibrium.

144

ISMAIL et al.: 137Cs AND 22SRa IN SEDIMENTS

High Dam Wall

Kalab sha

U,

Tornas

El Abu Simble

Sarra

r'(Wodi HQI/Q Arnkat~Am2nd CotarQct

t ~ Arnka George Murshed ~ Jumi Semna ~...-~ K ~ i A ttirx~- e ~ - El Dwishat

/"~,,~ Matek El Nasser J" | Okma

El Dokko

EGYPT SUDAN

Fig. I. High Dam lake Samples are numbered from E1 Dakka No. I to Kalabsha, No. 45, and from right to left

The y-lines at 295.2 keV (18.6%) and 351.9 keV (35.8%),

both from 214pb, and 609.0 keV (45.0%) from 214Bi were

used. 228Th (HL=1.913 y) was determined using the y-

line of the decay product 208TI at 583.1 keV (86.0%)

35.9% branching from 212Bi, while 232Th (1.405xi0 I0 y)

was determined using the y-line of 228Ac (HL=6.J3 h)

at 911.1 keY (29.0%), assuming equilibrium with its

parent 228Ra (HL=5.75 y).

145

ISMAIL et al.: 137Cs AND nSRa IN SEDIMENTS

Fig. 2. Nile basin and its tributaries

RESULTS

Figure 2 shows the Nile b~sin and its main tribu-

taries. This map indicates that the main source of sedi-

ments of the Nile is the Ethiopian area, where the sedi-

ments are transported down the Nile according to their

grain gravity and the water velocity 3'4 On the other

hand, the Nile also carries old sediments reflecting

the geological and soil formations from the Ethiopian

tributaries to the lake. In general, the type of sedi-

ments and clays, reaching Sudan and Egypt (High Dam lake)

is significantly different at flood time 5. At this time

146

ISMAIL et al.: 137Cs AND 228Ra IN SEDIMENTS

'02

10

r I = 0.668

rf-0.716

Oo Sample number

Fig. 3. Distribution of 137Cs in sediment

large amounts of sediments from the Ethiopian tribu-

taries reach the lake. Entz 6 indicates that the center

of sedimentation is located near the second cataract at

Wadi Halfa. A former analysis of the sediment composi-

tion of the lake shows that at Abu Simbel (280 km from

the High Dam) it is 70% clays, 17% silt~ and 13% sand

(Fig. 3). 35% of the clay is montmorillonite, 35%

kaolinite, and 18% illite, while the remaining 13% con-

sists of other type of minerals 7. This indicates that

the transport of sediments in the lake is completely

controlled by sediment particle size, and according to

the slow water flow in the lake itself, It was found

also that the distribution of rare earth elements in

the lake follows the distribution of the clays in the

sediments 8 .

