Pollution and risk assessment of heavy metals in water, sediment and fish (Clarias gariepinus) in a fish farm cluster in Niger Delta region, Nigeria

The release of a wide range of toxic metals into the aquatic environment such as rivers, lakes and streams is one of the most baleful forms of pollution, and this has been a matter of great concern globally, especially in areas with high population growth and economic activity. Freshwater environments are rapidly polluted due to activities such as modernised agricultural systems, industrialisation, population growth, unplanned urbanisation and destruction of natural areas (Demirak et al. 2006). Ponds are stagnant water bodies derived from naturally flowing or from freshwater resources, and they exhibit high biodiversity and potential for ecosystem function. However, ponds face many threats from anthropogenic activities like agricultural runoffs, pollution (Omuku et al. 2012), over-abstraction of water for human use, land drainage, inappropriate or lack of management (Chakravorty et al. 2014). Fish farming practised in earthen ponds, which is a large source of fish for food, is becoming the most predominant activity around flowing rivers in the Niger Delta area. The Niger Delta region houses a lot of industrial operations in Nigeria, some of which are crude oil refineries, fertiliser, glass and cement industries, steel plants, power plants and food processing plants. Most of these industries discharge their wastes directly into surrounding streams and rivers which empty into River Niger located in the Niger Delta region. These waste waters are known sources of trace metals in the aquatic environment (Abugu et al. 2013; Kinuthia et al. 2020). Fish farming requires a constant supply of good quality water to provide a conducive environment for fish development and survival. This has led to the establishment of various fish ponds within the banks of freshwater, estuaries and coastal ecosystems in the Niger Delta region, Nigeria (Abugu et al. 2021); since the river system is a major source of water supply for fish farming activities. However, Singh et al. (2012) have reported that the cultivation of fish in unwholesome environmental conditions could be worrisome. Globally, issues pertaining to water quality are of great concern since they are a threat to fish.

In aquatic systems, the ambient environment (water and sediment) is usually the medium and receiving point of these metal pollutants (Gilbert & Avenant-Oldewage 2014). According to Brady et al. (2015), these metal pollutants interact with sediment, giving rise to sequestration. Thus, higher concentrations of these heavy metal pollutants are observed in sediment that serves as a reservoir for metals compared to the receiving water medium (Brady et al. 2015; Bai et al. 2018). Pollution assessment based only on water analyses is an inaccurate and insufficient method of identifying metal inputs in aquatic systems (Ismail et al. 2016). Metal concentration in sediment is also an indispensable tool in water quality evaluation (Abdel-Khalek et al. 2016). Pollution by heavy metals is one of the problems of ecological systems owing to their ability to accumulate and biomagnify along water, sediment and aquatic food chains, resulting in sub-lethal effects or death in local fish populations (McGeer et al. 2000; Xu et al. 2004). The amount of the toxic elements in fish is dependent on the concentrations of these elements in the food and fish habitats and the detoxification rate of metals (Urena et al. 2007).

Clarias gariepinus is one of the most preferred cultivable fish species in Nigeria and Africa, owing to its ease of breeding in captivity, toughness to a wide range of adverse water quality conditions and fast growth (Dahunsi et al. 2012; Ogamba et al. 2016). The C. gariepinus species is noted for its ability to adapt to harsh environmental conditions (Kumar et al. 2011). However, it is a bottom feeder and is exposed to heavy metal contamination, thus making them good specimens for heavy metal bioaccumulation studies (Ibhadon et al. 2014; Madu et al. 2017). Fishes are a major component of the human diet. Consumption of cultured fish may present a pernicious source of certain heavy metals, which may have a far-reaching health risk. Thus, this study was designed to determine the concentrations of cadmium, chromium, copper, manganese, lead and zinc in Anwai River water, sediment and fish from a fish farm cluster. The settlement has earthen ponds dispersed all around the banks of the river. The river water serves as a water supply source for the fish farm but also receives effluents from a cassava milling plant, an abattoir point and a car wash facility. Also, the pollution status of the water and sediment was investigated, and the non-carcinogenic health risks of the metals by consumers of farmed fish were estimated.

