Heavy Metal Concentration and Pollution Status of Cross River in Afikpo Catchment Area, Ebonyi State, Nigeria

Heavy metals are notable chemicals that threaten river ecosystems and freshwater supplies from surface waters. The levels of some heavy metals-manganese (Mn), iron (Fe), zinc (Zn), lead (Pb), copper (Cu), chromium (Cr) and arsenic (As)-in Cross River were monitored over twelve months in 2018 to establish the pollution status and potability of the river. Water and sediment samples were collected from three stations of the river and analysed according to standard procedures. The result revealed that the respective mean concentrations of the metals in the water and sediment were 0.26 mg/l and 3.28 mg/kg (Zn), 2.22 mg/l and 43.73 mg/kg (Fe), 0.22 mg/l and 3.37 mg/kg (Mn), 0.42 mg/l 0.39 mg/kg (Pb), 0.036 mg/l and 0.096 mg/kg (Cu), 0.53 mg/l and 0.92 mg/kg (As), and 0.043 and 0.23 mg/kg (Cr). The decreasing order of heavy metal concentration in the water was Fe > As > Pb > Zn > Mn > Cr > Cu, while that of the sediment was Fe > Mn > Zn > Pb > As > Cr > Cu. The bottom sediment was observed to contain higher concentrations of heavy metals than the surface water, signifying the accumulation of these chemicals in the bottom sediment. Most of the heavy metals (Fe, Mn, Pb, As and Cr) were detected at higher levels above the permissible limits of the Standard Organization of Nigeria, World Health Organization, and United State Environmental Protection Agency, hence, the water was polluted with these metals indicating that there may be some ecotoxicological risk to pelagic and benthic organisms living in the river.


Introduction
River and other freshwater resources are essential for human survival; however, due to the increase in human population, and the corresponding increase in water consumption and urbanization, water quality is facing serious challenges [19]. Water pollution has become one of the main concerns of the world today. Human development, urban life, industrialization, and agricultural production have contributed to the degradation and pollution of surface waters and adversely affecting aquatic ecosystems and ultimately human health [42]. Many pollutants, including heavy metals, threaten river ecosystems and freshwater supplies from surface waters. Some of these pollutants enter Nigerian rivers through stormwater runoff, agricultural runoff, discharge of raw sewage into surface waters, and chemical wastes dumped by industries and governments. Stormwater from a watershed and municipal wastewater are notable sources of water to surface waters that are mostly polluted with hazardous metals and organic chemicals [15]. Insecticides and pesticides that we use on our farms are dissolved in water, and some amounts of these dissolved substances are washed into water bodies and pollute them [34]. From a public health or ecological view, a water pollutant is any biological, chemical or physical substance that in identifiable excess is known to be harmful to man and other desirable living organisms. Water is said to be polluted when it is negatively affected by natural or anthropogenic contaminants and either does not support human use, like serving as drinking water and/or undergoes a marked shift in its ability to support its constituent biotic communities, such as fish [3,36].
Heavy metals are non-biodegradable and toxic chemicals in the environment [6], and they are considered one of the major contaminants in aquatic habitats. Though heavy metals occur naturally in aquatic environments in concentrations that are not threatening to aquatic ecosystems, most of them are released into rivers in quantities that are hazardous to the environment and impact humans adversely [30]. Anthropogenic activities due to population growth and industrialization have been implicated as considerable sources of heavy metal pollution in rivers globally [6]. Waste from industrial effluents, combustion of coal and lignite, and industrial waste disposal appear to be some of the major contributors of heavy metals in aquatic environments, and heavy metal contamination alters soil sediment and water quality [22]. The negative impact of heavy metal contamination results from their toxicity to biological processes and their bioaccumulative characteristics. Different scholars have examined the degree of heavy metal contamination of Nigerian Rivers: Aba River [22,26], Otamiri River [25], Calabar River [10], Kaduna River [18], and Ogun River [43]. Aba River and Ogun River were reported to have been contaminated with heavy metal pollutants from domestic and industrial wastes deposited in the rivers and their surrounding environments [26,43]. Keke et al. [18] reported very high pollution of Kaduna River with Fe and Mn, while Yahaya et al. [41] noted that Ni, As, Pb, Cr and Cd were detected in Ogun river above the WHO permissible limits. Manganese, iron, nickel, zinc, lead, copper, chromium and arsenic were some of the heavy metals detected at disturbing levels in Nigerian rivers and wastes deposited in them [10,25].
There is an increasing concern regarding the roles and fates of heavy metals in aquatic environments. Much of these concerns arise from the low level of available information on the concentration of these metals within the environment. Given the impact of heavy metals in aquatic systems, especially in the Cross-river system, some researchers have reported the heavy metal concentration status of Cross River but most of their works did not focus on Afikpo Basin [2,5,29]. Ayotunde et al. [5] reported that the levels of Cu, Fe, Co, Pb, Cd and Cr in the water and sediment of the river along the Cross River State axis were below the maximum allowable levels set by the WHO, FEPA, and USEPA. Akpan and Thomspon [2] assessed the heavy metal content of the Cross River sediment in Cross River State, while Odoemelam et al. [29] documented the heavy metal levels in surface water and sediment of Lower Cross River System in Akwa Ibom State, Nigeria. However, in 2013, Odoemelam et al. reported the heavy metal status of the surface water, sediment and fish of the river in the Afikpo area. Since a waterbody can be polluted by chemicals from point and nonpoint sources and to varying degrees at different times, there is a possibility that the Cross River system in the Afikpo area can be contaminated by pollutants from different sources not captured by previous studies. Hence, there is a need for localized and periodic monitoring of the cross river along the Afikpo axis to establish its pollution status and aquatic productivity. This study was designed to provide scientific and hydrological information from twelve-month monitoring studies on the heavy metal pollution status of Cross River at Afikpo basin.

