Distribution of radionuclides in soil samples in and around Dhaka City

Appl. Radiat. Isot. Vol.49, No. 1-2, pp. 133-137, 1998 © 1997ElsevierScienceLtd. All rights reserved Printed in Great Britain PII: S0969-8043(97)00232-7 0969-8043/97 $19.00+ 0.00

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Distribution of Radionuclides in Soil Samples in and Around Dhaka City F. K. M I A H ~, S. R O Y *~, M. T O U H I D U Z Z A M A N 2 a n d B. A L A M 2 ~Health Physics Division, Atomic Energy Centre, Post Box No. 164, Dhaka-1000, Bangladesh and 2Department of Physics, University of Dhaka, Dhaka-1000, Bangladesh (Received 26 April 1997; in revised form 26 May 1997) The concentrations of the natural radionuclides of the uranium and thorium series and of*°K and a fission product t37Cshave been determined by gamma-ray spectrometry in soil samples collected from Dhaka city and its neighbouring environs. Values of the radionuclides present in the samples were greatly influenced by the geomorphological conditions in the area. The results have been compared with other global radioactivity measurements and evaluations. The typical concentration of ~37Cs found in near-surface samples ranged from 5 + 1 to 10 ___1 Bq kg -~, this being greater than for samples taken at greater depths, i.e. from 12.5-17.5 cm and 27.5-32.5 cm. © 1997 ElsevierScience Ltd. All rights reserved

Materials and Methods

Introduction Knowledge of the distribution pattern of both anthropogenic and natural radionuclides is essential in maintaining some sense of control of prevailing radiation levels. Measurement of natural and fallout radioactivities in soils gives information on natural sources, cumulative deposition from nuclear device testing and nuclear accidents. All rocks, soils and minerals contain naturally occurring radionuclides such as 4°K, 238U, 232Th and their progeny (Siotis and Wrixon, 1984). The reported radio toxicity of the radionuclides varies from moderate (222Rn) to high (234Th,228Ac, 137Cs) and to very high (226Ra, 22STh) (IAEA, 1973). Deposition of radioactive fallout including 137Csat any given site is related to factors such as latitude, precipitation and local topography (Cox and Fankhauser, 1984; Dejong et al., 1982; Whicker and Schultz, 1982). The fission product ~37Cs is strongly absorbed and retained by soil particles as are the natural radionuclides which are found randomly distributed at different depths of the soil. This investigation reports measurement of decay products of the uranium and thorium series, and of the primordial radionuclide 4°K and the fission product ~37Csin soil samples taken at various depths and locations in and around Dhaka city. Dhaka is expected to change from being a city to a megacity by the end of this century (Islam, 1996), and baseline data of this type will almost certainly be of importance in making estimations of population exposure.

*To whom all correspondence should be addressed.

Sample collection Ten locations in Dhaka city, the capital city of Bangladesh (20°34 ' N-26°39 ' N and 88°01' E92°41 ' E) and its neighbouring environs were chosen for collection of soil samples. The locations, shown in Fig. 1, are plain land where the soils are of clay type and believed to be undisturbed. The samples were collected from three layers of different depths, i.e. 0-5 cm, 12.5-17.5 cm and 27.5-32.5 cm, employing the template method (IAEA, 1989). Sample pretreatment The 30 soil samples which were obtained were oven dried at 105°C, homogenized, ground and then screened (IAEA, 1970) with a laboratory test sieve of aperture 425/~m (mesh no. 40, serial no. 238523). Sample aliquots of 400 g each were transferred to a separate plastic pot (diameter 5.4 cm, height 6.5 cm) and sealed. The activity of these was measured after one month (Martinez Lobo and Palomares, 1991; Ibrahiem et al., 1993) to enable an approach to equilibrium of 226Ra with its decay products in the uranium series and 228Ra with its daughters in the thorium series. Measurement procedure The high resolution gamma-spectrometry system which was used comprised a HPGe (97 c.c.) coaxial detector of 20% efficiency relative to a 7.62 cm × 7.62 cm NaI(TI) scintillator. The measured resolution of the HPGe detector was 2.3 keV at the 1332 keV full-energy peak of 6°Co.

133

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The samples were counted for 5000 s for a dry weight sources comprising various gamma-ray energies in a of 400 g. The lower limits of detection (LLD) of simulated soil matrix. 228Ra, 222Rn, 22STh, 4°K and t37Cs, determined from the background radiation for the same counting time, Activity calculation are given in Table 1. The energy efficiencies of the The results were analyzed by a PC using an detector were determined by using 6°C0, 137Cs, ~3~I M C A card 9308/A containing a commercial software liquid solutions and KCI salt mixed with 348 g of E M C A P L U S Version 1.012 supplied by Silena, analar grade A1203 so as to make composite reference Germany. The value obtained for the 22SThpresent in

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Radionuclides in soil samples Table 1. Lower limits of detection (LLD) for different full-energy peak of the radionuclides Radionuclides corresponding Energy LLD Radionu¢lides to full-energy peak (keY) (Bq) 22SRa mAc 911.1 6.15 968.9 6.57 n2Rn ~4Pb 352 1.56 214Bi 609 2.42 ~Th ~STI 583 1.28 '~K ~K 1460 57.50 I~Cs ~Cs 662 0.48

the samples was c o m p u t e d from the 583 keV peak of roT1, a n d t h a t for 226Ra was f o u n d from the m e a n of 2~4Pb (352 keV) a n d 2~4Bi (609 keV). The average values o f 2:SAc (911 keV a n d 969 keV) yielded the 22SRa activity (Michel et al., 1981).