Table I shows the values of 137Cs, 226Ra 228Th

and 40K in the analyzed sediments. It was found that

147

ISMAIL et al.: 13~Cs AND 12gRa IN SEDIMENTS

TABLE I

Distribution of 40K, 226Ra , 228Th and 137Cs i n t h e sediment

40 K 226Ra 228Th 137Cs

Sample size 45 45 45 45

Average 310.2 19.1 24.8 8.9

Median 311.0 19.4 26.0 8.8

Std. 29.1 2.8 5.4 4.2

Standard error 4.3 0.4 0.8 0.6

Minimum 221.0 9.8 6.6 0.1

Maximum 363.0 23.8 33.8 22.3

Range 142.0 14.0 27.2 22.2

Lower quartile 303.0 18.1 23.2 7.1

Upper quartile 330.0 20.7 28.0 11.9

Skewness -1.3 -1.6 -1.7 0.4

Kurtosis 2.8 3.8 3.9 1.6

the 137Cs concentration is very low where the sediments

consist mainly of sand and increases with increasing

clay content. The mean value of 137Cs in the lake is

8.95 Bq kg -I, with a maximum value at Kalabsha (22.3

Bq kg-1). Clays adsorb 137Cs and cause its transport in

the Nile and the lake. The source of 137Cs in the lake

is atomic weapons testing. The ratio of 137Cs (HL=30.25

y) to 134Cs (HL=2.06 y), used to distinguish between

the 137Cs from the atomic tests and Chernobyl, was 2.0

at the time of this accident. The absence of 134Cs in

the sediment samples indicates that the 137Cs originates

from an old deposition (atomic weapons tests in the

atmosphere 1950-1983). The question is why, after all

these years, 137Cs is present in fresh sediments. The

148

ISMAIL et al.: 137Cs AND 228Ra IN SEDIMENTS

samples were directly collected from the top sediment

at the bottom of the lake. The only interpretation

available now is that this sediment is not fresh mud ma-

terial carried directly from the Nile sources but old

sediment and soil carried over very long distances from

Sudan and the Ethiopian tributaries. On the other hand,

the lake covered a large cultivated area (Nubia), after

its formation in 1964.137Cs could be due to the ensuing

transformations of the soil composition as well as to

the weathering of sediments of the many islands in the

river. A further study of the values and movements of

137Cs along the Nile to the lake could help us to follow

the source of sediment and mineral transportation down

the pathway of the Nile.

Figure 3 shows the distribution of 137cs along the

lake, in comparison with the distribution of clays. The

correlation coefficient for all the samples (rl) was

0.668, while for the samples of the main bottom stream

only that of (r2) was 0.716.

Figure 4 shows the distribution of 40K 228Th and

a26Ra in the lake. This distribution shows a slight

increase in the concentration of 228Th, and 226Ra in

the sediments towards the High Dam. In contrast to

137Cs, which is an artificial (man-made) nuclide, these

isotopes are natural, formed during formation of the

earth. These radioisotopes can be divided into two

groups: the first, comprising the primordial activity,

have half-lives comparable to the age of the earth,

i.e. , 238U (4.5xi09 y), and 232Th (1.39xi0 I0 y). The

second group contains the secondary radioactive isotopes

resulting from the decay of the primary nuclides, i.e.,

226Ra (1600 y), and 228Th (1.9 y). The mean values of 40K,

226Ra and 228Th in the sediment were 310.2, 19.1 and

149

t03

I0 z

40 K

10

ne]. h

~ ~ l , t -_ _ .c" L-%%..%"~-: -: - 1p

ISMAIL et aL: 137Cs AND 228Ra IN SEDIMENTS

I . . . . ] . . . . . . . ] , 1 . . . , :

1 0 10 2O 30 40 50

Somple number

Fig. 4. Di~tributiQn of 226Ra, 228Th and 40K

24.8 Bq kg -1, respectively. The highest val~e of 228Th

was found is sample number 45 (Kalabsha) with 33.8

Bq kg -I , while the distribution of 226Ra alonc the lake

is more uniform. The distribution of these nuclides in

the sediments reflects the geochemical and weathering

processes of the U, Th and the heavy minerals from the

different rock formations in the area of the Nile path-

way. A knowledge of this distribution helps to follow

the equilibrium between the nuclide content in the sedi-

ments and in the water system of the lake, which is of

particular interest with respect to the use of Nile

water for drinking. Figure 5 shows the y~spectra of

samples 19 and 45.

Figure 6 shows the plot of the first two component

weights by applying principal component analysis 9 to

the sediments using the values for clays, sand, silt,

as well as the values for 137Cs, 226Ra, 228Th , and 40K.

150

ISMAIL et al.: t37Cs AND nSRa IN SEDIMENTS

o NIL 45

20 ~

i[ /

Somple moss : 47.69 Counting time: 87 lOOs

|

o

I

I _ .i

I~0 2OOO

Energy, keV

Fig. 5. Gamma spectra of samples 19 and 45

1.0

~0 .8 E o

0.6

0.4-

02-

0

-0.2 -

-0.4 DSand I I -0.42 -0.22 -0.02

eSi l t

I 018

137Cs m

~6Rae 2~Th

~eClay 0.38 0.58

Component 1 Fig. 6. PCA of the first two component weights

The results show that 3 factors, representing 86.2% of

the data variance, affect the classification of the

variables of the samples. The fact that these sediments

ISMAIL et al.: 137Cs AND 22SRa IN SEDIMENTS

2,0 N

3

I -E -4.3 corr )onent 1

6 I 4

-23

5

20 16 15

9 8

N 5 183~2 38

31 35 39

I 1 -0.3 12

45

Q. 1.2 E

o u 08

0.4

0

-0.4

-0 .8

-1.2

-1 .6

- 2D 3.7

Fig. 7. PCA of the sediment samples

belonging to the same group with respect to these radio-

nuclides and clays indicates that the former are trans-

ported with clays and that the latter probably have the

same particle size (<2 ~m). Figure 7 shows sample classi-

fication according to these variables, and indicates

that the sediment in the lake can be divided into three

groups. The first group characterizing the sediments at

the beginning of the lake (500 km from the High Dam)

