Water Quality and Heavy Metals Contamination of Artificial Lakes in Heipang and Rayfield, Plateau State, Nigeria

Water is very important to life, making up about 70% of adult human body, with some lower and aquatic animals containing as high as 95 % to 98 % [1]. It is widely distributed throughout the ecosystem and its quality is an issue of great concern to scientists and policy makers. Lake water is part of the 3 % of global fresh water supply. Less than 1 % of the earth’s water resources are accessible to humans as fresh water, in the form of either surface or ground water [2]. Water quality is often an indicator of environmental quality because of the importance of water in sustaining human health and ecosystem [3].

On the Plateau in the Middle Belt of Nigeria, artificial lakes are a major source of municipal water supply and irrigation farming. Most of the lakes were inadvertently created as a result of mining activities during the colonial era. On the fields adjoining the lakes farming activities take place every year, which means leaching of agrochemicals into the lakes is expected. Illegal mining activities still take place around some of these locations leading to heavy metals contamination [4, 5], yet not much literature exist of the heavy metals status of these lakes [6]. Lakes are susceptible to anthropogenic contamination without manifesting obvious signs [7], except in situation of eutrophication. Unlike rivers they tend to accumulate more sediments as records of past changes in climate and environment [8]. Heavy metals are among the most toxic and persistent contaminants that accumulate in aquatic habitats and their concentration increases through biomagnification [9-10]. Industrial and agricultural activities located at the catchment area are the main sources of lake water pollution, in addition to natural sources [11]. Human activities have increased the level of metal ions in natural water system [12]. Sources such as mine drainage, industrial (paints, leather, textile, fertilizer, pharmaceuticals), domestic wastes and agricultural runoff have all contributed to increase metal load in natural water bodies, being ultimately incorporated into aquatic sediments [13]. In the aquatic environment the heavy metals bioaccumulate in aquatic organisms, migrate through the food chain to humans. They are found even in the biological systems of most people. Factors that determine their toxicity include chemical species, dose, and route of exposure; also, the nutritional status, genetics, age, and gender of exposed individuals [14]. Heavy metals with most carcinogenic effects are Cr, Ni, Pb, Cd, and As. Cr (VI), Ni and Cd; As and Cd are also teratogenic, while Pb has been reported to have neurological impairment and malfunctioning of the central nervous system [15].

Heavy metals occur naturally throughout the earth’s crust, however most environmental contamination and human exposure occur as a result of anthropogenic activities involving metal ores mining and processing operations, industrial production and use, and domestic and agricultural use of metals and metal-containing compounds [16]. Metal-based activities, such as, mining, foundries and smelters are most responsible for point source pollutions [17]. Other sources of heavy metal contamination include metal corrosion, atmospheric deposition, soil erosion of metal ions and leaching of heavy metals, sediment re-suspension and metal evaporation from water resources to soil and ground water [18]. As society continues to advance technologically, heavy metal contamination is likely to increase correspondingly.

3.1 Sample collection

Samples were collected from each of the two lakes at Sampling Stations as shown on Figure 2. Heipang Lake was designated as sample station 1 coded SS1 while Rayfield Lake was designated as sample station 2 coded SS2. At each lake five samples were collected from the four corners of the lake – north, south, east, west and the middle, 10 – 20 cm below the water surface and pooled for a composite sample. Samples were collected with 500 mL glass bottles, acidified with conc. HNO₃ and transferred into glass bottles with lids. The samples were then stored in icebox and transported to the laboratory and stored in freezer until analysis within 2 days.

Two matured fish samples of Tilapia zilli (average weight 83.34 ± 0.2 g and average length 16 ± 2 cm) were caught from the lake at Heipang, Barkin Ladi Local Government Area using a hook and line. They were transported home alive and stored in a freezer to maintain their freshness. The gills, intestines and muscles were dissected and the samples were oven-dried to a constant weight at 105 ℃. Separate samples (gills, intestine and muscles) were milled with mortar and pestle to fine sizes, then they were stored in labelled plastic containers in a desiccator. Two mature fish samples of Clarias gariepinus (average weight and average length) were equally caught from the Rayfield Resort Lake and prepared as described above. About 5.0 g of each of the samples were kept aside for an alternative method of analysis, Energy Dispersive X-RAY Fluorescence Spectrometry (ED XRFS).

