Introduction

Dumpsite leachate is the most well-known source of pollution in shallow groundwater aquifers. Leaching of both organic and inorganic contaminants from dumpsites is a severe environmental problem for both surface water and aquifers over a long time, posing long-term health and environmental consequences (Macdonald et al. 2012; Bjerg et al. 2014; Oyeyemi et al. 2018, 2019). Since soil and rocks usually are good conductors of electricity, infiltration of leachate or polluted water may include a valuable number of dissolved ions, causing their conductivity to rise from poor to a moderate anomaly up to peak value. Humans are heavily reliant on groundwater, and predominantly, all sub-Saharan African communities obtain their drinking water from privately owned water wells; some of which are only a few meters deep or even deeper and in most cases prone to near-surface contaminants. Groundwater constitutes a higher percentage of the freshwater resources readily available in sub-Saharan Africa, and it forms the largest reservoir in the crystalline basement complex of Nigeria (Bernstone et al. 2000; Karlik and Kaya 2001; Macdonald et al. 2012; Olasehinde and Raji 2007; Abubakar et al. 2014; Adabanija and Alabi 2014; Ayolabi et al. 2015).

Contamination of groundwater can take place through migration of pollutants many meters in either vertical or horizontal distance, depending on the topography (via surface run-off), hydrology situation, and rock type within proximity or locale and poses a major threat to biotic communities (Dimitriou et al. 2008; Mepaiyeda et al. 2019; Moreira and Cesar 2021; Aziz et al. 2010; Fatta et al. 1999; Xing et al. 2013). Noninvasive and cost-effective geophysical techniques such as electrical resistivity tomography, very low-frequency electromagnetic (VLF-EM), and water quality assessment are appropriate for groundwater contamination investigation. The approaches have been effectively utilized to characterize geo-electric layers, landfill evaluation studies, map leachate movement around dumpsites, and groundwater contamination by examining their physical and chemical properties, also bacterial and heavy metals content (Bernstone et al. 2000; Karlik and Kaya 2001; Ogilvy et al. 2002; Porsani et al. 2004; Olasehinde and Raji 2007; Sundararajan et al. 2007; Boudreat et al. 2010; Adewoyin et al. 2019; Akinbiyi et al. 2020).

For identifying the occurrence and movement of leachate and to verify the geotechnical stability, induced polarization methods, electrical conductivity (EC) logging, and seismic surveys have been well utilized and documented (Karlik and Kaya 2001; Porsani et al. 2004; Boudreat et al. 2010; Ramalho et al. 2013; Zume et al. 2006; Osinowo and Olayinka 2012; Naudet et al. 2014), while much attention has been given to the investigation of effect leachate has on the qualities of the soils they are located in, and how these soil features contribute to the movement of the leachate plume, either by assisting or disrupting it (Ogilvy et al. 2002; De Carlo et al. 2013; Moreira et al. 2013; Ayolabi et al. 2015; Park et al. 2016; Maurya et al. 2017; Raji and Adeyoe 2017; Morita et al. 2020). However, hydrochemical analysis for trace element concentration and other microbiological effects is essential for the exact determination of groundwater contamination, and it corroborates the results of mechanical alteration in soil and leachate (Aziz et al. 2010; Ayolabi et al. 2015; Fatta et al. 1999; Xing et al. 2013; Kamble et al. 2020; Farzaneh et al. 2021; Igboama et al. 2021).

