Environmental Pollution (Series B) 11 (1986) 315-322
A Baseline Study of Mercury in Fish and Sediments in the Niger Delta Area of Nigeria
S. E. K a k u l u & O. Osibanjo* Department of Chemistry, University of Ibadan, Ibadan, Nigeria
ABSTRACT The baseline levels of mercury in fish and sediments from the Niger delta area of Nigeria have been examined. The measured eoneentrations infish varied from less than 10 #g kg- : to 410 #g kg- 1 wet weight and those in the sediments varied between O"024 and 1.54 ug g- 1dry weight. The levels found in the samples from this area were generally low in comparison with levels found in other regions of the worm and the 0.51~gg-I Hg in .fish recommended for human consumption by the Worm Health Organisation. These results indicate that the Niger delta area of Nigeria is relatively unpolluted with mercury.
INTRODUCTION The natural levels of heavy metals in surface waters, sediments and biological organisms are often influenced by the release of industrial, agricultural and domestic wastes into aquatic ecosystems. Mercury has generally been regarded as the most toxic heavy metal and has attracted global attention as a pollutant, especially after the Minamata disaster in Japan, in which severe mercury intoxication resulting from ingestion of fish and shellfish with elevated mercury concentrations caused the deaths of several people (Kurland et al., 1960). Thereafter, surveys of mercury in fish have been carried out in the developed countries (Holden, 1973; *To whom all correspondence should be addressed. 315 Environ. Pollut. Ser. B 0143-148X/86/$03.50 ~) ElsevierApplied SciencePublishersLtd, England, 1986. Printed in Great Britain
S. E. Kakulu, O. Osibanjo
Newberne, 1974), fish being the most important dietary source of mercury intake. Vegetable and fruits generally contain between 20 and 50/~g kg- 1 (Jervis et al., 1970). A mercury pollution scare occurred in North America in 1970, triggered by the Canadian discovery of mercury-contaminated fish in Lake Ontario. Mercury levels in excess of the 0.5/~gg -1 Hg World Health Organisation limit for human food were even discovered among beluga whales in Hudson Bay (Hodges, 1973). Generally, fish from fresh waters are reported to have particularly high mercury levels compared with those from coastal waters whereas the mercury content of unpolluted freshwater sediments varies between 0.05 and 0.30#gg-1 dry weight, which is slightly lower than the levels found in marine sediments (UNEP, 1982). Furthermore, mercury has been a proven problem in certain isolated regions with significant releases of mercury from misuse or excessive usage of organomercuric pesticides. Other important pathways of mercury release result from the combustion of fossil fuels, coal and crude oils, some of which have been reported to contain significant amounts of mercury (Filby & Shah, 1975). In Nigeria, a typical developing country at the dawn of its industrial revolution, industrial effluents are invariably discharged raw without any regard for environmental quality sustenance (FMHE, 1982). Owing to
I'OLOMOFNF ('( #
20 40 60 80 100kin. I
Fig. I. Nigerdelta samplinglocations.
Mercury in Niger delta fish
non-existent national environmental protection laws, organomercury compounds are still used as fungicides in agriculture and slimicides in pulp and paper mills. The Niger delta (Fig. l), which is the main oilproducing area of Nigeria, is situated at the southern end of Nigeria, bordering the Atlantic Ocean, and extending from about Longitude 3 ° to 9 ° E and Latitude 4 ° 3' to 5 ° 20' N. The construction of a chlor-alkali plant for the production of caustic soda and a petrochemical complex industry are in progress; these are likely to lead to the discharge of high mercury levels in the Niger delta area. Any future pollution control measures in the area will be meaningful only when applied in the light of adequate understanding of the system being protected, based on locally generated data. Furthermore, the coastal population in the area depends on fish consumption as the major source of animal protein. In view of the paucity of environmental quality data on the Nigerian environment, this paper describes the baseline levels of mercury in fish and sediments (natural sinks for metals deposition in aquatic systems) from some rivers in the Niger delta area of the country.
MATERIALS A N D M E T H O D S
Sample collection and preparation Most of the fishes analysed were economic species and were bought from artisanal fishermen in different locations in the Niger delta area (Fig. I). Others were bought directly from the local fish markets. The fishes were washed with distilled water and then frozen until analysis. Surface sediments were taken by divers. All samples were stored in polyethylene bags below 0 °C prior to analysis. The bags were previously cleaned by soaking in nitric acid (1:4) for 24 h and rinsed with distilled water. The sediment samples were air-dried in an oven at about 60 °C for 16-20 h and sieved through a nylon sieve of 200 mesh size. The particles of 200 mesh size and above in diameter were used in the analysis.
