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210Pb and 137Cs fluxes in a sediment core from Guanabara Bay, Brazil
The Science of the Total Environment, 58 (1986) 195-198 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
195
21°pb A N D 137Cs F L U X E S I N A S E D I M E N T C O R E F R O M G U A N A B A R A BAY, BRAZIL
R.-D. WILKEN GKSS Research Center, D-2054 Geesthacht (Federal Republic of Germany)
I. MOREIRA and A. REBELLO PUC/RJ Department of Chemistry, 22453 Rio de Janeiro (Brazil)
ABSTRACT The activities of Pb-210 and Cs-137 were measured in a sediment core collected from the north-west region of Guanabara Bay, Brazil. On the basis of Pb-210 geochronology, a sedimentation rate of 2 cm year-1 was estimated for the area in recent times. The Cs-137 activity was below the detection limit (0.2~.8 mBq g 1) in all depths of the core. INTRODUCTION G u a n a b a r a Bay, a s h a l l o w e s t u a r i n e system n e a r Rio de J a n e i r o , has a t o t a l a r e a of 400 k m 2, of w h i c h 80% is no deeper t h a n 10 m. This e s t u a r y receives the i n p u t of u n t r e a t e d d o m e s t i c a n d i n d u s t r i a l sewage from a densely p o p u l a t e d a n d i n d u s t r i a l i z e d area. The p o l l u t i o n by h e a v y metals has been i n v e s t i g a t e d [1] and decreases w i t h d i s t a n c e from w e l l - k n o w n p o i n t sources. In o r d e r to e s t i m a t e the c h r o n o l o g i c a l d e v e l o p m e n t of h e a v y m e t a l p o l l u t i o n d u r i n g r e c e n t times, a 90 cm s e d i m e n t core was i n v e s t i g a t e d for its Pb-210 and Cs-137 content. Lead-210 has been used by m a n y i n v e s t i g a t o r s as a g e o c h e m i c a l t r a c e r for d e t e r m i n i n g s e d i m e n t a t i o n r a t e s in r e c e n t sediments [2-6] and for i d e n t i f y i n g a n t h r o p o g e n i c i n p u t s a s s o c i a t e d with i n d u s t r i a l i z a t i o n over the past 100 y e a r s [7-9]. In this w o r k the Pb-210 d a t i n g t e c h n i q u e was used to estimate the s e d i m e n t a t i o n r a t e in G u a n a b a r a Bay. Caesium-137 is a fission product. The Cs-137 d a t i n g t e c h n i q u e is r e l a t e d to the n u c l e a r tests t h a t were c a r r i e d o u t in the a t m o s p h e r e b e t w e e n 1952 and 1962, w h i c h were p e r f o r m e d p r e d o m i n a n t l y in the n o r t h e r n hemisphere. Bec a u s e of t h e 1963 m a x i m u m in global fallout, this t e c h n i q u e has been used as a t r a c e r in o c e a n i c m i x i n g studies [10] a n d in s e d i m e n t a t i o n studies [3, 5, 9]. T h e r e are o n l y v e r y few m e a s u r e m e n t s on Cs-137 in the s o u t h e r n hemisphere. H a r d y et al. [11] e s t i m a t e d the r a t i o of the Pu-239/240 f a l l o u t b e t w e e n the n o r t h e r n a n d s o u t h e r n h e m i s p h e r e s as 5:1. The r a t i o for Sr-90 is 4:1, a c c o r d i n g to F e e l y et al. [12], b u t t h e y r e p o r t e d no d a t a for Cs-137. K o i d e et al. [13] f o u n d a c o n c e n t r a t i o n of 3 5 0 d p h k g -1 ( 0 . 0 9 7 m B q g -i) for Cs-137 in a d a t e d ice core from the Ross ice shelf, A n t a r c t i c a , for 1962/63. I f a v a l u e of 4 0 m B q g 1 is a s s u m e d for a s e d i m e n t of the same age from the 0048-9697]86/$03.50
© 1986 Elsevier Science Publishers B.V.
