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Heavy metal content in coastal sediments off Dar es Salaam, Tanzania
Environment International, Vol. 18, pp. 409-415, 1992 Printed in the U.S.A. All rights reserved.
0160-4120/92 $5.00 + .00 Copyright © 1992 Pergamon Press Ltd.
HEAVY METAL CONTENT IN COASTAL SEDIMENTS OFF DAR ES SALAAM, TANZANIA
John F. Machiwa Department of Zoology and Marine Biology, University of Dar es Salaam, P.O. Box 35064, Dares Salaam, Tanzania
El 9112-191 M (Received 28 December 1991; accepted 19 March 1992)
Anthropogenic input of Fe, Mn, Cu, Zn, Pb, Cr, Cd, and organic carbon into the Dares Salaam coastal sediments is assessed in relation with human activities in the watershed and within the respective water body. The geochemically available fraction of the sediments from the D a r e s Salaam harbor channel is enriched with iron and lead compared with the location off the harbor. Other elements such as copper, zinc, manganese chromium, and cadmium in the harbor sediments show moderate to weak signals in comparison with sediments outside the harbor area. Surprisingly, the sediments close to the outer anchorage zone for ships waiting to off-load cargo is strongly contaminated with Cd and Cr. In the case of Cd, the concentration is more than double that of the harbor sediments. There is a high flux of organic matter into the sediments in the harbor area.
INTRODUCTION Incidents of heavy metal pollution of coastal waters by anthropogenic inputs are nowadays not uncommon in almost all developing countries. Tanzania, being one of the less industrialized nations in the western coast of the Indian Ocean, is presently experiencing impacts of environmental degradation, probably, with the same magnitude both on land and within her exclusive economic zone (UNEP 1982, 1989; Bryceson et al. 1990). Unlike the industrialized countries, where the analytical instruments are readily available, most developing countries face acute shortages of both equipment and qualified environmental personnel. The harbor of D a r e s Salaam is within the major industrial area and largest city of Tanzania. The level of industrial activities related to metal processing has been steadily increasing in the city during the present three decades, vast open areas of the city have been converted into either residential or in-
dustrial places. Uncontrolled/haphazard disposal of domestic as well as industrial wastes is not uncommon. The harbor is situated in a narrow bay; this sheltered inlet is almost protected from free include flushing by seawater circulation. Activities in the harbor area include loading and off-loading of international cargo, dry docking, and other repair works concerning maritime activities. Also, there is a heavy traffic of local fishing and ferry boats and some industrial and municipal outflows discharge in the harbor area. The influence of the hinterland on the composition of near shore sediments has been documented in Chester (1969), therefore, it can be assumed that most suspended material associated with city runoff most probably settles within the shore area. The inshore deposition of the land-based particulate matter can be the result of the complex patterns of coastal winds and currents. The concentration of heavy metals in the sediments of such areas, therefore, should
409
410
J.F. Machiwa
low as reasonably achievable level" will ever come to practice. Ecologists in developing countries, therfore, are concerned with establishing baseline levels in order to be ready for environmental perturbations that are likely to occur in future.
be a good indicator of contaminant fluxes of terrigenous origin. Because of the growing flux of heavy metals from both industries and land denudation to the marine environment, areas with restricted circulation, such as the harbor of Dar es Salaam, material input can significantly cause a build-up of pollutants such as heavy metals, PAH, and PCB residues. Despite the enacted legislation safeguarding aesthetic value of water bodies, it is still beyond imagination whether or not the waste discharge at "zero level" or at "as
STUDY AREA
The sampling locations are within near-shore waters of Dar es Salaam, approximately between lat.
!
0
6"M'~"
N goyo Is.
INDIAN OCEAN
6.
0ubr
~n,~ ~ko~uml~ b.
Tabara
D ~.anz|bor )Oarts /
~
W~Q
Fig. I. Sketch map of the Dares Salaam coast showing the sampling locations.
Heavy metals
at coast of Dares Salaam
6°42'S to 6°50'S and longitude 39°14'E to 3 9 ° 2 2 ' E (Fig. 1). The hydrography of the D a r e s Salaam coastal zone can be summarized as: for the currents, it is predominantly the north-bound equatorial current almost the year round. This current is strongest during the southeast monsoon when the wind force is relatively high. However, in the harbor channel, both wind and tidal motion may influence the direction of localized inshore currents. Other oceanographic parameters such as seawater temperature, density, and salinity are typical of tropical coastal water (e.g., McClanahan 1988). Most of the storm drain outfalls in the city are located in the inner portion of the harbor channel, and the mouths of two streams are located in the channel. The Gerezani stream drains the small scale industrial organization (SIDO) area. The oil refinery is situated within the harbor bay. The area around the outer anchorage zone for ships is likely to receive particles from the Msimbazi river, which is the major transporter of industrial effluent and solids from the former dumping area of the city (Ak'habuhaya and Lodeneus 1988). The anchorage zone may be under the influence of particulate matter from the sewer outfall as well. MATERIAL AND METHODS
General description of the sediments The sediment cores were collected between April
and August 1991 at five sampling locations. Cores JB-1, JB-3, and JB-5 were taken at Stations 1, 3, and 5, respectively. These sample locations are characterized by sulfidic sediments, a thin superficial layer composed of a brownish floc; the rest of the core length is blackish to gray silty-clay ooze. Core JB-2 was taken at Station 2 near the mouth of the Gerezani stream. The sediment is sulfidic, mainly silty-sand. The above cores were retrieved aboard a 6-m inboard engine plank boat by hand coring by SCUBA diving. Core AMJ-10 was retrieved from a depth of 31 m (Station 6) aboard MV Mzalendo (a stern trawler) by a gravity corer with a plastic coring tube. Station 6 is close to the ships' outer anchorage zone and the single mooring buoy for tanker off-loading; the sediment is grayish, silty-clay. The samples were either dried immediately after sectioning or stored at -20°C until required.