Study Area
The study area is Cross River at Afikpo catchment area (Ndibe), Afikpo North L.G.A., Ebonyi State, Nigeria. The catchment area is within the geographical coordinates of latitude 5°50' north and longitude 7°56' east and with an elevation of about 38 m. The River in the Afikpo basin has its source from a hill in the Northern Cross River State [28]. Palm trees, bamboo trees and grasses are the predominant vegetation around the river. Also seen around the study area are minor recreation centres, and farmlands ranging from vegetables to food crops where fertilizers are being used for optimum crop yield. Cross River in the Afikpo catchment area, popularly known as Ndibe Beach by the riverine dwellers, is an important freshwater system in Afikpo with rich fisheries production. The water is used for domestic purposes (such as drinking, bathing, and cooking), agricultural purposes (such as irrigation), commercial fishing, transportation, recreation, and as a sand dredging site. Ndibe is a popular location for tourism in Ebonyi State (Fig. 1).

Study Stations
The study site was divided into three sampling stations: stations 1, 2 and 3. Station 1 which is located upstream (05°50.571' north and 07°56.989' east), is about 7 m close to the River bank where there are riverine dwellers and thatch houses of artisan fishermen and boatmen. Station 2 is located downstream (05°50.487' north and 07°56.771' east), 7 m away from the shoreline. It is the area for the landing of boats used in the transportation of goods. Station 3 (05°50.509' north and 07°56.862' east) is the open area of the river. The sampling stations were selected after preliminary surveys based on such factors as sediment composition, accessibility, anthropogenic influence and the various activities going on around and within the stations.

Water and Sediment Sampling
Water and bottom sediment samples were collected from the stations monthly for 12 months (January-December 2018) to cover both dry and rainy seasons. Each sampling station was subdivided into three sub-stations such that water and sediment samples were randomly collected in triplicates from each station. Water samples were collected at depths of about 30 cm with 0.5 L sterilized plastic containers. To reduce the adherence of heavy metals to the walls of the containers and also stop possible microbial activities in the sampled waters, the water samples were fixed immediately after collection with 2.5 ml of a 15 M concentrated nitric acid [31]. Sediment samples were collected using Eckman grab sampler (for surface sediment extraction) and then transferred into labelled plastic bags. Each month, sampling, preservation and transportation of the samples to the laboratory were done according to the standard method specified by APHA [1].