196 n o r t h e r n hemisphere, the r e p o r t e d v a l u e of 0 . 0 9 7 m B q g 1 w o u l d seem to be r e m a r k a b l y low. METHODS S e d i m e n t samples were collected u s i n g a 20 kg g r a v i t y c o r e r with plexiglass tubes of 100 cm l e n g t h and 3.6 cm i n t e r n a l diameter. The sediment core was X-rayed to reveal the presence of sediment s t r u c t u r e s . F r o m the surface to a depth of 50 cm t h e r e were no s t r u c t u r e s ; in the 5 0 ~ 0 cm l a y e r shell f r a g m e n t s were found. The first w e a k sediment s t r u c t u r e s were seen below 60 cm. The u p p e r 10 cm of the core was divided into slices of 1 cm, the next 40 cm l a y e r was divided into s e g m e n t s of 5 cm, and the last p a r t of the core was cut into sections of 10 cm. A f t e r d e t e r m i n i n g the w a t e r c o n t e n t , a b o u t 1.5 g of dried s e d i m e n t was placed in a small tube and inserted into the detector, a bore-hole HP-Ge crystal, w h i c h allowed the m e a s u r e m e n t of 7-energies from a b o u t 40 keV. It was t h e r e f o r e possible to e v a l u a t e the 46.5 keV line of Pb-210 w i t h o u t a n y r a d i o c h e m i c a l s e p a r a t i o n of Pb-210 or Po-210. The Ra-226 a c t i v i t y was m e a s u r e d by the 185.7keV 7-line for c a l c u l a t i n g the excess Pb-210. F o r the Cs-137 d e t e r m i n a t i o n the 661.6 keV 7-line was evaluated. RESULTS AND DISCUSSION No Cs-137 a c t i v i t y could be detected in the core from G u a n a b a r a Bay. The d e t e c t i o n limits, w h i c h depend on the a m o u n t of m a t e r i a l measured, the backg r o u n d c o u n t and the c o u n t i n g time, were lower t h a n 0.8 mBq g 1 (see Table 1). T h e y were c a l c u l a t e d after C u r r i e [14]. A v a l u e of a b o u t 10 mBq g-1 was expected in the 1962/63 region, given the a s s u m p t i o n s m e n t i o n e d in the I n t r o d u c t i o n . This period is r e a c h e d at a b o u t 40 cm d e p t h ( a c c o r d i n g to Pb-210 dating, see below) and here the d e t e c t i o n limit
excess 210 Pb 2
3
&
5
6 7 8g[dpm]
Depth 10 I cm] 20 30 4-0 50 60 70 BO
Fig. 1. Distribution of excess Pb-210 with depth in a sediment core from Guanabara Bay. The solid line represents the best fit for all values, giving a sedimentation rate of (o = 2 cm year- 1.
197 TABLE 1 Radionuclides in a s e d i m e n t core from G u a n a b a r a Bay, Brazil Depth (cm)
1 2 4-5 8-9 10-15 1~20 30-35 40-50 60-70
Water content (°/o)
Dpm per g r a m dry m a s s Ra-226
Pb-210
87 83 83 83 82 81 79 72
0.95 1.44 0.73 0.62 0.70 0.87 0.94 0.98
7.98 9.47 8.05 7.54 7.37 5.79 6.26 3.03
_+ 0.04 _+ 0.06 + 0.02 + 0.02 ± 0.02 _+ 0.03 ± 0.03 + 0.03
± ± ± ± ± ± ± ±
Cs-137 (mBq g - 1) Excess Pb-210 0.34 0.37 0.39 0.23 0.18 0.19 0.19 0.10
7.03 8.03 7.32 6.92 6.67 4.92 5.32 2.05
+ 0.34 + 0.37 _+ 0.39 ± 0.23 ± 0.18 _+ 0.19 + 0.19 + 0.10
~<0.8 ~<0.6 ~<0.3 ~<0.3 ~<0.3 ~<0.3 ~<0.3 ~<0.2
was about 0.3mBq g 1. We assume therefore t h a t the Cs-137 activity in the s o u t h e r n hemisphere could not be estimated from Pu and Sr data. The results for Ra-226, Pb-210, and excess Pb-210 are given in Table 1 and Fig. 1. The values down to 40 cm exhibit a linear relationship which yields a sedimentation rate of about 2 cm y e a r - 1. The first low value at a depth of 1-2 cm can be explained by bioturbation, the beginning of the anoxic zone or by compaction. The values below 40 cm deviate significantly from a linear relationship. An explanation for this could be the accelerated industrialization and the large increase in the population of this area which began about 30 years ago. The presence of shell fragments in this part of the core can be seen as an indication of a change in the environment, which engendered the extinction of some of the biota. Mussels, for example, are no longer encountered in this area. The deposition rate presumably changed as a result of the increased input of suspended solids from u n t r e a t e d industrial sewage and rivers, denudation of the land and the dredging of many river beds. An important source of suspended materials is the abundant organic m at t er generated in situ in the bay. Ponciano [15] reported values of productivity of 2 g C m -2 da y- < The high sedimentation rate observed in the bay thus stems from the massive deposition of particulates derived from both a u t o c h t h o n o u s and allochthonous sources. ACKNOWLEDGEMENT
We t h a n k W. J u n g e and R. Pepelnik, GKSS, for carrying out the y-measurements.
