1 Introduction

Water is considered one of the most essential substances in life and is the most abundant [1,2,3]. Underground water quality monitoring is essential, especially when the water is normally used for drinking and domestic purposes [3]. Unsafe water is a universal threat to public health, putting lives at risk due to a host of infectious diseases as well as chemical intoxication [4,5,6,7]. The pollution of underground water from anthropogenic activities such as indiscriminate dumping of refuse and discharge of untreated leachates to the environment causes much concern [8, 9].

Rapid population increase, industrialization, and urban growth have all led to an increase in waste in developing cities [10, 11]. The consequent failure of solid waste management in cities is a universal fact [12,13,14]. In Nigeria, open/unlined dumpsites are the oldest and most common waste disposal preference in cities [15, 16]. Unlined dumpsites are mainly used due to low budget allocation and the nonavailability of skilled personnel for waste disposal [17]. The environmental impact of leachate on groundwater has attracted much interest because of its overwhelming environmental significance [16, 18]. The Pollution of underground water through leachate infiltration from unsanitary dumpsites threatens the surrounding environment through domestic use.

Studies have shown that groundwater systems can be contaminated due to poorly designed waste disposal facilities, which can lead to acidification, nitrification and microbial contamination [18, 19]. Contamination occurs through leakages, which are formed when rainwater penetrates the dumpsites and softens the waste, which produces leachates as a result of chemical and biochemical processes [20]. These leachates create the challenges of groundwater pollution through the percolation of pollutants [21, 22]. The generated leachates can contain a high level of ammonium, total dissolved solids, nitrate, chloride, alkalinity, and even toxic matter that can degrade the quality of groundwater [19,20,21,22].

High nitrate levels have damaging effects on infants [23]. Additionally, high concentrations of ammonia–nitrogen (> 10 mg N/L) can create numerous problems for the environment, such as the eutrophication of groundwater if the waste dumpsite is not lined [22]. Several water quality indices have been used by different researchers to determine the overall quality of water [4, 24]. Water quality indices are one of the most effective ways of determining the quality level of water [25, 26]. The negative effect on water quality caused by the use of an unlined dumpsite practice calls for a broad method for assessing the effect on underground water [7].

In the literature surveyed thus far, only a few studies have performed a groundwater pollution assessment around unlined waste refuse dumps in Nigeria. Egbueri et al. [14] studied the assessment of the quality of groundwater proximal to the dumpsite in Awka and Nnewi, Anambra State. The results of the study indicated that the boreholes could not be used for drinking water due to the leaching of pollutants from the dumpsites. Ubechu et al. [28] studied the effect of leachates from an open unlined landfill site in Aba, Abia State, Nigeria. In their study, the important parameters for leachates such as ammonia and chemical oxygen demand (COD) recorded values higher than their acceptable limits in groundwater. This suggested that the continuous disposal of wastes leading to the production of leachate in the dumpsite will pose a serious risk to the groundwater through leaching.

However, to the best of our knowledge, no study has been conducted in the Okpunoeze Nnewi dumpsite to determine the effects of leachate on the borehole water around the dumpsite using the water quality index. The main objective of the work is to determine the water quality of the borehole water around the dumpsite. The pollution index and WQI were evaluated to determine the degree of pollution of the borehole water and its suitability to be used for drinking water and other domestic activities. The results will also help establish institutional actions for preventing human exposure through the prohibition of groundwater use in contaminated zones.

2 Materials and methods

2.1 Study area

The sampling areas for the study (Fig. 1) are situated around Okpunoeze Otolo Nnewi dumpsite (latitude 6 \(^\circ \) 00′43.4″N and longitude 6 \(^\circ \) 54′28.2″E) in Anambra State. The coordinates of the sampling locations are indicated with D1–D16 as the 16 dumpsite area sampling points, while C1–C16 are the 16 control area sampling points. The potentiometric map of the study area is shown in Fig. 2.

Fig. 1
figure 1

Map of the dumpsite and sampling points

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Fig. 2
figure 2

Potentiometric map of the study area

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Nnewi is a metropolitan city and a commercial hub that has witnessed a high rate of population growth, industrialization and urbanization. According to the 2006 census, the population of Nnewi is 391227 inhabitants [27]. Nnewi is home to many major indigenous manufacturing industries such as automobile, plastic, food processing, battery, soap, and oil/lubricant industries. The wastes disposed of by these industries are deposited in the dumpsite, with no prior sorting of the waste materials. It is estimated that approximately 3000 tonnes/day of solid waste is disposed of on the dumpsite, which would have increased over the years [18].

