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CHLORINATED HYDROCARBON SUBSTANCES

by

O. Osibanjo, C. Biney, D. Calamari, N. Kaba, I.L. Mbome, H. Naeve, P.B.O. Ochumba and M.A.H. Saad

1. INTRODUCTION

Chlorinated hydrocarbon substances (CLHCs) are synthetic organochlorine compounds and include pesticides (e.g. DDT and derivatives, hexachlorocyclohexane (HCH), aldrin, dieldrin, heptachlor and endosulfan) as well as some commercial or industrial chemicals such as polychlorinated biphenyls (PCBs), dioxins (PCDDs) and hexachlorobenzene (HCB). These chemicals are lipophilic with low volatility and low water solubility and have been in use in both developed and developing countries for several decades. The organochlorine pesticides (OCPs), for example, are deliberately introduced into the environment and have played a major role in increasing worldwide food production and protecting human health and natural resources, eg DDT and aldrin (Ware, 1989). The industrial chemicals such as PCBs, however, are introduced indirectly into the environment (Alford-Stevens, 1986).

It has been recognised that the persistence and bioaccumulative tendency of these substances, their metabolites and residues in the environment make them not to remain where they are applied but instead partition between the major environmental compartments in accordance with their physico-chemical properties and may thereby become transported several kilometres from the point of their original release (Haque and Freed, 1975). Such environmental distribution may lead to exposure of living organisms including man, that are far removed from intended targets.

These substances are micro-organic pollutants and are included in the priority list of pollutants (Keith and Telliard, 1979) because of their toxicity, ecological effects and toxicological hazards (Khan, 1977) including human disaster episodes associated globally with their use and accidental release into the environment. The manufacture and use of some CLHCs (e.g. DDT and PCBs) have therefore been banned or restricted in the developed countries. Nonetheless, the use of these chemicals still thrives in most developing countries due to lack of appropriate national regulatory control and their relatively low prices, compared to the more expensive alternative, less persistent, chemicals.

Population explosion, rapid urbanisation and industrialisation have increased reliance on the use of CLHCs in Africa in agriculture, public and animal health (e.g. DDT) and in electric transformers and capacitors (e.g. PCBs) for power generation. Data exist on the production and use of these chemicals in the developed countries (WHO/UNEP, 1990; Miller, 1982), but such data are not readily available in Africa. Nonetheless, pesticide usage in Africa is much less compared to India, Europe and North America. For example 60,000 tonnes of DDT were produced globally in 1974 with the United States of America (USA) consuming about 66% of these (40,000 tonnes) (WHO, 1979).

Estimated total global production and use of PCBs is 2 million tonnes (Hansen, 1987). Balk and Koeman (1984) had reported the following total pesticide consumption for 1977: Indonesia 5,400, Thailand 20,600, Gambia 50, Côte d'Ivoire 1,030 and Sierra Leone 30 tonnes. They also predicted similar growth rates which means that the same trends hold for the 1990s. Although most of the pesticides used in Africa are imported, there are a few production facilities in some countries for OCPs, e.g. Nigeria, Senegal, South Africa, Côte d'Ivoire and Egypt. It is estimated that about 25,000 tonnes of OCPs are in use in the region (see chapter 2).

Chlorinated hydrocarbons have low solubility in water (e.g. DDT: 1.2μg/l). Being lipophilic, these substances can be concentrated to harmful levels in the aquatic environment through bioaccumulation, biomagnification and biogeochemical processes (Edwards, 1977). Consequently, aquatic organisms that are commercially exploited for human food may pose a risk to man. Hence the levels of CLHCs in the aquatic environment including organisms have been continuously measured in surveillance and monitoring programmes in most developed countries (Suzuki et al., 1977; Picer and Picer, 1979; Wegman and Greve, 1980; Gayner et al., 1984; UNEP/WHO, 1988).

Protection of the aquatic environment and associated resources is included in chapters 17 and 18 of Agenda 21, the United Nations Programme of Action from the Rio Earth Summit, for achieving sustainable development for the rest of this and into the next century (United Nations, 1993). It has been estimated that 80% of illness in developing countries is attributed to unsafe water supplies (Harries et al., 1990). Therefore, national, regional and international strategies for implementing Agenda 21 have to be put in place. However, the lack of scientific and ecotoxicological data on chemical pollutants, effective for the control and prevention of aquatic pollution has been recognised in Africa (see pages 7 ff of this publication). In particular, the paucity of data on CLCH levels in environmental media in the region was highlighted at the Workshop on marine pollution monitoring in West and Central African Region, held in Senegal, 1985 (IOC-Unesco, 1985).

Meaningful development of management policies and regulatory framework for the protection of the aquatic environment in the region can only be achieved on the availability of reliable and adequate scientific data generated in the region. To this end, various coastal and marine pollution monitoring programmes and research studies were initiated in the early 1980s in various universities and research institutes in Africa under the aegis of UNEPs Regional Seas Programme. The pertinent programmes are the Mediterranean Pollution Research Programme (MEDPOL) in North Africa, the West and Central Africa Marine Pollution Research Programme (WACAF 2) and the Eastern Africa Marine Pollution Research Programme (EAF/6) respectively.

As Africa embarks on the transition to sustainable development for the present and future generations, this review on levels of chlorinated hydrocarbons in the aquatic environment (streams, rivers, lakes, estuaries and coastal waters) including sediments and biological organisms complements an earlier review of trace metal levels in the African aquatic environment (see pages 32 ff of this publication) aimed at fostering a holistic approach towards the formulation of management and regulatory policies on the protection of the aquatic environment and its resources in the region.

2. SOURCES AND PATHWAYS OF CHLORINATED HYDROCARBONS

Anthropogenic activities provide the primary point source of chlorinated hydrocarbons input into the aquatic environment. The organochlorine pesticides (OCPs) enter the aquatic environment mainly by deliberate application or accidentally, while PCBs entry into the aquatic environment is indirect and principally accidental.

Agricultural production of food for the continent's rapidly growing population (3–4% annually) and cash crops for economic buoyancy as well as disease vector control activities since the 1940s, represent major anthropogenic sources of OCP inputs into the aquatic environment. These substances are sometimes applied directly to water bodies to control aquatic pests, snails, weeds and mosquito larvae. Misuse of these chemicals for killing fish in streams and rivers is also practised. However, the types, quantities and usage pattern of OCPs vary across the region.

About 2500 tonnes of OCPs mostly DDT, toxaphene and endosulfan were used annually on cotton plantations in the 1970s in Sudan (Elzorgani et al., 1979). More than 3500 tonnes of DDT were used on cotton plantations in Uganda between 1965 and 1972 (Dejoux, 1988). In Côte d'Ivoire in 1976, about 600 tonnes of lindane were used for cocoa and 320 tonnes of DDT were applied on cotton. In 1981, about 350 tonnes of lindane, dieldrin, heptachlor and endrin were used for timber protection. In Zimbabwe, about 300 tonnes of DDT applied at the rate of 2–3 kg/ha were used in agriculture between 1981 and 1982. In Burkina Faso, in Houndé-Dédougou region, 30 tonnes of DDT and 30 tonnes of endosulfan annually were used on cotton.

In the areas north of the Sahara (Mauritania, Mali, Niger and Sudan), the control of desert locusts for several years was by OCPs (dieldrin and lindane). Limited amounts of dieldrin were still used during 1986–1988 while presently organophosphates are used which are less persistent than the OCPs (FAO, 1988).

Ground spraying and aerial application of OCPs especially DDT, dieldrin and endosulfan to control vectors for human and livestock diseases are also an important source of contamination of aquatic ecosystems. Since 1944, DDT had been used largely for black fly larvae control (Simuliidae) in many regional programmes in Africa. About 60 tonnes of DDT were used annually in the continent to control Simuliidae from 1966 to 1970 (Dejoux, 1988). Tsetse fly control and eradication programmes involving the spraying of DDT, dieldrin and endosulfan have taken place in different parts of the region over the last 20 to 30 years as well (Dejoux, 1988).

These applications of OCPs cause the accumulation of their residue in the environment of the application area which is of ecological and public health concern. The main anthropogenic sources of these substances into the aquatic environment are:

  1. Deliberate application e.g. spraying of pesticides to eradicate trash fish, and control aquatic weeds, snails and insects.

  2. Dumping of wastes/containers from public health, agricultural and industrial usage.

  3. Domestic and industrial effluents - effluents from pesticide manufacturing or formulating industries or industries using CLHCs, e.g. textile factories, food industry and thermal power plants.