are nearly 100% quartz. This applies to the samples rep-

resenting the bottom of the main stream at distances of

487, 475, and 466 km from the High Dam, respectively,

and to samples 4 and 6 representing both the eastern

and western banks of Malk E1 Nasser which, however, con-

tain small amounts of clays. The second group comprises

the samples up to the Second Cataract (357 km from the

High Dam), and contains 20-30% clays, and high silt

minerals. The third group comprises the last part of the

lake, where the sediments are very rich in clays (50-80%

152

ISMAIL et al.: 137Cs AND 228Ra IN SEDIMENTS

100

el e

60

40

20

0 I I Area-1 Ares-2

Fig. 8. Distribution of the clays in the lake

by wt). Figures 8-9 show the distribution of clays,

137Cs, and 228Th in the lake according to these classi-

fications, i.e. Area I for the region between Ei-Dwaishat

and the Second Cataract, Area 2 for the rest of the lake

up to the High Dam. Apart from the clay content being

high in the last third of the lake, the median value of

137Cs is slightly higher (12.3 Bq kg -I) than in the rest

of the lake (8.2 Bq kg-1).

CONCLUSION

The distribution of the artificial radionuclide 137Cs

in the sediment of the High Dam lake follows the distri-

bution of clays, and shows the highest values in the

Egyptian part of the lake. A further study is necessary

for a deep profile of the lake sediment to investigate

153

ISMAIL et al.: ~37Cs AND ngRa IN SEDIMENTS

=o, 24 i -

g 20

1E

12

8

4 -

/% %l I

Are(a -1

;3k s �9

I Area - 2

36

m

33

30

27

4

21

18

228 Th

- r 7

I !

Areh-1 AreQ-2

Fig. 9. Distribution of 137Cs and 228Th

the Source and the transport of this nuclide in the Nile

and the lake from 1964 up to now in order to be able to

use this element as geochemical indicator.

~!54

ISMAIL et al.: 137Cs AND naRa IN SEDIMENTS

One of the authors (Dr. Ismail) would like to thank

Professors R.M. Awadallah, M. Ei-Magraby, Science Faculty

Aswan, Egypt, for their encouragement and support and

Dr. I. Grass for her linguistic assistance during the

last 10 years.

REFERENCES

I. W. Kampe, Xrztl-Praxis, 38 (1986) 1809.

2. Deutsche Strahlenschutzkommission, Auswirkungen des Reaktorunfalls in Tschernobyl auf die BRD, Gustav Fischer Verlag, Stuttgart, 1987.

3. M.A.M. Saad, Water Supply & Management, 4 (1980) 81.

4. M. Ei-Dardir, Geochemical and sedimentiological studies on the sediments of Aswan High Dam reservoir, Ph.D. Thesis, A1 Azhor Univ., Cairo, Egypt (1984).

5. M.K. Sherief, R.M. Awadallah, F. Grass, J. Radioanal. Chem., 60 (1980) 267.

6. B. Entz, Water Supply & Management, 4 (1980) 63.

7. R.M. Awadallah, S.S. Ismail, A.R. Arfien, S.M. Nourel-Din, "Minerals composition of the High Dam lake sediments", to be published, Chem. Erde, 1993.

8. S.S. Ismail, A. Ghods, R.M. Awadallah, F. Grass, Th, U and trace elements determination in Egyptian lake sediments by INAA and Laser Fluorimetry, 3rd Intern. Conf. Nuclear and Radiochem., Vienna, September 1992.

9. K. Varmuza, Pattern Recognition in Chemistry, Springer, Berlin, 1980.

155