REVIEW OF HEAVY METALS (continued)

7. LEVELS OF HEAVY METALS IN DIFFERENT ENVIRONMENTAL COMPARTMENTS (continued)

Table VI
Mean metal concentrations in inland water fish (μg/g fresh weight)

* Range values

** Action level adopted in many countries

Table VII
Mean metal concentrations in marine fish (μg/g fresh weight)

* Range values

** Action level adopted in many countries

In support of what has been reported in several studies (Hellawell, 1986; Kakulu and Osibanjo, 1986; Kakulu et al., 1987a; Institute of Aquatic Biology, 1990), shellfish had higher concentrations of most metals. The highest concentrations of cadmium, copper and zinc occurred in Crassostrea sp. which has a great capacity to accumulate contaminants and is a biological indicator of pollution.

7.4 Concentration of Metals in Aquatic Flora

Aquatic plants have been shown to accumulate heavy metals in their tissues and therefore have been used as biological indicators for metal pollution monitoring in the aquatic ecosystem. Table VIII shows the distribution of heavy metals in aquatic plants. Generally, the levels in aquatic plants from inland waters were higher than in those from coastal waters. The variability in the levels of heavy metals in different regions could be ascribed to biological variation between the species rather than environmental factors. Nonetheless, higher concentrations of cadmium were found in Ceratophyllum from industrial areas in Egypt compared to relatively unpolluted areas (Fayed and Abd El-Shafy, 1985). Furthermore, it is significant that an excessively high value of lead (78.0 μg/g) was found in blue-green algae from Lake Mcllwaine, Zimbabwe (Greichus et al., 1978) compared to the rest of the region.

7.5 Comparison between metal contents in sediments and biota

Comparisons between heavy metal concentrations in sediments and biota of selected African waters are shown in Table IX. In Egypt all metals except cadmium showed higher values in the sediments than in fish (Saad, 1985a, 1987). In Ghana only iron and lead followed this pattern, whereas the other metals gave higher values in certain flora and fauna (Biney, 1991a). In Kenya the metals accumulated in higher concentrations in sediments than in fish (Wandiga and Onyari, 1987). In Southern Africa the same pattern occurred with few exception (Greichus et al., 1977). The levels of accumulation of metals in the different flora and fauna did not follow the same pattern.

7.6 Comparison between different sub-regions in Africa

In Tables X and XI are presented the trace metal concentrations in sediments and in fish muscle and shellfish from the major African sub-regions, Northern, Western, Eastern and Southern Africa. The data presented are ranges of means based on Tables IV and V for sediments and Tables VI and VII for fin- and shellfish. Hot-spots, i.e., abnormally high concentrations were excluded form the calculations since the objective is to compare actual background levels from the different sub-regions in Africa.

An inspection of Tables X and XI shows inadequacy of data especially for the coastal and marine areas. In spite of this, the four regions exhibit comparable concentrations of trace metals in both inland and coastal fish and sediment. Mercury for example, occurred in finfish within the narrow ranges of 0.01 to 0.053 μg/g fresh weight for inland fishes and 0.06 to 0.17 for marine fishes. Corresponding values for cadmium were 0.004 to 0.19 and 0.004 to 0.36 μg/g fresh weight.

Table VIII
Mean metal concentrations in aquatic plants (μg/g dry weight)

Table IX
Comparison of trace metal concentrations in sediments, fauna and flora (μg/g dry weight)

* Range values

Table X
Metal concentrations in sediment from the major African sub-regions (μg/g dry weight)

Table XI
Metal concentrations in fish from the major African sub-regions (μg/g fresh weight)

Table XII
Comparison of metal concentrations in sediment from Africa and other areas of the world (μg/g dry weight)

* ND - Not Detected

Table XIII
Comparison of metal concentrations in fish from Africa and other areas of the world (μg/g fresh weight)

* ND - Not Detected

Where comparable data were available, coastal fishes showed slightly higher maximum values of trace metals than inland fishes. This was also true for sediments and may be due to the data originating mainly from coastal lagoons which are normally heavily influenced by anthropogenic activities.