  4. Accidental spillage from agricultural and industrial sites, road and rail vehicles and ships.

  5. Drainage and run-off from treated farm lands, garbage and industrial solid wastes dump.

  6. Dumping of sewage sludge, municipal and industrial solid wastes.

  7. Atmospheric input e.g. in dry deposition and wet precipitation; burning or/and incineration of domestic, municipal or industrial solid wastes; industrial emissions e.g. through vaporisation of paints, varnishes, lacquers etc.

The relative importance of the foregoing point and non-point sources of pollution by CLCHs depends on the aquatic system under consideration (Table I). Spray contamination by drift, drainage and agricultural run-off as well as domestic/industrial effluents are significant point-and non-point (diffuse) sources of pollution of lakes/ponds, streams/rivers and coastal waters/estuaries. For CLHCs, atmospheric input seems to be the major source of oceanic pollution compared to land-based sources (GESAMP, 1989).

Table 1
Relative importance of sources of chlorinated hydrocarbon pollution for different aquatic systems (From Edwards, 1977. © Plenum Press, New York)

 SprayingRun-offEffluentsAtmosphere
Lakes/Ponds++++-+
Streams/Rivers+++++++
Estuaries+++++
Oceans-+++++

Co-disposal of industrial, municipal, domestic and medical wastes in open dumps, or non-sanitary landfills or open burning is commonly practised. Though these waste disposal methods may be cheap and convenient, they are not environmentally safe and sound and therefore not acceptable. Leachate from open dumps and landfills (Shuster, 1976; Arebun, 1990), are therefore recognised also as sources of CLHC pollution of surface and ground water.

A recent report (UNEP/FAO/WHO/IAEA, 1990) indicated that land-based sources contribute a total input of organochlorine pesticide estimated at about 90 tonnes/annum to the Mediterranean Sea; information for PCB loads is not available. Similar exercises are in progress in the WACAF and EAF sub-regions especially as some CLHCs such as PCBs, DDT and dieldrin have become global pollutants due to (a) their volatilisation from the sites of application; (b) atmospheric transport and deposition and (c) transport via rivers and ocean currents (Eisenreich et al., 1979; Croll, 1991).

3. FATE OF CLHCs IN THE AQUATIC ENVIRONMENT

On entry into the aquatic environment through various pathways, these non-polar, toxic, semi-volatile and fairly persistent substances may remain within the water body unchanged for a period of time, undergo degradation to simpler compounds which may be more toxic or/and more persistent than the parent compounds (e.g. DDE, dioxin) or get reversibly transferred into the atmosphere by volatilisation, Fig. 1 (Edwards, 1977; Keith, 1976).

Fig.1

Fig.1   Pathways and movement of chlorinated hydrocarbon substances in the aquatic environment. (Adapted from Edwards, 1977)

The ultimate fate of these pollutants including partitioning into various aquatic environmental compartments (water, suspended solids, sediments and biota), will depend on a number of factors including: concentration, dilution, water solubility, biogeochemical processes taking place, adsorption to soils, suspended particulates and sediments, lipohilicity, and bioaccumulation in living organisms (Khan, 1977).

The hydrophobic nature of CLHCs makes their presence in water to be at ultra-trace level (ng/l) and their accurate determination difficult. The adsorption of these compounds to particulate matter and sediments is an important mechanism for their removal from the water column. Consequently, the sediment component of aquatic ecosystems can be ultimate sink of CLHCs similar to metals (see pages 32 ff of this publication) and petroleum hydrocarbons (Adekambi, 1989); suspended particulates entering slow moving waters such as larger water bodies settle out, and their associated CLHCs are added to the existing sediments component.

Being hydrophobic, CLHCs have a high potential for bioaccumulation in aquatic plants, fish and shellfish and undergo bio-magnification along trophic levels (Vandenbroek, 1979; Osibanjo and Bamgbose, 1990). The bioaccumulation potential of CLHCs can be predicted based on the I-octanol/water equilibrium partition coefficient values, i.e. Kow values (Karichhoff et al., 1979; Mackay, 1982; Gossett et al., 1983 with log Kow values ranging from 3.8 to 6.9. The accumulation of these recalcitrant CLHCs in birds and mammals feeding on contaminated aquatic biota may occur which can then result in their transport over great distances (migratory birds, fish and mammals).

Air has been recognized as an important medium of long-range transport of these substances (GESAMP, 1989).

4. ECOLOGICAL AND PUBLIC HEALTH IMPACT OF CLHCs IN THE AQUATIC ENVIRONMENT

The organochlorine pesticides (OCPs), e.g. DDT, dieldrin and industrial chemicals like PCBs, their metabolites or conversion products have been reported to be ecologically harmful and toxic to humans as well. The acute toxicity of OCPs to aquatic organisms has been become evident in the past by significant fish kills associated with the accidental release of DDT, toxaphene, dieldrin, aldrin and heptachlor into the aquatic environment (Eichelberger and Lichtenberg, 1971; Heydorn, 1970). Nowadays, contamination of water with these recalcitrant chemicals often results in bioaccumulation in fish and other biota, sometimes to biologically active levels. Hence, these chemicals have been suspected to be cancer causing agents in fish and other aquatic organisms (GESAMP, 1991).

Residues of these toxic chemicals found in water, sediments, fish and other aquatic biota can pose a risk to aquatic organisms, to predators and to humans. In order to minimise health risk from the ingestion of food contaminated with these chemicals, environmental protection agencies and public health authorities including WHO have set Maximum Residue Levels (MRL) or Maximum Allowed Concentrations (MAC) for CLHCs in water, fish and shellfish. (Smeets and Amavis, 1981; Walter and Graham, 1988; UNEP/FAO/WHO, 1988)

Fish eating birds are at risk of population decrease and even of extinction, as a consequence of reproductive failures resulting from eating aquatic organisms contaminated with these chemicals. This is because CLHCs can cause egg shell thinning or impair the process of formation of the egg shell of several species of birds by interfering with the deposition of calcium (Trape, 1985; Matthiessen and Roberts, 1982; Koeman et al., 1978).

5. ANALYTICAL METHODS FOR CLHC ANALYSIS IN AQUATIC ENVIRONMENTAL SAMPLES

Chlorinated hydrocarbons are present at parts per trillion level (ng/l) in water samples and at parts per billion or parts per million levels (ng/g or μg/g) in sediments and biota, thereby requiring highly specific, sensitive and reliable analytical methods for carrying out such trace and ultra-trace measurements.

The basic analytical steps involve (Roberts, 1985; Alford-Stevens, 1986):

  1. Sample collection and preparation;

  2. Extraction or removal of the compounds of interest from the sample matrix into one that can be analyzed;

  3. Clean-up of extracts or enrichment of the concentration relative to that of other sample components;

  4. Separation or isolation of extracted components;

  5. Determination (preceded by derivation, if necessary);

  6. Identification of positive residues and measurements of compounds of interest.

Since these compounds are non-polar, the use of single solvents, e.g. hexane, dichloromethane, or binary mixtures of non-polar and polar solvents (e.g. hexane + diethylether; hexane + acetone, hexane + isopropanol, hexane + benzene) is employed to extract these compounds from aquatic environmental samples, namely water (APHA/AWWA/WPCF, 1985; Kahanovitch and Lahav, 1974; Nwankwoala and Osibanjo, 1992), sediments (El-Dib and Badway, 1985; Jensen et al., 1977; Frank et al., 1977) and fish (Jensen et al., 1972; UNEP/FAO/IOC/IAEA, 1986; Haahti and Perttilä, 1988).

The need for sample extract pre-concentration of CLHCs results from the fact that the detection system of the analytical instrument may not have the necessary selectivity, sensitivity or freedom from matrix interferences. It also offers analytical isolation as well as enrichment factors.

Most pre-concentration techniques fall into two classes:

  1. Solvent extraction followed by solvent reduction, or

  2. Sorbent trapping with subsequent elution or thermal desorption.

The Rotary Evaporator and the Kuderna-Danish concentrator with Snyder Column have been widely used for solvent reduction (UNEP/FAO/IAEA, 1986; Nwankwoala and Osibanjo, 1992).

A simple method for sample clean-up after solvent reduction is the use of acid-base (Waliszewski and Szymezynski, 1982; Haahti and Perttilä, 1988) depending on the stability of the analytes to this treatment. Another common method of sample clean-up involve the use of adsorbents: macro-reticular resins, polyurethane foams, activated carbon, Tenax-GC, Silica gel, Florisil, alumina (Keith, 1976, UNEP/FAO/IOC/IAEA, 1986) because of the difficulty in separating DDT and metabolites from PCBs (Armour and Burke, 1970).