Analytical techniques The sediment samples were dried to a constant w eight at 50+2°C in the oven, ground, and homogenized with a pestle and mortar. A well homogenized sub-
411
sample, approximately 0.5g, was weighed into a 100 ml PTFE beaker. In order to initially break down the organic matter, the sample was treated with 10 ml of 30% H202, which was added in small portions on a hot plate and finally heated to dryness at 96°C (MAFF 1978). For further decomposition of organic matter and dissolution of the reducible metal fraction and other geochemically available sediment phases, the sample was refluxed with fresh aqua regia (10 ml) and then completely dried (Jacob's and Keeney 1974; Agemian and Chau 1976). Then, 6M HCI (1 mL) were added in order to redissolve the metals, and the supernatant was diluted to 25 ml after separation of the residue by centrifugation. The metals were analyzed by flame atomic-absorption spectrophotometry (Perkin-Elmer Model 2380). The internal precision (expressed as coefficient of variation between replicate samples) was better than 5% for Cu, Zn, Pb, and Ni; better than 10% for Fe, Mn, and Cd; and 12% for Cr. The organic carbon c o n t e n t of the s e d i m e n t was d e t e r m i n e d by the dichromate oxidation technique (Gaudette et al. 1974). RESULTS AND DISCUSSION
Heavy metal concentrations in the geochemically available fractions and organic carbon results are presented in Tables la-e. The organic carbon results show high input of organic matter in the sediments of Stations 1 and 5. Organic substances in the harbor mainly result from allochthonous sources, such as direct fuel usage, deliberate disposal of organic wastes, and input from rerrestrial sources. Despite the fine grains of sediments from Station 6, they have relatively low organic content suggesting minimum input of terrigenous organic waste material. Iron and manganese
The geochemically available iron in the D a r e s Salaam coastal water varies between 0.06 and 2.27%, the average concentration is 0.91%. Low values are associated with silt-sandy sediments and high values occur in clay or silty deposits in accordance with the grain-size variation. Within a core, the distribution is more or less uniform, suggesting that mixing of the sediment is taking place throughout the sampled depth. The concentration-depth profile for iron does not indicate any diagenetie remobilization of the element in these sediments, indicating that iron is deposited in a reduced state. Generally, the Fe level is high in harbor sediments and the concentration rapidly decreases outside the harbor zone. The sources of iron in the harbor sediments possibly are: anchored
412
LF. Machiwa
Table la-c. Heavy metal concentrations in the total available fraction and the organic carbon content of bulk sediment samples. (C-org, Fe are quoted in percentage; other elements in Ixg/g)
(a)
Station
Sediment depth
no.
1
C-org.
Fe
Mn
Ni
Cd
Cr
Cu
Zn
Pb
(cm)
0.0
- 2.0
3.12
1.49
259
15.6
1.56
26.9
25.3
94.8
44.3
2.0
- 4.0
2.96
1.47
238
15.4
1.47
29.9
23.9
86.2
48.3
4.0
- 6.0
2.02
1.52
263
16.2
1.32
35.6
25.6
101.7
46.3
6.0
- 8.0
2.10
1.30
256
13.3
1.4926.022.4
87.7
45.0
8.0
- I0.0
2.35
1.33
254
14.2
2.1629.521.8
83 5
55.9
93 5
48.3
113 8
61.1
i0.0
- 15.0
2.39
1.45
264
14.3
1.7328.826.0
15.0
- 20.0
2.37
1.36
247
13.9
1.42
20.0
- 25.0
2.19
1.50
252
15.6
1.6422.534.2111
3
116.4
25.0
- 30.0
2.09
1.54
251
15.1
1.6233.331.3216
9
176.9
30.0
- 35.0
1.98
2.27
241
18.5
1.4623.677.3188
4
104.9
(b)
Station
Sediment depth
no.
30.1
29.3
2
C-org
Fe
Mn
Ni
Cd
Cr
Cu
Zn
Pb
(cm)
0.0
- 2.0
0.40
0.23
38
4.2
0.52
11.9
4.5
19.0
13.3
2.0
- 4.5
0.13
0.16
17
2.0
0.06
6.5
3.5
11.8
7.4
4.5
- 7.0
0.08
0.09
7
1.8
0.58
5.01.5
12.5
12.5
7.0
- 9.5
0.20
0.06
ii
2.8
0.i0
3.9
i0.0
5.8
9.5
- 12.0
0.27
0.16
22
3.4
0.52
4.63.5
14.7
11.9
1.8
12.0
- 17.0
0.23
0.12
18
3.0
0.39
7.42.7
15.5
8.8
17.0
- 22.0
0.17
0.16
i0
3.8
0.49
4.55.0
18.7
22.2
22.0
- 27.0
0.33
0.17
13
3.2
0.72
5.92.8
13.1
12.6
27.0
- 32.0
0.27
0.16
18
4.5
0.22
7.43.0
13.3
28.0
32.0
- 37.0
0.24
0.13
14
3.5
0.77
6.50.9
8.9
8.5
Heavy metals at coast of Dar
es
Salaam
413
(c) s t a t i o n
Sediment
no.