The study area is Camp 74 fish farm settlement, a fish farm cluster situated in a small community called Anwai in Asaba, Delta State in the Niger Delta region of Nigeria. The settlement lies between latitude 06° 14′N and longitude 06° 42′E (Figure 1). Camp 74 fish farm settlement derives its water supply from a course (River Anwai) flowing between latitude 06° 15′N and 6° 20′N and longitude 06° 23′E and 06° 06′E (Ezemonye et al. 2016). The settlement has earthen ponds dispersed all around the banks of the Anwai River. The river is used as a water supply source for the fish farm. However, the River Anwai receives effluents from anthropogenic sources such as a cassava milling plant, an abattoir point, a car wash facility, and also drains through the Anwai community and Asaba town (the capital city of Delta State), into the popular River Niger. The study area (Figure 1) is the capital of Delta state and is located within the northern flank of the Niger Delta Basin. The area is well drained by two rivers, the River Atakpo and the River Anwai, and the drainage pattern is basically dentritic (Chinyem 2013). Because the soil is porous, the surface water is recharged by runoff and is infiltrated. Subsurface water is also available from the underlying formations: The Pleistocene to Recent Alluvium, the Eocene Ogwashi – Asaba Formation and the Oligocene – Miocene Ameki formation (Chinyem 2013). Alluvium is presently the most exploited aquifer and all others are untapped (Akpoborie & Etobro 2006).

Five sampling sites (upstream, MC, Pond A, Pond B and Pond C) and a control site were demarcated with the aid of a GPS (global positioning system) device as presented in Figure 1. The upstream site and MC sites were established along the upper reaches of the River Anwai (the water supply source) after identifying the point sources of pollution in the study area. Pond A, Pond B and Pond C sites are fish farms in the farm cluster established along the banks of the lower reaches of the Anwai River. These sampling sites were chosen, especially Ponds A, B and C randomly on the basis of the proximities of the sampling points to the Anwai River, earthen ponds from the Anwai River and a longer period of culturing (7 months).

Site 1: Control site (for reference water collection) established before the fly-over, which experiences a lot of traffic prior to the upstream site. Co-ordinates: 06° 14′ 30.896″N and 006° 42′ 05.737″E.

Site 2: Upstream site (labelled UPS) is the first effluent discharge point along the flowing river (Anwai) which acts as a water supply source for the fish farm cluster. Co-ordinates: 06° 14′ 32.459″N and 006° 42′ 07.781″E.

Site 3: MC site, which lies in close proximity to the Upstream site, part of the upper reach of the River Anwai. This site also receives an inflow of effluents from the cassava processing mill, abattoir and rendering spot slab. Co-ordinates: 06° 14′ 33.357″N and 006° 42′ 11.83″E.

Site 4: Pond A (the first earthen pond in the fish farm cluster) is sited along the lower reach of the Anwai River. Co-ordinates: 06° 14′ 34. 842″N and 006° 42′ 14.857″E.

Site 5: Pond B (earthen pond in the fish farm cluster) is in close proximity with Pond A. Co-ordinates: 06° 14′ 37.617″N and 006° 42′ 15.21″E.

Site 6: Pond C (earthen pond in the fish farm cluster): and is the last point along the bank, downstream of the flowing river (Anwai), prior to the point where the river empties into the River Niger. Co-ordinates: 06° 14′ 38.672″N and 006° 42′ 17.805″E.

Water and sediment samples were collected from the five sampling sites and one control site for metal contamination and evaluation. Fish samples were also collected from earthen ponds (Pond A, Pond B and Pond C). A systematic sampling method was applied where the individual ponds were selected at regular intervals from the whole fish farm. This ensured appropriate representation of the whole farm that will reveal the exact contamination level of the whole farm.

Three water samples were collected at each of the sampling sites using acid prewashed 500 mL plastic bottles. Prior to water collection, the bottles were submerged 20 cm below the water surface. This was done to prevent surface water from interfacing with impurities and contamination from air. The collected water samples were acidified with 2 mL of concentrated nitric acid (69%) to minimise the adherence of heavy metals to the walls of the container and to avoid microbial growth. The water samples were stored in an icebox in order to maintain their freshness and later transported to the laboratory, stored at 4 °C, and analyzed within 4 days of collection. Three bottom sediment samples were collected from each of the corresponding water sampling sites (with the exception of the control site) using a stainless steel hand trowel and immediately placed into pre-cleaned polythene bags. Cultured C. gariepinus samples were collected with a drag net from three randomly selected earthen ponds within the farm cluster; chosen on the basis of a longer period of culturing (7 months). At each pond, three fish samples were obtained, varying from 0.78 to 1.20 kg and 42 to 44.50 cm in weight and length, respectively. An inclusion criterion was used for the sample selection as the ponds house only one type of fish (C. gariepinus). All specimens were immediately sealed in pre-cleaned polythene bags and placed inside an icebox pack and transported to the laboratory and stored at −20 °C until analysis. The fish samples were thawed and oven-dried at 105 °C for 24 h to constant weight. The dried fish samples were pulverised using a porcelain mortar and pestle and stored in clean, dried plastic bottles in a desiccator prior to digestion (Abah et al. 2016; Daniel & Matthew 2016). The sediment samples were also oven-dried at 105 °C to a constant weight, pulverised and sieved through 2 mm of stainless steel mesh.