Considering the increased population of an ancient rural settlement Malete, which serves as a host for Kwara State University and other federal developmental projects, the increased rate of municipal dumps on the community’s open dumpsite necessitates aggressive preventive and restorative management measures on leachate impacts on surface and groundwater quality of the area. Municipal waste disposed in such open dumpsite is subject to either infiltration from precipitation or groundwater underflow that could result in accumulation of organic and inorganic compounds at bottom of the dumpsite and eventually penetrates through the soil to contaminate the groundwater (Taonameso et al. 2019). The resident of Malete depends on shallow hand-dug wells with low water recharge potential and susceptible to contamination and a few very deep wells (generally above 100 m) for domestic and commercial use. Thus, the aim of this study is to determine the lateral and vertical extent of leachate migration through the sub-base soil and its effect on the quality of the underground water in agreement with the World Health Organization (WHO 2017) standard for drinking water and Nigerian standard of drinking quality water (NSDWQ 2007), using the electrical resistivity tomography, soil sample, and physicochemical analysis.

Location and geology of the area

Malete falls within Moro local government area of Kwara State, bounded by geographical coordinates 8° 40′ to 8˚ 45′ N and 4° 25′ to 4° 30′ E, in the southwestern region of Nigeria. The climate is that of Guinea Savannah region with relative humidity of 50–60%, temperature range within 20.8 to 38.7 °C, sufficient untapped agricultural resources and agglomeration of buildings where people live and work. The rocks are of the Precambrian basement complex of southwestern Nigeria, predominantly Migmatite-Gneiss complex (MG) comprising mainly the biotite gneisses, porphyritic and microgranite, migmatite, and grey gneiss which are made up of minerals such as the quartz, biotite, plagioclase feldspar, microcline feldspar, hornblende, and other accessories which signify rocks in the low grade metamorphic facies (Fig. 1). These rocks trend in NE-SW and slightly dipping to the west. The heap of the dumpsite is about 8-m high above the average relief of 308 m above sea level. It has population of about 102,780 people who depends on River Laonu which flows through the southeast, as the main source of water and near surface water tapped from the major NE-SW deformational trend documented in Nigeria (Burke and Dewey 1972); (Obaje 2009). The dumpsite is within a built-up area, accessible by minor roads and several footpaths from the major road, about 25.5 km from Ilorin.

Fig. 1
figure 1

Site description (a) geologic map of Malete and its environ, (b) hip of the dumpsite, and (c) survey and sampling points

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Methodology

Electrical resistivity tomography

The direct current electrical resistivity method involving the Wenner electrode configuration, which allows for better resolution of the subsurface resistivity distribution (Hamzah et al. 2006), was used to determine the mode of occurrence of leachate in the dumpsite area and the near-subsurface earth model. Three profiles were established NW–SE crosscutting the major structural trend and having spread length within 60 to 90 m. Equidistance electrode spacing “a” of 1, 3, 5, 10, and 15 m were deployed and the resistance “R” readings were transformed to apparent resistivity “\({\rho }_{a}\)” data using the geometric factor “K’” (Eq. 1). This method can measure alterations that are associated with rock parameters such as porosity and permeability and are related to discontinuity, fracture, and faults. Direct current is injected through current electrodes, and the potential difference is calculated and measured laterally and vertically by potential electrodes (Loke and Barker 1996). The apparent resistivity data was topographically corrected, and the resultant parameters were processed using Res2DINV resistivity modeling software, which generated two-dimensional “2D” inverse resistivity models of the subsurface of the dumpsite (Fig. 2).

Fig. 2
figure 2figure 2

a 2D resistivity tomography of traverse 1, showing the resistivity distribution at Malete dumpsite. b 2D resistivity tomography of traverse 2, showing the resistivity distribution at Malete Dumpsite. c 2D resistivity tomography of traverse 3, showing the resistivity distribution at Malete dumpsite

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$${\rho }_{a}=KR$$
(1)
$$K=2\pi a$$
(2)