3.2 Sample analysis

About 100 mL of acidified water sample was measured in a 250 mL conical flask. Exactly 5 mL of conc. HNO₃ was added to the conical flask and covered with watch glass. The flask was heated on hot plate at 60 °C for 20 minutes until the solution appeared light colour, and then cooled to 45 °C.  Flask and watch glass were rinsed with distilled water and filtered into a clean conical flask. And the filter was rinsed with 3 mL distilled water. Filtrate was transferred to a 100 mL volumetric flask, and the conical flask was rinsed with 5 mL distilled water into the volumetric flask. The sample was made up to 100 mL mark with distilled water, agitated to mix thoroughly and stored for analysis.

About 5 g of the milled sample was weighed into a round bottom flask and 10 mL conc. HNO₃ and 2 mL HClO₄ added. The sample was digested at 65 – 70 °C. The residue was filtered, dissolved and diluted with 0.2% (v/v) HNO₃ to 20 mL. Digest were stored in pre-cleaned polyethylene bottles until AAS analysis.

3.2.1 Atomic absorption spectrometer

The heavy metal concentrations were determined using FS240AA Agilent Atomic Absorption Spectrometer. The standard was run for the working curves, followed by the blank to overcome instrument drift.  After thoroughly mixing the sample by shaking, 100 mL of the sample was transferred into 250 ml beaker. The sample was aspirated into the machine equipped with mono-elemental hollow cathode lamps. The metal concentrations were read in ppm from the extracted standard calibration curve. The analyses were conducted in triplicate for each element and the average values calculated.

3.2.2 Energy disperse x-ray fluorescence spectrometer

After initial size reduction of samples as described above, the samples were further ground to a fine powder that passed through 250 mesh sieve. The samples were then dried in oven at 105 °C for 1 hour and cooled in a desiccator. The samples were then mixed with binder in the ratio of 5:1 (5.0 g sample to 1.0 g cellulose, w/w) and pelletized at a pressure of 10 – 15 tons/inch² in a pelletizing machine. The ED XRFS machine was switched on and allowed to warm for 2 hours. Appropriate programmes for the various elements were employed to analyse the sample material.

Cr and Cd were observed to be within acceptable limits set by WHO and Nigerian Standard for Drinking Water Quality, while Pb, As and Hg were much higher than the recommended limits. The levels of Pb in SS1 (3.52 mg/kg) and SS2 (5.63 mg/kg) indicate active contamination of the lakes, which might be due to illegal mining activities within the areas. Two methods were used to determine the levels of bioaccumulation of heavy metal in the fish sample – Atomic Absorption Spectrometry (AAS) and Energy Dispersive X-Ray Fluorescence Spectrometry (ED XRFS). The AAS proved to be a better method than ED XRFS in the determination of heavy metals in the tissues of fish because it detected trace amounts in samples that ED XRFS could not. In both SS1 and SS2 the concentrations of Cd, As, Cr and Hg in the fish were higher than the concentrations in the water medium. Concentrations of Pb in the two lakes were higher in the water than in the fish. However, Pb concentration in the fish was still higher than the other elements, which means that Pb is the highest contaminant in the two lakes. Based on the procedures described in 3.2.1 and 3.2.2, AAS technique was a better method of determining heavy metals in the fish samples than XRFS. Thus, this study concludes that there was heavy metal contamination of the two lakes, Tilapia zilli and Clarias gariepinus are good bio-indicators for monitoring heavy metals contamination at Heipang and Rayfield lakes respectively, and AAS is a better method than ED XRFS in determining heavy metals in fish tissues.