Water samples were prepared following standard methods for examination of water and wastewater (APHA 2005). Each of the unfiltered water sample (100 mL) was digested with 5 mL HCl (37%) at 90 °C until the volume was reduced to 20 mL. The digest was cooled, filtered and made up to the mark in a 100 mL standard flask. Acid digestion of the sediment and fish samples followed standard methods (APHA 2005; Kisser 2005; Daniel & Matthew 2016). The fish and sediment samples were dried at 105 °C for 4 h and grounded to a fine powder using an agate mortar and pestle. 0.5 g of each ground fish and sediment sample were placed into a borosilicate beaker and 12 mL of aqua regia (3:1 HCl/HNO₃) was added. The beakers were covered with watch glasses and left for 16 h at room temperature. The samples were heated for 2 h on a hot plate at about 80 °C. After the first 15 h of heating, the watch glasses were removed and small amounts of 1% v/v HNO₃ were periodically added to avoid drying of the samples. The samples were allowed to cool and then filtered through Whatman 41 filter papers. The samples were made up to 50 mL. Deionised water was used in all the preparations.

Fish, sediment and water digest samples were analyzed for Cd, Cr, Cu, Mn, Pb and Zn with an Atomic Absorption Spectrophotometer (GBC Scientific Equipment SensAA-Pty Ltd, Australia) equipped with an air acetylene flame (APHA 2005). AAS standard heavy metal (Cd, Cr, Cu, Mn, Pb and Zn) solutions (1,000 mg/L) were employed to calibrate the instrument after appropriate dilutions to produce calibration curves from which the metal concentrations were read out. A Deuterium lamp was used to calibrate the wavelength for background purposes. Standard and blank solutions were prepared by using 1% (v/v) HNO₃. Three different concentrations were prepared by adding a suitable volume of stock standard solution (1,000 mg/L) to a 100.0 mL capacity graduated flask and it was made up to the mark with deionised water.

Acceptable recoveries for all the metals analyzed were in the range of 98.60–99.60% (sediment); 96.75–100.60% (water) and 97.35–101.15% (fish).

The data generated were computed as mean ± SD. The results from the different sampling sites were subjected to one-way analysis of variance (ANOVA) at the 95% confidence level (α = 0.05) to test the significance between mean metal concentration in water, sediment and fish samples. P ≤ 0.05 was considered significant, and where the means differed significantly, a post hoc test was carried out using Tukey's Honest Significant Difference. A Pearson correlation analysis was also carried out to assess the relationship between the fish metal content and those of their ambient environment (water and sediment). All statistics were done with SPSS software version 23 for Windows.

The BAF was calculated using the mean concentration values of each of the elements present in the fish sample and sediment. A bioaccumulation factor of 1 and above indicates that the metal is bioaccumulated or biomagnified (Ibhadon et al. 2014).

The concentrations of heavy metals in the whole body tissue of the studied fish were employed to assess the human health risk from fish consumption by computing the estimated daily intake of metals (EDI), the hazard quotients (HQ) and the Total hazard index (THI). A hazard quotient (HQ) is an estimated non-carcinogenic risk level of a particular pollutant through the consumption of fish (Abdel-Khalek et al. 2016).

Summations of HQ give the total potential of non-carcinogenic effects posed by more than one heavy metal (chronic hazard index), otherwise known as THI. HQ and THI exceeding unity shows there is concern about potential human health risk caused by exposure to non-carcinogenic elements.

The heavy metal concentrations in the river Anwai's water and sediment as well as the fish collected from the ponds which receive its water from the river revealed higher values in the upstream and MC water samples (Table 2). Cadmium, copper, zinc and manganese were below the detection limit of the AAS in the major canal. For the upstream samples, chromium, copper and manganese were below the detection limit of the AAS.

Table 3 shows the pollution assessment indices of water and sediment from the Camp74 fish farm cluster. The single-factor index assessment revealed that water from UPS and Pond A sites was contaminated with cadmium (I > 1). The results also showed that the samples in UPS, MC and ponds A, B and C were severely polluted.

A highly significant positive association was observed between whole fish body tissue and sediment (Table 4). It was also observed between water and sediment. Consequently, chromium, lead, zinc and manganese may have common origins in water and sediment. A negative correlation was observed between whole fish body tissue and water, between sediment and water, and showed the metals may exhibit similar behaviour and suggests less uptake of one metal in the presence of another (Table 5).