Gas chromatography with electron capture detector (GC-ECD) using single or mixed stationary phases on a packed glass column or a single phase on a glass or quartz high resolution capillary column, is the major technique used for the determination of chlorinated hydrocarbon compounds in environmental samples (Alford-Stevens, 1986; UNEP/FAO/IOC/IAEA, 1986; Sarkar and Sen Gupta, 1988; Cairns and Siegmund, 1981).

In recent years, the use of GC-mass spectrometry (GC-MS) especially in the selective ion monitoring mode (SIM) has proffered the most selective and sensitive detector for the simultaneous determination and confirmation of CLHCs (Alford-Stevens, 1986; Roberts, 1985).

Apart from the dearth of competent experts in trace CLHCs determination in environmental samples, the choice of analytical methods for CHLCs in the African region has been influenced largely by the restricted availability of materials, apparatus and functional modern selective and sensitive equipment, underscored by the desirability of obtaining accurate analytical data with reasonable cost-effectiveness. Consequently, most of the CLHC determinations in the region have been carried out by packed column gas chromatography with 63-Ni electron capture detector.

6. OVERVIEW OF CHLORINATED HYDROCARBON DISTRIBUTION IN AFRICA

6.1 North Africa

Organochlorine pesticides, especially DDT and its derivatives were the first generation of chlorinated hydrocarbons to be used for control of agricultural pests and human diseases (vector control) in the North African sub-region since the 1950s.

The River Nile and the main irrigating and drainage canals, ending directly or indirectly via coastal lagoons into the Mediterranean Sea, are the main sources of pesticide contamination of the Egyptian coast.

The limited information available in the sub-region is based largely on studies that have been carried out since the late 1970s in Egypt on the residual levels of CLHCs in different environmental compartments of inland and coastal water bodies. El-Sebae and Abu-Elamayem (1979), in one of the earliest studies on the River Nile, the connected canals and drainage systems reported the detection at quantifiable levels of lindane, heptachlor, o,ρ'-DDT and ρ,ρ' -DDT at concentrations ranging from 100–950 ng/l in all the water samples. The relatively low levels of chlorinated insecticides in the Mahmoudiah Canal (drinking water source for Alexandria) and the waste water might be due to the cessation of use of this group of insecticides in Egypt since 1971 because of their persistence.

Saad et al. (1985) analyzed composite sediment samples taken in 1968 from Lake Manzalah and in 1970 from Lake Mariut and Nozha Hydrodrome for DDT and PCBs. Lake Manzalah, the largest of the Nile delta lakes, receives drainage water contaminated with pesticides and fertilizers from a much larger agricultural area than Lake Mariut which is also polluted by sewage and industrial wastes. The Nozha Hydrodrome was separated artificially from Lake Mariut in 1939 and is fed by contaminated Nile water. While Lake Mariut and Nozha Hydrodrome showed relatively low levels of total DDT (29.8 and 54.1 ng/g respectively), Lake Manzalah was heavily polluted with 877 ng/g. PCB levels were 17.8, 21.4 and 71.2 ng/g respectively (Table III). The total DDT/PCB ratio was in all three lakes much greater than 1, indicating agricultural inputs of DDT rather than industrial discharges as the main source of pollution by organochlorine compounds.

During 1978/79 Lake Mariut and Nozha Hydrodrome were revisited and analyses done for organochlorines in water by Saad et al. (1982) and in sediments by Abu-Elamayem et al., (1979). The major substances detected in water of Lake Mariut were lindane, ρ,ρ'-DDE, o,ρ'-DDT and ρ,ρ'-DDT, with mean residue values of 2,091, 4,493, 9 and 134 ng/l, respectively (Table II). Since sediments are sinks for pollutants, much higher values were found compared to the levels in water. Mean values reported for the Lake Mariut sediments were 89.1 (lindane), 768.0 (ρ,ρ'-DDE), 19.3 (o,ρ'-DDT) and 86.3 (ρ,ρ'-DDT) ng/g dry weight respectively (Table III). Contamination levels in fish were 34.98 ng/g (lindane), 38.96 ng/g (ρ,ρ'-DDE), 17.36 ng/g (ρ,ρ'-DDT) and 60.76 ng/g (total DDT) (Table IV). In Nozha Hydrodrome, lindane, ρ,ρ'-DDE and ρ,ρ'-DDT were detected in its water and sediments. The mean concentrations of these compounds found in water were 1,100 (lindane), 1,540 (ρ,ρ'-DDE) and 600 (ρ,ρ'-DDT) ng/l. Values for the Hydrodrome sediments were 119.5 (lindane), 840.4 (ρ,ρ'-DDE) and 91.0 (ρ,ρ'-DDT) ng/g dry weight.

Saad (1981) reported concentrations of organochlorine pesticides in water, sediment and fish in Lake Mariut and the Nozha Hydrodrome for the following two-years period (1979–81). The mean concentrations of total DDT (21,440 ng/l) found in the water of Lake Mariut was markedly higher than that in the Hydrodrome water (13,610 ng/l), reflecting the effect of pollution with DDT compounds on Lake Mariut resulting from huge discharges of agricultural drainage waters. α-HCH, lindane, o,ρ'-DDE, ρ,ρ'-DDE, o,ρ'-DDT and ρ,ρ'-DDT were found in Lake Mariut water with mean values of 120, 1,310, 1,380, 6,630, 2,690 and 9,820 ng/l respectively (Table II). The concentrations of pesticides in fish samples were higher than those in the lake water; mean values for α-HCH, lindane, o,ρ'-DDE, ρ,ρ'-DDE, o,ρ'-DDT and ρ,ρ'-DDT were 2.03, 80.06, 29.0, 10.1, 9.3 and 31.8 ng/g respectively. The mean value of total DDT was 84.5 ng/g (Table IV). Fish from the Hydrodrome were found to be less contaminated. The detected pesticides in Lake Mariut sediments were α-HCH, lindane, ρ,ρ'-DDE, o,ρ'-DDT and ρ,ρ'-DDT, with average values of 65.5, 327.0, 1,672.8, 82.8 and 270.5 ng/g dry weight, respectively (Table III). In the Hydrodrome sediments, only lindane and ρ,ρ'-DDE were found, with mean values of 423.5 and 456.0 ng/g, respectively. The pesticide levels in the lake sediments were much higher than those in the lake water and fish.

Tayel (1981) investigated the seasonal distribution of chlorinated pesticide residues in the water and fish of Abu-Kir Bay (Mediterranean Sea). The mean concentration found in the water were 4.88, 18.05 and 24.26 ng/l for α-HCH, lindane and total DDT, respectively (Table II). These pesticides showed regional and seasonal variations in the bay, due to changes in meteorological conditions, discharges of freshwater from the Rosetta Branch of the Nile and of the brackish water from lake Edku, as well as due to industrial wastes from El-Tapia pumping station. These discharges are significant pollution sources for the bay. Residues of these compounds in the sediments of the bay gave values several times higher than the corresponding concentrations in the bay water: 0.09, 0.05 and 1.73 ng/g for α-HCH, lindane and total DDT, respectively (Table III). Analysis of six fish species in the bay showed the presence of α-HCH, lindane and total DDT in much higher concentrations compared to those in the bay water. Concentrations differed between various fish species and even within specimen of the same species, concentrations also varied seasonally and with body size and weight.

Ernst et al. (1983) monitored the levels of organochlorine compounds in various aquatic organisms from the Egyptian coastal area in the vicinity of Alexandria. DDT and its major degradation products, DDE and DDD, α-HCH, γ-HCH, dieldrin and PCBs were the major compounds detected. The results indicated that primarily the western coast of Alexandria seemed to be polluted with organochlorine compounds. With increasing distance from Alexandria towards Rosetta, the level of PCBs tended to decrease.

Macklad et al. (1984a) also monitored the levels and distribution of chlorinated hydrocarbons in some fish species from Lake Edku and Abu-Kir Bay. The data indicated that DDT and its metabolites (DDE and DDD), hexachlorocyclohexane (α- and γ- isomers), endrin and PCBs (as Aroclor 1260) were the major chlorinated hydrocarbons detected in fish from Lake Edku and in the region of lake-sea connection. DDE was the major DDT degradation compound detected. DDD was detected only in Mugil; its level in the lake-sea connection was higher than inside the lake, reflecting the effect of industrial pollution. Also, the levels of chlorinated pesticides in Tilapia species from the lake-sea connection were higher than those from this lake, again reflecting the effect of industrial pollution. PCBs such as Aroclor 1260 was detected for the first time in two fish samples from Lake Edku. Its level was positively correlated with the fat content of the fish. However, PCBs were detected more frequently in Abu-Kir fish samples than in those from Lake Edku. This indicated that the PCB pollution originated essentially from industrial wastes in Abu-Kir Bay.