Sample Preparation and Analysis
The concentration of the heavy metals (Fe, Zn, Mn, Pb, Cu, As and Cr) in water were estimated using Atomic Absorption Spectrophotometer (AAS; Unicam 919, Analytical Technology Inc., Cambridge, USA). The sediment samples were oven dried to constant weight at 105 °C, ground to fine powder, sieved with a 2 mm mesh sieve, and then digested according to the procedures described by Wangboje and Oronsaye [37]. One gram (1 g) of the dried samples were then transferred into an acid-wash 250 ml extraction bottle. Thereafter, 9 ml of concentrated hydrochloric acid (HCl), 3 ml of concentrated nitric acid (HNO 3) and 2 ml of perchloric acid (HClO 4 ) were added. The mixture was digested for 5-6 h on a mechanical shaker hotplate. After the digestion, 20 ml of distilled water was added and the solution was filtered using a Whatman No. 42 filter paper and finally made up to 100 ml. The filtrate/extract was then analysed for the heavy metals using the AAS instrument. Each estimation was done monthly in triplicate, and the mean values were recorded.

Quality Control Analysis
To ensure high analytical data quality during the analysis, laboratory quality assurance and quality control methods were implemented, including calibration of the AAS instrument (Unicam 919, Analytical Technology Inc., Cambridge, USA). AAS standard heavy metal (Fe, Zn, Mn, Pb, Cu, As and Cr) solutions (1000 mg/l) were used to calibrate the instrument after appropriate dilutions to produce calibration curves from which the metal concentrations were read out [9]. Standard and blank solutions were prepared by using 1% (v/v) HNO 3 . Five standard solutions which ranged from 0.1 to 1.0 mg/l were prepared from the stock solutions. A reagent blank was run at intervals of every five samples analysed to minimize equipment drift. The water samples as well as the sediment extracts and blanks were then analysed using the AAS to obtain the absorbance values, and the concentrations of each heavy metal were then automatically estimated from the equation of the calibration curve of the AAS. The detection limits of the heavy metals were 0.0001 for Mn, Fe, and Zn, 0.0002 for Cu and 0.0003 for Pb, Cr and As. Each sample was analysed in triplicates to enhance accuracy and precision. A clean laboratory environment was maintained during the preparation of solutions and analysis. The glass wares and reagent bottles used during the analyses were thoroughly washed with distilled water and detergent, rinsed and dried in an oven. All the reagents used were formulated from pure analytical-grade chemicals and of high purity.

Statistical Analysis
The Statistical Package for Social Science (SPSS) version 20 was deployed for the statistical analysis of data obtained from this study. The values were represented as mean ± standard error (SE). Analysis of variance (ANOVA) was used for inter-station comparisons, while Duncan Multiple Range (DMR) test was carried out to ascertain the point of significant difference. The level of significance was set at p < 0.05.

Result
The mean concentrations of Fe, Mn, Pb and As measured in the surface water exceeded the permissible limits stipulated by SON, WHO, EU and USEPA for potable water but Zn and Cu did not; however, Cr did exceed the limit in some of the months (Table 1). All the metals but copper showed significant monthly variations in the water. Fe was the most abundant heavy metal in the water and bottom sediment with the widest concentration range of 0.008-8.685 mg/l and 22.456-98.88 mg/kg respectively (Tables 1 and 2). Spatially, there was no significant difference in the heavy metal concentrations of the water (Table 3). Also, there was no significant seasonal variation (p > 0.05) in the concentrations of all the heavy metals in the surface water except in zinc (station 3) (p = 0.030) and manganese (station 1) (p = 0.047) ( Table 4). No significant difference (p > 0.05) was noticed in the concentrations of most of the heavy metals in the sediment during the dry months (Table 4). Fe was the most abundant heavy metal detected in the sediment in all the stations during the dry and wet seasons, while Cr (dry season) and Cu (wet season) were the least. Whereas the decreasing order of heavy metal concentration in the water was Fe > A s > Pb > Zn > Mn > Cr > Cu, that of the sediment was Fe > Mn > Zn > Pb > As > Cr > Cu.
Spatially, all the heavy metals in the bottom sediment but Pb and Cu showed significant concentration variations (p < 0.05), with the bottom sediment of station 1 having the highest load of the metals, while that of station 2 has the least. (Table 3). The rainy season values of zinc, manganese, lead, copper and chromium obtained in the bottom sediment were higher than the values obtained during the dry season in all the stations; nonetheless, this difference was not statistically significant (p > 0.05), except for zinc in station 1 (p = 0.031) and chromium in station 1 (p = 0.036) (Table 3). Contrarily, higher arsenic concentrations were detected in the bottom sediments during the dry season in all the stations. The bottom sediment of the river had significantly (p < 0.5) higher concentrations of heavy metals than the surface water. However, the mean concentration value recorded for Pb in the water was slightly higher than that of the sediment, although the difference was not statistically substantial (p = 0.825) ( Table 5).

