Characterization of the Oum Er Rbia (Morocco) high basin karstic

Applied Radiation and Isotopes 56 (2002) 985–990
Characterization of the Oum Er Rbia (Morocco) high basin
karstic water sources by using solid state nuclear track
detectors and radon as a natural tracer
N. Khalila, M.A. Misdaqb,*, S. Berrazzoukb, J. Maniac
b
a
Laboratory of Hydrogeology, Faculty of Sciences Semlalia, BP 2390, University Cadi Ayyad, Marrakech, Morocco
Nuclear Physics and Techniques Laboratory, Faculty of Sciences Semlalia, University Cadi Ayyad, B.P 2390, Marrakech,
Morocco
c
Sciences and Technology University of Lille, France
Received 27 April 2001; received in revised form 19 December 2001; accepted 26 December 2001
Abstract
Uranium and thorium contents as well as radon a-activities per unit volume were evaluated inside different water
samples by using a method based on calculating the CR-39 and LR-115 type II solid state nuclear track detectors
(SSNTDs) detection efficiencies for the emitted a-particles and measuring the resulting track density rates. The validity
of the SSNTD technique utilized was checked by analysing uranyl nitrate (UO2(NO3)26H2O) standard solutions. A
relationship between water radon concentration and water transmission of different water sources belonging to two
regions of the Middle Atlas (Morocco) water reservoir was found. The influence of the water flow rate as well as the
permeability and fracture system of the host rocks of the sources studied was investigated. r 2002 Elsevier Science Ltd.
All rights reserved.
Keywords: Radon; SSNTD; Detection efficiencies; Water sources; Fracturation; Water transmission; Karstic aquifers
1. Introduction
Due to the population explosion and climate change
(causing long periods of drought) in the world, many
countries have intensively increased their use of water
sources for supplying potable water to populations and
for their agricultural (irrigation) and industrial developments. Rivers constitute important water sources. So, it
is necessary to study and characterize the sources of
these water reservoirs. Radon (222Rn) is a chemically
inert and very mobile gaseous decay product of uranium
(238U) which is found in all rocks and soils. Radon is
very soluble in water. Radon has been used as a natural
tracer in geophysical and hydrogeological studies
*Corresponding author. Tel.: +212-4-4434649; fax: 212-44436769.
E-mail address: misdaq@ucam.ac.ma (M.A. Misdaq).
(Monnin and Seidel, 1991; Tidjani et al., 1990; Monnin
and Seidel, 1992; V"arhegyi et al., 1992).
Radon concentrations in water samples have been
evaluated by liquid scintillation and by Lucas cell
counting of alpha-scintillation (Salonen and Hukkanen,
!
1997; Yu, 1994; Drane et al., 1997; Gomez
Escobar et al.,
!
1996; Theodorsson,
1996; Kitto and Kuhland, 1995).
The influence of pollution and soil nature on radon
emanation from different water sources has been studied
by using a method based on determining the probabilities for a-particles emitted by the radionuclides of the
uranium and thorium series to reach and be registered
on two different types of solid state nuclear track
detectors (SSNTDs) (Misdaq and Satif, 1996). This
technique has also been utilized for studying the
influence of hydrological and hydrogeological parameters of karstic aquifers on radon emanation from
underground waters (Misdaq and Elharti, 1997) as well
0969-8043/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 9 6 9 - 8 0 4 3 ( 0 2 ) 0 0 0 5 8 - 1
986
N. Khalil et al. / Applied Radiation and Isotopes 56 (2002) 985–990
as evaluating the hydraulic exchange between two main
water reservoirs in Morocco (Misdaq et al., 2000a). In
the present work, we developed a method based on
calculating the detection efficiencies of the CR-39 and
LR-115 type II SSNTDs for a-particles emitted by the
radionuclides of the uranium and thorium series for
characterizing an important Moroccan water reservoir
by using radon as a natural tracer. The relevant ranges
of a-particles emitted by the radionuclides of the
uranium and thorium series, in the studied water
samples and SSNTDs were calculated by means of a
TRIM programme (Biersack and Ziegler, 1992).
using a method described in detail by Misdaq et al.
(1999). The average value of y0c for detecting a-particles
emitted by the radionuclides of the uranium and
thorium series inside the materials studied is of 46721.
