Abstract: Selected environmental samples such as water, fishes (Tilapia mariae and Clarias gariepinus), earthworm (Libyodrillus violaceous) and soils were obtained from the Otokutu end of Warri River, digested with acid mixtures and analyzed for trace metal concentrations using the atomic absorption spectrophotometry. The trace metals measured include; zinc, lead, copper, arsenic, iron, cadmium and mercury. The results obtained showed variations in the concentrations of metals in the entire samples analyzed. Lowest metal concentrations were recorded in the water samples. Trace metal concentrations in Tilapia mariae, Clarias gariepinus and Libyodrillus violaceous were higher than levels recorded in water and soil samples, respectively. The elevated concentrations of lead, arsenic, iron, cadmium and mercury were traceable to anthropogenic wastes and activities of industries operating in Warri and its environs.
INTRODUCTION
The major sources of pollution in streams and rivers are the effluents from industries and untreated wastes (Okafor and Nwajei, 2006). High levels of cadmium were found in tissues of fish which may be traceable to industrial pollution in Asaba and Onitsha areas of the River Niger (Nwajei and Oruvwuje, 2001). Human exposure of mercury via global cycle is likely to be greatest in the Arctic where natives depend on freshwater marine fish and marine mammals for much of their diet (Gobeil et al., 1999). As river receive discharges from the oil and gas industries, urban water storm and agricultural water runoff from the farming communities along the river course. Contamination of the sediment matrix by heavy metals may be accumulated in fishes and other aquatic resources which may eventually get into human food chains (Iwegbue et al., 2007). Heavy metal pollution in the Rio San Juan is a major concern in the Rio Bravo as well (Schmandt et al., 2000; IBWC, 1994). De La Garza (1995) observed that 89% of discharges to the Rio San Juan by the 10000 industries in the Rio San Juan watershed exceeded the standards. Therefore, degradation of water quality in the Rio San Juan watershed may involve two factors: (1) Increasing water withdrawal demands to meet domestic and industrial water supplies and (2) Increasing loads of pollutants into the river. The aquatic communities of fish, insects and other organisms are already under stress. The susceptibility of fish to damage by toxic substances such as heavy metals increases when the level of dissolved oxygen depletes (Brooks et al., 1992). The accumulation of heavy metals or their compounds in an aquatic environment has direct consequence to man and to the ecosystem (Pickering and Owen, 1994).
Trace elements and organic matter pollutants are predominantly transported in association with suspended particulates with heavy metals and organic matter pollutants in low turbulent river, leading to the formation of highly polluted river bottom sediments in industrialized areas (Haag et al., 2001). Study has revealed that nearly all metal contents in aquatic environment reside in water sediments while the fractions in biota are small (Ademoroti, 1996).
Earthworms constitute the largest terrestrial faunal biomass (Vandecasteele et al., 2004). Earthworms were found to have a high potential for cadmium accumulation in polluted floodplains (Hendriks et al., 1995). They have been considered useful for assessing heavy metal pollution in soils (Menzie et al., 1992) because earthworm biomass and abundance were found to be more sensitive to pollution in comparison with other indicator taxa (Spurgeon et al., 1996).
MATERIALS AND METHODS
The samples collected from Otokutu River include water, Tilapia mariae, Clarias gariepinus (fish) Libyodrilus voilaceous (earthworm) and soil. These samples were collected in the day time for the period covering October 2008 to March 2009. Two water sample locations were chosen (i.e., sample station A and B). The water samples were collected at a depth of about 15 cm in clean white polyethylene stoppered bottles which have been washed with soap solution, rinsed three times with pure water and then rinsed another three with 1% HNO3. The water to be collected was used to rinse the bottles and stoppers for three times before the final collection. The water samples were stoppered and labeled A and B, respectively. Filtration was done on location by passing water samples through a 0.45 μm millipore membrane filter. The membrane filters had been washed with 1% HNO3, followed by rinsing in high purity water prior to filtration (Bordin et al., 1992). The water samples were taken to the laboratory and stored in cold at 4°C prior to digestion. Then, 100 mL of each water sample was transferred into a beaker and 5.0 mL of concentrated HNO3 was added. The beaker containing the solution was placed on a hot plate and evaporated to near dryness making sure that the sample does not boil. The beaker containing the residue was cooled. Then, 5.0 mL of the concentrated HNO3 was further added. This was returned to the hot plate until digestion was completed. Then 2.0 mL of concentrated HNO3 was added and the beaker was warmed slightly to dissolve the residue (APHA, 1990; Ahn et al., 1996). The digested sample was filtered. The filtrate was made upto 50 mL with deionised water. The solution was returned to the laboratory and kept in the refrigerator prior to metal analysis. The blank was also prepared using the same procedure of digestion as in samples.
Soil sample was collected at depth of 0-15 cm by the bank of Otokutu River. The soil was air-dried and was sieved to pass through a 2 mm sieve. The 5.0 g of dried surface soil sample was accurately weighed into a clean dry beaker. The 10 mL of concentrated HNO3 was added and left for 10 min. 5 mL of concentrated HClO4 was added and left for another 10 min. The digest was heated on a hot plate regulated to 120°C for 30 min, stirred to attain a uniform mixture with the liberation of hydrogen chloride gas from the sample. The digest was transferred to fume cupboard where it cooled over night (Darmody and Marlin, 2002) and filtered through Whatman filter No. 42 study into 100 mL volumetric flask and made to mark with deionised water. A blank sample was also prepared using the same procedure. The solution was stored in the refrigerator prior to metal analysis.
