Investigation of Groundwater Pollution Around Angwan Gwandara Dumpsite, Garaku: Vertical Electrical Sounding (ves)

INTRODUCTION

Groundwater is a critical source of potable water for domestic, industrial, and agricultural use, especially in developing countries where surface water supply is limited [1]. In Nigeria, many communities depend on groundwater obtained from wells and boreholes due to inadequate water infrastructure and increasing population growth [1]. Despite its importance, groundwater quality is increasingly threatened by anthropogenic activities such as improper waste disposal, industrial effluents, and agricultural practices [1 - 3]. Open dumpsites remain one of the most common waste disposal methods in many Nigerian communities [4 - 5]. These dumpsites often lack engineered containment systems, allowing rainwater to infiltrate waste materials and generate leachate [5 - 6]. The resulting leachate typically contains dissolved organic matter, nutrients, heavy metals, and microorganisms that can migrate into surrounding soils and groundwater systems [6 - 7].

The vulnerability of groundwater to contamination depends largely on geological characteristics, aquifer depth, permeability, and the distance from pollution sources [7]. In basement complex terrains such as those found in Nasarawa State, Nigeria, groundwater occurs mainly within weathered regolith and fractured bedrock [8]. These formations provide secondary porosity that enhances groundwater storage but also increases susceptibility to contamination [8 - 9]. Geophysical methods have proven effective for investigating subsurface structures and assessing groundwater conditions. Electrical resistivity techniques are useful because subsurface resistivity varies with lithology, moisture content, and dissolved ionic concentration [9, 10]. Vertical Electrical Sounding (VES) is widely applied for identifying aquifer zones and evaluating groundwater potential [11].

Hydrochemical analysis complements geophysical investigations by providing direct information on water quality and contaminant concentrations. The integration of geophysical and hydrochemical methods therefore provides a comprehensive approach for assessing groundwater vulnerability [11 – 14].

Garaku, located in Kokona Local Government Area of Nasarawa State, has experienced population growth and increased waste generation. Waste from nearby settlements is commonly disposed of in open dumpsites, including the Angwan Gwandara dumpsite. The absence of engineered waste management systems raises concerns about potential contamination of groundwater sources [15 - 18] used by local communities. Despite the reliance on groundwater in the area, limited studies have evaluated the impact of dumpsite activities on groundwater quality. Understanding the context of contamination and identifying vulnerable aquifer zones is therefore essential for sustainable groundwater management [19 - 20].

This study aims to assess groundwater contamination around the Angwan Gwandara dumpsite using integrated geophysical and hydrochemical techniques, with objectives to determine subsurface resistivity variations using Electrical Sounding, identify aquifer systems within the study area, evaluate groundwater quality through hydrochemical analysis, and assess the potential impact of the dumpsite leachate on groundwater resources.

MATERIALS AND METHODS

Study Area

The study area is located around Angwan Gwandara dumpsite in Garaku, Kokona Local Government Area of Nasarawa State, North-central Nigeria. The region lies within the Nigerian Basement Complex characterized by crystalline rocks such as granites, gneisses, and schists [21 - 23]. The climate is tropical with distinct wet and dry seasons. The rainy season occurs between April and October, while the dry season extends from November to March. Annual rainfall ranges between approximately 1100 and 1400 nm [24 - 25].

Groundwater in basement terrains typically occurs within weathered and fractured rock zones [25]. These formations provide secondary permeability that allows groundwater storage and movement [26]. However, shallow aquifers in such environments are often vulnerable to contamination from surface activities [26].

The presence of an open dumpsite within the study area raises concerns about possible infiltration of leachate into surrounding groundwater systems.

Sampling Strategy and Hydrochemical Analysis

The groundwater samples were collected from seven wells and boreholes located approximately 0.5 – 2 km from the dumpsite. Sampling was conducted during the dry season to minimize dilution effects from rainfall.

Water samples were stored in clean polyethylene containers and transported to the laboratory for analysis. The parameters analyzed are: pH, electrical conductivity, turbidity, chloride, nitrate, sodium, and calcium.

Standard analytical procedures were used to determine parameter concentrations, and the results were compared with recommended drinking water standards.

Analytical Methods for Heavy Metals

Groundwater samples collected from boreholes and hand-dug wells around the Angwan Gwandara dumpsite were analyzed for heavy metals using standard hydrochemical procedures.

The metals analyzed included: Lead (Pb), Zinc (Zn), Copper (Cu), and Iron (Fe).

Sampling and analytical procedures were carried out following internationally accepted methods recommended by the American Public Health Association (APHA, 2017).

Sample Collection and Preservation

Groundwater samples were collected in 1 L acid-washed polyethylene bottles.

