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Convenient detecting system for radioactive tracers in underground water studies
Int. J. Appl. Rodiat. Isot. Vol. 33, pp. 475 to 478, 1982 Printed in Great Britain 0020-708X/82/060475-04$03.00/0
Convenient Detecting System for Radioactive Tracers in Underground Water Studies S. H. AL-MUSALLAM, F. MACHALI, M. EL-FIKI, M. FAGIEH and M. H. SALEH Umm Al-Qura University, Makkah, Saudi Arabia (Receiued 16 December 1980; in revisedform 3 March
1981)
The radioactive tracer technique with i3’I was used for the determination of the effective porosity of ground-water bearing formation in Wadi Na’Man, Makkah, Saudi Arabia. The detecting system consisted of a scintillation detector and a multichannel analyzer. An average value of 0.127 f 0.006 for the effective porosity was obtained, in good agreement with previous measurements on the same site.
radio-assayed using a NaI(Tl) crystal. The pulses from the photomultiplier, coupled to the cathode follower, were fed to the amplifier. A scaler was used to count the received pulses. From the time of arrival of the activity to the pumped well, the effective porosity could be determined. HAZZM et a1.t2*3’ used a set of four GM detectors coupled to a ratemeter and a chart recorder as a detecting system for the activity of the pumped water. In other experiments the same authors’4.5’ used a scintillation detector connected to a ratemeter and a chart recorder. Due to the fact that such kind of experiments need a long period of pumping, it was difficult to distinguish between pulses from the injected radioactivity and from the background, although a large amount of activity was involved. These factors became more significient if the observation wells are somewhat far from the pumping wells. The addition of a multichannel analyzer to our previous detecting system will make it possible to increase the ‘-“I 364 keV signabbackground ratio. In the present work this system is used to determine the effective porosity in Wadi Na’Man, Makkah, Saudi Arabia.
Experimental (a) Locality
Introductioo IN PREVIOUSexperiments,“’ the authors investigated the effective porosity of the water-bearing formation using radioactive isotopes in a locality in Wadi Na’Man, Makkah, Saudi Arabia. The radioactive tracer is injected into an observation well at the bottom of the aquifer under investigation. The water is pumped continuously at a constant rate from a neighbouring well and the pumped water is
The Wadi Na’Man Zone to be studied is located immediately uostream of the Jeddah-Taif road biidae (Fig. 11 (road by-p&sing Holy Makkah). The Wadi lies &km-east of Makkah and about 30 km west ofTaif. Thecatchment area of the Wadi is a basin whose upstream section slopes very considerably (about 15%). The slope is more gradual along the course of the Wadi( < 1%). The geological structure of the catchment area is such that run-off is good in the upstream section of the basin,
KINGDOM DF SAUDI ARABIA
FIG. la Key map. 475
476
Technical Note
/ .
FIG. lb. Location of Wadi Na’Man area, Makkah, Saudi Arabia.
which is formed from granites and diorites. In the middle section, at the confluence of two wadis namely Yardij and Majarsh, there are considerable quaternary rock terraces, which can act as reservoirs, storing run-off in their aquifers. (b) The radioactive tracer injection and detection “‘I (half-life t+ = 8.05 days) was usedJ3’ because it does not constitute a health hazard to users of the water, and it is not adsorbed to a great extent on the water bearing formations. The injection was performed by using the injector previously constructed and used by HAZZAA.‘@ This system was tried several times before the radioisotope was used and it functioned reliably. The injection was carried out after the steady rate has been reached in the aquifer, i.e. the level of water in both the pumped and observation wells is stable. The i3’I activity was detected in the field using a 1 x 1 in. NaI(TI) crystal coupled to a RCA-6342A photomultiplier and equipped with a multichannel (1024) analyzer. The detector was enclosed in a thin, black, water proof plastic envelope to protect it against any unexpected leakage of water. The detector is held vertically in a 0.5 m3 container into which the pumped water poured continuously. The container was provided with an adjustable opening to avoid overflow. The stability and sensitivity of the detecting set-up was examined and found to be considerably high. (c) The experiment To carry out the present experiment, a well (I,) located in the downstream zone of Wadi Na’Man and associated with a piezometer 7.85 m apart from it was selected. This well was provided with an electrically driven pump 6 in. in diameter. An activity of 50 mCi was injected in the piezometer at a depth of 40 m below the ground surface (18 m below the water level). The pumped water poured continuously into the container avoiding overflow, consequently, the passage of the activity through the container could be promptly detected and was recorded every 40s by the multichannel analyzer. Another 50 mCi was injected in the piezometer at a depth of 45 m 49 h after the first injection.