Earthworm (Libyodrillus violaceous) sampling was done on the bank of Otokutu River according to the combined method of Bouche and Aliaga (1986). While, the fish (Tilapia mariae and Clarias gariepinus) samples were collected using the method adopted by Ney and van Hussel (1983). Earthworm and fishes collected were taken to the Department of Animal and Environmental Biology Delta State University Abraka for identification. The samples were dissected with a clean stainless steel knife and were then oven-dried at 60°C. The whole body of Libyodrillus violaceous, Tilapia mariae and Clarias gariepunis was crushed in each case, in clean mortars with pestle. Five gram each of the dried Libyodrillus violaceous, Tilapia mariae and Clarias gariepunis were weighed in clean beakers and digested concentrated acid mixture containing 20 mL HNO3 and 5 mL HClO4. The resultant solution was placed on a hot plate with constant stirring for 30 min until the liberation of nitrous oxide gas from the sample was observed. The digest was transferred into the fume chamber overnight. On cooling, the digest was filtered through Whatman filter No. 42 into 100 mL volumetric flask and made to mark with deionized water (Heikens et al., 2001). The blanks were prepared for each case following the same procedure but omitting the samples using atomic absorption spectrophotometry (Perkin Elmer Analyst 200, Norwalk, CT, USA).
RESULTS AND DISCUSSION
Environmental samples such as water, soil, Libyodrillus violaceous, Tilapia mariae and Clarias gariepinus obtained from Otokutu River were analyzed for selected metals (zinc, lead, copper, arsenic, iron, cadmium and mercury). The results are presented in Table 1. The results revealed that there were variations in metal concentrations in all the samples studied. The results further revealed that metal concentrations in Clarias gariepinus, Tilapia mariae, Libyodrillus violaceous and soil were elevated when compared to metal concentrations in water samples. The aquatic vertebrates and invertebrates metal values exceeded those in the soil sample. The bioaccumulation of trace metals in the aquatic vertebrates (fishes) and invertebrates (earthworm) may have accounted for the elevated values.
Tilapia mariae contained higher concentration of trace metals compared to those obtained in Clarias gariepinus from the same river. Also, the trace metal concentrations in Libyodrillus violaceous (earthworm) were lower than those obtained in Tilapia mariae and Clarias gariepinus except for iron, cadmium and mercury concentrations which were considered high. This study revealed that vertebrates and invertebrates in rivers are the major indicator for metals accumulation.
The concentrations of zinc, lead, copper, arsenic, iron, cadmium and mercury in water from the two sampling stations (samples 1 and 2) were below the water quality guideline values for metals in water (WHO, 2006).
| Table 1: | Mean and range of trace metal concentrations in water, soil earthworm and fishes from otokutu river |
| Table 2: | Concentration range of metals in water, soil, earthworm and fishes in different rivers in Nigeria |
| ND: No detected, -: Parameter not measured | |
The concentrations of zinc, copper, iron and cadmium in water, in this study were lower than those values obtained in Ora River around Nigerian cement factory by Okafor and Nwajei (2007a). Table 2 represent the results of the present study in comparison with published data on the metal concentrations in water and fish species from some Nigeria waters.
The concentrations of lead and cadmium in Tilapia mariae, Clarias gariepinus, Libyodrillus violaceous and soil in this study were lower than those values obtained by Nwajei and Okrija (2004) from Warri River. The accumulated metal concentrations in the tissues of Tilapia mariae and Clarias gariepinus may be passed on through the food chain to man thereby causing food poison. These high concentrations of trace metals are traceable to various activities of industries operating in Warri and its environs.
In general, cadmium is the most important pollutant for food chain transfer whereas copper is the most element for earthworm survival. The knowledge of trace metal concentrations in earthworm is essential for risk assessment of trace metal biomagnifications. Earthworms have been considered useful for assessing heavy metal pollution in soils (Menzie et al., 1992). However, trace metal pollution has two major effects on the ecosystem. For example, accumulation of cadmium as measured in this study, can lead to risks of poisoning whereas low concentrations of copper in soil can lead to earthworm disappearance, thereby, causing food scarcity for earthworm predators such as fishes.
CONCLUSIONS
Water, soil, Libyodrillus violaceous, Tilapia mariae and Clarias gariepinus samples obtained from Otokutu River were analyzed for metals such as zinc, lead, copper arsenic, iron, cadmium and mercury. The results obtained revealed that there were metal variations among the samples measured. Lowest metal levels were obtained in water samples in comparison with other samples studied. The results further revealed that Libyodrillus violaceous depends on soil for survival. Secondly, it serves as food for predators such as fishes. This is an indication that metal pollution through the food chain can be passed to human. Soil metal levels being lower than the metal concentrations in earthworm and fishes being lower than that of water are indication of bioaccumulation. This implies that vertebrates and invertebrates can serve as bioaccumulators of trace elements in Nigerian rivers. The presence of trace metals in Otokutu River are traceable to anthropogenic wastes and activities of various industries operating in Warri and its’ environs.
ACKNOWLEDGEMENT
The author is thankful to Mr. G.C. Ndinwa for his assistance in sample collection and metal analysis.