Sampling and laboratory analysis

Soil sample analysis

Permeability is one of the important physical properties of soil, as some of the major problems of soil mechanics are directly connected with it (Rowe et al. 1995; Daniel 1987; Kayadelen 2007). Optimum permeability range to be possessed by geological barriers which are clay rich geological units (soil liners and covers) for reduction of contaminants in dumpsites are considered within 10−6 to 10−8 cm/s (Lambe and Whitman 1991; Allen et al. 1997). In order to determine the porosity, permeability, and the rate of settlement of the sub-base soil of the dumpsite, four (4) disturbed soil samples were collected at selected anomalous zones on the ERT using a hand auger and were labeled appropriately “A, B” for deeper (0.5 to 1.0 m) and “C, D” for shallow (0 to 0.5 m) depths of the two drill pits “PAC and PBD” respectively. The samples were manually sorted and filtered and prepared for grain size, liquid limit, plastic limit, specific soil gravity, linear shrinkage, and compaction analysis at the Geology Departmental Laboratory of University of Ilorin, Nigeria, using the BS ISO 11277:2020 standard. Both mechanical and hydrometer sieve analysis were adopted for the grain size distribution, in order to determine the coarse and fine grain fraction of the soil, useful to characterize the typical soil type. Mass of the soil retained on each dry sieve was used to determine the percentage passing, while the temperature at each hydrometer reading was recorded and then a statistical data sheet was produced showing the results of the analysis, and the clay and silt percentage in the samples were then calculated from the graph obtained by plotting percentage passing against the grain diameters (Fig. 3). The degree of cohesion and adhesion between the soil particles as related to the resistance of the soil to rupture (Atterberg limits) was determined using the clay minerals content, the plasticity, and shrinkage limits (Fig. 4). The soil were further classified into normal and or abnormal (over normal or under normal) specific gravity, calculating the void ratio of the soil and also the determination of porosity of the soil using the Pycnometer method (Table 1).

Fig. 3
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Grain size analysis stack showing distribution of the various grain sizes that make up the soil, and helps for silt and clay classification

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Fig. 4
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Plasticity chart for the shallow and deeper pits (PAC and PBD)

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Table 1 Soil characteristic for Malete dumpsite
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The coefficient of permeability is obtained from Eq. 3:

$$\frac{2.3026}{A}al\times \mathrm{log}{H}_{1}-\frac{\mathrm{log}{H}_{2}}{{t}_{2}-{t}_{1}}$$
(3)

where H1 and H2 signifies the initial height and final height of the water in the standpipe above that which is in the container at time t1 and t2 respectively. A signifies the cross-sectional area of the sample, while a is cross-sectional area of the standpipe, and l as the height of the sample.

Water sample analysis

Six (6) samples of water (3 samples from shallow (about 3 m depth) hand-dug wells and 3 samples from boreholes (above 100 m depth)) were acquired around the dumpsite. The well water were labeled AJW1 (x;y: 8.707953°; 4.464948°), AJW2 (x;y: 8.703558°; 4.470159°), and AJW3 (x;y: 8.709678°; 4.465798°), while the boreholes’ are AJB1 (x;y: 8.707801°; 4.464702°), AJB2 (x;y: 8.707508°; 4.466025°), and AJB3 (x;y: 8.70826°; 4.465474°). The in situ parameters viz; temperature, turbidity, electrical conductivity, appearance, odor, and taste (Fig. 5); preservation and transportation of water samples for chemical and heavy metal parameters laboratory analysis were done according to standard prescription by the American Public Health Association (APHA 2005). The hydrogen ion concentration in ground water and the acidity or alkalinity of the water are both key indicators of pH. Water’s acidity is measured by its ability to quantitatively reduce a strong base to a particular pH level, and it is typically caused by carbon dioxide, mineral acids, and hydrolyzed salts like ferric and aluminum sulfates. Acids can have an impact on a variety of activities, including biological processes, chemical reactions, and corrosion (APHA 2005). The main contributors to water alkalinity are the ions hydroxide (OH−), bicarbonate (HCO3−) carbonate (CO32−), or a combination of two of these ions. When softening water, the amount of lime and soda needed must be calculated based on the alkalinity of the water. All of the titratable bases together make up the alkalinity of water, which is its ability to neutralize acids (APHA 2005). It has also been found that temperature affects the metabolic processes of aquatic organisms (Igwemmar et al. 2013), while the chemical processes will be accelerated in a body of water as its temperature rises. Due to the insolubility of gasses like oxygen, effects like a foul odor and taste will result. The turbidity (cloudiness of water) measures how well light can travel through water (APHA 2005). Electrical conductivity measures the water’s ionic content and is directly related to the total amount of dissolved particles.