Macklad et al. (1984b) monitored the levels of chlorinated pesticides in two fish species from Lake Mariut and the Nozha Hydrodome. The data of these fish (Mugil and Tilapia species) indicated that DDT and its metabolites (DDE and DDD), HCH and endrin were the major chlorinated pesticides detected.

In the Hydrodrome fish, the level of γ-HCH was higher than the other detectable isomers α and β. DDE levels in Mugil species ranged from 3.13 to 822.0 and from 3.0 to 1,320.0 ng/g wet weight in fish muscles and liver, respectively. DDT is still detected in aquatic organisms although it was banned several years ago. Endrin was detected in all fish samples from the Hydrodrome. The bioconcentration factor for chlorinated pesticides in liver samples was higher than in muscles.

Also in fish from Lake Mariut DDE was the major degradation product of DDT, as in case of the Hydrodrome. DDD was detected in five samples. Generally, the levels of chlorinated pesticides in Lake Mariut fish were lower than in those from the Hydrodrome. Bioconcentration factors of these compounds in different organs of Tilapia galilaea increased from muscle over gonads to liver.

6.2 East Africa

The catchment areas of the East African lakes and the Indian Ocean are sites of intensive agriculture, urbanisation and industrialisation.

Most of the studies in this sub-region have been carried out in Kenya as part of the monitoring activities for the assessment of the ecological and human health impacts of pesticides application in agriculture and disease vector control activities. Only few cases of water and sediment analyses were reported. Most of the studies centre on analysis of biota and wild life samples.

Sudan

The only available studies outside Egypt concerning the accumulation of organochlorine insecticides in fish from the Gezira of Sudan (central Sudan) and Lake Nubia (southern 180 km stretch of High Dam Reservoir) are those of El-Zorgani (1976) and Elzorgani et al., (1979), respectively.

In the Gezira Research Farm, 28 specimens representing 5 types of fish have been analyzed for residues of organochlorine insecticides (Table IV). All fish samples contained residues of ρ,ρ'-DDE, ρ,ρ'-DDD, ρ,ρ'-DDT and total DDT with mean values of 670, 1,470 and 2,950 ng/g fresh weight respectively (El-Zorgani, 1976). This high accumulation reflects the increasing use of organochlorine insecticides over the last 50 years in central Sudan, where the amounts largely exceeded 1000 tonnes/year of various organochlorine insecticides.

Muscle and liver tissues of twenty-nine fish specimens belonging to seven different species were collected and analyzed by Elzorgani et al. (1979) for DDT isomers and metabolites. Only 10 out of 58 samples analyzed were found to contain detectable levels of OCP residues. ρ,ρ'-DDE was found in all the ten samples (49 (3–153) ng/g fresh weight) while ρ,ρ'-DDT was found in only 3 muscle samples (8 (5–14) ng/g fresh weight). The range of total DDT in fish muscle was from 6–184 ng/g; the highest residue value of 184 ng/g was found in a muscle sample of a specimen of Hydrocynus forskali (Table IV). This fish was proposed as a suitable indicator species for monitoring OCPs in the aquatic environment. The most likely source of chemical contamination of Lake Nubia is the cotton growing region along the Blue and White Niles in central Sudan. In this area, the annual pesticide usage amounts to about 2,500 tonnes of mostly organochlorine compounds, especially DDT, endosulfan and toxaphene. Furthermore, the intensive application of pesticide in the Gezira and along the White Nile in central Sudan were implicated as the source of chemical contamination of the lake which could adversely affect the fish population, thereby endangering plans for the development of a fisheries industry in the area.

Tanzania

The use of pesticides in agriculture is not widespread (Alabaster, 1981). Cotton, sugar cane and coffee are grown near the northern shore of Lake Tanganyika which are aerially sprayed with pesticides at a rate of about 45 t/year. In the marine environment, DDT, endrin, aldrin, toxaphene and other pesticides reach the Indian Ocean via rivers and from the major towns of Dar-Es-Salaam, Tanga, Lindi and Zanzibar (Bryceson et al., 1990).

Paasivirta et al. (1988) described the results of chlorinated insecticide and PCB residues analysis of six sediments samples, two aquatic plant samples (leaves and roots of Pistia stratiotes) and one fish (Tilapia) from a small man-made lake, Nyumba ya Mungu Reservoir in Tanzania. PCB was not detected in any sample. Three insecticides, DDT and its metabolites DDE and DDD, lindane and dieldrin were detected and quantified. Mean concentrations found in sediments were for DDE and DDD 1 ng/g, for DDT 3 ng/g, for lindane 1 ng/g, for dieldrin 4 ng/g and for Tanzadrin, a photometabolite of dieldrin, 131 ng/g (Table III). The fish sample contained (per gramme dry weight) 14 ng DDE, 4 ng DDD, 6 ng DDT, 3 ng lindane, 10 ng dieldrin and 11 ng Tanzadrin (Table IV). Mean values in aquatic plants per gramme dry weight were 15 ng DDE, 18 ng DDT, 4.5 ng lindane, 27 ng dieldrin and 25 ng Tanzadrin (Table VI).

Burundi

The principal use of pesticides in Burundi is on crops grown in the plains of the Ruzizi River (Autrique, 1977). The river drains into the northern, shallow part of Lake Tanganyika. Samples of fish have been examined in 1971 and 1972 for pesticides (Deelstra et al., 1976; Alabaster, 1981). The flour made from dried fish was analyzed and quantities of DDT and DDE combined was estimated at 700 ng/g in Limnothrissa, 750 ng/g in Stolothrissa, and 380 ng/g in young Luciolates. The total quantity of DDT and its metabolites varied in whole dried fish between 450 and 2,390 ng/g. Seasonal variation was apparent, showing a maximum during the dry season after the cotton harvest. Apart from this, the quantities decrease with distance from the opening of the Ruzizi plain at the inflow to the lake.

Uganda

In Uganda, pesticides have been used to protect cash and food crops and thus increase their yield (Bugenyi, 1984). The application of pesticides has been mismanaged by the locals with the consequent threat to the aquatic environment due to lack of advice on the safe use and disposal of pesticides. The quantity and concentration of pesticides used is unknown (Bugenyi and Balirwa, 1989). They are mainly applied as aerosols, smokes, sprays, paints, tree injections and granules. On the Victoria Nile, weekly treatments of 0.4 mg/l DDT was used to control blackfly larvae, Simulium damnosum (Corbet, 1958); in the Mount Elgon region in Uganda, however, DDT also killed predators of Simulium (Hynes and Williams, 1962). Sserunjoji (1974, 1976) determined dieldrin in Ugandan lakes (Nakivari, Mburo, Kazuma, Kachera, Kyesama, Mishera, Karunga, Itara and Kijanebarora). In fish fillet dieldrin was found at levels between 2 and 27 ng/g fresh weight with a mean of 5, while in sediments (semi-dried on blotting paper) the range was 2 to 39 ng/g with a mean of 10 and in plants (water cabbages and water lilies) the range was 1 to 180 ng/g fresh weight with a mean of 21 ng/g Tables III, IV and VI).

Malawi

Pesticides are mainly used on cotton grown along the shore of Lake Malawi and on tobacco, maize and tea grown in the Shire Valley (Alabaster, 1981). Between 1973 and 1978 fish from the Shire Valley were sampled and analyzed for DDT (Pickering et al., 1980). Concentrations of DDT and its metabolites in fish muscle were below 50 ng/g but were relatively high in the ovaries of Clarias gariepinus (2,700 ng/g).

Kenya

The use of pesticides in Kenya dates back to 1946 when DDT was used in aerial spraying for control of mosquitoes in the Lake Victoria region.

Approximately 5,000 tonnes of pesticides/year are used on coffee, maize, cotton, wheat, tea, sugar cane, horticultural crops and for insect control (Mbote, 1979). Details of specific pesticides can be found in Calamari et al. (1994). Pesticides and their metabolites have been found in water and aquatic organisms including fish (Koeman and Pennings, 1970; Koeman et al., 1972; Kallqvist and Meadows, 1977; Greichus et al., 1978b; Lincer et al., 1981; Mitema and Gitau, 1990; Mugachia et al., 1992b). There is concern that concentrations are approaching levels that may have adverse effects (Alabaster, 1981).

