4.1 Heavy metals in Cross River
The range of zinc concentration (0.000–1.058 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 2). 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 WHO (2003) 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. (2013). The increase in zinc level of the sediment over the years agrees with the findings of Odoemelam (2013) 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 (2014) 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; it may even induce rust formation on plumbing fixtures (Theis and Singer, 1974). 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 (Eddy et al., 2004). 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 (Begun et al., 2009). 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 (2014) that manganese is often found in waters containing iron. Manganese concentration in the river didn’t vary significantly spatially and seasonally (Tables 2 and 3); 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 5) which is upstream of the river, over the other two stations seems to support the autochthonous input opination. Furthermore, manganese has been reported to dissolve in rivers in substantial amounts by water percolating through rock and soil (USEPA, 1994). Industrial effluents, such as waste from the milling industry, could have also added to the manganese concentration of the water. Khopkar (2004) noted that wastes from milling factories, alloy industries, mines and minerals depots, and glass industries are significant sources of manganese in rivers. 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 the 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); it can also serve to activate many other enzymes (e.g. transferases, kinases, hydrolases, decarboxylases) (IPCS, 2002). 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 (UNEP, 1992). Detergents and soaps can hardly remove these stains, and the use of chlorine bleach may intensify the stains (Dvorak and Skipton, 2014).
The concentration of lead (Pb) in Cross River didn’t 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. (2019) 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 (Khopkar, 2004). Lead has found wide usage in paint pigments because lead-based paints adhere very well to wood and lead imparts brightness to colour. WHO (1995) reported that human activities are the chief factors that disperse lead throughout the environment. Additionally, leakage of petrol used in powered boats into the water is a possible intrinsic source of lead in the water. The use of gasoline has been associated with the contamination of environments with lead (Wojciechowska et al., 2019). 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) (Odoemelam et al., 2013) and for the Cross River Estuary at Oron (10.68 mg/kg) (Eddy et al., 2004). Generally, a wider range of lead concentration values was obtained in the surface water (0.00–2.54 mg/l) than in the bottom sediment (0.00–1.78 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 (Huang, 2003; Martinez et al., 2004); therefore, more of the lead might have bioaccumulated in the tissues of the aquatic animals than it accumulated in the bottom sediment. Lead, when ingested, affects the liver, kidney and nervous system, but the nervous system is the most susceptible target of lead poisoning (ATSDR, 1999).
The range of copper concentrations observed in the bottom sediment (0.00–0.347 mg/kg) was slightly higher than the range observed in the surface water (0.00–0.324 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 (Kapustka et al., 2004). 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. (2013) in their study also reported that the river isn’t polluted with copper. Nriagu (1979) has documented that copper is a naturally occurring trace metal that is generally present in surface waters. However, the WHO (2004) 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 didn’t show any significant spatial and seasonal variations (p > 0.05) but those of 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 (IPCS, 1981; Bissen and Frimmel, 2003). 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 (0.000–4.669 mg/l) than at the bottom sediment (0.000–3.604 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 0.000–4.669 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 reduce the production of red and white blood cells, and cause damage to blood vessels (DHWA, 2008).
In this study, the chromium concentration range measured in the surface water (0.000–0.232 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 not alarming because the levels were slightly exceeded only in some of the months of study. Besides, chromium didn’t 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 months, the all-year-round potability of the water is not assured because chromium is reported to be toxic and carcinogenic, owing to its oxidizing potential and the ease with which it permeates biological membranes (Świetlik, 1998). Just as in the case with copper and arsenic, the chromium content of the water didn’t 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 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) and by the submission of Eisler (1986) that elevated chromium levels can be noticed in waters within the environment of electroplating and metal finishing industries, and publicly owned municipal treatment plants.
4.2 Heavy metals in 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 7). This observed significant difference in the levels of the heavy metals is congruous with the documentation of Odiete (1999) that bottom sediment is the major depository of metals, in some cases holding more than 99 per cent of the total amount of a metal present in the aquatic systems. WHO (2003) and Zheng et al. (2012) also reported that metals can be retained over time in the bottom sediment and afterwards resorbed in the surface water during the re-suspension episode. Furthermore, other studies have also made similar observations that heavy metals concentrations of bottom sediments are usually higher than their surface water counterparts (Eddy et al., 2004; Malik and Maurya, 2015; Liu et al.,2019). 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 (Oboh and Edema, 2007). Through biomagnifications and the associated food chains, the heavy metals present in the sediment can be transferred to man and other terrestrial organisms (Eddy et al., 2004)
Heavy metals have been regarded as important contaminants of the aquatic sediment if present at levels greater than the natural concentrations (Bing et al., 2016). Pollution of bottom sediment with heavy metals can harm benthic communities and disrupt the aquatic food chain. Ademoroti (1996) revealed that the assessment of harmful and toxic substances in the bottom sediment of waterbodies gives more valuable information concerning the pollution status of the waters 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.
4.4 Potability of the surface water of Cross River at Afikpo Catchment Area
Potable water is water that is sufficiently of high quality so that it’s safe to drink and to be used for domestic purposes such as cooking (Brenniman, 1999); it's water that is drinkable and safe. 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. 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 couldn’t 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 wasn’t 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 long-term health risk as a result of bioaccumulation due to long-time exposure. Fidelis et al. (2013) have maintained that no concentration of heavy metal greater than zero should be regarded as safe. In 2007, Egboh and Emeshili reported that most rural communities in Nigeria lack access to basic drinking water services just like some riverine communities at Afikpo. Odoemelam et al. (2013) have opined that without potable water of adequate quantity, sustainable development will not be possible. World Health Organization (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 economic productivity of people.