The global track density rates (tracks cm2 s1) due to aparticles emitted by the radionuclides of the uranium
and thorium series registered on the CR-39 and LR-115
II detectors, after subtracting the corresponding backgrounds, are respectively given by (Misdaq et al.,
2000b):
rCR
G ¼
2. Methodology
Different water samples have been collected from the
Oum Er Rbia high basin karstic sources (Morocco)
(Fig. 1). Each water sample was separately placed in
close contact with disk shaped Pershore Mouldings CR39 (500 mm thickness) and Kodak LR-115 type II (12 mm
cellulose nitrate on 100 mm polyester base) SSNTD films
of 4 cm diameter in a hermetically sealed cylindrical
plastic container for one month and a half. During this
exposure time a-particles emitted by the nuclei of the
radionuclides of the 238U and 232Th series bombarded
the SSNTD films. After the irradiation, the films were
developed in a NaOH solution (2.5 N at 601C for
120 min for LR-115 II films and 6.25 N at 701C for 7 h
for the CR-39 detectors). The resulting track densities
registered on the SSNTDs were counted using an optical
microscope.
Backgrounds of the CR-39 and LR-115 II SSNTD
have been evaluated by placing these films in the same
empty well closed plastic containers used for analysing
water samples for one month and a half and counting
the resulting track density rates. This operation was
repeated ten times: track density rates registered on the
CR-39 and LR-115 II detectors were found similar
within the statistical limits. Since the system was sealed
(there is no escape of either radon or thoron) and the
exposure time is 45 days, one can assume a secular
equilibrium between uranium and thorium and their
corresponding decay products. For our experimental
conditions, the residual thickness of the LR-115 II
SSNTD is 5 mm which corresponds to the lower
(Emin ¼ 1:6 MeV) and upper (Emax ¼ 4:70 MeV) energy
limits for registration of tracks of a-particles in LR-115
type II films (Hafez and Naim, 1992). All a-particles
emitted by the radionuclides of the uranium and
thorium series that reach the LR-115 detector at an
angle smaller than its critical angle of etching y0c with a
residual energy between 1.6 and 4.70 MeV are registered
as bright track-holes. The CR-39 is sensitive to all
a-particles reaching its surface at an angle smaller than
its critical angle of etching yc y0c and yc are calculated by
pq2
CðUÞds
2Sd
"
#
8
7
X
CðThÞ X
0
0 0
CR
CR
ð1Þ
AU
kj Rj ej þ ATh
kRe
CðUÞ j¼1 j j j
j¼1
and
rLR
G ¼
pq2
0 CðUÞds
2Sd
"
#
8
7
X
CðThÞ X
0
0 0
LR
LR
AU
kj Rj ej þ ATh
kRe
; ð2Þ
CðUÞ j¼1 j j j
j¼1
where Sd and Sd0 are respectively the surfaces of the
CR-39 and LR-115 II films, CðUÞ (ppm) and C(Th)
(ppm) are the uranium and thorium concentrations of
the water sample, AU ðBq=gÞ and ATh ðBq=gÞ are the
specific activities of the sample for a 238U content of
1 ppm and a 232Th content of 1 ppm, dS is the density of
the water sample (g cm3), Rj and R0j are the ranges, in
the water sample, of an a-particle of index j and initial
energy Ej emitted by the nuclei of the radionuclides of
the uranium and thorium series, respectively, kj and kj0
are respectively the branching ratios corresponding to
the disintegration of the nuclei of the radionuclides
0
LR
of the uranium and thorium series and eCR
j ; ej CR; ej
0
LR
and ej are respectively the detection efficiencies of the
CR-39 and LR-115 II detectors for the emitted
a-particles.
Assuming a secular equilibrium between uranium,
thorium and their corresponding decay products radon
(222Rn) and thoron (220Rn) a-activities per unit volume,
3
A220
and A222
can be deterc
c ; respectively, in Bq cm
mined from the following equations (Misdaq et al.,
2001):
A220
c
¼
A222
c
P8
CR
CR
LR
LR
j¼1 kj ej Rj rG =rG
j¼1 kj ej Rj
P
P7
0
7
CR
LR
0
0
0
0
0
0
rG =rG
j¼1 kj ej CRRj Sd =Sd
j¼1 kj ej LRRj
Sd0 =Sd
P8
ð3Þ
and
¼
A222
c
pq2 ds
hP
8
j¼1
2Sd0 rLR
G
220
222
kj eLR
j Rj þ Ac =Ac
P7
j¼1
kj0 e0j LRR0j
i: ð4Þ
N. Khalil et al. / Applied Radiation and Isotopes 56 (2002) 985–990
987
Fig. 1. Map of Morocco showing the Zaouit Echeik and Khenifra regions (a); location of the Zaouit Echeik region (b) and Khenifra
region (c) water sources.