3
C-org
Fe
Hn
Ni
Cd
Cr
Cu
Zn
Pb
depth
(cm)
0.0
2.0
1.55
0.85
180
13.3
1.88 24.9
19.9 67.2
44.3
2.0 - 4.0
1.22
0.97
143
11.3 2.56 27.5
14.5 56.0
57.9
4.0 - 6.0
0.92
0.90
132
12.3 2 . 0 3 21.9
17.0 51.3
43.2
6.0 - 8.0
0.96
0.83
130
11.9 2 . 1 6 21.2
16.1 58.0
78.9
1.61
1.21
168
16.1
15.0
1.59
1.04
140
18.0 2.04 28.8
16.7
15.0 - 20.0
1.05
0.96
132
12.0
20.0
1.02
0.89
119
C-org
Fe
Hn
0.0 - 2.0
2.90
2.0 - 4.0
-
93.1
40.4
62.7
45.5
1.58 26.4
30.4 88.0
128.1
11.6
1.93 24.1
17.I 8 4 . 3
70.7
Ni
Cd
Cu
1 22 214
16.7
1.45 32.0 20.3
88.3
43.0
2.89
1 23 196
13.0
1.76 32.0 21.2
91.2
51.5
4.0 - 6.0
3.30
1 09 181
17.5
1.23 31.9 21.5
86.7
6O. 4
6.0 - 8.0
2.59
1 30 220
16.0
1.89 34.9 23.9
98.3
47.4
8.0 - i0.0
2.37
1 25 212
12.9
1.77 33.8 23.5
119.0
50.9
I0.0-
15.0
2.57
1 10 182
15.1
1.30 29.5
18.4
84.4
47.4
15.0
- 20.0
2.42
1 18 229
15.8
1.96 31.3 22.6
91.2
53.4
20.0
-
25.0
1.99
1 06 185
15.3 2.82
25.5 25.5
87.2
56.8
25.0
-
30.0
1.48
1 11 169
15.3 2.49
29.1
20.7
87.8
59.7
30.0
-
35.0
1.67
1 19
167
25.0
30.1 25.0
119.8
98.1
35.0
-
40.0
1.23
0 80
132
11.2 2.22
22.8
16.0 321.1
467.2
40.0
-
42.0
1.16
0 80
127
12.7 2.17
27.2
18.7
111.0
C-org
Fe
Mn
N i
Cr
Cu
0.0 - 2.0
0.56
0.76
104
2.0 - 4.0
0.64
0.77
4.0 - 6.0
0.56
6.0 - 8.0
8.0
I0.0
-
i0.0
-
- 25.0
(d) S t a t i o n Sediment depth
no.
1.93 29.2 88.9
5
Cr
Zn
Pb
(cm)
(e) S t a t i o n Sediment
no.
1.39
76.6
6 Zn
Pb
18.3 4.51 36.7 7 . 3
16.9
30.7
97
22.2
3.15 41.5 7.4
11.5
20.0
0.79
100
22.4
3.79 42.3 7.4
13.7
23.4
0.57
0.74
90
19.1 3.31
6.9
14.9
22.3
8.0 - i0.0
0.59
0.72
93
20.5 3.36 33.6 7.0
12.0
27.6
10.0-
15.0
0.81
0.78
102
2 1 . 3 3.89 4 2 . 3 7.0
10.2
30.1
15.0 - 20.0
0.61
0.78
91
19.1 3.20 3 9 . 3 7.0
11.2
30.9
20.0
0.73
0.75
97
19.6 3.44
10.2
25.5
depth
Cd
(cm)
- 25.0
37.1
36.1 7 . 3
414
vessels, ship and boat repair, and deliberate disposal of metallic wastes. The concentration of manganese varies between 7 and 263 mg/kg of dry sediment, the average level is 142.2 mg/kg. The high values are found in the fine-grained harbor sediments. In all of the samples Fe/Mn ratios do not suggest a common source for the two elements. The likely source of Mn is not known, hence, the observed low concentration of Mn can be its background level.
Copper, zinc, and nickel Copper, zinc, and nickel are among the ubiquitous elements in effluents from industries. Copper is a common element in electrical and water pipe manufacture as well as in weed and pest control; mostly zinc is used in metal plating, and nickel is present in fuel oils, coal, tobacco, and chemical and metal industries. The concentrations of these elements in the D a r e s Salaam sediments are quite variable. The total available copper varies between 0.9 and 88.9 mg/kg, the average concentration is 18.4mg/kg. Sediments from Stations 1 and 3 contain a high Cu content compared with sediments off the harbor. Similarly, the zinc level in the samples range between 9 and 321 mg/kg, the average concentration is 69.3 mg/kg. The higher concentration values are from Station 1, 3, and 5, the latter sampling site containing the highest level. The levels are consistently low (below 17 mg/kg) off the harbor. The nickel content of the sediments is generally low throughout the stations. The lowest value is 2, the highest is 22, and the average concentration is 13.1 mg/kg, The most likely sources of copper and zinc in the harbor area are the Gerezani stream and/or deliberate disposal within the harbor. The surprisingly low concentration of the elements outside the harbor zone indicate minimum discharge (from industrial and municipal sources) and mobilization by transporting agents.
