Heavy metal assessment of groundwater quality in part of Karu, Central Nigeria

Groundwater remains an essential resource for sustainable development with respect to livelihood and food security. Groundwater resource all over the world is under threat due to contaminant load introduced into it through urbanization, industrialization, agriculture and exploitation of natural resources (Ravindra & Mor 2019). These are all basic activities that are associated with everyday sustenance in the world today but can introduce unwanted substances into the environment, especially toxic metals considered hazardous to human health (Nasrabadi 2015). Heavy metals, due to their stability and ability to accumulate in tissues of animals and plants, and the fact that they are not readily biodegradable, persist in the environment especially in groundwater. Heavy metals have been given important consideration globally (Adeyemi & Ojekunle 2021). They occur in trace but significant amounts in the environment and have adverse health effects even at such low concentrations (Hosseinpour et al. 2014).

Heavy metals such as Cr, Pb, Hg, Cd, As and Co have no useful effects in the body system; long-time exposure may cause more acute interruptions in the normal operations of the human organ systems where the metals are deposited (Mominul et al. 2018). Other metals, such as Cu, Zn and Fe for example, are considered micronutrients required for normal growth and functioning of the human body; but at higher concentrations, they become toxic (Wang et al. 2019). The key anthropogenic sources of trace metals in groundwater are natural matters leached into the soil or rocks, residue from agrochemicals, controlled release from the sewage treatment plant and industrial run-off, and unrestrained releases or escape from landfill spot and chemical accidents or calamities.

Heavy metals in groundwater are sourced from atmospheric precipitation, agricultural wastes, discharge of industrial wastewater, agro-pesticides leaching, and urban sewage, mineral mining, and infiltration of surface runoff. Groundwater is exposed to these pollutants due to it being a component of the water cycle, which includes the atmosphere, ground surface, rocks and surface water. More than 50% the world's population depends on groundwater for survival (Rajankar et al. 2009). As, Ag and Pb are toxic even in trace amounts and are released as effluents from industrial activities (Adekunle & Akinyemi 2004). Carcinogenic risks associated with consumption of groundwater from Ogun State in Nigeria contaminated by heavy metals was reported by Adekunle & Ojekunle (2021).

The study area is a fast-growing semi-urban settlement, and has recorded increases in industrial and agricultural activities mainly poultry and fish farms, dimension stone quarry and processing plant. These activities can stress groundwater resources and impact negatively on its quality since wastes (solid and effluent) produced that contains heavy metals will eventually come in contact with groundwater. Studies linking heavy metal contamination of groundwater from anthropogenic activities in the area are lacking, the need to investigate groundwater quality in the area then arises. Assessment of the distribution of heavy metal load in groundwater can assist in linking this to the sources as well as design and implementation of prevention and mitigation measures. The objectives of this study were to (1) determine the distribution of heavy metal load in groundwater (2) evaluate groundwater contamination by these heavy metals using certain indices and (3) from the distribution pattern attempt to link heavy metal contamination to land use (anthropogenic activities) practices in the study area. Groundwater quality assessment based on heavy metal concentration will be achieved using the indices: contamination index; heavy metal pollution index and heavy metal evaluation index as described and used in: Backman et al. (1998); Prasad & Bose (2001) and Edet & Offiong (2002). These indices were also used to evaluate groundwater contamination: anomalous concentrations of heavy metals in groundwater (Panda et al. 2020); heavy metal contamination by hydrocarbons (Nnoli et al. (2021) and source of groundwater pollution by heavy metals (Vesali Naseh et al. 2018). The present study will evaluate heavy metal pollution (contamination) using the indices mentioned earlier to establish any risks posed to human health, plants or animals in the study area. Groundwater in the study area is essentially used for domestic purposes as well as providing the major source of water; heavy metal content thus affects its suitability for the intended use.

The study area is centered on Tudun Wada town, Karu, Central Nigeria and lies within latitudes: 8 °53′N and 8 °56′N, and longitudes: 7 °38′E to 7 °44′E. The area is accessible through the Keffi–Abuja express road and has a network of other minor roads in between isolated settlements while footpaths provide accessibility to remote areas. The area has isolated hills mainly towards the northwestern part; it is drained by a major stream flowing generally towards the south with smaller streams originating from the northwestern and north eastern parts of the study area. The area is underlain by the Basement Complex rocks of North Central Nigeria; main rock units are gneiss (including banded gneiss) and schist.