N. Khalil et al. / Applied Radiation and Isotopes 56 (2002) 985–990
988
Measuring rCR
and rLR
track density rates and
G
G
0
LR
0
calculating eCR
;
e
CR;
e
and
e
j
j
j
j LR detection efficiencies
220
222
one can evaluate the Ac =Ac ratio (Eq. (3)) and the
thoron A220
and radon A222
a-activities per unit volume
c
c
in a given water sample (Eq. (4)).
3. Results and discussion
In order to test the validity of our method, different
uranyl nitrate (UO2(NO3)26H2O) solutions were prepared by adding different distilled water volumes to a
concentrated uranyl nitrate solution and their uranium
contents were measured. Data are shown in Table 1.
From the statistical error on track counting one can
determine the error on track density per unit time and
then evaluate the relative uncertainty of the uranium
and thorium content determinations which is about 7%.
Tables 2 and 3 show results obtained for the uranium
CðUÞ and thorium CðThÞ contents and radon a-activities
per unit volume A222
c for water samples collected from
the Oum Er Rbia high basin karstic sources of the
Zaouit Echeik and Khenifra regions, respectively.
Statistical uncertainty of the uranium and radon aactivity determination is 7% whereas that of the thorium
concentration determination is 10%. The studied water
samples were uraniferous: their uranium to thorium
ratio is larger than 1.8. According to 18O measurements
we performed at the ‘‘Laboratoire d’Hydrologie et de
G!eochimie Isotopique’’ of the University of Paris-Orsay
(France) (Khalil, 2000) the Zaouit Echeik sources
(Fig. 1b) were supplied by rain and snow in the (1100–
1800 m) altitude zone, whereas the Khenifra sources
(Fig. 1c) were supplied by rain and snow in the (1600–
2000 m) altitude zone.
We noticed from results shown in Table 2 and
represented in Fig. 1b (Zaouit Echeik region) that:
*
*
For the same lithology (sources S2 and S3, S4 and
S22 and S24 and S26), radon a-activity increased
when the water flow rate (Q) increased due to the
increased amount of water in contact with the host
rocks. This means that the S2, S4 and S26 sources
presented higher water transmissions than the S3,
S22 and S26 sources (Table 2), respectively;
even though the S1 and S2 and S24 and S25 sources
presented the same lithology and different water flow
rates, their A222
activities were similar within the
c
uncertainties. This was due to the fracture systems
being more important for the host rocks of S1 and
S25 than for those of S2 and S24, respectively,
Table 1
Data obtained for uranium contents for different prepared uranyl nitrate standard solutions
Standard solution
(uranium content in ppm)
Concentrated
Concentrated
Concentrated
Concentrated
Concentrated
solution (474.1)
solution+100 ml
solution+150 ml
solution+200 ml
solution+225 ml
of
of
of
of
distilled
distilled
distilled
distilled
water
water
water
water
(31.3)
(3.87)
(0.48)
(0.1)
6
rLR
G 10
(tr cm2 s1)
6
rCR
G 10
(tr cm2 s1)
CðUÞ
(ppm)
303687304
1969739
25077
15579
37.371.6
941417940
61047122
776715
481719
11577
47477
31.070.4
3.970.1
0.5370.03
0.11070.008
Table 2
Data obtained for uranium CðUÞ; thorium CðThÞ contents and radon a-activities per unit volume A222
c for water samples collected from
sources of the Zaouit Echeik region
Source name
(water sample)
Water flow rate
(Q) (l s1)
Lithology
CðUÞ
(ppm)
CðThÞ
(ppm)
A222
c
(Bq m3)
Boulmattene (S1)
Tiguimate (S2)
Nougziza (S3)
Igly (S4)
Tamda (S21)
Ouarnfaa (S22)
Boudilite (S23)
Ait Abdellah (S24)
Ait Laadi (S25)
Tameskourte (S26)
7
53
4
84
62
45.5
34
24
8
33
Limestone and dolomite
Limestone
Limestone
Travertine
Travertine and limestone
Travertine
Scree
Limestone
Limestone
Limestone
0.4770.03
0.4770.03
0.3970.02
0.3770.02
0.3370.02
0.12670.007
0.2070.01
0.1870.01
0.1970.01
0.2270.01
0.2570.02
0.2470.02
0.2070.02
0.1970.01
0.1770.01
0.06570.005
0.10670.09
0.09670.008
0.10270.002
0.1170.01
58337372
57927370
48267247
45647247
40827247
1552786
25217126
22667126
24177127
26637121
N. Khalil et al. / Applied Radiation and Isotopes 56 (2002) 985–990
989
Table 3
Data obtained for uranium CðUÞ; thorium CðThÞ contents and radon a-activities per unit volume A222
for water samples collected from
c
sources of the Khenifra region
Source name
(water sample)
Water flow rate (Q)
(l s1)
Lithology
CðUÞ
(ppm)
CðThÞ
(ppm)
A222
c
(Bq m3)
Itnaknouine (S6)