Cadmium, chromium, and lead The concentration of Cd varies between 0.06 and 4.51 mg/kg, and the average value is 1.82 mg/kg. Surprisingly, Cd is low in the harbor area compared with Station 6 sediments (core AMJ-10) where, indeed, elevated levels have been recorded. Even though the Cd level in the harbor sediments can not be regarded as purely background concentration, the contamination level is too low to trigger any environmental alarm. Apparently, the enrichment of Cd in sediments off the harbor is not well documented, and the available data is too scanty to reveal any accept-
J.F. Machiwa
able trend. Ak'habuhaya and Lodenius (1988), for example, have reported low concentrations of Cd (0.1-2.7 mg/kg) in sediments along the Msimbazi river in the Msimbazi creek, suggesting minimum input from some major industries in the city. Pollution studies in the city of D a r e s Salaam by Haskoning and M. Konsult (1988) revealed high levels of CD (7 mg/kg) contamination of the Luhanga river upstream of the former Tabata clump site; the level decreases downstream to values less than 1 mg/kg. The analysis of Cd in algal samples by Wekwe et al. (1989), as well, has indicated low Cd concentrations (0.05-0.48 g/kg) along the D a r e s Salaam coast. The contribution of the sewer outfall to the Cd amount in the Dar es Salaam sediments has not been fully investigated. So far, deliberate disposal of Cd-containing wastes close to Station 6 can be considered as the source of the metal in the sediment. There is no likelihood of mobilization of such elevated Cd levels by streams other than Msimbazi; eolian transport is dismissed. The chromium level in the sediments varies between 4 and 42 mg/kg, the average concentration is 25.5 mg/kg. Like Cd, high Cr concentrations were recorded at Station 6. Stations 1, 3, and 5 showed intermediate levels. Following the Chester and Hughes (1969) report that about 90% of Cr in the deep sea sediments is structually bound in the lattice positions of detrital minerals, the observed authigenic Cr concentrations in the sediments strongly suggest a significant anthropogenic flux. In fact, the bicycle factory in the city is one of the possible sources of Cr in the marine environment. The effluent from the factory drain through the Kawe stream, possibly containing Cr dust, may settle in the region of Station 6. This, however, does not implicate the factory for Cd contamination. The geochemically available lead in the bulk sediment samples varies between 7 and 467 g/kg; the average concentration is about 57.3 mg/kg. Sediments fron Station 6 show relatively low Pb values ( 20 mg/kg). The data indicate a prominent flux of Pb from cultural activities in the watershed. Some probable sources of authigenic lead in the harbor zone include the heavy traffic of outboard and inboard ferrying and fishing boats, ships, deposition of leaded dust from the atmosphere, effluents from SIDO (Gerezani) and storm drain outfalls, dicharging water containing lead from building and vehicle paints as well as petrol stations. In summary, the analyzed elements show uniform distribution down the length of the cores, possibly due to mixing of the sediment column caused by physical mixing agents rather than bioturbation.
Heavy metals at coast of Dares Salaam
Only Cd and Cr are slightly enriched in sediments off the harbor area, other elements are present at more or less background levels. The source of Cd in sediments from Station 6 has not been established; however, the low level of organic carbon in the sediment suggests that runoff may not be involved in the Cd transport. The bicycle factory, on the other hand, is suspected to be the cause for the Cr contamination of the sediments. The elevated levels of iron and lead in the harbor sediments, may be mainly a result of harbor activities and a small proportion is imported from the city. With the available data, it is not possible to evaluate the Pb contributions from different transporting agents. There is a need for studying the local current patterns at the Dar es Salaam coast in relation with the distribution of metals in order to determine the areas of deposition. REFERENCES A'khabuhaya, J.; Lodenius, M. Metal pollution of River Msimbazi, Tanzania. Environ. Int, 14:511-514; 1988. Agemian, H.; Chau, I.S.Y. Evaluation of extraction techniques for the determination of metals in aquatic sediments. Analyst: 761-767; 1976. Bryceson, I.; De Souza, T.F.; Jehanger, I.; Ngoile, M.A.K.; Wynter, P. State of marine environment in the Eastern African region.
415
UNEP regional seas report and studies No. 113. United Nations Environment Programme, Nairobi, Kenya; 1990.. Chester, R.; Hughes, M.J. The trace element geochemistry of a North Pacific pelagic clay core. Deep-Sea Res. 16: 639-654; 1969. Chester, R. Marine geochemistry. London: Unwin Hyman; 1990: 698p. Gaudette, lt.E.; Flight, W.R.; Toner, L.; Folger, D.W. An inexpensive titration method for the determination of organic carbon in recent sediments. J. Sediment. Petrol. 44:249-253; 1974. Haskoning and M - Konsult, Consulting Engineers. Study on solid waste management and pollution caused by sewage systems in Dar es Salaam. Report to the Ministry of Water, Department o f Sewerage and Sanitation, Dares Salaam, Tanzania; 1988. Jacob's, L.W.; Keeney, D.R. Aqua regia for quantitative recovery of mercuric sulphide from sediments. Environ. Science Technol. 8: 267-268; 1974. McClanahan, T.R. Seasonality in East Africa's coastal waters. Mar. Ecol. Prog. Ser. 44: 191-199; 1988. MAFF (Ministry of Agriculture, Fisheries and Food), Directorate of Fisheries Research, Lowestoft, U.K.; Fisheries Research Technical Report No. 47: 1-15; 1978. UNEP (United Nations Environment Programme) Environmentalproblems of the East African region. UNEP Regional seas, reports and studies No. 12; 1982. UNEP (United Nations Environment Programme) Coastal and marine environmental problems of the United Republic of Tanzania. UNEP Regional seas, reports and studies No. 106; 1989. Wekwe, W.W.; Othman, O.C.; Khan, M.R. Seaweeds as heavy metal pollution indicators. In: Khans, M.R; Gijzen, H.L, eds. Prec. workshop on environmental pollution and its management in Eastern Africa. Sep. 1989: 241-246. Available from: Facuhy of Science, University of Dares Salaam, Dares Salaam, Tanzania.