Results and Discussion The effective porosity 02, was calculated on the assumption of radial flow by applying the equation
f-z= nxQt where Q is the constant pumping rate; t is the time difference between injection and detection of the tracer; x is the distance between the two wells; b is the thickness of the aquifer. Figure 2 shows some recorded spectra from one of the performed runs, from the beginning of the experiment (a) to the moment of maximal activity (e). The number of counts under the 364 keV peak (corrected for background) as a function of time is given in Fig. 3 and it is clear that the first arrival was 24.0 h after the first injection, while the second arrival was 71.0 hr after the first injection, i.e. 22.0 h after the second injection. Values of the effective porosity gCwere calculated from equation (1) and given in Table 1 for the two runs in the same locality. The obtained value for the effective porosity of the main water-bearing formation of Wadi Na’Man which is 0.127 + 0.006 is in good agreement with the previous data”.‘) on the same site, which are 0.12 and 0.13 respectively. The use of a multichannel analyzer makes it possible to inject and record different kinds of radioactive tracers simultaneously in a piezometer to examine the influence of
TABLE1. Effective porosity
1st injection 2nd injection
Rate of pumping (m3 h-i)
Time of arrival (h)
Calculated effective porosity
32 32
24 22
0.133 0.121
Distance of. the piezometer from the pumped X = 7.85 m, thickness of the aquifer, b = 30 m. -. I he average value 0, = 0.127 k 0.006.
well,
Technical
Note
Number of chanels FIG. 2. Some recorded spectra from the instance of the injection (a) to the moment of
maximal activity (e).
the adsorption of tracers on to water-bearing formation particles as a function of the velocity in aquifers under pumping conditions. Acknowledgemenrs-The authors wish to express their gratitude to Saudi Arabia National Centre for Science and Technology (SANCST) for supporting this study under the SANCST contract AR-l-48. They are grateful for the keen collaboration and help of the Water and Sewage Depart-
ment, Western Region. They are also indebted to the goveming body of King Abdulaziz University in Jeddah and Makkah for continuous encouragement and help.
References 1. AL-MUSALLAM S. H., MACHALIF., EL-RKI M., FAGIZIH M. and SALEHM. H. Isorope Radiat. Res. (In press).
Technical Note
Makkah Wadi Na’Man well ( I , 1
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l9
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5
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Sk
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!%cond Injaectlon
Firrt injection I 20
/
I
I
40
60
lima
I 60
I 100
I
after injection, h
FIG. 3. The activity-time curve.
2. HAZZAAI. B., SMD K. F., G~RGISR. K., BAKR A. A. and SWAILEMF. M. Int. J. Appl. Radiat. ht. 16, 261 (1965). 3. HAZZM I. B., Gm~ts R. K., SAAD K. F., $WAILBM F. M. and BAKR A. A. Int. J. Appl. Radiat. Isot. 17, 621 (1966). 4. HAZZAAI. B., GIRGIS R. K., SAAD K. F., SWA~LEMF. M. and BAKRA. A. Bull. I.A.S.H. XII’ Annee No. 3, 55 (1967).
5. HAZZM’I. B., SHAHANA. M. and SHUHAIBERY. K. Isotope Radiat. Res. 3, 1 (1970). 6. HAZZM I. B. Isotope Radiat. Res. 3, 1 (1970). 7. Unpublish report on “Underground Dam Studies on Wadi Na’Man”, Water and Sewage department, Western Region, Saudi Arabia, by SGGREAH, consuiting engineer, January (1980).
