Assessment of Metal Concentration and Bioaccumulation Factor in Surface Water and Fishes Sampled From Four Creeks in the Bonny River Estuary, Niger Delta Region of Nigeria

The indiscriminate disposal of industrial and urban waste has added to the concerns of global water pollution. Anthropogenic activities and activities of oil companies (oil spills) have also contributed to the pollution of the aquatic ecosystem in the Niger Delta region of Nigeria. This study was therefore designed to evaluate the concentration of Iron, Nickel, Lead, Cadmium and Chromium in our surface waters and our marine life. Samples of surface water were collected into 50 cl sterile plastic bottles and labelled accordingly. Twenty medium-sized tilapia fishes from about a one-kilometre radius of each site were caught and also labelled accordingly. Water and fishes from each site were then kept in a cooler, iced and immediately transported to the laboratories for analysis for heavy metals after these fishes were homogenised using a blender. Atomic absorption spectrophotometer was used for the metal analysis. Standards were prepared using tracecert of each of the above-mentioned metals. The results of these study sites (Akpajo, Okujagu, Woji and Elelenwo) had high concentrations of metals. Fishes from all these sites also had high concentration of these metals. The trend of metal concentration in the surface water as follows: Okujagu–Pb>Cr> Cd>Fe = Ni, Akpajo–Cd>Fe>Pb>Cr>Ni, Elelenwo–Pb>Cr>Fe>Cd>Ni, Woji–Cr>Cd>Pb>Fe>Ni. Results obtained also revealed the trend of metal concentration in fish muscles as follows: Okujagu–Pb>Cr> Cd>Fe = Ni, Akpajo– Cd>Fe>Pb>Cr>Ni, Elelenwo–Pb>Cr>Fe>Cd>Ni and Woji–Cr>Cd>Pb>Fe>Ni. Fishes and surface water sampled from these four creeks in the Bonny estuary in the Niger Delta region of Nigeria had high concentrations of iron, nickel, cadmium, chromium and lead in relation to their concentration in surface water. The results of this study revealed that metal concentration in surface water had some relative effect on their concentration in the fishes. This, therefore, means consumption of these fishes may lead to the accumulation of these metals in man to a toxic concentration. This may be one of the reasons for the increase in some heavy metal-related health issues (diabetes, renal diseases) in this region.


Introduction
Water pollution is a global threat that has augmented various advanced and emerging countries, undermining economic growth, as well as the physical and environmental well-being of the world's population (Mateo-Sagasta et al., 2017).A major environmental concern due to the indiscriminate disposal of industrial and urban waste generated by anthropogenic activities is the contamination of soil and water (Khayatzadeh & Abbasi, 2010).Anthropogenic activities such as deforestation, industrialization, agricultural and domestic activities, etc., result in the contamination of aquatic ecosystems through the introduction of pollutants such as metals, crude oil, fertilizers, plastics, sewage, organic and inorganic particles (Mateo-Sagasta et al., 2017).
The main sources of water pollution are industrialization, agriculture, and human settlements (Dibofori-Orji et al., 2019;Dibofori-Orji & Marcus, 2012).In the Niger Delta region of Nigeria, petroleum hydrocarbon spill is a major contributor to aquatic ecosystem contamination (UNEP, 2011).Petroleum hydrocarbons from oil spills are harmful to all life forms and have caught the attention of many researchers due to their severe impact on marine life (Wolska et al., 2012;Zeneli et al., 2019).Most aquatic ecosystems have the habitual tendency to absorb pollution to some extent, but continuous contamination of the ecosystem results in changes in the flora and fauna of the community (Fan et al., 2017;Singh et al., 2017).In recent years, aquatic population, especially in urban and semi-urban cities in Nigeria have become an issue of great concern as many rivers in the country are used as waste disposal dumps for both solid wastes and untreated water, and as a result of this, the problem of getting good drinking water from surface water bodies is increasing (Isaiah et al., 2019).
Several studies have shown a relationship between petroleum hydrocarbon pollution and metal contamination in the environment (Abu-Elgheit et al., 1981;Akpoveta & Osakwe, 2014;El-Tokhi & Mostafa, 2001;Udoetok et al., 2011).Although trace metals exist naturally in the environment (Bruland et al., 2014), the concentration over the years has increased due to anthropogenic activities (Rajaram & Ganeshkumar, 2019).Galloway et al. (1982) predicted that based on the rate of emission, atmospheric concentration and trends in deposition, Ag, Cd, Cu, Pb, Sb, Se and Zn are expected to show great increases in the atmosphere.Studies of anthropogenic activities have found that Cd, Cu, Hg, Pb and Zn were being deposited in some areas at levels toxic to humans and other organisms.
Abandoned boats and other sea vessels along coastal regions have been identified as major environmental threats to aquatic ecosystems (Turner & Rees, 2016).These water vehicles leach hydrocarbons, plastics, metals etc. into aquatic ecosystems resulting in major environmental risk (Lord-Boring et al., 2004).According to Soroldoni et al. (2018) concentration of metals such as Cr, Cu, Mn, Sn and Zn can be elevated above background concentrations in the aquatic environment as a result of antifouling paint particles used on aquatic vessels.
Therefore, this study is designed to determine the concentrations of Fe, Ni, Pb, Cd and Cr in surface water and fish sampled from four creeks in the Bonny River estuary and to determine the bioaccumulation factor for each metal.