Sample preservation procedures included; Filtration through 0.45 µm membrane filters, Acidification using concentrated nitric acid to prevents precipitation and adsorption of metals onto container walls, Adjustment of pH to less than 2, and Storage at 4°C prior to analysis.

Sample Digestion

Samples were digested before instrumental analysis.

Procedure:

1. 100 mL of water sample transferred into digestion flask;

1. 5 mL concentrated HNO₃ was added;

1. It was then Heated on hot plate at 95°C;

1. It was Reduced to 20 mL;

1. Diluted to 100 mL using deionized water; and

1. Filtered before analysis

The digestion was to converts metals into dissolved ionic form.

Atomic Absorption Spectroscopy (AAS)

Heavy metals were quantified using Flame Atomic Absorption Spectroscopy.

Instrumentation

Atomic Absorption Spectrometer equipped with hollow cathode lamps for each element.

Analytical Conditions

The metals analyzed include Pb, Fe, Zn, and Cu, and are presented as shown in Table 1.

Table 1 Metals with their typical wavelengths, Method and LOD

S/No	Metal	Wavelength (nm)	Method	LOD (mg/L)
1	Pb	217.0	AAS	0.01
2	Fe	248.3	AAS	0.01
3	Zn	213.9	AAS	0.005
4	Cu	324.8	AAS	0.005

Principle:

The free atoms produced in a flame absorb radiation from element-specific lamps. Metal concentration is proportional to absorbance (APHA, 2017; Skoog et al., 2014; Welz and Sperling, 1999).

Quality Assurance and Quality Control (QA/QC)

Strict QA/QC procedures were implemented to ensure accuracy and precision.

Field Quality Control

Field Blanks: Field blanks consisted of deionized water transported to the sampling site. Field blanks were treated identically to groundwater samples.

Purpose: Detect contamination during sampling (USEPA, 2007)

Trip Blanks

Trip blanks remained sealed during sampling.

Purpose: Detect contamination during transportation (Keith, 1996)

Laboratory Quality Control

Calibration Standards: Calibration standards prepared from certified reference materials.

Concentration range of 0.01 – 5 mg/L, and Correlation coefficient of R² ≥ 0.995 (Skoog et al., 2014) was used.

Analytical Blanks: Reagent blanks analyzed with each batch to Identify contamination from reagents (APHA, 2017).

Duplicate Samples: Ten percent of samples analyzed in duplicate.

Precision evaluated using: The Relative Percent Difference (RPD), with acceptable precision of RPD < 10% (USEPA, 2007) was done.

Detection Limits: Detection limits determined from blank measurements was done following Miller and Miller, 2010.

Geophysical Data Acquisition

A PASI Terrameter SAS 1000 was used with Schlumberger and Wenner electrode arrays.

1. A total of 20 VES stations were occupied along four traverses, with AB/2 spacing up to 300 – 400 m.

1. Control VES stations were positioned near boreholes for lithological correlation.

1. Instrument calibration was performed using a standard test resistor prior to each field season.

1. Error margins for apparent resistivity readings were within 5 %, with repeated measurements taken to ensure reproducibility.

Fig. 1. Terrameter, electrodes, cables and the hammer for electrical resistivity survey

Schlumberger Array: The potential electrodes remained fixed while the current electrodes were progressively moved outward. When the voltage between P1 and P2 became too low to measure accurately, the potential electrode spacing was increased.

The distance between the currents electrodes was sufficient with the depth to which the corresponding current penetrates the subsurface. For depth sounding, the measured resistance was made at the shortest separation and then at the progressively large spacing.

Figure

Fig. 2. Vertical electrical method array configuration

Wenner Array: All four (4) electrodes were moved outward simultaneously at equal increments.

The measured resistance at each electrode separation was used to calculate apparent resistivity, incorporating the geometric factor specific to each electrode configuration. The resistivity survey was conducted along four (4) traverses, using Schlumberger for depth sounding and Wenner for lateral variations. Twenty (20) VES soundings were carried out, with current electrode separations ranging from 300 to 400 m. A PASI Resistivity Terrameter, supported by a 12V 60Ah battery was used for data acquisition. At each separation, a value of apparent resistivity was calculated using the measured resistance in conjunction with the appropriate geometeric factor for the electrode configuration and separation used. In Wenner array, all the four electrodes were moved to a new position while in Schlumberger array, the potential electrodes were placed at a fixed spacing which is no more than one-fifth of the current electrode half spacing .

Data Processing and Integration

1. VES data were initially interpreted using curve matching with auxiliary master curves (H, K, A, Q types) and refined with IPI2WIN inversion software.