For the present study groundwater was sampled from 20 water points (Figure 1) consisting of 17 boreholes and 3 hand-dug wells. The hand-dug wells had depths in the range of 10 m–15 m while boreholes were deeper, reaching 120 m. Locations for the wells included households, dimension stone quarry and processing plant, irrigated farms but mostly boreholes from poultry and fish farms were sampled; Figure 1 shows the sampling stations within the study area. Sample collection points were centered on potential sources of heavy metals.

Groundwater samples were collected in 100 mL polyethylene bottles to prevent unpredictable changes in characteristics. The collected samples were treated with a few drops of HNO₃ (to keep metals in solution) and kept at a temperature of 4 °C for further analysis. Prior to the sampling, physical parameters were measured using Sartorius potable meter (PT-10), i.e. pH, electrical conductivity (EC) and total dissolved solids (TDS). Elevation and coordinates values were taken for each sample location with the aid of a GPS. Concentrations of heavy metals (As, Zn, Pb, Ni, Fe and Cu) in water samples were determined using ICP-OES method.

To evaluate heavy metal pollution/contamination in the study area, the following indices were used: contamination index or degree of contamination (Cd; Backman et al. (1998), heavy metal pollution index (HPI; Prasad & Bose 2001) and heavy metal evaluation index (HEI; Edet & Offiong 2002). Panda et al. (2020) used the indices to evaluate trace metal anomalies in groundwater from a foothill aquifer in Tamil Nadu, India; anomalous values of the indices identified areas of groundwater contamination by the trace metals. Nnoli et al. (2021) used the indices to gain insights into contamination levels in the oil spill-ravaged Ogoni land in southern Nigeria; elevated levels of the indices were observed in water. Low levels of the indices, indicative of low contamination were observed by Vesali Naseh et al. (2018) while assessing groundwater pollution sources in Ghaen plain, Iran.

In the equation, H_c is the concentration of each heavy metal as measured in water and H_mac is the maximum allowable concentration of the particular heavy metal in water.

Physical parameters measured in-situ at water points in the study area are: pH, EC and TDS (Table 2). PH ranged from 6.4 to 8 with a mean value of 7.11 ±0.09; the values fall within the permissible range for drinking water (6.5–8.5; WHO 2011). Given the values of pH measured, groundwater is slightly acidic to basic, the slight acidity will keep metals in solution. TDS values are as low as 30 mg/L to 620 mg/L, with a mean value of 183.50±32.68 mg/L; all values are within the permissible limit for drinking water (500 mg/L; WHO 2011) with the exception of two points, one adjacent a dumpsite and the other water point belonging to a dimension stone processing plant. The EC of water increases with concentration of ions and, therefore, dissolved solids.

Concentrations of the trace metals in groundwater from the study area are presented in Table 3. The elements: Pb, As, Fe, Ni, Zn and Cu had mean concentrations of 81.10±11.54; 854.50±82.02; 860.15±152.72; 62.65±20.24; 2,537.20±335.13 and 1,527.40±175.29 μg/L respectively. The concentrations in most water points exceed the limits set by the World Health Organization (WHO 2011). Concentrations of Pb and As in all samples exceed the WHO limit of 10 μg/L. Concentration of Fe in all but two locations exceed the WHO limit of 300 μg/L. Concentration of Ni in the samples are within the WHO limits except for five locations which exceed the limit of 70 μg/L. For Zn and Cu, water samples from 6 points had concentrations above the set limit of 3,000 and 2,000 μg/L respectively.

Groundwater was classified using the scheme proposed by Ficklin et al. (1992) and modified by Caboi et al. (1999). The scheme uses the pH of groundwater and the combined metal load computed as the sum of the individual concentrations of the heavy metals for each sample point. Based on this scheme, all samples plot in the field for near-neutral high metal (Figure 3).

Computed values for Cd provide insights into the level of contamination by these trace elements. According to the classification scheme presented in Edet & Offiong (2002), Cd can be grouped into three categories as follows: Cd<10 (low), Cd=10–20 (medium) and Cd>20 (high). For the study area, all samples had Cd values much greater than 20 (Table 4) indicating that there is a high degree of contamination by trace elements in groundwater of the study area.

