The distribution and occurrence of heavy metals in water column of the Anambra River were investigated in the dry season adopting standard chemical methods. Following the establishment of the presence and appreciable concentration of heavy metals in various stations (P<0 .05="" against="" and="" bioassays="" biological="" column="" copper="" effects="" fish="" heavy="" i="" in="" investigated="" laboratory="" metals="" of="" prevalent="" river="" species="" the="" two="" water="" were="" zinc="">Oreochromis
niloticus and Clarias gariepinus) based on the toxicity scale of 96hLC50 to evaluate possible risk effect of the metals on the aquatic species and human population depending on the river. Toxicity ranking for zinc and copper in the water revealed copper to be consistently more toxic to the test species with Oreochromis niloticus being more sensitive. The binary mixtures in predetermined ratios (1:1 and 1:2) of zinc and copper acting against the test animals showed significant departures when compared to levels of toxicity attained by the individual constituent metals when acting alone against the same species. In a majority of cases, the resultant interactions (Zn-Cu against fish species) were in conformity to the model of synergism while fewer instances portrayed antagonism and additive action. There is high risk of heavy metal poisoning among the human population depending on the river and heavy metal related public health effects observed in a survey are circumstantial evidence. The work recommends setting more realistic and effective safe limits and standards to minimize and properly manage heavy metal polluted water bodies.

1.1       Background of the Study
Amongst the several heavy metals that pose some risk to our environment, thirty- five of them have been listed or known to have significant occupational problems in man.

Interestingly, small amounts of these elements are common in our environment and diet. Moreover these elements are actually necessary for good health but large amounts of any of them may cause acute or chronic toxicity (Glanze, 1996).

Heavy metal toxicity can result in damaged or reduced mental and nervous functions, ATP depletion and damage to blood composition, lungs, kidneys, liver and other vital organs (Wright, Welbourn and Martin, 1991). Long term exposure may result in slowly progressing physical, muscular, and neurological degenerative processes that mimic Alzheimer’s disease, Parkinson’s disease, muscular dystrophy and multiple sclerosis (Wright et al., 1991). Repeated long term contact with some metals or their compounds may even cause cancer (International Occupational Safety and Health Information Centre, 1999).

For some heavy metals, toxic levels can be just above the background concentrations naturally found in nature (Dupler, 2001). Therefore, it is important for us to inform ourselves about these metals and to take protective measures against excessive exposure. Based on the above background, most widely used/encountered heavy metals are the focus of this work and these are mercury, arsenic, cadmium, iron, manganese, chromium, zinc, copper, lead, sodium and potassium. In most parts of the world, heavy metal toxicity is an uncommon medical condition (International Safety and Health Information Centre, 1999). When these metals are found in our immediate environmental media unrecognized or inappropriately treated, toxicity can result in significant illness and reduced quality of life (Ferner, 2001).

Industrial activities worldwide are characterized by the production of a wide variety of end products that are useful to mankind; however all production pathways are accompanied by emission of waste products that enter the environment (Adetoro, 2004). Industrial wastes have been shown to be a complex admixture of several classes of pollutants such as synthetic chemicals of various types, hydrocarbons and heavy metals (Oyewo, 1998; Otitoloju, 2003). Among the pollutants which find their way into the Anambra River, heavy metals are important (Igwilo, Afonne, Maduabuchi, and Orisakwe, 2006). Also important to mention is that they are non-biodegradable and remain in the environment long after the elimination of source (Otitoloju, 2002). Although most of these metal pollutions are anthropogenic, there are a few examples of localized ones resulting from natural weathering of ore bodies (Hager and Abrahamsen, 1990) mining and smelting (Walker, Hopkin, Sibly and Peakau, 2001). They are considered priority pollutants because they easily bioaccumulate in the food chain (Chukwu, 1991). Heavy metals, although usually considered pollutants, are natural substances with the exceptions of radioisotopes produced in man-made nuclear reactions (Walker et al., 2001). Heavy metal contents of Anambra River have been earlier reported by Igwilo et al. (2006) and Obodo (2004) attributing the loading to various anthropogenic activities in and around the river.

