Introduction

Fresh water is a finite resource, essential for agriculture, industry and even for human survival. Without fresh water of adequate quality and quantity sustainable development will not be possible. Because of this reason, governments of most countries see an imperative need to become involved in its over-all management. Yet in many developing countries, water resources are not being managed efficiently and in a manner that will sustain the natural qualities. As a result of the increasing level of industrialization, urbanization, agricultural practices etc it is obvious that man’s interaction with his aquatic environment has impacted negatively on the latter. One consequence of this is that aquatic resources and indeed food chain have been adversely affected as a result of the bioaccumulation of pollutants. Pollution of the aquatic environment occurs when humans introduce either by direct discharge to water or indirectly through atmospheric or water management practices, substances or energy that result in deleterious effects such as hazards to human health, harm to living resources, hindrances to aquatic activities such as fishing, impairment of water quality with regards to its use in agriculture, industry or other economic activities or reduction in its aesthetic values17.

As a result of the increasing rate of industrialization in Nigeria, a lot of harmful substances, including heavy metals, are now being discharged into the environment particularly water bodies. This has resulted in an increase in the concentration of various heavy metals in the aqueous environment beyond their natural levels26. In water bodies, heavy metals may become bound to silt particles, complexed with compounds and settle to the bottom or adsorbed to sediment26. They may be ingested by sea animals or absorbed by plants and thus enter food chain where they bioaccumulate.

In Nigeria, most rural communities do not have access to safe drinking water12 and adequate sanitation services26. Such rural communities inhabiting the banks of rivers avail themselves of this opportunity to obtain water for domestic purposes from nearby rivers and streams. This exposes them to water borne diseases which are known to pose serious health problems.

Several authors have reported on the pollution status of some aquatic resources in Nigeria1845242528231532. However, there is dearth of information on the pollution status of Cross River watershed at Afikpo. This study was therefore designed to estimate the physicochemical parameters and levels of toxic metals in water, fish and sediment from Cross River at Afikpo North Local Government Area, Nigeria.

Materials and Methods

Collection of Water Sample

The Cross River is the largest river in the South-south geopolitical zone of Nigeria. It has its source from a hill in the Northern Cross River State. It is located between longitude 7o191E and 4o451E and between latitude 4o491N and 5o561N, covering an estimated area of more than 2300 km2. Sampling was carried out from March, 2004 to October, 2004. The river was divided into four sampling stations, approximately 4 km apart. The stations were located at Ndebe beach, Anofia (Itim-Ukwu) beach, Kpogirikpo beach and Unwana beach in Afikpo North Local Government Area, Nigeria. Sampling points were chosen where the river mixes properly and where perturbation from human activities was minimal. Water samples were collected in triplicates from depths of 20 cm. The samples were stored in plastic containers at 4oC (in a refrigerator) until required for analysis. Samples for heavy metal determination were preserved with 2 cm3 of concentrated nitric acid.

Collection of Sediments

Sediment samples were collected at four different points in each of the sampling stations. A composite sediment sample for each sampling station was obtained by pooling together the four sediment samples from each sampling station. The sediment samples were then dried, ground to fine powder and stored in labeled plastic bags until required for analysis.

Collection of Fish Samples

Fish samples were collected from the Cross-River at Afikpo North Local Government Area. The fishes were identified and authenticated by experts from the Fisheries Department, Michael OKpara University of Agriculture, Unudike, Nigeria. The scales were removed and the fishes were eviscerated and dried at 80oC to a constant weight for a period of about 36 hours. The fish sample was then pulverized and finally preserved (in a refrigerator) in a clean dried polyethylene bottle until required for analysis.

Digestion of Samples and Analysis

Fish and sediment samples were digested as previously described29. The concentrations of the heavy metals in the samples were determined using atomic absorption spectrophotometer (Unicam 919, Analytical Technology Inc., Cambridge, U. K.). Sodium and potassium were determined by flame photometer. Electrical conductivity and pH were determined at source using portable conductivity meter and pH meter, respectively. Physicochemical parameters of the water samples were determined using a spectrophotometer (Hach, model DREL/5, Hach Company, Colorado, U. S. A.).

Results and Discussion

The result of physicochemical analysis is presented in Table 1. The values obtained for some of the parameters varied widely between samples. The electrical conductivity of the water samples ranged from 4.05 ms/cm (Station 2) to 19.4 ms/cm (Station 3) indicating that the river is not heavily loaded with ions. Conductivity is a measure of the tendency of a liquid to conduct electricity61. Begun et al.,7 reported that electrical conductivity is a useful parameter of water quality for indicating salinity. It is related to the amount of total dissolved solids (notably certain ionic salts) in the water body. Thus the observed conductivity of the water is due to the presence of soluble salts of some metals in the water. Dara8 has reported that natural water contains reasonable amounts of dissolved salts and that the concentration of these salts varies from one water body to another. Conductivity depends on the temperature as well as on the concentration of cations and anions in the water196. There was a positive correlation between the conductivity of the water and the temperature. This suggests that as conductivity of the water increases the temperature also increases. It was observed that the sample with the highest electrical conductivity (Station 3) also had the highest concentrations of sodium, calcium and chloride ions as well as the highest level salinity.

Table 1. Mean Concentration of Physicochemical Parameters in the Four Sampling Stations.

Salinity of the water samples ranged from 71.00% to 89.00%. Salinity changes in estuaries may be due to seasonal regimes of river discharge and precipitation. Eddy et al.,11 reported that in the Cross River estuary, changes in river discharge significantly increased salinity. High salinity value at a point in a river may be as a result of salt water intrusion near the point. The high value of salinity at Station 3 might be due to the fact that Station 3 is located close to an estuary where there is a continuous intrusion of salty water. Consequently, the low value of salinity recorded in other stations shows there is a reduced river discharge and salt water intrusion at those stations. Eddy and Etuk10 have suggested that changes in physicochemical properties of a water body may be due to tidal mixing of water. The salinity of the river was found to correlate positively with most of the measured physicochemical parameters.