Results and Discussion The activity levels of n a t u r a l radioactive elements a n d o f ~37Cs are given in T a b l e 2, where the levels were f o u n d to be n o r m a l l y distributed except t h a t of ~37Cs. T h e c o n c e n t r a t i o n s of ~°K f o u n d in the soil samples ranged from 402 + 78 to 750 __. 82 Bq kg -~ with a n average value o f 574 ___ 111 Bq kg -~ with the exception o f the value o f 165 + 89 Bq kg -~ f o u n d at the R a y e r b a z a r High School (location no. 7) at a d e p t h o f 12.5-17.5 cm. Low values of concentrations o f the naturally occurring radionuclides o f u r a n i u m a n d t h o r i u m series at this particular d e p t h at the same location are also to be noted. R e p o r t e d 4°K activities in soil samples from the C h i t t a g o n g hills

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in Bangladesh range from 128 + 16 to 610 + 49 Bq kg -~ with a n average value o f 351 + 113 Bq kg -~ (Nurul A l a m et al., 1990), being o f the same order as t h a t of the present m e a s u r e m e n t s in D h a k a . C o n c e n t r a t i o n s of 4°K in other parts of the world, given in Table 3, are also c o m p a r a b l e with those o f the present study. As m e n t i o n e d above, in addition to '°K, the other naturally occurring radionuclides which were measured were 226Ra, 228Ra, 22STh (Table 2). 226Ra concentrations varied from 21 _ 6 to 43 _+ 7 Bq kg -~ (an exception being t h a t at Rayerbazar, location no. 7, with the value o f 13 __ 5 Bq kg -~ at a d e p t h of 12.5-17.5 cm). These values are again c o m p a r a b l e with a n u m b e r of clay samples (Ibrahiem et al., 1993; Ackers et al., 1985; P a n et al., 1984; Beretka a n d Mathew, 1985, respectively) a n d soil samples o f Louisiana (U.S.A.), C h i t t a g o n g (Bangladesh), Savar (Bangladesh), Milos (Agean Sea) a n d Greece (Delaune et al., 1986; N u r u l A l a m et aL, 1990; M o U a h et al., 1994; F l o r o u a n d Kritidis, 1991; E.R.L., 1989, respectively). T h e concentration o f 228Th in the soil samples ranged from 9 __ 2 to 22 _+ 2 Bq kg -t, being comparable with those of clay samples in Greece a n d soil samples of Savar (Bangladesh), C h i t t a g o n g (Bangaldesh) a n d Egypt (Papastefanou et al., 1983; M o U a h et al., 1994; N u r u l A l a m et al., 1990; I b r a h i e m et al., 1993, respectively). The concentrations of 228Ra in the soil samples were found to range from 34 + 12 to 81 _+ 15 Bq kg -t, which were c o m p a r a b l e to those of Louisiana (Delaune et al., 1986).