The calculated THI in sediment, fish and water revealed that the cumulative heavy metal exposure via consumption of farmed fish from the studied farm cluster was less than one (THI < 1) for the adult population of consumers, with values of 0.9380, 0.9064, 0.6923 for Pond A, Pond B and Pond C, respectively. With the exception of lead which at any concentration is toxic (USEPA 2020b), the HQ determined for the individual metals showed that no significant health risk (HQ < 1) would be experienced by a 70 kg adult consumer with regard to daily consumption of farmed C. gariepinus, either prepared fresh or grilled. However, the HQ of copper is much higher than that of the other metals.

The mean total metal concentrations in water are presented in Table 2. Cadmium in surface water was detected only in two sites (UPS and Pond A). The observed concentrations at these sites could be attributed to the influence of car wash effluents and channels that drain the city and abattoir waste near the sites. The concentrations of cadmium were comparable to 0.15–0.27 ppm reported by Akaahan et al. (2015) in the River Benue, Odum et al. (2021) in vegetables; and 0.006–0.106 ppm reported by Alinnor et al. (2016) in Oguta Lake. Total Chromium was detected in MC, Pond A, Pond C and Control sites since there was no metal speciation test that was conducted for the study. The mean concentrations obtained in all the sites were much higher than the value obtained at the control site (0.006 ppm). This could be as a result of cassava processing and cloth washing effluents and abattoir waste near these sampling sites. Comparable chromium concentrations have been reported by Akaahan et al. (2015) in River Benue and Alinnor et al. (2016) in Oguta Lake. The mean cadmium and chromium concentrations in the water samples exceeded 0.003 ppm cadmium and 0.05 ppm chromium set by the UNEP/WHO for protection of freshwater aquatic ecosystems and human health (UNEP 2008).

Copper was detected only in the control site and Pond B. The mean concentration (0.067 ppm) in Pond B was five times the value (0.013 ppm) reported in the control site. This could be attributed to copper-rich algaecide applied in an abandoned pond before Pond B and runoff from rich agricultural land near Pond B which may have been extensively treated with copper-based fertiliser/herbicide. Lead was detected in all the sampling sites except in Pond C. The appreciable lead levels may have resulted from wastewater effluent from the car wash facility and polluted urban runoff which empties into the water body. Lead has no known biological function, but is widely used in industrial products such as pipes, lead batteries, paints and also as petrol additives (Khidr et al. 2012; Jimoda et al. 2014; Sfakianakis et al. 2015; Ihedioha et al. 2021). In Nigeria, leaded petrol is still in use with about 0.6–0.74 g/L of tetraethyl lead remaining the only upgrading option given priority in Nigeria, due to negligence (Ugwu et al. 2011; Faruq et al. 2012; Galadima et al. 2012). The mean lead and copper concentrations in this study were lower than 0.01 ppm Pb and 2 ppm copper set by UNEP/WHO for protection of freshwater aquatic ecosystems and human health (UNEP 2008). The concentrations of copper and lead in water were comparable to the 0.093–0.280 ppm copper and 0.536–0.740 ppm lead reported by Alinnor et al. (2016) in Oguta lake. Even though the concentration of lead is lower than 0.01 ppm, recently, WHO has withdrawn the weekly tolerable intake of 25 μg/kg body weight since it has been agreed that there is no known safe threshold from which lead does not cause problems in humans (USEPA 2020a).

Zinc was detected in all the sites except in samples from the MC. The values recorded in Ponds A and C were higher than 0.06 ppm reported by Mohanty et al. (2009) for lowering significantly the growth and feed intake of fish. Manganese was only present in Pond A with mean a value of 0.163 ppm and could be attributed to its nearness to the abattoir waste and husk dumpsite. The mean concentrations of zinc and manganese were comparable to 0.030–0.054 ppm and 0.097–0.109 ppm reported for zinc and manganese in Nkisa River (Alinnor & Alagoa 2014). The mean zinc and manganese concentrations were below 3 ppm zinc and 0.5 ppm manganese set by UNEP/WHO for protection of aquatic life and human health (UNEP 2008). The heavy metal concentration in the water sample showed an un-patterned distribution similar to that observed by Chimela et al. (2016) on the Nun River. This may be attributed to factors such as variations in flow rates, reversal effects and high mixing rate (Seiyaboh et al. 2013).