The Okpunoeze Otolo Nnewi dumpsite is the most common dumpsite used by the inhabitants of the city for the disposal of waste materials and is still active. It is an open dumpsite, poorly constructed with no protective liners. The dumpsite is located at the centre of the city and is surrounded by hospitals, industries, households, markets and motor parks. Generally, different types of waste are found in the dumpsite, which include electronics, industrial, hazardous, plastic, hospital and household wastes.

The communities living around the study area use groundwater water for cooking, bathing, washing and drinking. Nnewi falls within the tropical rainforest region of Nigeria with two major seasons (wet and dry) [28]. Many deforestation activities have taken place due to urbanization, which has exposed the hydrologic environment to harsh weather [11, 28]. The area has an average annual rainfall of approximately 2000 mm [11, 28].

The geology of the study area falls within the Anambra Basin in southeastern Nigeria. Nnewi metropolis lies beneath the Nanka Formation, which is Eocene in age [29]. Generally, the lithology of the area is composed of sandy clays, sandstones and loose sands [29]. On the other hand, sandstones are porous in nature because of their high permeability potentials, which makes it difficult to obstruct the infiltration of leachates into the aquifer. Nnewi lies along the Mamu River basin with an altitude of 105 m to 300 m above sea level [29]. Nnewi metropolis is underlain by unconfined aquifers with an average depth to the water table of approximately 110 m and an average static water level of 120 m [28, 29].

2.2 Collection and treatment of borehole water samples

A total of 16 samples of borehole water were collected monthly in the dumpsite and control areas for four months of the wet season (May–August 2018) and three months of the dry season (December–February 2019). The collection of the samples was performed around the dumpsite area (dumpsite area sampling points) and about 685–935 m away from it (control area sampling points). The samples were collected in a clean polyethylene container, which was thoroughly washed and rinsed with distilled water.

During sampling, the sampling bottles were rinsed 3 times with the water samples. The samples from 16 points per sampling site were combined to form a composite sample. The temperature and pH readings were taken onsite. Samples were acidified with 10% HNO3, ice-cooled and transported to the laboratory. It was stored in a refrigerator at 4 °C before analysis.

2.3 Physicochemical analysis

Turbidity was determined using a turbidity meter (Thermo Fisher Scientific, Model: AQ4500). The pH and conductivity were determined using a pH meter (Hanna Instrument, Model: HI991300) and conductivity meter (Yantai Stark Instrument, Model: DDS-307A) respectively. Total suspended solids (TSS), total solids (TS) and total dissolved solids (TDS) were measured using gravimetric analysis [30, 31]. Argentometric titration procedures were applied for the determination of chloride. Total alkalinity and total acidity were determined by volumetric analysis.

Total hardness, magnesium hardness and calcium hardness were measured using a complexometric titration of EDTA solution. Sulphate, nitrate, nitrite and phosphate were measured using a UV–visible spectrophotometer (Bioevopeak, Model: 721G-100). The COD was determined using the dichromate digestion procedure [31]. The temperature was measured using a temperature/TDS meter (HM Digital, Model: TDS-3).

The Winkler method was adopted for the determination of dissolved oxygen (DO) and bacterial oxygen demand (BOD) [31]. The physicochemical parameters of the borehole water values were analysed in triplicate using APHA [30] and APHA [31] standard methods. In all, the analyses were carried out using analytical grade reagents. The blank samples and certified reference materials (CRMs) from the National Water Resources Institute (NWRI) were analysed to ensure the accuracy, reliability and reproducibility of the laboratory measurement processes [21,22,23]. The results were found within ± 5% of the certified values.

2.4 Pollution index

The pollution index (PI) is defined as the ratio of the concentration of individual parameters to the reference point standard. It provides data on the relative pollution contributed by individual parameters [4, 6]. The threshold value is 1.0; values > 1.0 indicate pollution, while values < 1.0 indicate no pollution [4, 6]. The pollution index is evaluated using Eq. (1) [4].

$$\mathrm{Pollution\,Index}(\mathrm{PI})=\frac{\mathrm{Concentration}}{\mathrm{Standard}}$$
(1)

2.5 Water quality index (WQI)

The overall water quality can be evaluated using the WQI [33]. It helps to transpose estimates of water quality (WQ) parameters into illuminating and definite values, which can simply be interpreted [32]. The water quality index using a weighted arithmetic index method gives a classification of the WQ concerning the level of purity by combining the most commonly assessed WQ parameters [33]. The method has been widely used in evaluating water quality globally and specifically [34, 35].

