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: [email protected] (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. 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