Studies on pesticides in Kenya's aquatic environment focused on the Rift Valley lakes, Lake Victoria and its catchment area, Athi and Tana rivers and the Indian Ocean. Van Someren (1950) reported massive fish kills in Lake Victoria following aerial spraying of DDT to control mosquitoes. Koeman et al. (1972) reported pesticides used in tsetse fly control treatments around Lake Victoria, where Tilapia, Alestes, and Clarias muscles had levels of 14 to 60 ng/g of dieldrin, and 10 to 25 ng/g of DDE (Table IV). Fish eating birds (kingfishers and cormorants) were contaminated via the food chain and contained 10 ng/g of dieldrin and 60 to 260 ng/g of DDE. Greichus et al. (1978b) detected DDE, DDD, dieldrin, DDT and PCBs in water, sediments and fish from Lake Nakuru. Very low levels of organochlorine pesticides were found in Tilapia grahami.

Munga (1985) also studied DDT and endosulfan residues in fish from Hola irrigation scheme in the lower Tana River basin where the pesticides were used for the control of cotton and maize pests. The concentration of residues varied according to the distance of the sampling site from the cotton fields, with fish closest to the site being the most contaminated. Fish samples from the Tana river itself were found to be least contaminated with pesticide residues. Of the four fish species studied, Clarias mossambicus (= C. gariepinus), a bottom feeding species, had the highest mean concentration of residues in muscle tissue, 400 ng/g fresh weight of total DDT and 110 ng/g fresh weight of endosulfan (Table IV). Strong correlation of pesticide residues in muscle tissue with fat content was found.

Lincer et al. (1991) found that the main use of pesticide use in the Rift Valley region was in urban centres for mosquito, weed and rodent control, dog washes, stored grain protection and household pest control. DDE residues were detected in in fish from Lakes Naivasha, Nakuru and Baringo at levels from 7 to 143 ng/g dry weight (Table IV); one predatory fish from Lake Baringo showed the highest level of DDE with 2,130 ng/g dry weight. Kanja (1988) estimated total DDT levels ranging from 31 to 367 ng/g in fish from Lake Victoria; HCH, lindane and dieldrin were also detected with levels ranging from 1.3 to 123 ng/g. Mitema and Gitau (1990) found in nile perch (Lates niloticus) from Lake Victoria total DDT levels ranging up to 4,510 and 460 ng/g in fat and fillet respectively. Residues of lindane, aldrin, dieldrin and α-HCH were also detected (Table IV).

Mugachia et al. (1992a, 1992b) detected DDE, DDD, DDT, HCH, heptachlor and lindane in fish from Lake Naivasha, Rivers Athi and Tana and from the Indian Ocean, and found residue levels higher than those reported in other Kenyan studies. The mean total DDT level found in sharks was 702 ng/g (highest value: 3,415 ng/g); residue levels of the HCH group ranged from 4 to 290 ng/g. DDT in freshwater fish ranged from 52 to 11,125 ng/g. The highest DDE level measured was 220 ng/g. Lindane levels ranged from 3 to 295 ng/g, α-HCH from 9 to 21 ng/g (Table IV).

Greichus et al. (1978b) detected PCBs in Lake Nakuru, the most widely studied surface water in the country. They reported concentrations in water of <1 ng/ml, in sediments of <20 ng/g dry weight and in fish of <500 ng/g dry weight (Tables II-IV).

A preliminary risk assessment for pesticides used in the Lake Victoria catchment area was done by Calamari et al. (1994). They concluded that most pesticides used in the area may not create toxicity problems and do not bioaccumulate in the lake biota; only a few of them like aldrin have a potential for bioaccumulation. DDT found in fish tissues was low in concentration and is not of recent origin.

6.3 West and Central Africa

Many chlorinated insecticides have been used in this sub-region for over three decades in agriculture, vector control and public health; data on quantities are not readily available. Chlorinated hydrocarbons have been detected and quantified in different compartments, i.e. water, sediments, plants and fish, only in Nigeria and Côte d'lvoire; in other countries of the sub-region studies of CLHCs have concentrated mainly on fish and shellfish from coastal waters.

Nigeria

Nigeria is the most populous country in Africa, with fairly active agricultural and vector disease control activities involving the use of chlorinated pesticides. It is the most industrialised country in the sub-region with about 70% of the manufacturing industries including electric power generating stations, petroleum refineries, pulp and paper mills, located in the coastal zone.

Agunloye (1984) and Tongo (1985) studied the occurrence and levels of chlorinated hydrocarbons in water of 17 rivers, 2 lakes and one dam in southern Nigeria. The overall range of values (ng/l) of the major CLHCs found were: lindane ND-167, aldrin ND-190, endosulfan ND-750, HCB ND-9.2, heptachlor ND-96 and PCBs ND-8991 respectively. DDT and metabolites have not been detected. Table II indicates the levels of these substances in some specific rivers. For example, concentrations (ng/l) in River Ogun which traverses three states and discharges into Lagos Lagoon were: lindane 1.4–41.9 (13.3), aldrin 5.1–49 (40), endosulfan ND-260 (116), heptachlor ND-0.8 (0.25) and PCBs ND-224 (87) respectively.

Ogunlowo (1991) studied the occurrence and levels of CLHCs in 9 rivers in Ondo State, a major cocoa growing area of Nigeria. He detected (in ng/l): lindane ND-6.4 (2.4), heptachlor ND-5.0 (2.1), endrin ND-21 (5.1), aldrin ND-3.5 (1.0) and dieldrin ND-2,150 (1,062). PCBs, DDT and metabolites were not detected (Table II).

Nwankwoala and Osibanjo (1992) reported the detection of 10 CLHCs residues including PCBs in surface waters in lbadan, the largest city in the sub-region. The concentration ranges in ng/l of some of the compounds quantified are α- and β-HCH 1–302, lindane 7–297, aldrin ND-40, dieldrin 17.8–657, endrin ND-19, heptachlor 4–202, endosulfan ND-430, HCB ND-92 and total DDT ND-1,266; PCBs were detected but not quantified (Table II). These results show higher loads of CLHCs in the water bodies compared to concentrations elsewhere. This study confirms that organochlorine pesticide residues are widely distributed in the surface waters studied, even at sites remote from point sources.

Okonna (1985) showed the presence of pesticide residues in Lagos Lagoon water, concentrations in ng/l were: lindane 85.3; aldrin 19.3; DDE 12; HCB 1.9; endrin 12.5 and dieldrin 28.0.

Sunday M. (1990) analyzed 20 sediments samples from streams and rivers in lbadan city, Oyo State. The concentration range (mean) in ng/g dry weight were: dieldrin ND-6 (1.4), α-HCH ND-1.6 (0.2), γ-HCH ND-2 (0.3), aldrin ND-0.04 (0.002), DDE ND-50 (6.8) and PCB ND-14 (1.8); heptachlor, endosulfan and endrin were not detected (Table III).

Ojo (1991) investigated the occurrence and levels of CLHCs in 23 bottom sediment samples from Lekki Lagoon in Lagos State. Eleven organochlorine pesticides and HCB were detected. PCBs were not detected (Table III). The ranges of concentration with means parenthesis, reported in ng/g dry weight were: lindane 0.11–4.9 (1.1), aldrin ND-347 (56), ρ,ρ'-DDE 11–555 (263), o,ρ'-DDD ND-348 (88), endosulfan 7–1,155 (30), heptachlor ND-1845 (64), β-HCH, ND-260 (66), α-HCH ND-116 (18.6), HCB ND-3.3 (0.4), endrin ND-129 (16.5), dieldrin 190–8,460 (4,560). Compared to other parts of the world, the sediments of Lekki Lagoon are to be considered fairly contaminated with organochlorine pesticides.

Fish samples from fresh water were found to contain significantly higher concentration of these chemicals than sediments and water. Amakwe (1984) detected 10 organochlorine pesticides, HCB and PCBs in 40 freshwater fish samples collected from various locations in Oyo and Ogun State. The relative occurrence of some of the CLHCs identified were lindane 100%, PCB and endosulfan 97%, DDT and metabolites 75%. The concentration ranges with mean in parenthesis in ng/g fresh weight were (Table IV): lindane 7–106.0 (25.6), ρ,ρ'-DDE 2.0–30.0 (3.4), ρ,ρ'-DDD 2.0–60.0 (7.8), ρ,ρ'-DDT 3.0–18 (2.9), total DDT 3.3–161 (20.6), PCB (Aroclor A1250) 8.0–130 (28.7), heptachlor 1.0–300 (50.0), endosulfan 3–904 (173), HCB 9.0–130.0 (12.7) and α-HCH 0.2–5.0 (1.3). Fayomi (1987) also detected and quantified 9 OCPs in south-eastern Nigeria. The relative occurrence of some of these compounds were PCBs, aldrin, lindane, and α-HCH 100%, endosulfan, δ-HCH, ρ,ρ'-DDD, ρ,ρ'-DDE and heptachlor 44.4%, 16.7%, 33.3%, 61.1% and 72.2% respectively. The concentration ranges with means in parenthesis in ng/g fresh weight were: α-HCH 0.2–7.4 (1.8), lindane 0.6–13 (4.4), heptachlor ND-1.0 (0.3), aldrin ND-14.9 (5.5), endosulfan ND-89.6 (14), ρ,ρ'-DDE ND-4.2 (1.8), DDD ND-8 (0.7), and PCBs 0.7–14 (3.8).