The WQI was calculated using Eq. (2) according to Tripaty and Sahu [36]. Seventeen water parameters were involved in the WQI calculation.

$$\mathrm{WQI}=\frac{\sum {W}_{n }\times {q}_{n }}{\sum {W}_{n}}$$
(2)
$${W}_{n}=\frac{1}{\left({S}_{n}\right)}$$
(3)
$${q}_{n}=100\frac{\left({V}_{n}-{V}_{i0}\right)}{\left({S}_{n}-{V}_{i0}\right)}$$
(4)

where \({W}_{n}\) represents the weightage unit of each parameter obtained as given in Eq. (3) based on WHO standard values;\({S}_{n}\) represents the WHO standard value for the nth parameter, and \({q}_{n}\) denotes the quality rating obtained using Eq. (4). \({V}_{n}\) represents the nth parameter of the given sampling station and \({V}_{i0}\) = the ideal value of the nth parameter in pure water. The ideal value used for all parameters was zero [36]. The water quality ratings are 0–25 (excellent), 26–50 (good), 51–75 (poor), 76–100(very poor) and > 100 (heavily polluted) [35].

2.6 Statistical evaluation

Correlation analyses were carried out using Pearson’s correlation to evaluate the interaction between the dumpsite area and control area parameters. A significant value of p < 0.05 was considered. SPSS statistics software was used for the analyses.

3 Results and discussion

3.1 Levels of physicochemical parameters of the borehole water samples

The borehole water physicochemical parameters are presented in Table 1. The borehole samples showed different concentrations of physicochemical characteristics in both study areas. The values of the mean physicochemical parameters were compared with WHO standard limits for drinking water shown in Table 1 and Fig. 3.

Table 1 Physicochemical properties of the borehole water samples
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Fig. 3
figure 3

Seasonal variation in the mean physicochemical parametersfor both study areas

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The total alkalinity mean values for the dumpsite area for both seasons ranged from 34.0 to 47.90 mg/L, while the control area values ranged from 29.40 to 31.67 mg/L. The values reported were higher than by [18]. The values of total alkalinity were below the WHO permissible limit.

The total hardness mean values ranged from 87.28 to 89.37 mg/L for the dumpsite area for both seasons and from 70.30 to 72.20 mg/L for control areas for both seasons. In this study, the obtained values were higher than by [18]. In this study, the obtained values were lower than by [37]. The total hardness values of the dumpsite area were higher than the control area, possibly due to the infiltration of the high content of calcium and magnesium ions through the migration of leachates to the dumpsite area [38]. The maximum concentration of total hardness was observed during the dry season for both locations.

The Mg hardness values for the dumpsite area for the wet season ranged from 22.3–37.0 mg/L to 20.8–36.9 mg/L for the control areas. The dry season values for the dumpsite area ranged from 25.2–41.8 mg/L to 25.2–29.1 mg/L for the control areas. The mean values of the dumpsite areas were predominantly higher than the control areas for both seasons. This can be attributed to the run-off of magnesium salt particles from the refuse dump [19]. The values obtained were in agreement with [40].

The average calcium hardness values for the dumpsite area for both seasons ranged from 51.20 to 54.30 mg/L, while the control area results for both seasons ranged from 44.57 to 47.63 mg/L. The wet season values were higher than the dry season values for both locations, with maximum values observed in the dumpsite area. This can be a result of the infiltration of calcium-related compounds by rainfall from industrial and municipal wastes disposed of in the refuse dump [20]. Additionally, the values were lower than by [37, 40].

The chloride concentration in water can be associated with contamination [37]. In the dumpsite area, the average chloride values for the wet and dry seasons ranged from 99.2 to 100.1 mg/L, while the control area values ranged from 69.3 to 73.0 mg/L for wet and dry seasons. The mean maximum value of 149 mg/L was reported by [14], which was higher than the present study. The values obtained in the dumpsite area were generally higher than the control areas.

The pH values for the dumpsite area for wet and dry seasons ranged from 7.1–7.3 to 6.9–7.2 respectively. The control areas ranged from 7.1–7.2 to 7.0–7.2 for wet and dry seasons respectively. The mean pH values for both locations and seasons were slightly alkaline. The dumpsite area values were higher than the control areas in both seasons. The mean pH values for both locations in the wet season were higher than the dry season values. The values were within the WHO permissible limit. Meanwhile, pH values obtained in this study were higher than by [40].

