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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