REFERENCES W. Haekel, F. Schroeder, V. Felsen, A. Rebello, I. M o r e i r a and R. Santelli, Cd, Pb, Cu, Cr and Hg in w a t e r s and sediments of the G u a n a b a r a Bay e s t u a r y . GKSS R e p o r t 85]E/21, G e e s t h a c h t (West Germany), 1985.
198 2 3 4
5 6 7 8 9
10 11 12 13
14 15
S. Krishnaswamy, D. Lae, J.M. M a r t i n and M. Meybeck, Geochronology of lake sediments, E a r t h Planet. Sci. Lett., 11 (1971) 407 414. J.A. Robbins and D.N. Edgington, Determination of recent sedimentation rates in Lake Michigan using Pb-210 and Cs-137, Geochim. Cosmochim. Acta, 39 (1975) 285-304. A. Walton and J.N. Smith, Sediment accumulation rates and geochronologies measured in the Saguenay Fjord using the Pb-210 dating method, Geochim. Cosmochim. Acta, 44 (1980) 22~240. J. Dominik, A. Mangini and G. Miiller, Determination of recent deposition rates in Lake Constance with radioisotopic methods, Sedimentology, 28 (1981) 653~77. F. P~iez-Osuna and E.F. Mandelli, Pb-210 in a tropical coastal lagoon sediment core, Estuarine Coastal Shelf Sci., 20 (1985) 367-374. K.W. Bruland, K. Bertine, M. Koide and E.D. Goldberg, History of metal pollution in Southern California Coastal Zone, Environ. Sci. Technol., 8 (1974) 425432. E.R. Christensen, J. Scherfig and M. Koide. Metals from urban runoff in dated sediments of a very shallow estuary, Environ. Sci. Technol., 12 (1978) 1168-1173. J. Dominik, U. Foerstner, A. Mangini and H.E. Reineck, 21°Pb and laTCs chronology of heavy metal pollution in a sediment core from the German Bight, Senckenbergiana Marit., 10, 212~227. J.D. Burton, Radioactive nuclides in the marine environment, in Y.P. Riley and G. Skirrow (Eds), Chemical Oceanography, Vol 3, Academic Press, London, 1975, pp. 91-191. E.P. Hardy, P.W. Krey and H.L. Volchok, Global inventory and distribution of fallout plutonium, Nature, 241 (1973) 444~445. H.W. Feely, H.L. Volchok, E.P. Hardy and L.E. Toonkel, Worldwide deposition o f ~ S r through 1976, Environ. Sci. Technol., 12 (1978) 808-809. H. Koide, R. Michel, E.D. Goldberg, M.M. Herron and C.C. Langway, Depositional history of artificial radionuclides in the Ross ice shelf, Antarctica, E a r t h Planet. Sci. Lett., 44 (1979) 205-223. L.A. Currie, Limits for qualitative detection and quantitative determination, Anal. Chem., 40 (1968) 586-593. C. Ponciano, Avaliacfio da produtividade prim~ria e da disponibilidade de nutrientes na Baia de Guanabara, Thesis PUC, Rio de Janeiro, 1985.