Nitrate in groundwater derives mainly from fertilizers, waste dumps, and manure storage pollution [38]. The nitrate values for the dumpsite area in the wet and dry seasons ranged from 19.6 to 29.6 mg/L, while those in the control areas ranged from 17.1 to 25.0 mg/L. The maximum values were observed in the dumpsite areas. The values of nitrate obtained from the dumpsite area could be attributed to the discharge of sewage and industrial wastes from the dumpsite. The run-off of water from agricultural fields making use of fertilizers might also have influenced the values of nitrate in the control areas by [19].The nitrate values were within the acceptable limit of the WHO. In the study, the nitrate values were higher than by [14, 18] and lower than by [37].

The nitrite mean values for the dumpsite areas varied from 3.13 to 4.03 mg/L in both seasons, while in the control areas, they ranged from 2.33 to 2.84 mg/L. The nitrite results for the dumpsite area in both seasons were higher than the WHO permissible limits, while the values in the control areas were lower than the permissible limit. The high nitrite values in the dumpsite areas were attributed to the infiltration of untreated leachates and fertilizer run-off from farms by [19]. The nitrite values were lower than the values obtained by [39].

The total solids values for the dumpsite areas for both seasons ranged from 81.6 to 95.1 mg/L. The control areas for both seasons ranged from 78.3 to 87.7 mg/L. The values of 74.9 to 88.4 mg/L were similar to the obtained report by [39]. The higher values of the total solids in the dumpsite area could be attributed to the leachate percolation of organic and inorganic particles from the refuse dump. The total solids were significantly greater in the wet season than in the dry season for both study areas due to the infiltration of leachates and run-off of soil during rainfall. The total solids were within WHO limits.

The total suspended solids of the dumpsite area values for the wet and dry seasons ranged from 13.8–18.9 mg/L to 15.9 – 17.1 mg/L respectively. The control area values for wet and dry seasons ranged from 14.2–17.1 mg/L to 13.4–15.8 mg/L respectively. The values obtained in this work were lower than the values obtained by [39]. TSS values were within WHO limits for both locations.

In the dumpsite area, the values of TDS for the wet and dry seasons ranged from 70.57 to 71.80 mg/L, while the control areas varied from 66.93 to 67.80 mg/L. The TDS values in the dumpsite area were slightly greater than the control areas, possibly due to the leaching of pollutants (organic and inorganic) from the refuse dump and the surrounding environment. The TDS values were lower than by [37, 40].

The BOD values ranged from 18.7–24.3 mg/L to 20.5–24.3 mg/L for the dumpsite areas in the wet and dry seasons and 12.3–16.2 mg/L to 18.3–22.3 mg/L for control areas in both the wet and dry seasons respectively. Lower BOD levels were reported by [39]. The levels of BOD might be ascribed to the infiltration of leachates, surface run-off from farms and wastewater infiltration, which might have polluted the underground water by [39].

The DO mean values in both seasons for the dumpsite areas varied from 2.9 to 3.3 mg/L, while in the control areas for both seasons, they varied from 3.5 to 3.8 mg/L. The low level of DO observed was an indication of oxygen depletion. This inferred the presence of pollutants from the infiltration of wastewater that uses up the oxygen in the water, through the decay by bacteria [37]. The DO values were similar to the reported result by [39] and lower than the value reported by [37]. The values were within the WHO allowable limits.

The COD mean values for the dumpsite area for both seasons varied from 27.37 to 31.35 mg/L, while in the control areas, they varied from 21.87 to 29.57 mg/L. The values of COD might have been caused by the percolation of leachates, wastewater infiltration and surface runoff of pollutants, which contains organic and inorganic pollutants by [37, 40].

Phosphate contamination of groundwater may occur as a result of the infiltration of leachates and fertilizer effluents by [37]. In this study, phosphate values for the dumpsite area ranged from 21.2 to 27.6 mg/L for both seasons, while control area values ranged from 16.4 to 26.1 mg/L for both seasons. The values of phosphate from the study were higher than by [14, 37].

Sulphate in the water can occur through the oxidation of sulphide in soils, mineral dissolution and runoff of fertilizers in the soil [37]. The sulphate mean values in the dumpsite area for both seasons varied from 23.83 to 26.48 mg/L, while in the control area for both seasons, it varied from 20.53 to 24.17 mg/L. In this study, the sulphate values were higher than the values reported by [14, 39].

The turbidity mean values in the dumpsite area for the wet season varied from 0.56 to 0.97 NTU, while the control area for both seasons varied from 0.38 to 1.07 NTU. Turbidity values were within the WHO limit. The turbidity value of 107.5 NTU higher than the present study was reported by [18].