Fig. 5
figure 5

Compaction result for soil a sample A, b sample B, c sample C, and d sample D, showing the maximum dry density (MDD) and optimum moisture content (OMC)

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In water, solids can be suspended or in solution. Glass fiber filter were to distinguish between two main categories of solids (Tchobanoglous et al. 2003) in the samples. First are dissolved solids, which pass through the filter with the water, and second are suspended solids, where particles and colloids are retained on the filter’s top (APHA 2005). One of the other key indicators of water quality is dissolved oxygen (DO), which serves as a crucial measure of water pollution (APHA 2005). Higher dissolved oxygen “DO” concentrations are related to better water quality. However, only very little oxygen is soluble in water, and this has great sensitivity to temperature (Ogbonnaya 2008; Wasiu et al. 2022). The chemical and heavy metals analysis in this study include: total suspended solid (TSS), total dissolved solids (TDS), total hardness (TH), calcium hardness, magnesium hardness, acidity, alkalinity, nitrate (NO−3), chloride (Cl), sulfate \({(SO}^{3-})\), dissolved oxygen (DO), biochemical oxygen demand (BOD), bicarbonate, carbonate, arsenic (As), cadmium (Cd), copper (Cu), chromium (Cr), iron (Fe), lead (Pb), and zinc (Zn). The gravimetric method was used to estimate the TDS from the resultant TSS, while the TH and calcium hardness were determined using teh Erichrome indicator and further used for Mg hardness calculation (Table 2). The acidity and/or alkalinity, NO−3 and Cl of the water samples were determined through titration. Presence of \({SO}_{4}^{2-}\) was determined using the gravimetric Method. Sulfate is precipitated in hydrochloric acid medium as BaSO4 by the addition of BaCl2 solution. The dissolved oxygen was determined using the iodometric titration method on content of the stored water sample kept in a dark room for 7 days, while the BOD was determined by subtracting the dissolved oxygen (DO) of the seventh day from the initial dissolved oxygen for the first day. The analytical technique used to determine the concentration of metal atoms/ions (As, Cd, Cu, Cr, Fe, Pb, Zn) in the water samples was determined after digestion, using the atomic absorption spectroscopy (AAS), and the results are presented in Table 3.

Table 2 Statistical values (mean and standard deviation) for the chemical parameters of the water samples
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Table 3 Statistical values (mean and standard deviation) for the heavy metals variables in water samples
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Result and discussion

Leachate mapping

The ERT sections (Fig. 2a–c) delineate the presence of decomposed/inorganic waste, topsoil with very low resistivity within \(1.8\times {10}^{-3}\) to \(6.4\times {10}^{-1}\) Ωm and fresh dump to lateritic cover with relatively high resistivity. The resistivity model corresponds to conductive material to about 2 m, underlined by a thin layer (about 0.5 m thick) of sandy-clay/weathered basement with a resistivity range within 87.033 Ωm to 222.13 Ωm and fresh basement downward with the highest resistivity contrast. The lowest resistivity signature at the western region on traverse 1 (Fig. 2a) is attributable to the topography as it is at a low elevation to other traverses and, thus, linked with the leachate percolation path. The ERT traverse 2 reveal the conspicuous silty-clay liner (depicted with irregular polygon) beneath the inorganic waste topsoil. The high resistivity contrast (above 100 Ωm) on traverse 3 (5 m away from the dumpsite; Fig. 2c) shows a typical basement complex lithology with top lateritic hardpan overlying the sandy-clay or weathered basement rock and the fresh near-surface rocks. The underlying weathered zones and/or fractured zones are considered insignificant to leachate migration in the area because they do not show surface manifestation and could not serve as conduit for contaminant.