Lablouane (S7)
Rive droite (S8)
Rive gauche (S9)
Source du Lac (S10)
Vieux Moulin (S11)
Source de l’Ecole (S12)
Petit pont (S13)
Assoul (S15)
Arrouggou (S16)
H. Hassan sal!ee (S17)
H. Hassan douce (S18)
Jnanes Imes (S19)
8.1
0.27
40
80
213
23
1.09
9
20
5.2
38
12
18
Limestone
Limestone
Limestone
Limestone
Limestone
Limestone
Limestone
Limestone
Limestone
Travertine
Travertine
Travertine
Travertine
0.5070.03
0.5370.03
0.15370.009
0.2970.02
0.1970.01
0.3370.02
0.1870.01
0.2670.01
0.3970.02
0.2670.01
0.2970.02
0.3770.02
0.2970.02
0.2670.02
0.2870.02
0.07970.007
0.1570.01
0.10170.009
0.1770.01
0.09570.008
0.1370.01
0.2070.02
0.1370.01
0.1570.01
0.1970.01
0.1570.01
62147373
65717372
18917111
35487245
23967126
40807247
22397124
31637122
48347248
31697122
35277243
45707247
35807247
*
*
*
*
*
compensating the effect of the water flow rate and
leading to similar water transmissions of the considered sources (Table 2);
even though the S4 and S22 sources showed higher
water flow rates than S3 and S23, respectively, their
A222
activities were lower than those of the S3 and
c
S23 sources. This was because the travertine lithology
of the former sources (S4 and S22) was more
permeable than the limestone lithology of the latter
sources (S3 and S23) leading to higher water
transmissions of the latter sources;
we noticed from results shown in Table 3 and
represented in Fig. 1c (Khenifra region) that:
for the S6 and S7 sources which have the same
lithology (limestone) their A222
activity decreased
c
when the water flow rate Q increased, because the
fracture system was more important for the S6 host
rocks than for S7 leading to a higher water
transmission for S7;
for the S8, S11, S10 and S9 sources which have the
same lithology (limestone), even though S8 and S10
showed higher water flow rates than S11 and S9,
respectively, the A222
activities of the former were
c
higher than those of the latter, because the fracture
system was more important for S11 and S9 than for
S10 and S8, compensating the water flow rate effect
and leading to higher water transmissions for S10
and S8;
for the S17 and S18 sources which have the same
lithology (travertine+scree), even though S17
showed a higher water flow rate than S18, the A222
c
activity of the latter source was higher than that of
the former. This was due to the fracture system being
more important for S18 than for S17: water
transmission of S18 was more important than that
of S17;
and
and
and
and
dolomite
dolomite
dolomite
dolomite
and dolomite
and Scree
and Scree
*
for the S15 and S16 sources, we noticed that A222
c
increased when the water flow rate increased: water
transmission of these sources was influenced by the
water flow rate.
For the same lithology (limestone) and water flow rate
(Tables 2 and 3), the S6 source belonging to the
Khenifra region presented a higher A222
activity than
c
that of the S25 source of the Zaouit Echeik region. This
was due to the fact that the host rocks of S6 were more
fractured than those of S25.
4. Conclusion
It has been shown by this study that by calculating the
CR-39 and LR-115 type II SSNTD detection efficiencies
for a-particles emitted by the radionuclides of the
uranium and thorium series and measuring the resulting
track density rates one can evaluate the 238U and 232Th
contents as well as radon a-activities per unit volume
inside different water samples. It has been shown that by
using radon as a natural tracer one can characterize
water sources in two regions of the Middle Atlas
Moroccan water reservoir. Furthermore, the water
transmission of these sources was influenced by the
water flow rate as well as the permeability and fracturing
of the host rocks. The water transmission of a source
was higher when radon a-activity in the corresponding
water was higher. It has also been shown that for the
same lithology and water flow sources belonging to the
Khenifra region presented higher water transmission
than those of the Zaouit Echeik region. This technique
which has the advantage of being simple, accurate,
inexpensive and does not need the use of any standard
for calibration is a good tool for studying drought
effects on water sources.
990
N. Khalil et al. / Applied Radiation and Isotopes 56 (2002) 985–990
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