Osibanjo and Bamgbose (1990) have reported contamination by CLHCs of Nigerian marine fish and shellfish, based on the analyses of 94 samples of 25 marine fish species over 1983 – 1985 and 14 samples of 7 shellfish species in 1987. The concentration ranges in ng/g fresh weight were found to be for HCB 0.04–9.48, lindane ND-5.30, endosulfan ND-4.95, DDT 0.15–18.6, aldrin ND-54.60 and for PCB 11.0–225 (Table IV). Fish contained higher concentrations of aldrin, heptachlor, HCB and lindane than shellfish, while the reverse was observed for DDT and PCBs. The concentrations of residues obtained were found to be lower than those reported in literature for industrialised countries. Furthermore, predator fish species were found to concentrate more residues in muscle tissue than plankton feeders. The DDT/PCB ratios were less than 1, indicating a predominance of industrial activities over agricultural activities as the source of contamination of the marine environment. The fish Galeoides decadactylus was proposed as a potential bio-indicator organism for chlorinated hydrocarbon pollution monitoring in the study area.

Thus, various compartments of the aquatic environment in Nigeria are greatly contaminated by several OCPs, PCBs and HCB.

Côte d'Ivoire

Marchand and Martin (1985) assessed the contamination of Ebrié Lagoon sediments with DDT and metabolites, lindane and PCB. The concentration ranges in ng/g dry weight were: lindane 0.5–19, DDE 0.2–149, DDD 0.2–803, DDT 0.2–354, PCB 2–213 (Table III). Two hot spots were highlighted: Bietry Bay and Marcory Bay which are highly polluted.

The analysis of surface sediments of bays from the urban area of Abidjan Lagoon (Kaba, in press) showed that these bays are polluted. Concentration ranges were (ng/g): α-HCH 0.01–13.4, lindane 0.07–19.81, β-HCH 0.32–157.32, δ-HCH 0.01–5.05, heptachlor ND-6.80, aldrin 0.07–62.1, dieldrin ND-125.75, endrin ND-15.06, total DDT 2.50–242.83, PCB 8.49–1,013.92 (Table III).

Fish caught by poisoning with lindane and fish contaminated with lindane (lethal dose 0.1 mg/l) in the laboratory were also analyzed. Concentrations of HCH found did not exceed international consumption standards for fish and fish products. Concentrations in the gills were higher than those in the guts, muscle tissue had the lowest concentrations.

Marine fish in Côte d'lvoire have very low concentrations of organochlorine substances (Kaba, 1992). The ranges for the species analyzed (Pagellus bellotii, Epinephelus aeneus, Cynoglossus canariensis, Pseudotolithus senegalensis, Sphyraena sphyraena and Panaeus notialis) were: lindane < 0.1–2.4, heptachlor ND-2.7, aldrin 0.1–3.9, dieldrin ND-2.1, endrin ND-<0.1, total DDT 0.4–12.9 ng/g dry weight.

Cameroon

Available data on organochlorine compounds from Cameroon concern fishery products from the coastal waters between Cape Limbo (around Limbe) and the River Wouri estuary and creeks in Douala (Mbi and Mbome, 1991, Mbome and Mbi, 1991; see Table V). The results obtained (ranges and means in ng/g fresh weight) are fish- lindane ND-7.31 (1.60), aldrin ND-13.3 (2.4), DDTs ND-393 (89.5), PCBs ND-983 (196); shrimp-lindane 0.28–1.76 (0.98), aldrin ND, DDTs 76–540 (244), PCBs ND-705 (342); oyster-lindane ND-5.3 (1.44), aldrin ND-12.0 (1.71), DDTs ND-181 (113), PCBs ND-716 (209).

The area studied is characterised by the presence of banana, rubber and palm plantations along the coastline, of a petroleum refinery around Limbe and intense industrial activities around the port of Douala. The Douala area is linked by creeks to Edea (southeast), a town with an aluminium processing complex, a hydroelectric plant and a paper pulp factory (closed down 2 to 4 years ago). Samples from Douala account for most of the high values of organochlorine compounds, especially DDT and PCBs.

Benin, Sierra Leone and Gambia

In Benin, Soclo and Kaba (1992) reported the following mean concentrations in ng/g fresh weight for fish: HCB <0.016, lindane 0.10, heptachlor 0.02, aldrin <0.006, ρ,ρ'-DDT 1.86 (Table V).

Portmann et al. (1989) in their assessment of the state of the marine environment of West and Central Africa reported mean total DDT and lindane levels of 46 and 12.4 ng/g fresh weight respectively for fish from the coast of Sierra Leone.

From the Gambia, the mean concentrations of lindane in ng/g fresh weight (Jallow, 1988) were: fish 0.029, shrimp 3.07 and oyster 1.74. Concentrations for heptachlor were 0.15, 0.74 and 0.18 ng/g respectively (Table V).

The low levels of CLHCs in marine organisms are probably due to the relatively low amounts of such substances used in these West African countries, compared to other developing countries.

6.4 Southern Africa

This Sub-region has a long history of chlorinated pesticides usage in agriculture, public health and disease control programmes. Two countries in the region, Zimbabwe and South Africa have heavy concentrations of primary and tertiary industries which could severely stress the aquatic ecosystem.

Zimbabwe

Greichus et al., 1978a reported the detection and quantification of chlorinated hydrocarbons including PCBs in different environmental compartments of Lake Mcllwaine, Zimbabwe, with concentrations in fish being much higher than in sediments and water respectively. The concentrations found were for PCBs <1,000 ng/l in water, 120 ng/g dry weight in sediments and 1,200–2,300 ng/g dry weight in fish, for total DDT < 100 ng/l in water, 57 ng/g dry weight in sediments and 180–450 ng/g dry weight in fish (Tables II–IV).

Matthiessen (1983) measured concentrations of DDT and metabolites in the environmental compartments of the principal rivers running into Lake Kariba. The values obtained were < 20–300 ng/l in water, 40–740 ng/g wet weight in sediments, 170 ng/g fresh weight in fish muscle and 150–740 ng/g fresh weight in shellfish (Tables II–IV). Residue levels were also measured in fish liver (440 ng/g wet weight) and fish ovaries (360 ng/g fresh weight). According to the author, DDT contamination is due to the use of this pesticide for tsetse fly control.

Mhlanga and Madziva (1990) reported the concentrations of HCH, aldrin, dieldrin and total DDT in different matrices of Lake Mcllwaine. The values obtained (range; mean) were in fish (ng/g fresh weight): α-HCH ND-240 (64.1), aldrin ND, dieldrin ND-24 (1.33), total DDT (66.6), in water (ng/l): α-HCH 26–270 (100), aldrin < 10–120 (100), dieldrin 10–530 (200), total DDT 30–700 (400) and in sediments (ng/g fresh weight): HCH 2.0–42 (16), aldrin ND-12 (1.0), dieldrin ND-16 (5.0), total DDT 32–146 (76). No data was given for PCBs (Tables II–IV).

Botswana

Mathiessen et al., 1982 have investigated the accumulation of endosulfan (highly toxic to fish with 24-h LC50 = 0.09–11.2 μg/l depending on species and temperature) residues in fish and their predators, after aerial spraying for the control of tsetse fly in the Okavango Delta, Botswana. The maximum total residue concentration found in the muscle of living fish was 190 ng/g wet weight while the maximum found in whole dead fish was 1,500 ng/g. The residue levels in fish were approximately proportional to their fat content, with lean fish therefore being more susceptible to poisoning than fat fish. The lack of persistence of endosulfan in the Okavango aquatic ecosystem is demonstrated by the fact that residue levels in fish predators (fish-eating birds and crocodiles) were similar to their prey, the risk to them was consequently low.