Using the classification for the HPI: HPI<100 – low; HPI=100 medium; HPI>100 – high (Edet & Offiong 2002); four locations had HPI less than 100 and are thus classified as having a low level of contamination (Table 4). On the other hand all other samples had HPI greater than 100 indicating a high level of contamination. The high HPI may be due to wastewater from industrial and agricultural activities and domestic sewage; land use in the study area is mainly poultry farming and dimension stone processing, while domestic regions are also prevalent. These anthropogenic activities may be contributing to the trace element load observed.

HEI focuses on heavy metals in water samples for estimating the water quality (Edet et al. 2003). The water quality index is classified into the categories: HEI<10 (low), HEI=10–20 (medium) and HEI>20 (high). All samples had HEI much greater than 20 and are thus categorized as having high level of contamination.

To assess relative contribution of the trace elements to contamination, Pearson's univariate correlation coefficient between the elements and the computed indices was calculated (Table 5). Very strong correlation was observed between the indices; especially so between the degree of contamination and HEI.

Surface plots of heavy metal concentration in groundwater and the assessment indices were created using Matlab (Rb2011) software and compared with land use activities in the study area.

Iron is the second-most abundant metallic element in the Earth's crust; in spite of this, its concentration in water is small (Ngah & Nwankwoala 2013). Iron in groundwater originates from dissolution from the aquifer framework and subsequent percolation into the saturated zone. It is an essential element in the metabolism of animals and plants, for nutrition and in the formation of mammalian haemoglobin; but if present at high concentration, it forms a red oxy-hydroxide precipitate that stains laundry and plumbing fixtures, dish wares and glasses owing to its very reactive nature. Iron actually presents no health hazards even in excess concentration except for imparting a metallic taste to water if the concentration is above 1,800 μg/L (Ngah & Nwankwoala 2013).

The distribution of iron in the study area (Figure 5(b)) is such that the concentration is highest in the north eastern and southern regions, coinciding with the area around a waste dumpsite and poultry farming respectively. Leachate from this dumpsite and poultry waste may have polluted groundwater within these regions.

Nickel in drinking water is primarily sourced by leaching from metals in contact with drinking water, such as pipes and fittings. However, nickel may also be present in some groundwater as a consequence of dissolution from nickel ore bearing rocks.. High concentration of nickel in the study area (Figure 7(b)) is observed in the regions of the dimension stone quarry and processing plant and the poultry farms, although significant concentrations are observed elsewhere.

Heavy metal concentration in groundwater was evaluated using contamination indices with the aim of establishing its contamination status. Concentrations of Pb and As in all samples exceed the WHO limit of 10 μg/L. Concentration of Fe in all but two locations exceeds the WHO limit of 300 μg/L. Concentration of Ni in 75% of the samples are within the WHO limit of 70 μg/L. For Zn and Cu, water samples from 6 points had concentrations above the set limit of 3,000 and 2,000 μg/L respectively. Degree of contamination, heavy metal pollution and heavy metal evaluation indices showed that groundwater is significantly contaminated by heavy metals. Very strong correlation was observed between the degree of contamination and heavy metal evaluation index; between Pb, Fe and all indices; between TDS and Pb; Pb and Fe, Fe and Zn, Zn and Cu, Ni and Cu/Zn. The elements Pb, Zn, Cu, Fe and Ni contribute to the contamination observed relative to the others. TDS/EC is a reflection of contamination in the study area. Groundwater contamination is attributed to anthropogenic activities within the study area especially poultry farming and dimension stone quarrying and processing. The highest concentration of heavy metals was recorded from water points close to poultry farms, the dimension stone quarry and processing plant and a waste dumpsite, highlighting the need for proper solid waste/effluent disposal practices i.e. containment and treatment or evacuation to safe waste disposal facilities. Contamination indices were highest within these regions. Of the three sources, the poultry farms contribute most to the heavy metal load of groundwater. Groundwater in the study area is therefore not suitable for drinking purposes, poultry and fish farming. Results obtained here present a clear case for an environmental impact assessment and routine monitoring of water quality.

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

The authors declare there is no conflict.