Convenient Detecting System for Radioactive Tracers in Underground Water Studies S. H. AL-MUSALLAM, F. MACHALI, M. EL-FIKI, M. FAGIEH and M. H. SALEH Umm Al-Qura University, Makkah, Saudi Arabia (Receiued 16 December 1980; in revisedform 3 March
1981)
The radioactive tracer technique with i3’I was used for the determination of the effective porosity of ground-water bearing formation in Wadi Na’Man, Makkah, Saudi Arabia. The detecting system consisted of a scintillation detector and a multichannel analyzer. An average value of 0.127 f 0.006 for the effective porosity was obtained, in good agreement with previous measurements on the same site.
radio-assayed using a NaI(Tl) crystal. The pulses from the photomultiplier, coupled to the cathode follower, were fed to the amplifier. A scaler was used to count the received pulses. From the time of arrival of the activity to the pumped well, the effective porosity could be determined. HAZZM et a1.t2*3’ used a set of four GM detectors coupled to a ratemeter and a chart recorder as a detecting system for the activity of the pumped water. In other experiments the same authors’4.5’ used a scintillation detector connected to a ratemeter and a chart recorder. Due to the fact that such kind of experiments need a long period of pumping, it was difficult to distinguish between pulses from the injected radioactivity and from the background, although a large amount of activity was involved. These factors became more significient if the observation wells are somewhat far from the pumping wells. The addition of a multichannel analyzer to our previous detecting system will make it possible to increase the ‘-“I 364 keV signabbackground ratio. In the present work this system is used to determine the effective porosity in Wadi Na’Man, Makkah, Saudi Arabia.
Experimental (a) Locality
Introductioo IN PREVIOUSexperiments,“’ the authors investigated the effective porosity of the water-bearing formation using radioactive isotopes in a locality in Wadi Na’Man, Makkah, Saudi Arabia. The radioactive tracer is injected into an observation well at the bottom of the aquifer under investigation. The water is pumped continuously at a constant rate from a neighbouring well and the pumped water is
The Wadi Na’Man Zone to be studied is located immediately uostream of the Jeddah-Taif road biidae (Fig. 11 (road by-p&sing Holy Makkah). The Wadi lies &km-east of Makkah and about 30 km west ofTaif. Thecatchment area of the Wadi is a basin whose upstream section slopes very considerably (about 15%). The slope is more gradual along the course of the Wadi( < 1%). The geological structure of the catchment area is such that run-off is good in the upstream section of the basin,
KINGDOM DF SAUDI ARABIA
FIG. la Key map. 475
476
Technical Note
/ .
FIG. lb. Location of Wadi Na’Man area, Makkah, Saudi Arabia.
which is formed from granites and diorites. In the middle section, at the confluence of two wadis namely Yardij and Majarsh, there are considerable quaternary rock terraces, which can act as reservoirs, storing run-off in their aquifers. (b) The radioactive tracer injection and detection “‘I (half-life t+ = 8.05 days) was usedJ3’ because it does not constitute a health hazard to users of the water, and it is not adsorbed to a great extent on the water bearing formations. The injection was performed by using the injector previously constructed and used by HAZZAA.‘@ This system was tried several times before the radioisotope was used and it functioned reliably. The injection was carried out after the steady rate has been reached in the aquifer, i.e. the level of water in both the pumped and observation wells is stable. The i3’I activity was detected in the field using a 1 x 1 in. NaI(TI) crystal coupled to a RCA-6342A photomultiplier and equipped with a multichannel (1024) analyzer. The detector was enclosed in a thin, black, water proof plastic envelope to protect it against any unexpected leakage of water. The detector is held vertically in a 0.5 m3 container into which the pumped water poured continuously. The container was provided with an adjustable opening to avoid overflow. The stability and sensitivity of the detecting set-up was examined and found to be considerably high. (c) The experiment To carry out the present experiment, a well (I,) located in the downstream zone of Wadi Na’Man and associated with a piezometer 7.85 m apart from it was selected. This well was provided with an electrically driven pump 6 in. in diameter. An activity of 50 mCi was injected in the piezometer at a depth of 40 m below the ground surface (18 m below the water level). The pumped water poured continuously into the container avoiding overflow, consequently, the passage of the activity through the container could be promptly detected and was recorded every 40s by the multichannel analyzer. Another 50 mCi was injected in the piezometer at a depth of 45 m 49 h after the first injection.