195
21°pb A N D 137Cs F L U X E S I N A S E D I M E N T C O R E F R O M G U A N A B A R A BAY, BRAZIL
R.-D. WILKEN GKSS Research Center, D-2054 Geesthacht (Federal Republic of Germany)
I. MOREIRA and A. REBELLO PUC/RJ Department of Chemistry, 22453 Rio de Janeiro (Brazil)
ABSTRACT The activities of Pb-210 and Cs-137 were measured in a sediment core collected from the north-west region of Guanabara Bay, Brazil. On the basis of Pb-210 geochronology, a sedimentation rate of 2 cm year-1 was estimated for the area in recent times. The Cs-137 activity was below the detection limit (0.2~.8 mBq g 1) in all depths of the core. INTRODUCTION G u a n a b a r a Bay, a s h a l l o w e s t u a r i n e system n e a r Rio de J a n e i r o , has a t o t a l a r e a of 400 k m 2, of w h i c h 80% is no deeper t h a n 10 m. This e s t u a r y receives the i n p u t of u n t r e a t e d d o m e s t i c a n d i n d u s t r i a l sewage from a densely p o p u l a t e d a n d i n d u s t r i a l i z e d area. The p o l l u t i o n by h e a v y metals has been i n v e s t i g a t e d [1] and decreases w i t h d i s t a n c e from w e l l - k n o w n p o i n t sources. In o r d e r to e s t i m a t e the c h r o n o l o g i c a l d e v e l o p m e n t of h e a v y m e t a l p o l l u t i o n d u r i n g r e c e n t times, a 90 cm s e d i m e n t core was i n v e s t i g a t e d for its Pb-210 and Cs-137 content. Lead-210 has been used by m a n y i n v e s t i g a t o r s as a g e o c h e m i c a l t r a c e r for d e t e r m i n i n g s e d i m e n t a t i o n r a t e s in r e c e n t sediments [2-6] and for i d e n t i f y i n g a n t h r o p o g e n i c i n p u t s a s s o c i a t e d with i n d u s t r i a l i z a t i o n over the past 100 y e a r s [7-9]. In this w o r k the Pb-210 d a t i n g t e c h n i q u e was used to estimate the s e d i m e n t a t i o n r a t e in G u a n a b a r a Bay. Caesium-137 is a fission product. The Cs-137 d a t i n g t e c h n i q u e is r e l a t e d to the n u c l e a r tests t h a t were c a r r i e d o u t in the a t m o s p h e r e b e t w e e n 1952 and 1962, w h i c h were p e r f o r m e d p r e d o m i n a n t l y in the n o r t h e r n hemisphere. Bec a u s e of t h e 1963 m a x i m u m in global fallout, this t e c h n i q u e has been used as a t r a c e r in o c e a n i c m i x i n g studies [10] a n d in s e d i m e n t a t i o n studies [3, 5, 9]. T h e r e are o n l y v e r y few m e a s u r e m e n t s on Cs-137 in the s o u t h e r n hemisphere. H a r d y et al. [11] e s t i m a t e d the r a t i o of the Pu-239/240 f a l l o u t b e t w e e n the n o r t h e r n a n d s o u t h e r n h e m i s p h e r e s as 5:1. The r a t i o for Sr-90 is 4:1, a c c o r d i n g to F e e l y et al. [12], b u t t h e y r e p o r t e d no d a t a for Cs-137. K o i d e et al. [13] f o u n d a c o n c e n t r a t i o n of 3 5 0 d p h k g -1 ( 0 . 0 9 7 m B q g -i) for Cs-137 in a d a t e d ice core from the Ross ice shelf, A n t a r c t i c a , for 1962/63. I f a v a l u e of 4 0 m B q g 1 is a s s u m e d for a s e d i m e n t of the same age from the 0048-9697]86/$03.50
© 1986 Elsevier Science Publishers B.V.