Sample Collection, Preparation and Analysis
With the aid of a boat and the support of fishermen, five surface water samples and twenty fishes were collected from a 1 km radius sample area at each creek.Adult tilapia fishes of approximately similar sizes were collected irrespective of sex; they were put into plastic sample bags and labelled.Surface water samples were collected from a depth of ≈50 cm using a sterile plastic bottle and labelled.Both fish and water samples were put into a cooler with ice and immediately transported to the laboratory for analysis.This study was carried out between May and August of 2021 (four months).
In the laboratory, fish muscles were collected using scalpel and forceps, weighed and homogenized using a blender.Homogenised fish and water samples were digested using nitric acid, and hydrochloric acid as described in the standard methodology ASTM: D4698-92 (ASTM, 2013) and ASTM D1971-16 (ASTM, 2016) for fishes and water samples respectively.
Samples were analyzed using Atomic Absorption Spectrophotometer (AAS) (instrument type-GBC SensAA Atomic).Standard reference materials, TraceCERT is a registered trademark of Sigma-Aldrich Co. LLC., were used to prepare standards for calibration for each metal.Lead (Pb), chromium (Cr), cadmium (Cd), nickel (Ni) and iron (Fe) concentrations were then analysed in surface water and sediment samples.Some samples were picked  randomly to be analysed in triplicated and blank samples were also added to ascertain the quality of the analytical method.

Data Analysis
Analysis of variance (ANOVA) was used to determine the spatial statistically significant difference in metal concentrations in both surface water and fish samples.Bioconcentration factor (BCF) was determined using the following equation:

BCF =
Metal concentration in the biota Metal cocentration in the surface water (1) BCF values were assessed and categorized according to the assessment criteria presented in Table I.
Mean concentrations of Ni across all sample locations in the present study were less than that measured in Woji Creek in 2019 (0.109 mg/l); similarly, Pb and Cr in Woji Creek in 2019 (3.252 mg/l and 1.590 mg/l, respectively) also exceeded those measured in the present study (Ihunwo et al., 2020).Pb and Cd in surface water sampled from Lake Manzala, Egypt, were less than those measured in the present study (Bahnasawy et al., 2009).Although Ni in surface water sampled from River Osse Benin City, Nigeria, was higher than that in the surface water sampled from the four creeks, Cr in the present study exceeded that measured from River Osse (Igbinedion & Oguzie, 2016).Heavy metal content was assessed in river water sampled from in Ife North Local Government Area of Osun State, Nigeria, Fe, Pb, and Cr exceeded those measured in the present study except for Cd (Oluyemi et al., 2010).
In 2019, Grey mullet (Mugil cephalus) from Woji Creek were sampled and assessed for the content of Cd, Cr and Pb; Cd and Cr in the fishes were below those in tilapia collected from Woji Creek in the present study (Ihunwo et al., 2020).Tilapia sampled from Lake Njuwa, Adamawa State, Nigeria, had lower concentrations of Pb, Cr and Cd when compared to the present study.However, Ni in tilapia fishes from the lake exceeded those in the present study (Ibrahim et al., 2018).The content of Pb and Cr in the present study exceeded those sampled from Egypt (Morshdy et al., 2021).Concentrations of Pb and Cd in the flesh of tilapia fish sampled from Lake Geriyo, Adamawa State, Nigeria, were much less than those in the present study fishes (Bawuro et al., 2018).
Heavy metals (such as manganese, platinum, mercury, chromium, copper, cadmium, iron, lead, zinc, gold, nickel, silver, cobalt, etc.) pollution in the aquatic ecosystem can be caused by various anthropogenic and natural processes (Bashir et al., 2020), such as weathering of the earth's crust, surface runoff, mining, industrial effluence, sewage discharge, etc. (Rashid et al., 2019).This category of metals is termed heavy metals, not because of their high density but because of their damaging effects on the environment and living organisms (Engwa et al., 2016).The progression of industrial and agricultural activities promotes a constant increase in heavy metal pollution (Bashir et al., 2020).Heavy metals are natural constituents of rocks and soils which enter into the environment as a result of weathering and erosion from rocks and soil into the water which has no serious lethal effects on human health (Rashid et al., 2019).When heavy metals are released into the water, they are bound to surrounding matter in the water, which settles down in the water beds or becomes part of sediment, which is later released into the water body under favourable conditions like pH change, temperature, etc. leading to further contamination of the water.These metals have high environmental importance because they are not removed from the water, therefore they accumulate Note: I-IV are assessment categories (Beek, 2002).
in large quantities in the water bodies, and thus entering the food chain (Loska & Wiechuła, 2003).Bioconcentration factor and assessment categories are presented in Table IV.Trend of bioaccumulation factor was as follows: Okujagu-Cr>Cd>Pb>Fe = Ni, Akpajo-Fe>Pb>Cd>Cr>Ni, Elelenwo-Fe>Cd>Cr>Ni, Woji-Fe>Ce>Cr>Pb>Ni.According to assessment categories, Fe was within category IV in fish samples from Akpajo, Elelenwo and Woji, indicating very high BCF.BCF category for Pb at Okujagu and Elelenwo was II indicating moderate BCF, however, it was III at Akpajo indicating high BCF.Moderate BCF was estimated for Cd at Okujagu and Akpajo, which high BCF was estimated at Elelenwo and Woji.Estimation of BCF for Cr indicated high BCF at Elelenwo and Okujagu, moderate BCF at Woji and low BCF at Akpajo.All metals are toxic at higher concentrations; however, some of these metals, such as cobalt, copper, iron, molybdenum, manganese, and zinc are essential nutrients that are required by the body for its normal functioning (Singh et al., 2017).Heavy metals can damage/alter the proper functioning of some metabolic functions in vital organs and glands in the human body such as the heart, kidneys, brain, liver, etc. from heavy metal accumulation, and they can also displace important essential nutrients from their original places, thereby limiting/disrupting their biological functions (Dar et al., 2019;Sparling, 2016).Also, heavy metals toxicity can either be chronic or acute as they can have several consequences on the body; it can damage the function of the central nervous system, leading to mental disorders, and can also damage many vital organs and blood constituents in the body (Jaishankar et al., 2014).Long-term exposure and accumulation of the metals in the body can progressively lead to severe muscular, physical and neurological degenerative processes which are similar to Parkinson's disease, and the continuous long-term exposure to some of these metals and their compounds may even lead to the development of cancer cells (Jaishankar et al., 2014).Studies have shown that the prevalence of renal disease in Nigeria is on the increase, and this was attributed to the main complications of diabetes and hypertension in adults and glomerulonephritis in children.(Odubanjo et al., 2011) The consumption of fishes with high concentration of heavy metals may possibly be an additional factor.This is in keeping with a study on the effect of heavy metals and kidneys (Lentini et al., 2017).This showed that increased plasma concentration of heavy metals can lead to both acute and chronic kidney disease.