South Africa

Concentrations and distributions of chlorinated hydrocarbon insecticides, PCBs and some metals were determined in bottom sediments, aquatic plants, aquatic insects, fish, fish-eating birds and their eggs from two South African lakes, Hartbeesport Dam and Voëlvlei Dam (Greichus et al., 1977). Mean values reported for the Hartbeespoort Dam ecosystem were in fish (ng/g dry weight): dieldrin 80, ρ,ρ'-DDE 77, ρ,ρ'-DDD 100, ρ,ρ'-DDT 67, total DDT 244, PCBs 920, in water (ng/l): dieldrin < 100, ρ,ρ'-DDE 100, ρ,ρ'-DDD 100, ρ,ρ'-DDT 100, total DDT 300, PCBs 2,000 and in sediments (ng/g dry weight): dieldrin 2, ρ,ρ'-DDE 10, ρ,ρ'-DDD 18, ρ,ρ'-DDT 13, total DDT 45, PCBs 320. For the Voëlvlei Dam ecosystem, the mean values were in fish (ng/g dry weight): dieldrin 27, ρ,ρ'-DDE 160, ρ,ρ'-DDD 20, ρ,ρ'-DDT 190, total DDT 370, PCBs 600, in water (ng/l): dieldrin < 100, ρ,ρ'-DDE < 100, ρ,ρ'-DDD < 100, ρ,ρ'-DDT < 100, ρ,ρ'-DDT < 200, PCBs < 1,000 and in sediment (ng/g dry weight): dieldrin < 1.0, ρ,ρ'-DDE 5.0, ρ,ρ'-DDD 2.0, ρ,ρ'-DDT 6.0, total DDT 13.0, PCBs 70.0.

Values for aquatic plants (water hyacinth and algae) are presented in Table VI. The insecticide residues most commonly found in both dams were DDE, DDD, DDT and dieldrin. Hartbeespoort had higher levels than Voëlvlei of insecticides and PCBs in all types of samples common to both lakes, concentrations of PCBs having six or more chlorines increased with an increase in the trophic level.

7. LEVELS OF CLHCs IN DIFFERENT ENVIRONMENTAL COMPARTMENTS

Tables II to VI provide data on CLHCs in different compartments in the aquatic environment.

A panoramic view of the study sites pinpoint the fact that the investigation of the presence and quantitative estimation of CLHCs have centred on lakes, rivers and streams in agricultural pests control or disease vector control activity areas as well as in urban areas, some of which are industrialised. Since environmental impact assessment was non-existent prior to the application of these chemicals coupled with the significant contribution of atmospheric transport of these chemicals to distances far away from point(s) of application, most of the so-called baseline studies really represent post-impact assessment data. In other words, these studies provide apparent baseline data rather than the true baseline data expected from pristine environments.

7.1 Concentration of CLHCs in Water

There are gaps in data available on the levels of CLHCs in water (Table II). Most of the studies have been carried out to assess the ecological impact of specific groups of OCPs which have been used, in particular DDT and isomers, lindane, endosulfan, dieldrin, aldrin and heptachlor. Only in a few isolated cases industrial chemicals such as PCBs and HCBs have been determined. More data are available on inland than on coastal waters, especially from Nigeria and Southern Africa.

Generally, African inland waters are contaminated by a broad spectrum of OCPs. However, HCH, aldrin, endosulfan, ρ,ρ'-DDE, ρ,ρ'-DDT and total DDT were not detectable in many Nigerian inland water samples despite fairly large scale use of these chemicals. Perhaps in the hot tropical climate, volatilisation of these chemicals and atmospheric transport are responsible for the trace concentration of these chemicals in water. The concentrations in water can also be correlated with water solubility and persistence in water of these chemicals.

The OCPs with the lowest concentrations are generally heptachlor (ND-11) and HCB if we leave out single high values. Lake Mariut in Egypt appears the most polluted water body, based on exceedingly high values for γ-HCH (lindane) of 1,310 ng/l, for ρ,ρ'-DDE of 6,630 ng/l and for total DDT of 21,440 ng/l (Saad, 1981). Lake Nakuru in Kenya, Lake Mcllwaine in Zimbabwe and the South African reservoirs are contaminated by PCBs indicating pollution from industries as source of contamination. According to data from Nigeria (Tongo, 1985) and Egypt (Tayel, 1981) coastal waters are just as contaminated as freshwaters except for the Egyptian lakes which have very high values.

7.2 Concentration of CLHCs in Sediments

From the available data (Table III) relatively more information on a wide range of CLHCs has been reported for West Africa, in particular on aldrin, lindane, dieldrin, DDT and metabolites and PCBs. The table also shows that most freshwater sediments in Africa are contaminated with CLHCs. Except for high levels of DDT and metabolites (1.73–877 ng/g dry weight), however, it can be inferred that in sediments of African inland waters contamination levels are low and therefore pose no ecological threat.

Coastal sediments exemplified by Abu Kir-Bay, Egypt are cleaner than inland water sediments. Lekki Lagoon, Nigeria, a semi-enclosed water body with several rivers draining into it provides a hot spot with very high mean values of some compounds, e.g. dieldrin (4,560 ng/g) and ρ,ρ'-DDE (263 ng/g dry weight). Another hot spot appears to be the Ebrié Lagoon, Côte d'lvoire, which has a mean PCB value of 355.5 ng/g dry weight.

7.3 Concentration of CLHC Residues in Fish

Since CLHCs are lipophilic and can bioaccumulate in fish tissues, Table IV indicates that inland water fish are highly contaminated by DDT and metabolites and lindane which reflect the heavy usage of these chemicals. In particular fish from Kenyan rivers (Tana and Athi) and even more from the South African reservoirs are heavily contaminated with levels of 370 ng/g for DDT (Voëlvlei Dam) and 920 ng/g fresh weight for PCBs (Hartbeespoort Dam). Data for CLHCs levels in marine fish (Table V) are available only for West and Central Africa. In general, marine fish are less contaminated than freshwater species. Data for marine fish from Cameroon, however, show high values for DDTs and PCBs.

An attempt was made to identify trends of occurrence of DDT and its derivatives in fish fillet by looking at the data in Table IV. There is no clear trend towards decreasing concentrations over the years; however, lower DDT concentrations are more frequently detected in recent samples. This may be due to reduction in use and more stringent controls. This is also supported by the fact that differences in concentrations of DDT and its metabolites were wider in earlier studies which indicated recent use while they were much closer in later studies indicating past use. Furthermore, marine fish species appeared less contaminated than freshwater species as already indicated.

7.4 Concentration of CLHCs in Aquatic Plants

Only scanty data are presently available for levels of these chemicals in aquatic plants, since most studies of biota concentrated on fish.

Table VI reports quantifiable levels of ρ,ρ'-DDE for aquatic plants in Lake Naivasha, Kenya whereas trace to non-detectable levels were reported for Lake Nakuru, Kenya.

The ubiquitous water hyacinth which is now a menace in many African inland waters was found to give detectable and quantifiable levels in Nigerian coastal waters for dieldrin of 43, for lindane of 48.6, for heptachlor of 52 and for PCBs of 2,700 ng/g dry weight. Lower values of CLHCs were recorded for that plant in Hartbeespoort Dam, South Africa.

8. REGIONAL COMPARISON OF CLHC LEVELS

Based on the levels of dieldrin, lindane, endosulfan, total DDT and PCBs, water and sediments of the North African lakes in Egypt are more polluted, followed by West African rivers in Nigeria, the East and Southern African waters are the least polluted by these organic pollutants.

However, the trend for fish contamination is reversed. Fish from East Africa is more contaminated followed by fish from North Africa while fish from West Africa is least contaminated. This disparity may be linked to a number of factors including relative fat content in fish, fish size and feeding habits, as well as to bio-geochemical and abiotic transformations in different aquatic ecosystems.

9. COMPARISON OF CLHCs IN THE AFRICAN AQUATIC ENVIRONMENT WITH OTHER PARTS OF THE WORLD

In the GEMS/WATER programme report (UNEP/WHO, 1988), the absence of data on CLHCs levels in African waters was highlighted. The lack of data on the levels of these substances in coastal and marine fish was also underscored at a pollution monitoring workshop (IOC-Unesco, 1985). Nonetheless, this review indicates the presence and detection of all the substances mandatory for monitoring in GEMS/WATER during 1979–1984 with the exception of mirex.

Concentrations above 100 ng/l were found for γ-HCH, DDE and total DDT in Egyptian lakes, lbadan streams, Nigeria (DDT only) and PCBs in Hartbeespoort Dam, South Africa respectively, indicating pollution by these chemicals. These results are similar to level of DDT reported in Colombia by GEMS/WATER and the γ-HCH values reported for some major rivers in the United Kingdom (Croll, 1969), Japan (Suzuki et al., 1974) and India (Ramesh et al., 1990). However, the majority of the African inland waters have γ-HCH concentrations below 10 ng/l which is the recommended guideline for the protection of freshwater aquatic life in Canada (Merriman and Metcalf, 1988), similar to the results reported for Niagara River, Canada (Oliver and Nicol, 1984).