The concentrations of heavy metals in sediment samples are also presented in Table 2. The sediments from the various sites had an appreciable cadmium concentration, with the lowest mean value at the UPS site (1.156 mg/kg) and the highest value at the Pond C site (3.329 mg/kg). These values were above the threshold effect level (TEL) (0.6 mg/kg) but within the possible effect level (PEL) (3.5 mg/kg) within which adverse effects will occasionally occur in aquatic life (CCME 2001). Rahman et al. (2012) noted that dietary intake of cadmium in an amount greater than 1 mg/kg can cause chronic toxicity in fish. The high cadmium content in sediments from the study area may have come from car wash sites at the banks of the Anwai River and drainage channels that passed through Asaba town which empties into the river. Chromium was detected in all the sites, with the lowest mean value recorded at Pond A (9.439 mg/kg) and the highest value at Pond C (14.373 mg/kg). The chromium concentration observed in sediments (14.373 ± 0.27) was below (37.3 mg/kg) the TELs according to CCME (2001). Copper concentration in sediments from MC, Pond A and Pond B sites were below the TELs ( < 35.7 mg/kg) (CCME 2001), while the values in the UPS site were within the possible effect range (>35.7 mg/kg but below the PELs (197 mg/kg)). Mean concentration of copper in the Pond C site was above the PELs (>197 mg/kg). Fishes in Pond C are likely to suffer frequent adverse effects owing to the concentration of copper in the sediment. C. gariepinus fish is a bottom feeder and may be exposed to copper. Shiau & Ning (2003) has reported that dietary intake of 20 mg/kg copper significantly reduces the weight gain of growing Tilapia fish. The high content of copper in sediments from the entire site may have resulted from its application in fish pond as algaecide used for the control of growth of phytoplanktons and filamentous algae and certain fish disease (Saria 2016). Manganese was present in sediments from all the sites, with the lowest mean value at the UPS site (3.143 mg/kg) and high values at the MC site (37.202 mg/kg). Manganese is an essential micro nutrient that plays an important role as a constituent and co-activator of several enzymes responsible for biological processes in fish and shows relatively low toxicity to aquatic biota (Maage et al. 2000). Sediment from the study recorded the lowest mean lead level (0.340 mg/kg) at Pond B and the highest mean lead value (1.537 mg/kg) at UPS site. Mean lead levels recorded in the sediment were below the 35.0 mg/kg TEL given by CCME (2001). The zinc concentration in sediments from Pond B (87.681 mg/kg) was below TELs (123 mg/kg) while values at UPS (274.4 mg/kg); MC (275.883 mg/kg) and Pond A (270.924 mg/kg) were within the possible effect range. Also, zinc levels in Pond C sediment exceeded the probable effect level (315 mg/kg), so there is a likelihood that fishes cultured in Pond C may suffer adverse zinc effects frequently (CCME 2001). The concentrations of cadmium, chromium, copper and zinc obtained in this study were higher than 0.068–0.58 mg/kg (Cadmium), 5.80–8.33 mg/kg (Chromium) and 6.65–12.65 mg/kg (Copper) reported by Nwajei et al. (2014) on sediments of Crayford Creek. However, Nwankwoala & Angaya (2017) reports of 4.11–7.82 mg/kg (Cadmium) and 6.04–16.5 mg/kg (Lead) in New Calabar River were appreciably high compared to the levels in sediments obtained in this study.

All sediment samples analysed had concentrations of chromium, copper, manganese and lead below their maximum allowed concentrations in soil/sediment set by the Department of Petroleum Resource (DPR 2002) with exception of Copper in Pond C (Table 2). In contrast, cadmium and zinc concentrations in sediment from the sampled sites exceeded the target value set by DPR with exception of Pond B which recorded Zinc concentration in sediment below the target value (DPR 2002). The higher value of copper in Pond C site may not be far from it being the last sampling site downstream, so it receives the cumulative of the discharged effluents into the aquatic system. Copper and zinc in the sediment from the study were appreciably high. This could be attributed to it being an essential component of fish feed, so pellets feed not consumed by fish settles on sediments and pollutes it. Copper enrichment of sediments has also been associated with crude oil exploitation and metal works activities in the communities near rivers (Alinnor & Alagoa 2014).

The mean concentrations of cadmium, chromium, manganese and zinc in sediment were significantly higher when compared with water (P < 0.05) among the sampling sites. This agrees with Mayerson et al. (1981) who reported that river sediments act as a reservoir which may either concentrate metals from the water or release them into the water. Similarly, the results from the study agree with reports from other studies (Alinnor & Alagoa 2014; Chimela et al. 2016). This is an indication that bottom sediment acts as a natural sink of heavy metals within an aquatic ecosystem, thereby leading to a reduction of bioavailable fraction in water (Gilbert & Avenant-Oldewage 2014). Similarly, Udosen et al. (2016) explains it to result from simple settlement from water column as well as preferential partitioning of the metals to the sediment, adsorption and complexation with organic matter in the sediment.