Analytical procedures The analytical procedure employed for the total mercury determination in the fish involved a wet acid digestion (Analytical Methods Committee, 1965). 5 g of the edible (muscle) tissue of the finfish was digested with 15 ml
S. E. Kakulu, O. Osibanjo
of nitric acid/sulphuric acid (2:1) mixture. The shellfish (prawns and crabs) were analysed whole. The digests were diluted to 50 ml with distilled water in volumetric flasks. The sediment samples for total mercury determination were extracted with concentrated nitric acid (Tuncel et al., 1980). 0.5 g o f sediment was digested with 3 ml of nitric acid in a high-pressure decomposition vessel at 140°C for 5 h. The extracts were diluted to 10 ml with distilled water in volumetric flasks. The organic carbon in the sediments was determined by wet oxidation with K / C r 2 0 7 acidified with concentrated sulphuric acid (Gross, 1971). For both the fish digests and the sediment extracts, total mercury determination was carried out by the cold vapour technique after reducing mercury salts with tin(II) to elemental mercury (Hatch & Ott, 1968). A Perkin-Elmer 460 atomic absorption spectrophotometer was used in the determinations, using the conditions stated in the operations manual.
RESULTS A N D DISCUSSION The digestion procedures for the determination of the total mercury in the fish and sediments gave a precision of 11.78 ~ and 12.03 ~o respectively. A summary of the results for total Hg concentrations in the fish and sediments is given in Tables 1 and 2. There was considerable variation in the mercury concentration between the species. The fish from the local waters exhibited a maximum of 4 1 0 p g H g k g -~ wet weight and a minimum of less than 1 0 p g H g k g -1 wet weight. The highest concentrations of Hg were found in the species Papyrocranus afar. Generally, most species analysed contained less than 65 pg Hg kg-1 wet weight. Like the fishes, the total Hg concentrations in the sediments from the rivers in the area studied were also low, showing that at present there is no serious problem. The levels varied between 0.024 and 1.540 pg Hg g- ~dry weight, with a mean of 0.325 pg Hg g - 1 dry weight. With the exception of the sediments from the river Asa near Uzere, with a Hg content of 1.54 pg g-x dry weight, the Hg concentrations in the sediments increased with increase in organic carbon content. Urban sewage, various small-scale industries, coupled with the use of various mercury compounds in agriculture and wastes from dental clinics
Mercury in Niger delta fish
I I 0
S. E. Kakulu, O. Osibanjo
Mercury in River Sediments from Rivers in the Niger Delta Area of Nigeria Sampling location
Warri Ughelli Port Harcourt (Okrika jetty) Olomoro Ase, Uzere Oron Calabar Ibeno
"/., organic' carbon
Mercury concentration (l~gg 1 dry weight)
2.96 1.13 3.43 0.32 0.83 1.65 2.16 1.79
0.104 0.025 0.378 0.024 1.540 0.145 0-277 0.109
and hospitals, contribute to the higher levels of Hg found in river sediments which receive drainage and run-off from the urban cities. This probably accounts for the high Hg content in the sediments from the Port Harcourt area since it is the most densely populated and has a vast number of small-scale industries. Comparison of the levels of Hg in the fish and sediments from the study area with levels in other regions of the world shows that it is relatively unpolluted (Table 3). For instance, fish from the Mediterranean contained mercury levels between 130 and 510/~g kg- 1 wet weight, with a TABLE 3
Comparison of Hg Levels in the Niger Delta Area with Some Other Regions of the World (First figure range, second figure mean) Region
This study Turkish Mediterranean coast Tuscany coast Mediterranean Off south-west England Lake Paijanne, Finland Unpolluted freshwater sediments
Fish ( p g k g i wet weight )
Sediments (pgg 1 do' weigh t)
0.024 1.540;0.325 0.028 0-460; 0.14 0-04 1-30
130-510:340 170-370; 260 50 4680