The nitrate concentration ranged from 0.15 mg/L to 9.20 mg/L. All the values, except the 9.20 mg/L recorded for Station 4, were within the maximum permissible limit recommended by Nigeria’s Federal Environmental Protection Agency16. Nitrite concentration in the water samples ranged from 0.15 mg/L to 3.67 mg/L. The presence of nitrite in water bodies may be due to the reduction of nitrate. The observed mean concentration of ammonium ion, (NH4+) in the water was above FEPA16 maximum permissible limit of 0.5 mg/l. Introduction of nitrogen-containing waste into the river and run-off water from farm lands where nitrogenous fertilizers had been applied might be the reason for the high NH4+ level81. Another reason could be that when nitrogen containing waste is oxidized, nitrate and NH4+ are produced. The mean concentrations of the anions indicate that chloride ion is the dominating anion with the highest concentration recorded for Station 3. Mean concentrations of the cations reveal that potassium is the most abundant with all the cations (except Mg) predominating in Station 3. The levels of silicate reported in this study ranged from 0.25 mg/L (in Station 2) to 0.38 mg/L (in Station 3) with a mean of 0.30 mg/L. This range is within FEPA’s limit for unpolluted water16.

The pH of water samples from Cross River at Afikpo varied from 6.80 (slightly acidic) to 7.49 (slightly alkaline) with a mean value of 7.16. These values are similar to pH values reported previously on Cross River13 and Calabar River estuary10 but slightly below the range of 4.84 to 7.2 reported on Imo River by Ezeronye and Ugbogu15. The pH of water samples at Stations 2 and 4 was low and this may be attributed to the discharge of acidic water into these stations by agricultural and domestic activities730.

The water samples from the four sampling stations differed slightly in the concentrations of Mg ion determined. Station 4 had the highest concentration of Mg ion (17.78 mg/L) while the lowest concentration (10.40 mg/L) was recorded for Station 1. The levels of Mg ion obtained in this study are well below the limit prescribed by FEPA16 for domestic purposes. The presence of magnesium in water is an indication of hardness of water. High levels of magnesium in water will lead to change in taste of the water and could even cause gastro intestinal irritation and scale formation. There is a positive correlation (r = 0.45) between magnesium and sulphate levels which indicates the preponderance of Mg2+ in the form of MgSO4. Except for ammonium ion, magnesium correlated negatively with other physicochemical parameters.

The dissolved oxygen ranged from 5.10 mg/L to 8.00 mg/L. The dissolved oxygen levels obtained in this study are similar to values reported for Calabar River estuary10 and Imo River15 but significantly higher than the values reported for River Ethiope12.

The levels of heavy metals in sediment, fish and water are presented in Table 2. Iron was found to be the most abundant heavy metal in sediment (99.78 mg/kg), fish (11.45 mg/kg) and water (4.85 mg/L). The concentration of heavy metals in the Cross River ecosystem investigated varied according to: sediment > Fish > water. Similar observation was made by Eddy et al.,11 who ascribed the high load of heavy metals in sediment to the latter acting as a sink or reservoir for heavy metals. By acting as a sink for heavy metals and other pollutants, sediments constitute a possible source of contaminants in aquatic systems. Metals interact with organic matter in the aqueous phase and so settle down thereby resulting in a high concentration of metals in sediment7. The fine particles of silt in sediment provide large surface areas and intermolecular forces that adsorb and accumulate heavy metals. Thus sediment acts as a sink for the metals which may remain there for a long time. Precipitation of these metals may occur in the form of insoluble hydroxides, oxides and bicarbonates whenever pH increases to the alkaline region. Begun et al.,7 have noted that metal mobilization in the sediment environment is dependent on physicochemical changes in the water at the sediment-water interface.

Table 2. Mean Concentration of Heavy Metals in Water, Sediment and Fish Samples.

Aquatic organisms, including aquatic plants and bivalves, that are unable to regulate metal concentration in their tissues tend to suffer metal accumulation in polluted water bodies. The concentration of lead in sediment was found to be 3.01 mg/kg whereas the composite fish sample and water had 0.78 mg/kg and 0.44 mg/L, respectively. Similarly cadmium had 3.12 mg/kg, 2.14 mg/kg and 0.27 mg/L in sediment, fish and water, respectively. Cadmium and lead have been documented to be the most toxic heavy metals in aquatic ecosystems33. The permissible limits for the concentrations of lead and cadmium in portable water are 0.01mg/L and 0.05mg/L, respectively. Hence with respect to these two metals, water from the Cross River between Ndebe beach and Unwana beach in Afikpo North LGA is not suitable for cooking and drinking contrary to the practice by the rural communities within this area. Dara8 reported that the major source of lead in the aquatic environment is industrial effluent. However, there are no industries in the study area using or producing lead. High levels of lead causes inhibition of the synthesis of haemoglobin, adversely affects the kidney, central and peripheral nervous systems and neurological problems such as aggressive, hostile and destructive behavior20. Cadmium peumonitis has been shown to result from inhaling cadmium dusts and fumes. This disease is characterized by death of the lining of the lung tissues because of excessive accumulation of fluid. Furthermore, growth of plants is reduced by high levels of these metals18. The observed level of copper in the water was below the permissible limit of 1.0 mg/L16. Hence the river is not polluted with respect to copper. However, significantly higher level of copper was observed in sediment. The same trend was observed with nickel and zinc.