Results and Discussion The effective porosity 02, was calculated on the assumption of radial flow by applying the equation
f-z= nxQt where Q is the constant pumping rate; t is the time difference between injection and detection of the tracer; x is the distance between the two wells; b is the thickness of the aquifer. Figure 2 shows some recorded spectra from one of the performed runs, from the beginning of the experiment (a) to the moment of maximal activity (e). The number of counts under the 364 keV peak (corrected for background) as a function of time is given in Fig. 3 and it is clear that the first arrival was 24.0 h after the first injection, while the second arrival was 71.0 hr after the first injection, i.e. 22.0 h after the second injection. Values of the effective porosity gCwere calculated from equation (1) and given in Table 1 for the two runs in the same locality. The obtained value for the effective porosity of the main water-bearing formation of Wadi Na’Man which is 0.127 + 0.006 is in good agreement with the previous data”.‘) on the same site, which are 0.12 and 0.13 respectively. The use of a multichannel analyzer makes it possible to inject and record different kinds of radioactive tracers simultaneously in a piezometer to examine the influence of
TABLE1. Effective porosity
1st injection 2nd injection
Rate of pumping (m3 h-i)
Time of arrival (h)
Calculated effective porosity
32 32
24 22
0.133 0.121
Distance of. the piezometer from the pumped X = 7.85 m, thickness of the aquifer, b = 30 m. -. I he average value 0, = 0.127 k 0.006.
well,
Technical
Note
Number of chanels FIG. 2. Some recorded spectra from the instance of the injection (a) to the moment of
maximal activity (e).
the adsorption of tracers on to water-bearing formation particles as a function of the velocity in aquifers under pumping conditions. Acknowledgemenrs-The authors wish to express their gratitude to Saudi Arabia National Centre for Science and Technology (SANCST) for supporting this study under the SANCST contract AR-l-48. They are grateful for the keen collaboration and help of the Water and Sewage Depart-
ment, Western Region. They are also indebted to the goveming body of King Abdulaziz University in Jeddah and Makkah for continuous encouragement and help.
References 1. AL-MUSALLAM S. H., MACHALIF., EL-RKI M., FAGIZIH M. and SALEHM. H. Isorope Radiat. Res. (In press).
Technical Note
Makkah Wadi Na’Man well ( I , 1
. .
. . h
G ”
. .
0.
l9
. .
;*i*
. . ” .
.
h . ”
. . .
..a
0’
.
.
:
&
. .
5
‘1
Sk
. .
”
.
.
l .
” .
: .”
/
m
.
!%cond Injaectlon
Firrt injection I 20
/
I
I
40
60
lima
I 60
I 100
I
after injection, h
FIG. 3. The activity-time curve.
2. HAZZAAI. B., SMD K. F., G~RGISR. K., BAKR A. A. and SWAILEMF. M. Int. J. Appl. Radiat. ht. 16, 261 (1965). 3. HAZZM I. B., Gm~ts R. K., SAAD K. F., $WAILBM F. M. and BAKR A. A. Int. J. Appl. Radiat. Isot. 17, 621 (1966). 4. HAZZAAI. B., GIRGIS R. K., SAAD K. F., SWA~LEMF. M. and BAKRA. A. Bull. I.A.S.H. XII’ Annee No. 3, 55 (1967).
5. HAZZM’I. B., SHAHANA. M. and SHUHAIBERY. K. Isotope Radiat. Res. 3, 1 (1970). 6. HAZZM I. B. Isotope Radiat. Res. 3, 1 (1970). 7. Unpublish report on “Underground Dam Studies on Wadi Na’Man”, Water and Sewage department, Western Region, Saudi Arabia, by SGGREAH, consuiting engineer, January (1980).