Tuncel et al. (1980) Renzoni et al. (1973) Stoeppler et al. (1979) Anon. (1977) Hattula et al. (1978) UNEP (1982)
Mercury in Niger deltafish
mean of 340#gkg-~ wet weight (Stoeppler et al., 1979), and the World Health Organisation maximum acceptable limit in fish for consumption is 0.5 #g g-~ wet weight compared with the range of 10-410 #g Hg k g wet weight and a mean of 34/~g Hg kg-1 wet weight from fishes in the study area. With the exception of the high value of 1.54/~g Hg g - 1 in the sediments of the river Ase with no identifiable point source discharge, most other sites show mercury levels of less than 0.30/~gHgg-~ dry weight for unpolluted freshwater sediments (UNEP, 1982). These results suggest that the Niger delta area of Nigeria is relatively unpolluted with mercury. With the construction of industries with potentially high mercury release in their effluents, e.g. the chlor-alkali plant and the petrochemical complex, the above results serve as valuable baseline data for the future mercury pollution status of the Niger delta area. A C K N O W L E D G E MENTS The authors wish to thank the International Seminar in Chemistry, University of Uppsala, Sweden, for a fellowship to S. E. Kakulu; and Dr R. G. Lichtenthaler of the Central Institute of Industrial Research, Oslo, Norway, for providing the facilities in his laboratory where part of this work was carried out. REFERENCES Analytical Methods Committee (1965). Report prepared by the Metallic Impurities in Organic Matter Sub-committee. The determination of small amounts of mercury in organic matter. Analyst, Lond., 90, 515-30. Anon. (1977). A baseline study of the level of contaminating substances in living resources of the North Atlantic. ICES Cooper Res. Rep., 69, 1-82. Federal Ministry of Housing and Environment (FMHE) (1982). The state of the environment in Nigeria. Monograph Series No. 1, Industrial Waste Management, Lagos, Environmental Planning and Protection Division. Filby, R. H. & Shah, K. R. (1975). Neutron activation methods for trace elements in crude oils. In The role of trace metals in Petroleum, ed. by T. F. Yen, 89-110. Michigan, Ann Arbor Publishing Inc. Gross, M. G. (1971). Carbon determination. In Procedures in sedimentary petrology, ed. by R. E. Carver, 589-90. New York, Wiley-Interscience. Hatch, W. R. & Ott, W. L. (1968). Determination of sub-microgram quantities of mercury by atomic-absorption spectrophotometry. Analyt. Chem.~ 40, 2085 7.
S. E. Kakulu, O. Osibanjo
Hattula, M. L., Sarkka, J., Janatuinen, J., Paasivirta, J. & Ross, A. (1978). Total mercury and methyl mercury contents in fish from Lake Paijanne. Environ. Pollut., 17, 19-29. Hodges, L. (1973). Environmental pollution. New York, Holt, Rinehart & Winston. Holden, A. V. (1973). Mercury in fish and shellfish: A review. J. Fd Technol., 8, 1-25.
Jervis, R. E., Debrun, D., LePage, W. & Tiefenbach, B. (1970). Mercury residues in Canadian foods,fish and wildlife: Progress report. National Health Grant 605-7-510, University of Toronto. Kurland, L., Faro, S. & Siedler, H. (1960). Minamata disease. World Neurology, 1,370-95. Newberne, P. W. (1974). Mercury in fish: A literature review. CRC Crit. Rev. Fd Technol., 4, 311-25. Renzoni, A., Bacci, E. & Falciai, L. (1973). Mercury concentration in the water, sediments and fauna of an area of the Tyrrhenian coast. Rev. Int. Oceanogr. med., 31--2, 17-45. Stoeppler, M., Bernhard, M., Backhaus, F. & Schulte, E. (1979). Comparative studies on trace metal levels in marine biota. 1. Mercury in marine organisms from the Italian coast, the strait of Gibraltar and the North Sea. Sci. Total Environ., 13, 209-23. Tuncel, G., Ramelow, G. & Balkas, T. I. (1980). Mercury in water, organisms and sediments from a section of the Turkish Mediterranean coast. Mar. Pollut. Bull., 11, 18-22. United Nations Environment Programme (1982). GESAMP (IMCO/FAO/ WHO/IAEA/UN/UNEP Joint Group of Experts on the Scientific Aspects of Marine Pollution): The health of the oceans. UNEP, Regional Seas Reports and Studies, No. 16.