Table 2. Concentrations of natural radionuclides and ~37Csin soil samples (Bq kg- t) from Dhaka city and its neighbouring environs Serial Depth of the no. Sample location soil (era) n2Rn (226Ra) 2:SRa(ngAc) 228Th(~T1) ~K ~37Cs 1. F.H. Hall (0-5) 32+6 56+16 125:2 7115:97 105:1 (12.5-17.5) 32 5:6 51 5:15 16 5:2 561 5:85 8 5:1 (27.5-32.5) 33 5:6 55 5:15 21 + 3 619 5:88 7 5: 1" 2. S.M. Hall (0-5) 40 + 7 70 5:18 21 5:3 464 5:93 10 + I (12.5-17.5) 43 5:7 69 5:16 17 5:2 741 5:94 ND (27.5-32.5) 37 5:6 59 + 15 19 5:2 697 5:88 ND 3. West End High School (0-5) 27 5:5 47 + 12 13 5:2 488 5:70 ND (12.5-17.5) 31 5:5 34 5:12 9 5:2 662 5:80 3 5:1 (27.5-32.5) 32 + 6 38 5:13 15 + 2 558 5:83 ND 4. Armanitola Government High School (0-5) 19 5:4 45 5:16 ND 750 + 82 ND (12.5-17.5) 27 5:5 38 5:14 17 5:2 464 5:82 7 5:1 (27.5-32.5) 33+6 455:16 115:2 4475:89 55:1 5. Nawabpur Government High School (0-5) 31 _+4 50 5:12 14 5:2 608 5:73 ND (12.5-17.5) 34 _+6 64 5:14 15 5:2 645 5:83 ND (27.5-32.5) 34 5:5 51 5:13 16 5:2 572 5:76 ND 6. Notre Dame College (0--5) 39 5:6 60 14 22 5:2 563 5:76 5+ 1 (12.5-17.5) 32 5:5 59 5:13 16 5:2 703 5:82 6 5:1 (27.5--32.5) 36 5:6 68 5:15 18 5:2 656 5:84 ND 7. Rayerbazar High School (0-5) 42 + 7 71 5:18 17 5:3 744 5:107 7 5: I (12.5--17.5) 13 5:5 17 5:15 3 5:2 165 + 89 ND (27.5-32.5) 38 + 7 65 5:18 18 5:4 621 5:99 ND 8. Dhaka College (0-5) 42 + 6 64 5:15 16 + 2 424 5:74 10 5- I (12.5--17.5) 40 __,6 57 + 15 17 5:2 402 5- 78 9 5:1 (27.5-32.5) 36 5:6 59 + 14 12 5:2 444 5:71 6 5: I 9. East Bengal Institution (Sadarghat) (0-5) 29 5:5 66 + 14 17 5:2 523 + 74 9 5:1 (12.5--17.5) 345-6 44+ 15 185:3 5745:92 ND (27.5--32.5) 34 + 6 41 5:14 13 5:2 560 5:86 ND 10. Adbarchandra High School (Savar) (0-5) 34 5:5 63 5:14 15 5:2 405 + 72 ND (12.5-17.5) 36 5:6 81 5:15 17 5:2 399 + 70 ND (27.5--32.5) 21 5- 6 73 5- 16 20 5:2 644 5:80 ND ND, not detectable. _ _ .

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Table 3. Concentrations of the natural radionuclides and a fission product in soil samples (Bq kg- ') from different parts of the world listed here for comparison with the present study (Table 2) SI. no. Location n6Ra nSRa (nSAc) nSTh 4°K '37Cs Reference 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

Louisiana (U.S.A.) (soil) Chittagong (Bangladesh) (soil) Savar (Bangladesh) (soil) Milos (Aegean Sea) (soil) Greece (soil) Cairo (Egypt) (clay brick) U.K. (clay) U.K. (clay) Norway (clay) Finland (clay) China (clay) Australia (clay) Netherlands (clay) Greece (clay) Nile Delta (Egypt) (soil) Present study (clay type soil)

43 95 15 81 32-49 21-101 21-80 24 52 65 104 78 41 41 39 49 17 33_+7

16-48

-----55_+14

The low c o n c e n t r a t i o n s o f 4°K a n d the o t h e r radionuclides o f the u r a n i u m a n d t h o r i u m series o b s e r v e d in the isolated R a y e r b a z a r sample (location no. 7) m a y be due to s o m e as yet unidentified structural a n d c o n s t i t u t i o n a l difference o f the soil at that particular d e p t h o f 12.5-17.5 cm. T h e a n t h r o p o g e n i c r a d i o n u c l i d e '37Cs, d e p o s i t e d in the soil o f B a n g l a d e s h p r e s u m a b l y as a result o f fallout o f radioactivity f r o m the a t m o s p h e r e following the nuclear p o w e r p l a n t accident at C h e r n o b y l o n 26 April 1986 a n d o t h e r previous a t m o s p h e r i c tests o f nuclear devices a r o u n d the world, was also a n a l y z e d in the study. T h e ~37Cs activities varied f r o m 3_+ 1 to 1 0 + 1 B q k g ' as c o m p a r e d with the ~37Cs levels in the soil s a m p l e s o f the C h i t t a g o n g hills w h i c h varied f r o m 1 to 4 Bq k g - ' (Nurul A l a m et al., 1990), that o f L o u i s i a n a soil, r a n g i n g f r o m 5_+ 1 to 5 8 + 2 B q k g - t (Delaune et al., 1986), a n d t h a t o f Savar (Bangladesh) soil, varying f r o m 2 to 3 Bq k g - ' ( M o l l a h et al., 1994). T h e c o n c e n t r a t i o n s o f the naturally occurring radionuclides d e t e r m i n e d in the soil s a m p l e s in D h a k a city a n d its n e i g h b o u r i n g e n v i r o n s as r e p o r t e d in the p r e s e n t study can be i n c o r p o r a t e d into baseline d a t a for the e s t i m a t i o n o f p o p u l a t i o n exposure. Acknowledgements--The authors are grateful to the Director, Atomic Energy Centre, Dhaka for his interest in this study. Thanks are also due to Dr. M. M. Rahman and Mrs. R. Samina for the preparation of the liquid sources and to Mr. A. Hoque and Mr. I. Ibrahimy for their technical support.

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50-190 7-39 19-29 2(~94 I(~75 24 44 48 62 62 52 89 41 24 19 16_+4

43-729 128-610 129-527 352-1796 337 1080 258 703 620 1058 962 714 681 560 670 316 574_+111

5-58 1-4 2-3 --------7_+2

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