The mean concentrations of the heavy metals in African catfish (C. gariepinus) are also presented in Table 2. Cadmium concentration in fish had the highest mean value at Pond A (1.405 mg/kg). Fishes from Pond A and Pond B exceeded the permissible limit (1 mg/kg) established for cadmium by FAO (2017) for fish. Cadmium is a non-essential element which causes reduction in calcium availability, endocrine disruption and infertility (Sobha et al. 2007). Cd poisoning could lead to anaemia, renal damage, bone disorder and cancer of the lungs (Das et al. 2017). Mean chromium concentrations in C. gariepinus in this study ranged (0.593–1.692 mg/kg), with highest occurring at Pond C. These values were comparable to 0.64 mg/kg chromium reported by Alinnor & Alagoa (2014) in C. gariepinus from Nkisa River. The values obtained in this study were lower than 12–13 mg/kg stipulated by USFDA (2017) for fish intended for consumption. Chromium is an essential metal; its trivalent form aids the action of insulin as well as helps in the metabolism and storage of carbohydrate, fat and protein (Abbas & Ali 2007). However, exposure to its hexavalent form causes skin irritation and in extreme cases, cancer of the nose and kidney problems (Sun et al. 2015). At high concentration, chromium can alter the function of DNA and can cause genotoxicity (Sabbir et al. 2018).

Copper concentrations in C. gariepinus ranged 175.882–245.321 mg/kg. These values were higher than 7.97 mg/kg reported by Wangboje et al. (2017) in C. gariepinus in Orogodo River. The copper concentrations in fish from this study exceeded the permissible limit 30 mg/kg Cu of fish recommended by Esilaba et al. (2020) (Table 6). Copper, although vital in cell function and haemoglobin synthesis may give rise to nausea, acute stomach pains, diarrhea, fever, etc., when ingested in excess amount (Rahman et al. 2012, 41). Its presence in the aquatic environment may arise from the accumulation of domestic and agricultural wastes (El-Moselhy et al. 2014). The high level of copper in the ponds could be attributed to the anthropogenic activities (abattoir waste effluents, produce feeds and algaecides applied in the ponds and runoff from agricultural lands) and possibly the interaction of the water with the local soil and rock debris within (Chibuike & Obiora 2014). Osakwe 2018, in the assessment of trace metals distribution in different geochemical phases of soils around automobile junk markets in the southern Nigeria revealed that nickel, chromium and iron were of lithogenic origin.

Manganese concentrations in C. gariepinus were detected in Pond A and Pond C with mean levels 3.326 and 1.946 mg/kg, respectively. These values exceeded the WHO permissible limit (1 mg/kg) for fish intended for consumption (Esilaba et al. 2020). The mean concentration of lead in C. gariepinus ranged 0.217–0.263 mg/kg. These values were higher than 0.19 mg/kg and 0.15 mg/kg reported in wild and farm raised C. gariepinus in Kaduna (Ibhadon et al. 2014). However, it has been agreed that there is no known safe threshold from which Lead does not cause problems in human and so, the weekly tolerable intake of 25 μg/kg/body weight has been withdrawn by the World Health Organization (USEPA 2020a).

The mean concentrations of zinc in C. gariepinus in this study ranges from 248.191 to 292.333 mg/kg. The mean concentration of zinc in the fish from all the ponds was more than twice the fish permissible limit (100 mg/kg) recommended by the WHO for consumption (Esilaba et al. 2020). Zinc is essential in biological function of several proteins and enzymes, as well as immune system, neuro-transmission and cell functions (Sfakianakis et al. 2015). Zinc, although an essential element in human metabolism and nutrition may have side effects when the recommended limit is exceeded. The appreciable values of some metals in fish could indicate bioaccumulation. The use of different fish produce feeds of varying compositions in the various ponds as well as its administration in varying ration may have contributed to heavy metal build up in fishes studied. Das et al. (2017) has reported appreciable levels of heavy metals in feeds. The high content of zinc, copper, manganese and cadmium in the fishes studied may also be due to the discharge of cassava waste effluent into the study area (Olaoye et al. 2018, 2020). Adewoye (2013) has reported bioaccumulation of metals in the order zinc > manganese > copper > cadmium > lead in C. gariepinus exposed to cassava effluent. Comparing metal concentration in fish with its ambient environment (water and sediment) shows variation followed the trend; sediment > fish >water. This recorded trend agrees with Yi et al. (2017), that concentration of heavy metals are lowest in water compared to that in sediment and fish. In contrast, copper and zinc concentrations observed in fish were greater when compared with sediment. copper and zinc also exhibited close and discernible elevation in the studied fish, and this may be due to their nutritive functions and which may have been ingested from fish feeds.