0160-4120/92 $5.00 + .00 Copyright © 1992 Pergamon Press Ltd.
HEAVY METAL CONTENT IN COASTAL SEDIMENTS OFF DAR ES SALAAM, TANZANIA
John F. Machiwa Department of Zoology and Marine Biology, University of Dar es Salaam, P.O. Box 35064, Dares Salaam, Tanzania
El 9112-191 M (Received 28 December 1991; accepted 19 March 1992)
Anthropogenic input of Fe, Mn, Cu, Zn, Pb, Cr, Cd, and organic carbon into the Dares Salaam coastal sediments is assessed in relation with human activities in the watershed and within the respective water body. The geochemically available fraction of the sediments from the D a r e s Salaam harbor channel is enriched with iron and lead compared with the location off the harbor. Other elements such as copper, zinc, manganese chromium, and cadmium in the harbor sediments show moderate to weak signals in comparison with sediments outside the harbor area. Surprisingly, the sediments close to the outer anchorage zone for ships waiting to off-load cargo is strongly contaminated with Cd and Cr. In the case of Cd, the concentration is more than double that of the harbor sediments. There is a high flux of organic matter into the sediments in the harbor area.
INTRODUCTION Incidents of heavy metal pollution of coastal waters by anthropogenic inputs are nowadays not uncommon in almost all developing countries. Tanzania, being one of the less industrialized nations in the western coast of the Indian Ocean, is presently experiencing impacts of environmental degradation, probably, with the same magnitude both on land and within her exclusive economic zone (UNEP 1982, 1989; Bryceson et al. 1990). Unlike the industrialized countries, where the analytical instruments are readily available, most developing countries face acute shortages of both equipment and qualified environmental personnel. The harbor of D a r e s Salaam is within the major industrial area and largest city of Tanzania. The level of industrial activities related to metal processing has been steadily increasing in the city during the present three decades, vast open areas of the city have been converted into either residential or in-
dustrial places. Uncontrolled/haphazard disposal of domestic as well as industrial wastes is not uncommon. The harbor is situated in a narrow bay; this sheltered inlet is almost protected from free include flushing by seawater circulation. Activities in the harbor area include loading and off-loading of international cargo, dry docking, and other repair works concerning maritime activities. Also, there is a heavy traffic of local fishing and ferry boats and some industrial and municipal outflows discharge in the harbor area. The influence of the hinterland on the composition of near shore sediments has been documented in Chester (1969), therefore, it can be assumed that most suspended material associated with city runoff most probably settles within the shore area. The inshore deposition of the land-based particulate matter can be the result of the complex patterns of coastal winds and currents. The concentration of heavy metals in the sediments of such areas, therefore, should
409
410
J.F. Machiwa
low as reasonably achievable level" will ever come to practice. Ecologists in developing countries, therfore, are concerned with establishing baseline levels in order to be ready for environmental perturbations that are likely to occur in future.
be a good indicator of contaminant fluxes of terrigenous origin. Because of the growing flux of heavy metals from both industries and land denudation to the marine environment, areas with restricted circulation, such as the harbor of Dar es Salaam, material input can significantly cause a build-up of pollutants such as heavy metals, PAH, and PCB residues. Despite the enacted legislation safeguarding aesthetic value of water bodies, it is still beyond imagination whether or not the waste discharge at "zero level" or at "as
STUDY AREA
The sampling locations are within near-shore waters of Dar es Salaam, approximately between lat.
!
0
6"M'~"
N goyo Is.
INDIAN OCEAN
6.
0ubr
~n,~ ~ko~uml~ b.
Tabara
D ~.anz|bor )Oarts /
~
W~Q
Fig. I. Sketch map of the Dares Salaam coast showing the sampling locations.
Heavy metals
at coast of Dares Salaam
6°42'S to 6°50'S and longitude 39°14'E to 3 9 ° 2 2 ' E (Fig. 1). The hydrography of the D a r e s Salaam coastal zone can be summarized as: for the currents, it is predominantly the north-bound equatorial current almost the year round. This current is strongest during the southeast monsoon when the wind force is relatively high. However, in the harbor channel, both wind and tidal motion may influence the direction of localized inshore currents. Other oceanographic parameters such as seawater temperature, density, and salinity are typical of tropical coastal water (e.g., McClanahan 1988). Most of the storm drain outfalls in the city are located in the inner portion of the harbor channel, and the mouths of two streams are located in the channel. The Gerezani stream drains the small scale industrial organization (SIDO) area. The oil refinery is situated within the harbor bay. The area around the outer anchorage zone for ships is likely to receive particles from the Msimbazi river, which is the major transporter of industrial effluent and solids from the former dumping area of the city (Ak'habuhaya and Lodeneus 1988). The anchorage zone may be under the influence of particulate matter from the sewer outfall as well. MATERIAL AND METHODS
General description of the sediments The sediment cores were collected between April
and August 1991 at five sampling locations. Cores JB-1, JB-3, and JB-5 were taken at Stations 1, 3, and 5, respectively. These sample locations are characterized by sulfidic sediments, a thin superficial layer composed of a brownish floc; the rest of the core length is blackish to gray silty-clay ooze. Core JB-2 was taken at Station 2 near the mouth of the Gerezani stream. The sediment is sulfidic, mainly silty-sand. The above cores were retrieved aboard a 6-m inboard engine plank boat by hand coring by SCUBA diving. Core AMJ-10 was retrieved from a depth of 31 m (Station 6) aboard MV Mzalendo (a stern trawler) by a gravity corer with a plastic coring tube. Station 6 is close to the ships' outer anchorage zone and the single mooring buoy for tanker off-loading; the sediment is grayish, silty-clay. The samples were either dried immediately after sectioning or stored at -20°C until required.