Heavy Metals in Cross River
The range of zinc concentration (BDL-1.058 mg/l) obtained in the surface water of the river is not objectionable since both the SON and WHO maximum permissible limits are set at 3 mg/l for drinking water. Also, the zinc level is not likely to impart an undesirable astringent taste to the water considering that the level was below 1.0 mg/l during all the sampling months for all stations (Tables 1 and 3). The higher significant concentration of zinc obtained in station 3 during the rainy season can be attributed to urban runoffs that contain wastes from commercial products and activities, wood ash and industrial effluents. The observation of higher concentrations of zinc in the bottom sediment as against those obtained in the surface water is in tandem with the documentation of Andarani et al. [4] that most zinc in rivers is deposited principally in sediment through adsorption and precipitation. The zinc level obtained in the bottom sediment of the river in this study is slightly higher than 1.75 ± 0.5 mg/ kg earlier observed in the river in 2004 as reported by Odoemelam et al. [28]. The increase in the zinc level of the sediment over the years agrees with the findings of Odoemelam [27] that bottom sediments act as a sink for heavy metals which may remain there for a long time.
Judging based on the 0.3 mg/l iron concentration limit stipulated by SON, WHO and USEPA, the iron concentration of the surface water of the river was on the high side (0.008-8.685 mg/l) and that can make the water objectionable for domestic and industrial uses. High iron concentration in water can affect the flavour and colour of drinking water and food. Dvorak and Skipton [11] have reported that dissolved iron in water can react with tannins in tea, coffee and some alcoholic beverages to impact an undesirable taste and appearance to the beverages. Again, iron in water can cause reddish-brown staining of laundry, dishes and utensils. The contamination of the river with iron might have resulted from scraps of iron metal thrown into the river, washing of rusted iron farm implements as well as deposition of iron-containing wastes into the water by the riverine dwellers [12]. The deposition of iron from the surface water to the bottom sediment must have caused the significantly higher iron values measured in the sediment. Iron, like other heavy metals, can interact with organic matter in the aqueous phase and settle down, thereby resulting in a high iron level in sediment [7]. The insignificant seasonal iron concentration variations noted in both the surface water and bottom sediment can be a pointer to the dominance of onsite iron inputs over allochthonous inputs. The simultaneous detection of manganese and iron in the water during most of the sampling months conforms to the proposition of Dvorak and Skipton [11] that manganese is often found in waters containing iron. Manganese concentration in the river did not vary significantly spatially and seasonally (Tables 3 and 4; this observation suggests that autochthonous input might have been the major source of manganese in the river. The significantly higher manganese level recorded in the bottom sediment of station 1 (Table 3) which is upstream of the river, over the other two stations seems to support the autochthonous input opination. Industrial effluents, such as waste from the milling industry, could have also added to the manganese concentration of the water. In some months, the manganese level of the river fell above the SON (0.2 mg/l) and WHO (0.08 mg/l) standards for drinking water; however, this is not alarming as the mean levels obtained in the three stations (0.217 ± 0.07 mg/l, 0.263 ± 0.07 mg/l and 0.176 ± 0.07 mg/l) deviated slightly from SON standard. The presence of manganese at a permissible level in the river can be a safe source of this important element for humans and animals. The human body requires it for the functioning of many cellular enzymes (e.g. manganese superoxide dismutase, pyruvate carboxylase). On the other hand, a high concentration of manganese in the water, as obtained in some of the sampling months, can impact objectionable and tough stains on laundry, dishes, utensils and plumbing fixtures. Detergents and soaps can hardly remove these stains, and the use of chlorine bleach may intensify the stains [11]. The concentration of lead (Pb) in Cross River did not conform to the SON, WHO and EU water standards (0.01 mg/l) for domestic purposes during some of the months of sampling. Judging based on the overall mean concentrations of lead (0.419 mg/l), the surface water is polluted with lead. The relatively high lead level obtained in the surface water can mainly be attributed to extrinsic factors such as the influx of surface runoffs from municipal and industrial effluents. This reason is believable as most of the high levels of lead detection occurred in the wet months (Table 1). Wojciechowska et al. [41] have reported stormwater runoff as one of the important means of heavy metal transference to bottom sediment. Lead finds wide application