The soil classification test corroborated the resistivity model with a result that shows the topsoil as poorly graded (≤ 50% fines and ≤ 3% gravel) inorganic silt-sand (Fig. 3) using specific gravity “SG” range 2.61 to 2.67 in concordance with the standard soil type for SG classification by Lambe and Whiteman (1969); Rowe et al. (1995); Elsbury et al. (1990); NRA (1995). Pit PAC (along traverse 2) revealed a lower plastic index within 22.5–22.6 (Fig. 4) which depicts the thick inorganic silty sand overburden with medium swelling potential and very low permeability within \(2.85\times {10}^{-6}\) to \(2.91\times {10}^{-6}\), while PBD has relatively high swelling potential due to the high PI and low permeability range \(3.28\times {10}^{-5}\) and \(3.01\times {10}^{-5}\) and (Table 1). The characteristics of the sub-surface soil (at greater than/equal to 2-m depth) are attributable to silty-clay with: maximum dry density (MDD) range from 1.75 to 1.87 g/cm3, optimal moisture content (OMC) range from 11.1 to 14.5% (depicted in Fig. 5), low permeability (low hydraulic conductivity), and acceptable liquid limit (> 20%) for liner materials, following the classification as compared to field need by Benson et al. (1994); Rowe et al. (1995); Daniel (1987); O’Flaherty (1998); Layade and Ogunkoya (2018).

Water quality

Physical parameters

The in situ sensory test revealed the groundwater samples as tasteless, colorless, and odorless, while the hydrogen ion concentration and the acidity/alkalinity of the water indicate a pH level range within 6.85 to 7.10 with a temperature range from 27.2 to 27.8 °C (Fig. 6). The pH is within the neutral point, and the temperature is barely below the standard room temperature, which is the permissible limit for portable water (FEPA 1991; WHO 2004; Ukpong and Okon 2013; Wasiu et al. 2022). The turbidity is higher in the samples from the shallow wells (within the range 2.04–2.56 NTU) than in the relatively low range of 1.20 to 1.38 NTU in the borehole samples. Colloidal particles made of clay and silt at shallow depths in the area within the wells’ depth level might be attributed to the increased turbidity of the near-surface water as compared with that of the deeper source. However, the turbidity is generally relatively low as compared with the estimated range of Sunday and Oyinade (2020) and in concordance with the acceptable range stated by WHO and the NSDWQ. The water’s ionic content (obtained from an electrical conductivity test) ranges from 58.50 to 243.00 µS/cm, with well AJW2 (about 2-m deep and near PAC), having the highest conductivity. The high conductivity is attributable to induced near-surface leachate migration of ionic chemicals into the well water, conspicuous in the electrical resistivity model of traverse 2.

Fig. 6
figure 6

Representation of the in situ physical parameters of water sample obtained from hand-dug wells and boreholes around Malete dumpsite