7.7 Comparison of African data with some other areas of the world

The levels of heavy metals in sediments and finfish from African inland and coastal waters are presented alongside data from some other areas of the world in Tables XII and XIII. The means and ranges for African waters were calculated from Tables IV and V for sediments and Tables VII and VIII for fish, excluding the hot spots.

Comparison of such data may be difficult since data calculated for the whole African region are being judged in relation to selected individual areas and sites of the world which may not be representative for their regions. Moreover, different species of fish and fractions of sediments were analyzed. Also, information on sex and weight is often lacking, and comparison is further complicated by the differences in data presentation. For example, analytical results may be presented as means or ranges on a dry or wet weight basis.

The above notwithstanding, the occurrence of trace metals in African aquatic systems is not excessive when compared to some other areas of the world. For example, mean mercury levels in fish were lower by an order of magnitude compared to values reported for mullets in the Tyrrhenian Sea, an area close to naturally occurring mercury deposits (Leonzi et al., 1981). They were, however, similar to levels in other tropical, less industrialized areas like Indonesia and Thailand (Gomez et al., 1990). The maximum cadmium concentrations were also low compared to fish from British rivers (Mason, 1987) and from the coast of the Philippines (Gomez et al., 1990), but they were within the same range as levels in other areas.

With the exception of lead, inland and coastal water sediments also had comparable or relatively low contents of Hg, Cd, Cu and Zn. Admittedly, these comparisons are based on data which exclude hot spots. However, the low occurrence of heavy metals in African aquatic environments indicate low inputs of contaminants containing trace metals, compared to the more industrialized regions.

8. CONTROL MEASURES

In view of the expected increase in industrialization and urbanization in most African countries, it is still important to formulate pollution control policies that take into account the need to regulate discharges of contaminants into aquatic systems.

In spite of the actual relatively low inputs of contaminants, there is increasing awareness of the need to control waste discharges into the environment. In general, many African countries, aware of the possible detrimental effects, have formulated various laws to control aquatic pollution, although many of these are not enforced.

Since trace metals originate from both domestic and industrial discharges, the control of these sources would also serve to control trace metal contamination of the environment. In addition to legislation, control measures should include the formulation of standards and criteria, effluent treatment, monitoring, environmental training and education programmes. These aspects have been treated in detail for African inland in this publication on pages 7 ff and 23 ff, respectively, and can equally well apply to coastal waters.

It is also important to add that, as part of the objectives of pollution control, emphasis should be placed on the need to minimize waste generation. Industries should be encouraged to adopt low- and non-waste technologies (LNWT) at all stages of a products life, i.e. raw material extraction, production, use and disposal. For planned and proposed industries the achievement of LNWT is possible through the assessment of their potential impacts on the environment and through the adoption of clean manufacturing processes in the design stage. Environmental auditing which involves self regulation should also be encouraged within the business community as part of an overall environmental management policy. The advantage here is that self-regulation is frequently more effective than reliance on official rules which may not cover every contingency.

9. CONCLUSIONS

This review of heavy metals in the African aquatic environment has shown that available data originate from only a few areas of the continent, are scattered and may be inconsistent in some cases. Besides, depending on the area, more information may exist on coastal than on inland areas or vice versa. It is also not possible to establish a trend in heavy metal accumulation since data cover only a narrow period of time. There is, therefore, a need to generate more data covering the different environmental compartments in all the African sub-regions.

Despite this inadequacy, some conclusions may be drawn from this review. Generally, lower concentrations of heavy metals occur in African aquatic systems compared to other areas of the world. Concentrations in inland and coastal environments exhibit no significant differences and on a continental level, the four geographical areas - Northern, Western, Eastern and Southern Africa - have similar low levels. There are however some hot-spots, such as lake Mariut, Lagos lagoon, Ebrié lagoon and Hartbeespoort dam.

With the expected increases in urbanization and socio-economic activities, there is the need to identify the sources and quantify the discharges of heavy metals into aquatic environments on a national basis. It is also important to formulate pollution control measures in each country which should cover legislation, standards and criteria, waste minimization, effluent treatment, monitoring, training, education and public awareness.