Bioaccumulation factor is important in predicting the relative contributions of abiotic media as a source of heavy metals accumulation in fish and the accumulation efficiency for any particular pollutant in fish whole body. According to Eneji et al. (2011), the rate of bioaccumulation of heavy metals in fish depends on the concentration of such metal in water body or surrounding sediment, its feeding habits as well as the ability of fish to digest the metal. Sediment serves as a natural sink of heavy metals in aquatic ecosystem, thereby leading to reduction of bioavailable fraction in water (Gilbert & Avenant-Oldewage 2014), and in due course gets transferred to fish (Eneji et al. 2011). C. gariepinus is benthic specie and an omnivorous fish. It is a bottom feeder and burrows into muds or sediments and spends a longer time at the bottom sediment. Madu et al. (2017) noted that C. gariepinus feed on plant and animal materials of both aquatic and terrestrial origin depending on availability, as influenced by season or water hydrology. They also tend to bioconcentrate heavy metals from the bottom sediments, because of their habitat and feeding preferences. Bioaccumulation factor of heavy metals in C. gariepinus were evaluated with respect to sediment as the medium (Figure 2). The order of BAFs were copper > cadmium > zinc > lead > manganese > chromium for Pond A; copper > zinc > lead > cadmium > chromium for Pond B; copper > zinc > lead > cadmium > chromium > manganese for Pond C. Copper has the highest bioaccumulation factor in C. gariepinus with values 7.37, 9.18, 1.44 for Pond A, Pond B and Pond C, respectively. These values were greater than one showing tendency of copper to accumulate in C. gariepinus. Similarly, zinc showed tendency to accumulate in C. gariepinus obtained from Pond B. The high BAF of copper and zinc in fish whole body tissue in relation to sediment agrees with Orata & Birgen (2016) study which reported high BAF for Cu and Zn in C. gariepinus and Protopterusaethiopicus fish tissues compared to other metals. This could be attributed to its essential role in fish species. Copper and zinc exhibit similar atomic structure and could therefore compete for the same active site (Alinnor & Alagoa 2014). These essential elements are supplemented through fish feed and they may bio-accumulate in fish.

MC, Pond A and Pond C were contaminated with chromium (I > 1). UPS, MC, Pond A and Pond B water were contaminated with lead whereas Nemerrow's index evaluations showed that water samples from UPS, MC, Pond A and Pond B sites were severely polluted (Table 3). The CF, degree of contamination (C_d) and (PLI) or sediment at different sampling sites shows that sediments from UPS and Pond A were moderately contaminated with cadmium while those from MC, Pond B and Pond C were considerably contaminated with cadmium. Sediments from all sampling sites showed low contamination with chromium, manganese and lead. MC, Pond A and Pond B sediments showed low copper contamination while UPS and Pond C sediments showed moderate, and considerate copper contamination, respectively. Pond B sediments showed low zinc contamination while UPS, MC, Pond A and Pond C sediments were moderately contaminated with zinc. Camp 74 fish farm settlement presented a low degree (Upstream, Pond A and Pond B) to moderate degree (MC and Pond C) of contamination. The PLI values for different sampling site investigated were below 1. This suggests that the sediment of the entire studied site in Camp 74 fish farm settlement was not polluted.

The possible inter-elemental relationship that exist between mean heavy metals concentrations in whole fish (C. gariepinus) tissue and its ambient environment (water or sediment), as well as metal pair relationship between water and sediment collected from the fish farm cluster were represented by Pearson's correlation coefficient (r) in Tables 4 and 5. Correlation between metals in different environmental compartments is influenced by physical, biological and chemical activities within the aquatic system (Abdel-Khalek et al. 2016). The distribution of metal loads as well as its behaviour in the aquatic environment is influenced by the anthropogenic input (Abdel-Khalek et al. 2016). Moderate, strong and very strong and significant positive inter-elemental associations denotes that their sources of origin are similar and may be due to anthropogenic input such as industrial effluents, municipal wastes and agricultural inputs (Bhuyan et al. 2017). However, a strong and very strong negative correlation of metal pairs between two media suggests lesser uptake of one metal in the presence of another and vice versa (Etim & Adie 2012). A high significant positive associations was recorded for manganese/cadmium (r = 0.671), manganese/chromium (r = 0.824), manganese/manganese (r = 0.670), manganese/zinc (r = 0.837) between whole fish body tissue/water, chromium/copper (r = 0.670) between whole fish body tissue/sediment and chromium/lead (r = 0.669), chromium/zinc (r = 0.680), zinc/manganese (r = 0.724), zinc/lead (r = 0.776) and zinc/zinc (r = 0.753) between water/sediment suggesting manganese, cadmium, zinc, chromium and copper were uniformly distributed and may have similar biogeochemical pathways for subsequent accumulation by fish (C. gariepinus) in the fish farm cluster. Consequently, chromium, lead, zinc, manganese may have common origin of contamination for water and sediment. The negative correlation recorded for manganese/Copper (r = −0.751) between whole fish body tissue/water; cadmium/cadmium (r = −0.873), cadmium/manganese (r = −0.873), cadmium/lead (r = −0.945), chromium/lead (r = −0.922), copper/lead (r = −0.780), manganese/copper (r = −0.912), lead/copper (r = −0.934) and zinc/copper (r = −0.932) between sediment and water shows the metals may exhibit similar behaviours and suggests lesser uptake of one metal in the presence of another.