Analytical techniques The sediment samples were dried to a constant w eight at 50+2°C in the oven, ground, and homogenized with a pestle and mortar. A well homogenized sub-
411
sample, approximately 0.5g, was weighed into a 100 ml PTFE beaker. In order to initially break down the organic matter, the sample was treated with 10 ml of 30% H202, which was added in small portions on a hot plate and finally heated to dryness at 96°C (MAFF 1978). For further decomposition of organic matter and dissolution of the reducible metal fraction and other geochemically available sediment phases, the sample was refluxed with fresh aqua regia (10 ml) and then completely dried (Jacob's and Keeney 1974; Agemian and Chau 1976). Then, 6M HCI (1 mL) were added in order to redissolve the metals, and the supernatant was diluted to 25 ml after separation of the residue by centrifugation. The metals were analyzed by flame atomic-absorption spectrophotometry (Perkin-Elmer Model 2380). The internal precision (expressed as coefficient of variation between replicate samples) was better than 5% for Cu, Zn, Pb, and Ni; better than 10% for Fe, Mn, and Cd; and 12% for Cr. The organic carbon c o n t e n t of the s e d i m e n t was d e t e r m i n e d by the dichromate oxidation technique (Gaudette et al. 1974). RESULTS AND DISCUSSION
Heavy metal concentrations in the geochemically available fractions and organic carbon results are presented in Tables la-e. The organic carbon results show high input of organic matter in the sediments of Stations 1 and 5. Organic substances in the harbor mainly result from allochthonous sources, such as direct fuel usage, deliberate disposal of organic wastes, and input from rerrestrial sources. Despite the fine grains of sediments from Station 6, they have relatively low organic content suggesting minimum input of terrigenous organic waste material. Iron and manganese
The geochemically available iron in the D a r e s Salaam coastal water varies between 0.06 and 2.27%, the average concentration is 0.91%. Low values are associated with silt-sandy sediments and high values occur in clay or silty deposits in accordance with the grain-size variation. Within a core, the distribution is more or less uniform, suggesting that mixing of the sediment is taking place throughout the sampled depth. The concentration-depth profile for iron does not indicate any diagenetie remobilization of the element in these sediments, indicating that iron is deposited in a reduced state. Generally, the Fe level is high in harbor sediments and the concentration rapidly decreases outside the harbor zone. The sources of iron in the harbor sediments possibly are: anchored
412
LF. Machiwa
Table la-c. Heavy metal concentrations in the total available fraction and the organic carbon content of bulk sediment samples. (C-org, Fe are quoted in percentage; other elements in Ixg/g)
(a)
Station
Sediment depth
no.
1
C-org.
Fe
Mn
Ni
Cd
Cr
Cu
Zn
Pb
(cm)
0.0
- 2.0
3.12
1.49
259
15.6
1.56
26.9
25.3
94.8
44.3
2.0
- 4.0
2.96
1.47
238
15.4
1.47
29.9
23.9
86.2
48.3
4.0
- 6.0
2.02
1.52
263
16.2
1.32
35.6
25.6
101.7
46.3
6.0
- 8.0
2.10
1.30
256
13.3
1.4926.022.4
87.7
45.0
8.0
- I0.0
2.35
1.33
254
14.2
2.1629.521.8
83 5
55.9
93 5
48.3
113 8
61.1
i0.0
- 15.0
2.39
1.45
264
14.3
1.7328.826.0
15.0
- 20.0
2.37
1.36
247
13.9
1.42
20.0
- 25.0
2.19
1.50
252
15.6
1.6422.534.2111
3
116.4
25.0
- 30.0
2.09
1.54
251
15.1
1.6233.331.3216
9
176.9
30.0
- 35.0
1.98
2.27
241
18.5
1.4623.677.3188
4
104.9
(b)
Station
Sediment depth
no.
30.1
29.3
2
C-org
Fe
Mn
Ni
Cd
Cr
Cu
Zn
Pb
(cm)
0.0
- 2.0
0.40
0.23
38
4.2
0.52
11.9
4.5
19.0
13.3
2.0
- 4.5
0.13
0.16
17
2.0
0.06
6.5
3.5
11.8
7.4
4.5
- 7.0
0.08
0.09
7
1.8
0.58
5.01.5
12.5
12.5
7.0
- 9.5
0.20
0.06
ii
2.8
0.i0
3.9
i0.0
5.8
9.5
- 12.0
0.27
0.16
22
3.4
0.52
4.63.5
14.7
11.9
1.8
12.0
- 17.0
0.23
0.12
18
3.0
0.39
7.42.7
15.5
8.8
17.0
- 22.0
0.17
0.16
i0
3.8
0.49
4.55.0
18.7
22.2
22.0
- 27.0
0.33
0.17
13
3.2
0.72
5.92.8
13.1
12.6
27.0
- 32.0
0.27
0.16
18
4.5
0.22
7.43.0
13.3
28.0
32.0
- 37.0
0.24
0.13
14
3.5
0.77
6.50.9
8.9
8.5
Heavy metals at coast of Dar
es
Salaam
413
(c) s t a t i o n
Sediment
no.