196 n o r t h e r n hemisphere, the r e p o r t e d v a l u e of 0 . 0 9 7 m B q g 1 w o u l d seem to be r e m a r k a b l y low. METHODS S e d i m e n t samples were collected u s i n g a 20 kg g r a v i t y c o r e r with plexiglass tubes of 100 cm l e n g t h and 3.6 cm i n t e r n a l diameter. The sediment core was X-rayed to reveal the presence of sediment s t r u c t u r e s . F r o m the surface to a depth of 50 cm t h e r e were no s t r u c t u r e s ; in the 5 0 ~ 0 cm l a y e r shell f r a g m e n t s were found. The first w e a k sediment s t r u c t u r e s were seen below 60 cm. The u p p e r 10 cm of the core was divided into slices of 1 cm, the next 40 cm l a y e r was divided into s e g m e n t s of 5 cm, and the last p a r t of the core was cut into sections of 10 cm. A f t e r d e t e r m i n i n g the w a t e r c o n t e n t , a b o u t 1.5 g of dried s e d i m e n t was placed in a small tube and inserted into the detector, a bore-hole HP-Ge crystal, w h i c h allowed the m e a s u r e m e n t of 7-energies from a b o u t 40 keV. It was t h e r e f o r e possible to e v a l u a t e the 46.5 keV line of Pb-210 w i t h o u t a n y r a d i o c h e m i c a l s e p a r a t i o n of Pb-210 or Po-210. The Ra-226 a c t i v i t y was m e a s u r e d by the 185.7keV 7-line for c a l c u l a t i n g the excess Pb-210. F o r the Cs-137 d e t e r m i n a t i o n the 661.6 keV 7-line was evaluated. RESULTS AND DISCUSSION No Cs-137 a c t i v i t y could be detected in the core from G u a n a b a r a Bay. The d e t e c t i o n limits, w h i c h depend on the a m o u n t of m a t e r i a l measured, the backg r o u n d c o u n t and the c o u n t i n g time, were lower t h a n 0.8 mBq g 1 (see Table 1). T h e y were c a l c u l a t e d after C u r r i e [14]. A v a l u e of a b o u t 10 mBq g-1 was expected in the 1962/63 region, given the a s s u m p t i o n s m e n t i o n e d in the I n t r o d u c t i o n . This period is r e a c h e d at a b o u t 40 cm d e p t h ( a c c o r d i n g to Pb-210 dating, see below) and here the d e t e c t i o n limit
excess 210 Pb 2
3
&
5
6 7 8g[dpm]
Depth 10 I cm] 20 30 4-0 50 60 70 BO
Fig. 1. Distribution of excess Pb-210 with depth in a sediment core from Guanabara Bay. The solid line represents the best fit for all values, giving a sedimentation rate of (o = 2 cm year- 1.
197 TABLE 1 Radionuclides in a s e d i m e n t core from G u a n a b a r a Bay, Brazil Depth (cm)
1 2 4-5 8-9 10-15 1~20 30-35 40-50 60-70
Water content (°/o)
Dpm per g r a m dry m a s s Ra-226
Pb-210
87 83 83 83 82 81 79 72
0.95 1.44 0.73 0.62 0.70 0.87 0.94 0.98
7.98 9.47 8.05 7.54 7.37 5.79 6.26 3.03
_+ 0.04 _+ 0.06 + 0.02 + 0.02 ± 0.02 _+ 0.03 ± 0.03 + 0.03
± ± ± ± ± ± ± ±
Cs-137 (mBq g - 1) Excess Pb-210 0.34 0.37 0.39 0.23 0.18 0.19 0.19 0.10
7.03 8.03 7.32 6.92 6.67 4.92 5.32 2.05
+ 0.34 + 0.37 _+ 0.39 ± 0.23 ± 0.18 _+ 0.19 + 0.19 + 0.10
~<0.8 ~<0.6 ~<0.3 ~<0.3 ~<0.3 ~<0.3 ~<0.3 ~<0.2
was about 0.3mBq g 1. We assume therefore t h a t the Cs-137 activity in the s o u t h e r n hemisphere could not be estimated from Pu and Sr data. The results for Ra-226, Pb-210, and excess Pb-210 are given in Table 1 and Fig. 1. The values down to 40 cm exhibit a linear relationship which yields a sedimentation rate of about 2 cm y e a r - 1. The first low value at a depth of 1-2 cm can be explained by bioturbation, the beginning of the anoxic zone or by compaction. The values below 40 cm deviate significantly from a linear relationship. An explanation for this could be the accelerated industrialization and the large increase in the population of this area which began about 30 years ago. The presence of shell fragments in this part of the core can be seen as an indication of a change in the environment, which engendered the extinction of some of the biota. Mussels, for example, are no longer encountered in this area. The deposition rate presumably changed as a result of the increased input of suspended solids from u n t r e a t e d industrial sewage and rivers, denudation of the land and the dredging of many river beds. An important source of suspended materials is the abundant organic m at t er generated in situ in the bay. Ponciano [15] reported values of productivity of 2 g C m -2 da y- < The high sedimentation rate observed in the bay thus stems from the massive deposition of particulates derived from both a u t o c h t h o n o u s and allochthonous sources. ACKNOWLEDGEMENT
We t h a n k W. J u n g e and R. Pepelnik, GKSS, for carrying out the y-measurements.