Fish consumption is said to be one of the major routes of heavy metal exposure by humans and is of immense concern (Madu et al. 2017). The metal concentration in the fresh weight of C. gariepinus and the estimated daily intake, hazard quotient and THI through C. gariepinus consumption are presented in Table 7. With the daily fish consumption rate of 0.0364 kg/day fresh weight for an adult Nigerian (Akoto et al. 2014), the EDI of fresh C. gariepinus showed Cu had the highest EDI ranging between 0.0197 mg/kg bw/day in Pond C to 0.0274 mg/kg bw/day in Pond B. Lead had the lowest EDI ranging from 0.0000243 mg/kg bw/day in Pond B to 0.0000294 mg/kg bw/day in Pond C. But it should be noted that Pb at any concentration is assumed toxic since there is no known safe threshold for lead in humans (USEPA 2020a). EDI values from this study were similar to the EDI reported by Akoto et al. (2014) through consumption of fish from the Fosu Lagoon. The findings of this study did not conform to that of Madu et al. (2017), who reported higher EDI values from manganese and lead exposure via consumption of C. gariepinus from the lower Niger River, Nigeria. With the exception of lead which at any concentration is toxic (USEPA 2020b), the HQ determined for the individual metals showed that no significant health risk (HQ < 1) would be experienced by a 70 kg adult consumer with regard to daily consumption of farmed C. gariepinus, either prepared fresh or grilled. However, the HQ of copper is much higher than that of the other metals. The HQ for most of the metals in this study is comparable to those reported by Madu et al. (2017) for C. gariepinus from the lower Niger River, Nigeria. The THI estimated showed that cumulative heavy metal exposure via consumption of farmed fish from the studied farm cluster was less than one (THI < 1) for the adult population of consumers, with values of 0.9380, 0.9064 and 0.6923 for Pond A, Pond B and Pond C, respectively.

Pollution studies of the water supply source (Anwai River) and fish farm cluster (Camp 74 Fish Farm Settlement) showed that the fish farm cluster had the highest concentrations of metal contaminants in water and sediment. Pond A, compared to the other ponds, recorded the highest concentrations of the metal pollutants. Water from Pond A and Pond B was severely polluted by lead, cadmium and chromium and was above the permissible limit by the UNEP/WHO. Low to considerable contamination was observed in the sediment with respect to zinc, copper and cadmium. These zinc, copper and cadmium concentrations were above the sediment quality guidelines set by the DPR. Cadmium, copper, manganese and zinc concentrations were above the standard limit intended for consumption as given by the FAO/WHO. The high value of most of the studied metals in sediment compared to fish and water confirms that sediment is the major uptake route and ultimate sink of heavy metal pollution that plays a crucial role in heavy metal uptake by fish. The variation of the studied metals among the samples mainly followed the trend: Sediment > Fish > Water. The health risk assessment showed that consumption of farmed catfish from the ponds is safe for a normal consumer (36.4 g/day), but not for children, pregnant women, and people with compromised immune systems. Nemerrow's index evaluations showed that water samples from UPS, MC, Pond A and Pond B sites were severely polluted, but PLI values for different sampling sites investigated were below 1, which suggests that the sediment of the entire studied sites in Camp 74 fish farm settlement was not polluted. Therefore, it is advisable that only moderate fish consumers should eat fish from the farm, whereas effort should be made by the government and policy makers to enforce the treatment of waste effluents from the potential pollution sources before discharge into the River Anwai.

All relevant data are included in the paper or its Supplementary Information.

There are no conflicts of interest.