Metals and other pollutants such as sewage, sawdust, hydrocarbons and organic wastes, when introduced into water bodies, can bring about alteration of physicochemical characteristics of such a water body and hence would affect species diversity (Akpata, 1987, Oyewo 1998, Saliu and Ekpo 2006).
In many instances, heavy metals occur in natural bodies of water at levels below their toxic thresholds. However due to their non-degradable nature, such sublethal concentrations may still pose risk of damage via uptake and subsequent bioaccumulation by organisms which cannot effectively metabolise and excrete the absorbed metals.

The main route by which heavy metals enter the aquatic environment is through the discharge of metal-laden municipal and industrial effluents, directly into the water bodies or indirectly via drainages and canals (Angino, Magnuson, Wangh, Galke, and Bredfeldt, 1970; Oyewo, 1998). The discharge of these metals into the environment also occurs during industrial processes and incineration of waste products containing metallic compounds.

Environmental research in any country conventionally should concentrate among other things on the pollutants that are considered as priority in terms of frequency and intensity of occurrence. Typically, the Anambra River acts as a sink or reservoir that receives waste water effluent from different parts of the locality and hinterland (Igwilo et al., 2006).

In response to this, there have been a few investigations on the occurrence and distribution of heavy metals in the Anambra River. Such authors have found measurable quantities of heavy metals although they failed to relate such levels of occurrence to biological action such as acute toxicity and sub lethal chronic action. Anambra River is the natural habitat of several fish species such as Clarias and Tilapia species. These fish species particularly the pelagic species are useful in assessing water quality in the field and laboratory because they are ubiquitous, sedentary in nature and have long life cycles; hence they act as continuous monitors of the water bodies they inhabit. Earlier works (Okonkwo and Obiakor, 2008; 2009; Odo, Didigwu, and Eyo, 2009) have provided information on the biology and ecology of these species. What was lacking in their report is detailed empirical data on the interaction of these locally important species with this important pollutant heavy metal detected in the water column of Anambra River.

1.2       Statement of the Problem
Surface water use has been of tremendous application across the globe for various purposes: domestic, industrial, agricultural etc. But studies by different people in both developing and developed countries of the world show that pollutants from various sources get to the surface water and pollute it. (Obodo 2004, Igwilo et al. 2006, Clark 1992, Matti 2001, Oyewo 1998 and Don-Pedro 1996). Otitoloju (2002) explained that industrial effluents released into the environment get to the surface water and pollute it with priority heavy metals; hence the author recommended the toxicity testing of the most sensitive species in the field to assess possible ecological damage that could result from these toxicants. Consequently, Igwilo et al. (2006) supported the above fact and advocated for a means to avert possible imbalance in the fish population structure and the residents around the river feeding on toxic metals.

In Nigeria, and indeed many other developing countries, there are host of unplanned and unorganized uses of river for various purposes, including solid waste dumping, effluents disposal, siting of wide scale toilet facilities, disposal of chemical wastes and wrong and frequent application of artificial fertilizers. These are done without putting the water quality of the river into consideration.

A casual inspection of Anambra River shows that there are unhealthy, unplanned and unorganized uses of the river for various purposes like solid waste dumping, effluents disposal, siting of wide scale toilet facilities, disposal of chemical wastes, use of poisonous chemicals in fishing, and indiscriminate application of fertilizers for farming activities around the banks of the river at various locations. These activities have negatively affected the quality and activities of the river in a number of ways.

Firstly, the aesthetic value of the river has long disappeared. In addition, the river is heavily silted, especially at the lower course. Consequently, the depth and width of the river have greatly reduced. Moreover, some activities which greatly depend on the quality of the river like swimming have been greatly affected. Furthermore, fishing in the river which used to be one of the means of sustaining livelihood for the inhabitants of the area has drastically reduced.
A preliminary survey conducted prior to the main eco-survey showed that various wastes are discharged into the river. Sewage dislodgers and various industries were seen discharging their wastewater/ effluents into the river.

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