REFERENCES W. Haekel, F. Schroeder, V. Felsen, A. Rebello, I. M o r e i r a and R. Santelli, Cd, Pb, Cu, Cr and Hg in w a t e r s and sediments of the G u a n a b a r a Bay e s t u a r y . GKSS R e p o r t 85]E/21, G e e s t h a c h t (West Germany), 1985.
198 2 3 4
5 6 7 8 9
10 11 12 13
14 15
S. Krishnaswamy, D. Lae, J.M. M a r t i n and M. Meybeck, Geochronology of lake sediments, E a r t h Planet. Sci. Lett., 11 (1971) 407 414. J.A. Robbins and D.N. Edgington, Determination of recent sedimentation rates in Lake Michigan using Pb-210 and Cs-137, Geochim. Cosmochim. Acta, 39 (1975) 285-304. A. Walton and J.N. Smith, Sediment accumulation rates and geochronologies measured in the Saguenay Fjord using the Pb-210 dating method, Geochim. Cosmochim. Acta, 44 (1980) 22~240. J. Dominik, A. Mangini and G. Miiller, Determination of recent deposition rates in Lake Constance with radioisotopic methods, Sedimentology, 28 (1981) 653~77. F. P~iez-Osuna and E.F. Mandelli, Pb-210 in a tropical coastal lagoon sediment core, Estuarine Coastal Shelf Sci., 20 (1985) 367-374. K.W. Bruland, K. Bertine, M. Koide and E.D. Goldberg, History of metal pollution in Southern California Coastal Zone, Environ. Sci. Technol., 8 (1974) 425432. E.R. Christensen, J. Scherfig and M. Koide. Metals from urban runoff in dated sediments of a very shallow estuary, Environ. Sci. Technol., 12 (1978) 1168-1173. J. Dominik, U. Foerstner, A. Mangini and H.E. Reineck, 21°Pb and laTCs chronology of heavy metal pollution in a sediment core from the German Bight, Senckenbergiana Marit., 10, 212~227. J.D. Burton, Radioactive nuclides in the marine environment, in Y.P. Riley and G. Skirrow (Eds), Chemical Oceanography, Vol 3, Academic Press, London, 1975, pp. 91-191. E.P. Hardy, P.W. Krey and H.L. Volchok, Global inventory and distribution of fallout plutonium, Nature, 241 (1973) 444~445. H.W. Feely, H.L. Volchok, E.P. Hardy and L.E. Toonkel, Worldwide deposition o f ~ S r through 1976, Environ. Sci. Technol., 12 (1978) 808-809. H. Koide, R. Michel, E.D. Goldberg, M.M. Herron and C.C. Langway, Depositional history of artificial radionuclides in the Ross ice shelf, Antarctica, E a r t h Planet. Sci. Lett., 44 (1979) 205-223. L.A. Currie, Limits for qualitative detection and quantitative determination, Anal. Chem., 40 (1968) 586-593. C. Ponciano, Avaliacfio da produtividade prim~ria e da disponibilidade de nutrientes na Baia de Guanabara, Thesis PUC, Rio de Janeiro, 1985.