The conductivity values of the dumpsite area for both seasons ranged from 105 to 127 mg/L, while control area values ranged from 103 to 127 mg/L. The values of the conductivity can be attributed to the presence of inorganic dissolved solids and agricultural runoff from soils by [38, 39].

Generally, the physicochemical parameters revealed that only the nitrite values in the dumpsite area for both seasons were above the WHO drinking water permissible limit. The wet season values were predominantly higher than the dry season values, while the dumpsite area values were predominantly higher than the control area. The values of the physicochemical parameters might have been influenced by the infiltration of pollutants from the dumpsite and surface run-off from the surrounding environment, which was aided by rainfall.

The correlation values of the physicochemical parameters (Table 2) for the borehole water showed a strong and positive correlation value between the dumpsite area and control area for the wet season (r = 0.990, p < 0.05) and dry season (r = 0.983, p < 0.05). The correlation values of the dumpsite area for both seasons (r = 0.994, p < 0.05) and control areas (r = 0.995, p < 0.05) showed a strong positive linear relationship.

Table 2 Pearson correlation between the study area parameters across both seasons
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It is important to note that the strong correlation observed in the study areas indicates a similar origin/source. Moreover, the pollutant source was attributed to anthropogenic activities such as the uncontrolled discharge of leachates from industrial and domestic wastes around the refuse dump. The control area may not be affected by the dumpsite based on the potentiometric map, the pollutant source was attributed to wastewater infiltration, run-off from farmlands, poor soakaway systems and other irregular disposal sites. Furthermore, the p-values were less than 0.05 (p < 0.05). This implies that the physicochemical characteristics are significant and dependent on the extent of accumulation of the pollutants in the borehole water samples in both study areas and seasons.

3.2 Pollution index

Table 3 shows the pollution index results of borehole water samples for both seasons. The pollution index for the wet season was 4.380 and 3.834 for the dumpsite area and control area respectively. The dry season pollution index was 4.073 for the dumpsite area and 3.516 for the control areas. The PI values obtained from the dumpsite area were higher than the values from the control area for both seasons. The pollution index for the wet season was higher than the dry season at both locations. This was attributed to the infiltration of leachates and wastewater, which was aided by rainfall by [39]. The results showed that the PI values were above 1.0 which was regarded as the critical value. The water samples were therefore regarded as critically polluted, and cannot be used for potable water.

Table 3 Pollution index values of the borehole water samples
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3.3 Water quality index

The WQI revealed that all the water samples were heavily polluted. The dumpsite areas were the most polluted in both the wet and dry seasons (Table 4). The dumpsite area WQI values were higher, possibly due to the migration of leachates to the underground water closer to the dumpsite. The control area's high WQI values might have been attributed to the leaching of pollutants from the surrounding environment, such as soakaway, indiscriminate dumping of refuse and surface runoff of contaminated soil. The WQI values from this study when compared to other regions showed that the WQI values were higher than the values found in Lagos, Nigeria [43]. However, WQI values higher than the study area results which ranged from (1243.9–3034.5) Uyo, Nigeria [44], 752.4–752.8 Rivers, Nigeria [45]. Therefore, underground water must be treated for use for drinking and domestic purposes due to the heavy pollution of the environment.

Table 4 Water quality assessment
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4 Conclusion

The physicochemical examination of the study area revealed that the dumpsite area and control area borehole water samples were polluted with different concentrations of pollutants through the percolation of the leachates from the refuse dump. The results showed that only the nitrite values in the dumpsite area for both seasons were above the WHO drinking water limits. The physicochemical parameters were higher in the wet season than in the dry season due to the infiltration of leachates from the refuse dump and runoff of pollutants from the soil aided by rainfall. The results showed a strong positive correlation between the dumpsite and control areas. The potentiometric map showed that the leachate from the refuse dump contaminated the dumpsite areas, while the control area may not be affected by the dumpsite, but was linked to wastewater infiltration, run-off from farmlands and other irregular disposal sites. The pollution indices for both study areas were above 1, which makes the water samples unfit for potable water. The WQI revealed that both study areas were heavily polluted with the most polluted being the dumpsite area in both seasons. Therefore, the government should encourage the use of sanitary landfills for proper waste collection and treatment, which will prevent environmental pollution through the leaching of pollutants. In addition, frequent quality monitoring of the underground water to ensure they are suitable for drinking water purposes should be conducted. The borehole water around the environment should be treated for domestic and drinking water purposes.