Exceedingly high dieldrin levels of over 1000 ng/l were found in some Nigerian surface waters. The results are also similar to levels reported for dieldrin under the GEMS/WATER in Colombia and Malaysia respectively but exceed several fold the 4 ng/l guideline set for Canadian freshwater aquatic life.

The PCBs values of < 1,000 ng/l obtained for dams in South Africa, lakes in Kenya, Zimbabwe and South Africa as well as Awba Dam in Nigeria are comparable to the high values reported for United Kingdom, Thailand, China and Japan in the 1979–84 GEMS/WATER programme.

Apart from a few hot spots due to localised pollution problems, African inland water sediments are relatively unpolluted by CLHCs and the values obtained are in most cases lower than concentrations reported for developed countries (Eisenreich et al., 1979, Oliver and Charlton, 1984).

Levels of CLHCs in fish from African waters are relatively low compared to values from other parts of the world and are similar to results obtained by GEMS/FOOD for 1984–1985 (UNEP/FAO/WHO, 1988), except in a few cases around pollution hot spots. Levels found are much lower than the permissible limits for CLHCs in fish for human consumption (Nauen, 1983; FAO, 1989).

10. REGULATORY CONTROL MEASURES

The occurrence of CLHCs in different compartments of the aquatic environment even at trace and ultra-trace levels should give cause for concern. Long-term exposure to sub-lethal concentrations of these substances through various pathways in the aquatic environment may cause far reaching ecological damage and health problems to man.

Institutional framework and regulatory mechanisms for the importation, transportation, storage, sale and application of these chemicals should be put in place in all countries. Reference is made to the International Code of Conduct on the Distribution and Use of Pesticides, adopted by the FAO Conference in 1985 and amended in 1989 (FAO, 1990). The objectives of this Code are to set forth responsibilities and establish voluntary standards of conduct for all public and private entities engaged in or affecting the distribution and use of pesticides, particularly where there is no or an inadequate national law to regulate pesticides.

11. CONCLUSIONS

The environmental impact of several decades of use of chlorinated hydrocarbons in Africa for multifarious purposes has been manifest in the contamination of different aquatic compartments with these substances, especially DDT and its metabolites, lindane, dieldrin and PCBs. The pattern of pesticide residues found in environmental samples based on the available data bears direct relationship to pesticides used in the continent.

This review has accomplished the difficult task of compiling data on CLHC levels in the aquatic environment of the region so far non-existent. The concentrations found in various aquatic environmental compartments, with few exceptions are lower than in other parts of the world, in particular in developed countries which have a longer history of high pesticide consumption and intense use. Generally, the coastal waters, sediments and biota are less contaminated than inland water environmental compartments, with the exception of a few hot spots.

The data available is limited and, therefore, forms a good basis for further monitoring work, involving more countries in the region. Nonetheless, the data on fish fillet are homogeneous and comparable, having been generated through coastal and marine pollution monitoring programmes and research studies initiated in the region in the 1980s under the aegis of the UNEP Regional Seas Programme. The data showed a tendency towards decreasing concentration of these substances over the years and a direct link with reduction in use and with more stringent controls on these classes of compounds by national governments.

Nevertheless, the occurrence of these synthetic micropollutants in different compartments of the aquatic environment, even at trace and ultra-trace levels, is of ecological and environmental health concern. The regulatory framework and control on the use of these chemicals should be put in place in the countries of the region as well as enforcing the International Code of Conduct on their proper use.

Table II
Concentration of chlorinated hydrocarbons in African inland and coastal waters (ng/l)

Location/water typeDieldrinαHCHHCBγHCH (Lindane)HeptachlorAldrinEndosulfanReference
INLAND WATERS        
North Africa - Egypt        
 Lake Mariut   2091   Saad et al., 1982
 Lake Mariut 120 1310   Saad, 1981
 Nozha Hydrodrome   1100   Abu-Elamayem et al., 1979
East Africa - Kenya        
 Lake Nakuru<100<100     Kallqvist and Meadows, 1977
West and Central Africa - Nigeria        
 Ibadan, Streams250 (17.8–657)150(1–302)17(ND-92)100(7–297)72(4–202)20(ND-40)98(ND-430)Nwankwoala and Osibanjo, 1992
 River Ogun   13.3(1.4–41.9)0.25(ND-0.8)40(5.1–49)116(ND-260)Agunloye, 1984; Tongo, 1985
 River Imo   0.2(ND-0.6)4(ND-11.4)13(ND-40)13(ND-41)Agunloye, 1984; Tongo, 1985
 Cross River  1.0 (ND-5)0.3 (ND-1.1)2(ND-8.6)36(ND-143)20 (ND-80)Agunloye, 1984; Tongo, 1985
 Awba Dam, Ibadan  0.8(ND-2.5)61(9–167) 18(12–29)20(ND-30)Agunloye, 1984; Tongo, 1985
 Kainji Lake   0.12 (ND-0.24)0.47 (ND-3.48)0.55 (ND-3.05) Adeniji et al., 1991
 R. Ero Dam, Ondo560  2.03.33.1NDOgunlowo, 1991
 River Ero, Ondo740  2.0NDNDNDOgunlowo, 1991
 River Osse, Ondo2150  2.05.0NDNDOgunlowo, 1991
 R. Owesse, Ondo1120  6.41.6NDNDOgunlowo, 1991
 River Apomu, Ondo1380  4.84.63.5NDOgunlowo, 1991
Southern Africa        
 Hartbeespoort Dam, RSA<100      Greichus et al., 1977
 Voëlvlei Dam, RSA<100      Greichus et al., 1977
 L. Mcllwaine, Zimb.<100      Greichus et al., 1978a
 L. Mcllwaine, Zimb.200(<10–530)100(26–270)   100(<10–120) Mhlanga and Madziva, 1990
COASTAL WATERS        
North Africa - Egypt        
 Abu-Kir Bay 4.88 18.05   Tayel, 1981
West and Central Africa - Nigeria        
 Lagos Lagoon8 (ND-24) 2 (0.8–4.1)182(16–634)ND53 (3–190)29 (ND-86)Tongo, 1985

ND = Not detected

Location/water typeρ,ρ'-DDEρ,ρ'-DDDρ,ρ'-DDTTotal DDTPCBReference
INLAND WATERS      
North Africa - Egypt      
 Lake Mariut4493 1345100 Saad et al., 1982
 Lake Mariut6630 982021440 Saad, 1981
 Nozha Hydrodrome1540 600  Abu-Elamayem et al., 1979
 Nozha Hydrodrome   13610 Saad, 1981
East Africa - Kenya      
 Lake Nakuru<100<100<100 <1000Greichus et al., 1978b; Kallqvist and Meadows, 1977
       
West and Central Africa - Nigeria      
 Ibadan, Streams   310 (ND-1266) Nwankwoala and Osibanjo, 1992
 River Ogun    87 (ND-244)Agunloye, 1984; Tongo, 1985
 River Imo    121 (ND-241)Agunloye, 1984; Tongo, 1985
 Cross River    120 (ND-470)Agunloye, 1984; Tongo, 1985
 Awba Dam, Ibadan    330 (ND-1000)Agunloye, 1984; Tongo, 1985
 Kainji Lake   3.88 (ND-10.05) Adeniji et al., 1991
 River Ero Dam, OndoNDNDNDNDNDOgunlowo, 1991
 River Ero, OndoNDNDNDNDNDOgunlowo, 1991
 River Osse, OndoNDNDNDNDNDOgunlowo, 1991
 River Owesse, OndoNDNDNDNDNDOgunlowo, 1991
 River Apomu, OndoNDNDNDNDNDOgunlowo, 1991
Southern Africa      
 Hartbeespoort Dam, RSA1001001003002000Greichus et al., 1977
 Voëlvlei Dam, RSA<100<100<100<200<1000Greichus et al., 1977
 Lake Kariba, Zimbabwe  (<20–300)  Matthiessen, 1983
 Lake Kariba, Zimbabwe   350 (<10–700) Mhlanga and Madziva, 1990
 Lake Mcllwaine, Zimbabwe100<100<100 <1000Greichus et al., 1978a
 Lake Mcllwaine, Zimbabwe   400(30–700) Mhlanga and Madziva, 1990
COASTAL WATERS      
North Africa - Egypt      
 Abu-Kir Bay   24.3 Tayel, 1981
West and Central Africa - Nigeria      
 Lagos Lagoon3 (ND-15)83 (ND-344)ND2500 (ND-9000) Tongo, 1985

ND = Not detected


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