3
C-org
Fe
Hn
Ni
Cd
Cr
Cu
Zn
Pb
depth
(cm)
0.0
2.0
1.55
0.85
180
13.3
1.88 24.9
19.9 67.2
44.3
2.0 - 4.0
1.22
0.97
143
11.3 2.56 27.5
14.5 56.0
57.9
4.0 - 6.0
0.92
0.90
132
12.3 2 . 0 3 21.9
17.0 51.3
43.2
6.0 - 8.0
0.96
0.83
130
11.9 2 . 1 6 21.2
16.1 58.0
78.9
1.61
1.21
168
16.1
15.0
1.59
1.04
140
18.0 2.04 28.8
16.7
15.0 - 20.0
1.05
0.96
132
12.0
20.0
1.02
0.89
119
C-org
Fe
Hn
0.0 - 2.0
2.90
2.0 - 4.0
-
93.1
40.4
62.7
45.5
1.58 26.4
30.4 88.0
128.1
11.6
1.93 24.1
17.I 8 4 . 3
70.7
Ni
Cd
Cu
1 22 214
16.7
1.45 32.0 20.3
88.3
43.0
2.89
1 23 196
13.0
1.76 32.0 21.2
91.2
51.5
4.0 - 6.0
3.30
1 09 181
17.5
1.23 31.9 21.5
86.7
6O. 4
6.0 - 8.0
2.59
1 30 220
16.0
1.89 34.9 23.9
98.3
47.4
8.0 - i0.0
2.37
1 25 212
12.9
1.77 33.8 23.5
119.0
50.9
I0.0-
15.0
2.57
1 10 182
15.1
1.30 29.5
18.4
84.4
47.4
15.0
- 20.0
2.42
1 18 229
15.8
1.96 31.3 22.6
91.2
53.4
20.0
-
25.0
1.99
1 06 185
15.3 2.82
25.5 25.5
87.2
56.8
25.0
-
30.0
1.48
1 11 169
15.3 2.49
29.1
20.7
87.8
59.7
30.0
-
35.0
1.67
1 19
167
25.0
30.1 25.0
119.8
98.1
35.0
-
40.0
1.23
0 80
132
11.2 2.22
22.8
16.0 321.1
467.2
40.0
-
42.0
1.16
0 80
127
12.7 2.17
27.2
18.7
111.0
C-org
Fe
Mn
N i
Cr
Cu
0.0 - 2.0
0.56
0.76
104
2.0 - 4.0
0.64
0.77
4.0 - 6.0
0.56
6.0 - 8.0
8.0
I0.0
-
i0.0
-
- 25.0
(d) S t a t i o n Sediment depth
no.
1.93 29.2 88.9
5
Cr
Zn
Pb
(cm)
(e) S t a t i o n Sediment
no.
1.39
76.6
6 Zn
Pb
18.3 4.51 36.7 7 . 3
16.9
30.7
97
22.2
3.15 41.5 7.4
11.5
20.0
0.79
100
22.4
3.79 42.3 7.4
13.7
23.4
0.57
0.74
90
19.1 3.31
6.9
14.9
22.3
8.0 - i0.0
0.59
0.72
93
20.5 3.36 33.6 7.0
12.0
27.6
10.0-
15.0
0.81
0.78
102
2 1 . 3 3.89 4 2 . 3 7.0
10.2
30.1
15.0 - 20.0
0.61
0.78
91
19.1 3.20 3 9 . 3 7.0
11.2
30.9
20.0
0.73
0.75
97
19.6 3.44
10.2
25.5
depth
Cd
(cm)
- 25.0
37.1
36.1 7 . 3
414
vessels, ship and boat repair, and deliberate disposal of metallic wastes. The concentration of manganese varies between 7 and 263 mg/kg of dry sediment, the average level is 142.2 mg/kg. The high values are found in the fine-grained harbor sediments. In all of the samples Fe/Mn ratios do not suggest a common source for the two elements. The likely source of Mn is not known, hence, the observed low concentration of Mn can be its background level.
Copper, zinc, and nickel Copper, zinc, and nickel are among the ubiquitous elements in effluents from industries. Copper is a common element in electrical and water pipe manufacture as well as in weed and pest control; mostly zinc is used in metal plating, and nickel is present in fuel oils, coal, tobacco, and chemical and metal industries. The concentrations of these elements in the D a r e s Salaam sediments are quite variable. The total available copper varies between 0.9 and 88.9 mg/kg, the average concentration is 18.4mg/kg. Sediments from Stations 1 and 3 contain a high Cu content compared with sediments off the harbor. Similarly, the zinc level in the samples range between 9 and 321 mg/kg, the average concentration is 69.3 mg/kg. The higher concentration values are from Station 1, 3, and 5, the latter sampling site containing the highest level. The levels are consistently low (below 17 mg/kg) off the harbor. The nickel content of the sediments is generally low throughout the stations. The lowest value is 2, the highest is 22, and the average concentration is 13.1 mg/kg, The most likely sources of copper and zinc in the harbor area are the Gerezani stream and/or deliberate disposal within the harbor. The surprisingly low concentration of the elements outside the harbor zone indicate minimum discharge (from industrial and municipal sources) and mobilization by transporting agents.