Conclusion
In the present study, tilapia and surface water were sampled from four creeks in the upper reaches of Bonny estuary in the Niger Delta region of Nigeria.Samples collected were analysed for iron, nickel, cadmium, chromium and lead; concentrations were also used to estimate the bioaccumulation factor of metals in fish muscles.The results of this study revealed that the trend of metal concentration in the surface water as follows: Okujagu-Pb>Cr> Cd>Fe = Ni, Akpajo-Cd>Fe>Pb>Cr>Ni, Elelenwo-Pb>Cr>Fe>Cd>Ni, Woji-Cr>Cd>Pb>Fe>Ni.
Results obtained also revealed the end of metal concentration in fish muscles as follows: Okujagu-Pb>Cr> Cd>Fe = Ni, Akpajo-Cd>Fe>Pb>Cr>Ni, Elelenwo-Pb>Cr>Fe>Cd>Ni, Woji-Cr>Cd>Pb>Fe>Ni. Except for surface water sampled from Woji and Okujagu, Fe exceeded NAAO screen value for acute and chronic concentrations in surface water at Akpajo and Elelenwo.Ni at Elelenwo Exceeded chronic NAAO and USEPA maximum permissible limits, while Pb and Cd exceeded acute and chronic values for both NAAO and USEPA maximum permissible limits.Pb and Cd detected in the fish tissues far exceeded FAO maximum permissible limit in fish, 0.3 and 0.05, respectively.Results also showed that the fishes' propensity to bioaccumulate high concentrations of metals from the creeks, given that the BCF were mostly very high to moderate for all metals except Ni.Therefore, the activities identified along the creek and the periodic petroleum spill on the surface water that leads to the introduction of metals in the aquatic system cause metal accumulation of fishes.When consumed, these fishes can pose a threat to human health since their concentration exceeds the FAO maximum permissible limit.
Fig. 1.Map of the study area.

TABLE I :
(Beek, 2002)tion Factor Range, Assessment Category and Comment(Beek, 2002) Assessment of Metal Concentration and Bioaccumulation Factor in Surface Water

TABLE II :
Mean Concentration of Metals in Surface Water Compared to NAAO Screen Values and USEPA Maximum Permissible Limit

TABLE IV :
Bioconcentration Factor and Assessment Categories