in industries such as paint and petroleum refining industries,effluents from these industries contain a good quantity of this heavy metal and can be a source of river contamination [43]. Lead has found wide usage in paint pigments because lead-based paints adhere very well to wood and lead imparts brightness to colour. Additionally, leakage of petrol used in poweredboats that navigate the river is a possible intrinsic source of lead in the water. The use of gasoline has been associated with the contamination of environments with lead [41]. The overall mean concentration of lead (0.290 ± 0.14 mg/ kg) detected in the bottom sediment is lower than the concentrations reported for the Cross River in the same catchment area in 2004 (3.10 ± 0.14 mg/kg) [28] and for the Cross River Estuary at Oron (10.68 mg/kg) [12]. Generally, a wider range of lead concentration values was obtained in the surface water (BDL-2.54 mg/l) than in the bottom sediment (BDL-1.78 mg/l) of the river suggesting a low rate of lead deposition at the bottom sediment. Lead has a high affinity for animal tissues where they are concentrated to varying levels [21], therefore, more of the lead might have bioaccumulated in the tissues of the aquatic animals than it accumulated in the bottom sediment.
The range of copper concentrations observed in the bottom sediment (BDL-0.347 mg/kg) was slightly higher than the range observed in the surface water (BDL-0.324 mg/l) indicating a low rate of copper deposition in the sediment. Considerably, uptake by aquatic biota of the little available quantity of the metal in the water might have contributed to comparatively low sediment deposition of the heavy metal. As a micronutrient, copper has been noted as an essential element in virtually all plants and animals at low concentrations [17]. Throughout the sampling period, all the copper levels detected in the river were within the permissible limit set by SON (1.0 mg/l), and WHO and EU (2.0 mg/l) indicating that the water isn't polluted with copper. Odoemelam et al. [28] in their study also reported that the river isn't polluted with copper. Yap et al. [44] have documented that copper is a naturally occurring trace metal that is generally present in surface waters. However, the WHO [38] has highlighted that surface waters which contain copper at concentrations above 2.5 mg/l can impart a light blue colour and detestable metallic bitter taste to drinking water.
The arsenic levels determined in the surface water did not show any significant spatial and seasonal variations (p > 0.05) but those of the bottom sediment did. Arsenic might have found its way into the river autochthonously through the dissolution of rocks and mineral ores, and allochthonously through atmospheric deposition and runoffs containing industrial effluents. The combined influence of the autochthonous and allochthonous input sources might have imparted evenly on the stations and seasons thereby resulting in the insignificant spatial and seasonal difference noticed in the arsenic content of the water. Generally, higher arsenic levels were detected at the surface water (BDL-4.669 mg/l) than at the bottom sediment (BDL-3.604 mg/ kg). This observation might have resulted from a higher resorption rate of the heavy metal from the sediment into the water column than the rate of its deposition back into the sediment. The arsenic range of BDL-4.669 mg/l measured in the water was higher than the permissible limit of 0.01 mg/l stipulated by SON, WHO and USEPA, implying that the water, at the period of study, was polluted with arsenic at concerned levels. It has been reported that exposure to high levels of arsenic can cause critical health problems such as skin and bladder cancers, and can also reduce the production of red and white blood cells [24].
In this study, the chromium concentration range measured in the surface water (BDL-0.232 mg/l) exceeded the 0.05 mg/l permissible limit set by SON and WHO, and 0.1 mg/l stipulated by USEPA; however, this condition is 1 3 not alarming because the levels were slightly exceeded only in some of the months of study. Besides, chromium did not occur in the water within a detectable limit in three out of the twelve-month study period (Table 1). However, following the detection of chromium at a high level in some of the months, the all-year-round potability of the water is not assured because chromium is reported to be toxic, and chromium (VI) can cause the increase of tumours in humans [10,36]. Just as the case of copper and arsenic, the chromium content of the water did not show any significant seasonal or spatial variation, but temporal variation was evident. The elevated chromium levels in some of the months could have resulted from the increased chromium content of the industrial and municipal runoffs during the wet months. This opinion is supported by the fact that higher chromium levels were generally recorded during the wet months (Tables 1 and 3). Davies and Ekperusi [10] also reported an elevated level of chromium in the sediment of the New Calabar river.