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Chemical parameters

The biochemical oxygen demand (BOD) is the quantity of oxygen required in a given volume of water to completely degrade or stabilize all biodegradable organic compounds. The “DO” and “BOD” of the water samples range between 88.00 to 129.60 mg/L and 19.20 to 56.00 mg/L respectively (Table 2). Both parameters are higher in the well water samples and quite above the permissible level, indicating some concentration of organic materials. The acidity and alkalinity levels of the water samples range between 0.20 to 0.48 mg/L and 2.80 to 4.40 mg/L, respectively, while the estimated total hardness ranged between 114.40 and 252.03 mg/L (Fig. 7). The concentration of calcium (Ca2+) and magnesium (Mg2+) ions is higher in the well water samples, characteristics of highly mineralized water which might have percolated primarily through the poorly graded inorganic silt-sand and/or silty-clay (with low permeability). Total suspended solids (TSS) viz; calcium, magnesium, sodium, and manganese chlorides, carbonates, sulfates, and nitrates concentrate range within 5.24 to 18.58 mg/L. The total dissolved solids (TDS) concentrate range within 16.41 to 23.78 mg/L which corresponds to fresh water. The samples generally possess low concentrations of total hardness. TDS and TSS are higher in borehole water samples with low nitrate, sulfate, and phosphate concentration, but within the acceptable range for drinking and all irrigation water usage, and could have no negative effects on the public’s health. In very small or trace amounts, water may contain a wide range of toxic inorganic substances. They may pose a risk to public health even in trace amounts (Rowe et al. 1995). Although many toxic substances come from industrial activities or improper management of hazardous waste, some toxic substances come from natural sources (Tchobanoglous et al. 2003). The heavy metal (Ar, Cd, Cr, Fe, Pb, Zn, and Cu) concentrations in the water samples (Table 3) are currently within the WHO standard permissible limit for potable water. However, the slight increase in Cd concentration (above 0.005) in the deeper wells (AJBs) suggests immediate treatment and prevention of prolonged exposure to the drinking water (Fig. 8).

Fig. 7
figure 7

Representation of the chemical parameters examined in the water samples. a Hand-dug well “AJW” and b borehole water “AJB”; Conc, concentration; T.Ac, total acidity; T.Alk, total alkalinity; T.H, total hardness; DO, dissolved oxygen; BOD, biochemical oxygen demand; TDS, total dissolved solids; TSS, total suspended solids; Cl, chloride; SO4, sulphate; N03, nitrate; P04, phosphate

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Fig. 8
figure 8

Representation of the heavy metals examined in the water samples a hand-dug well “AJW” and b borehole water “AJB”

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Conclusion

Integrated 2D electrical resistivity tomography, soil classifications, and physicochemical parameters of water samples have been used to determine the near-surface occurrence of leachate plume within a dumpsite in a Precambrian basement rock area, Malete, Nigeria. High resolution of the subsurface resistivity distribution “ERT” revealed leachate dispersion beneath decomposed/inorganic waste topsoil to about 2 m, hoisting low resistive moisturized soil within range \(1.8\times {10}^{-3}\) to \(6.4\times {10}^{-1}\) Ωm and underlain by a thin layer of about 0.5-m thick sandy-clay and weathered basement downward. The corresponding soil classification results indicate the thick overburden as poorly graded inorganic silt-sand with a specific gravity range within 2.61 to 2.67, medium swelling potential, and a very low permeability range within \(2.85\times {10}^{-6}\) to \(2.91\times {10}^{-6}\). The subsurface soil is characterized as silty-clay with a maximum dry density range from 1.75 to 1.87 g/cm3, optimal moisture content range from 11.1 to 14.5%, low permeability, and acceptable liquid limit (> 20%), which indicate a suitable liner material capable of impeding the flow of leachate to prevent groundwater contamination.

The in situ sensory test on six water samples around the dumpsite revealed acceptable physical characteristics of drinking water which are tasteless, colorless, and odorless, neutral pH level range within 6.85 to 7.10, and at the normal room temperature, a temperature range from 27.2 to 27.8 °C. The increased (but within the acceptable limit) turbidity, conductivity, DO, and BOD within the shallow wells are attributed to induced near-surface leachate migration of ionic chemicals or organic materials which have percolated primarily through the poorly graded inorganic silt-sand (topsoil) but prevented by the impervious silt–clay liner from vertical migration to the deeper water source. Also, the concentrations of As, Cd, Cr, Fe, Pb, Zn, and Cu in the water samples are below the recommended concentration approved by the WHO as the permissible limit for potable water. Therefore, increased TDS and TSS in samples of deeper water sources with low nitrate, sulfate, and phosphate concentration are within the acceptable range of drinking water that could have no negative effects on the public’s health.