Cadmium, chromium, and lead The concentration of Cd varies between 0.06 and 4.51 mg/kg, and the average value is 1.82 mg/kg. Surprisingly, Cd is low in the harbor area compared with Station 6 sediments (core AMJ-10) where, indeed, elevated levels have been recorded. Even though the Cd level in the harbor sediments can not be regarded as purely background concentration, the contamination level is too low to trigger any environmental alarm. Apparently, the enrichment of Cd in sediments off the harbor is not well documented, and the available data is too scanty to reveal any accept-
J.F. Machiwa
able trend. Ak'habuhaya and Lodenius (1988), for example, have reported low concentrations of Cd (0.1-2.7 mg/kg) in sediments along the Msimbazi river in the Msimbazi creek, suggesting minimum input from some major industries in the city. Pollution studies in the city of D a r e s Salaam by Haskoning and M. Konsult (1988) revealed high levels of CD (7 mg/kg) contamination of the Luhanga river upstream of the former Tabata clump site; the level decreases downstream to values less than 1 mg/kg. The analysis of Cd in algal samples by Wekwe et al. (1989), as well, has indicated low Cd concentrations (0.05-0.48 g/kg) along the D a r e s Salaam coast. The contribution of the sewer outfall to the Cd amount in the Dar es Salaam sediments has not been fully investigated. So far, deliberate disposal of Cd-containing wastes close to Station 6 can be considered as the source of the metal in the sediment. There is no likelihood of mobilization of such elevated Cd levels by streams other than Msimbazi; eolian transport is dismissed. The chromium level in the sediments varies between 4 and 42 mg/kg, the average concentration is 25.5 mg/kg. Like Cd, high Cr concentrations were recorded at Station 6. Stations 1, 3, and 5 showed intermediate levels. Following the Chester and Hughes (1969) report that about 90% of Cr in the deep sea sediments is structually bound in the lattice positions of detrital minerals, the observed authigenic Cr concentrations in the sediments strongly suggest a significant anthropogenic flux. In fact, the bicycle factory in the city is one of the possible sources of Cr in the marine environment. The effluent from the factory drain through the Kawe stream, possibly containing Cr dust, may settle in the region of Station 6. This, however, does not implicate the factory for Cd contamination. The geochemically available lead in the bulk sediment samples varies between 7 and 467 g/kg; the average concentration is about 57.3 mg/kg. Sediments fron Station 6 show relatively low Pb values ( 20 mg/kg). The data indicate a prominent flux of Pb from cultural activities in the watershed. Some probable sources of authigenic lead in the harbor zone include the heavy traffic of outboard and inboard ferrying and fishing boats, ships, deposition of leaded dust from the atmosphere, effluents from SIDO (Gerezani) and storm drain outfalls, dicharging water containing lead from building and vehicle paints as well as petrol stations. In summary, the analyzed elements show uniform distribution down the length of the cores, possibly due to mixing of the sediment column caused by physical mixing agents rather than bioturbation.
Heavy metals at coast of Dares Salaam
Only Cd and Cr are slightly enriched in sediments off the harbor area, other elements are present at more or less background levels. The source of Cd in sediments from Station 6 has not been established; however, the low level of organic carbon in the sediment suggests that runoff may not be involved in the Cd transport. The bicycle factory, on the other hand, is suspected to be the cause for the Cr contamination of the sediments. The elevated levels of iron and lead in the harbor sediments, may be mainly a result of harbor activities and a small proportion is imported from the city. With the available data, it is not possible to evaluate the Pb contributions from different transporting agents. There is a need for studying the local current patterns at the Dar es Salaam coast in relation with the distribution of metals in order to determine the areas of deposition. REFERENCES A'khabuhaya, J.; Lodenius, M. Metal pollution of River Msimbazi, Tanzania. Environ. Int, 14:511-514; 1988. Agemian, H.; Chau, I.S.Y. Evaluation of extraction techniques for the determination of metals in aquatic sediments. Analyst: 761-767; 1976. Bryceson, I.; De Souza, T.F.; Jehanger, I.; Ngoile, M.A.K.; Wynter, P. State of marine environment in the Eastern African region.
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UNEP regional seas report and studies No. 113. United Nations Environment Programme, Nairobi, Kenya; 1990.. Chester, R.; Hughes, M.J. The trace element geochemistry of a North Pacific pelagic clay core. Deep-Sea Res. 16: 639-654; 1969. Chester, R. Marine geochemistry. London: Unwin Hyman; 1990: 698p. Gaudette, lt.E.; Flight, W.R.; Toner, L.; Folger, D.W. An inexpensive titration method for the determination of organic carbon in recent sediments. J. Sediment. Petrol. 44:249-253; 1974. Haskoning and M - Konsult, Consulting Engineers. Study on solid waste management and pollution caused by sewage systems in Dar es Salaam. Report to the Ministry of Water, Department o f Sewerage and Sanitation, Dares Salaam, Tanzania; 1988. Jacob's, L.W.; Keeney, D.R. Aqua regia for quantitative recovery of mercuric sulphide from sediments. Environ. Science Technol. 8: 267-268; 1974. McClanahan, T.R. Seasonality in East Africa's coastal waters. Mar. Ecol. Prog. Ser. 44: 191-199; 1988. MAFF (Ministry of Agriculture, Fisheries and Food), Directorate of Fisheries Research, Lowestoft, U.K.; Fisheries Research Technical Report No. 47: 1-15; 1978. UNEP (United Nations Environment Programme) Environmentalproblems of the East African region. UNEP Regional seas, reports and studies No. 12; 1982. UNEP (United Nations Environment Programme) Coastal and marine environmental problems of the United Republic of Tanzania. UNEP Regional seas, reports and studies No. 106; 1989. Wekwe, W.W.; Othman, O.C.; Khan, M.R. Seaweeds as heavy metal pollution indicators. In: Khans, M.R; Gijzen, H.L, eds. Prec. workshop on environmental pollution and its management in Eastern Africa. Sep. 1989: 241-246. Available from: Facuhy of Science, University of Dares Salaam, Dares Salaam, Tanzania.