Heavy Metals in the Bottom Sediment of Cross River
Generally, significantly higher concentrations of heavy metals were observed in the bottom sediment of the river than in the surface water (Table 5). This observed significant difference in the levels of the heavy metals is congruous with the documentation of Moruf and Akinjogunla [23] that bottom sediment is the major depository of metals, holding more quantity than water. Furthermore, other studies have also made similar observations that heavy metal concentrations of bottom sediments are usually higher than their surface water counterparts [12,20]. Heavy metals in sediment may be ingested by benthos (or pelagic organisms when the metals are reabsorbed into the water column) or absorbed by plants and thus enter the food chain where they bioaccumulate [27]. Through biomagnifications and the associated food chains, the heavy metals present in the sediment can be transferred to man and other terrestrial organisms [12]. Heavy metals have been regarded as important contaminants of aquatic sediment if present at levels greater than the natural concentrations [8]. Pollution of bottom sediment with heavy metals can harm benthic communities and disrupt the aquatic food chain. Overah et al. [32] noted that the assessment of toxic chemicals in the bottom sediment of a waterbody gives more valuable information concerning the pollution status of the water than the analyses of the surface water will do. This is because these toxic substances show considerably higher temporal and seasonal fluctuations in surface water than they do in the sediment.

Potability of the Surface Water of Cross River at Afikpo Catchment Area
Potable water is water that does not contain biological or chemical agents that are directly detrimental to human health [16], it is water that is sufficiently of high quality so that it is safe to drink and to be used for domestic purposes.
Considering the deviation of most of the heavy metals' concentrations from the SON and WHO drinking water standards, the potability of the water was objectionable (Table 1). During some months of sampling, iron, manganese, lead, arsenic and chromium were detected in the water at levels unsafe for drinking purposes; therefore, the water was polluted with these heavy metals and could not be dependent to serve as a potable water source to Nigerians. Contrarily, it was noticed that during the months of sampling, some fishermen, farmers, boatmen and riverine dwellers used the water for drinking purposes as safe pipe-borne water was not easily accessible within the surrounding environment. This practice can pose a short-term threat to their health due to the toxicity of the metals and a long-term health risk as a result of bioaccumulation due to long-time exposure. Fidelis et al. [14] have maintained that no concentration of heavy metal greater than zero should be regarded as safe. WHO/ UNICEF [40] reported that although providing access to potable water has become a human right through various international treaties and declarations, not many human populations have access to potable water. Unfortunately, access to safe drinking water is still a critical global issue. Odoemelam et al. [28] have opined that without potable water of adequate quantity, sustainable development will not be possible. World Health Organization, in 2019 acknowledged the scarcity of potable water when they reported that 758 million people lack access to a basic drinking water source, including 144 million people who are dependent on surface water. Safe water sources will make fewer people fall sick, reduce expenditures on health and enhance the economic productivity of people.

Conclusion
This study reveals that the heavy metals detected in the surface water and bottom sediment of Cross River in the Afikpo Catchment Area showed temporal patterns of variation. Spatially, there was no significant difference in the heavy metal concentrations of the water, suggesting the homogeneity of the surface water. The bottom sediment was observed to contain higher concentrations of heavy metals than the surface water, signifying the accumulation of these chemicals in the bottom sediment. Most of the heavy metals (Fe, Mn, Pb, As and Cr) were detected at higher levels above the SON, WHO, EU and USEPA permissible limits, thus the river was