1. The hydrochemical data were compared against the NSDWQ (2007) and WHO (2010) standards.

1. The integration was carried by statistical correlation (Pearson’s r) between electrical resistivity anomalies and chemical exceedances (EC, nitrates, and chloride).

1. The pseudo-sections and geoelectric cross-sections were generated with WinResist and surfer 11 software.

RESULTS AND DISCUSSION

Resistivity Survey

The vertical electrical sounding curves revealed 3 – 5 geoelectric layers, with resistivity values ranging from 10 Ω.m for saturated fractured/weathered basement aquifers, and greater than 200 Ω.m for fresh basement or mineralized veins. The depth to aquifers varied between 5 – 50 m. while the borehole lithology confirmed that productive aquifers occur in fractured zones at 20 – 60 m (Table 2).

The summary of resistivity values, depths, and lithological classifications for the various locations is presented in Table 2.

Vertical Electrical Sounding (VES) Result and Interpretation Table 2: The VES points at different layers

VES	LAYER 1	LAYER 2	LAYER 3	LAYER 4	LAYER 5
	R (Ω.m)	H (m)	R (Ω.m)	H (m)	R (Ω.m)	H (m)	R (Ω.m)	H (m)	R (Ω.m)	H (m)
1	487	1.12	15.3	1.19	43.9	-	-	-	-	-
2	204	1.97	18.8	1.74	274	-	-	-	-	-
3	197	0.667	7.47	0.779	9827	2.87	844	-	-	-
4	342	0.444	20.6	2.83	30.36	-	-	-	-	-
5	251	0.539	5.72	0.235	1047	1.64	30.7	3.77	256	-
6	307	3.19	27.9	2.22	58933	-	-	-	-	-
7	861	0.587	193	4.45	33.3	4.35	32283	-	-	-
8	304	0.386	38.8	5.64	2701	-	-	-	-	-
9	487	1.12	15.3	1.19	43.9	-	-	-	-	-
10	204	1.97	18.8	1.74	274	-	-	-	-	-
11	197	0.667	1.47	0.779	9827	2.87	844	-	-	-
12	342	0.444	30.6	2.83	3036	-	-	-	-	-

Apparent resistivity values calculated from the field (i.e. resistance values multiplied by dielectric constant K) were calculated and the product ohm/m was then used against the current distance spreading , to generate a curve on a bi-log graph. The interpretation is finally confirmed by using the IPI2WIN software. The software is able to generate modelled curves and interpret geo-electric layers. Rock resistivity of most rock forming minerals is 108–10-1016 ohm/m. However, measurement. Insitu gives the following resistivity range values; sedimentary rocks 5-1000ohm/m, metamorphic/crystalline rocks: 100-1050 ohm/m. The range of the resistivity is due to the fact that rocks usually have pores or fractures that could be filled with fluids or crystallized ores hosting gold, and other precious stones, metals, e.t.c.

Table 2 along with the curves in Figures 3-11 give information about resistivity layer and curve fitting.

For VES 1 (Figure 3), the curve outlines three layers. The first layer has resistivity of 487 Ω.m and thickness of 1.12 Ω.m, representing lateritic topsoil. The second layer has resistivity of 15.3 Ω.m with thickness of 1.19 m, and basement occurs at 2.5m, indicating saturated clay. The low resistivity in the second layer suggests high moisture content. The interpreted layer from 2.5 to 50 m represents a fractured basement, indicating a potential groundwater reservoir.

Fig. 3. VES 1 Log-log plot (N8o52’2.154’’,E808’8.346’’) HK

For VES 2 (Figure 4), the curve outlines three layers. The first layer has resistivity of 204 Ω.m with thickness of 1.97 m. The second layer has resistivity of 18.8 Ω.m with thickness of 1.74 m, and basement occurs at 4.5 m. The fractured Zone occurs at 41 m depth in the third layer and extends 60 -70 m, indicating fractured lithology and suggesting good groundwater potential.

Fig. 4. VES 2 Log-log graph (N8050’31’’, E7046’15’’)

For VES 3 (Figure 6), the curve outlines four layers. The first layer has resistivity of 197 Ω.m with thickness of 0.667 m. The second layer has resistivity of 7.47 with thickness of 0.778 m. The third layer has resistivity of 9827 with thickness of 2.87 m, and basement occurs at 4.7 m. A thin surface layer of about 1.5 - 2.0 m is observed. The fractured zone is overlain by a hydrothermal vein at 41 - 50 m depth, suggesting mineralization that may host gold or other precious metals.

For VES 4 (Figure 6), the curve outlines three layers. The first layer has resistivity of 342 Ω.m with thickness of 0.444 m. The second layer has resistivity of 20.6 Ω.m with thickness of 2.83 m, and basement occurs at 3.5 m. The second layer represents a clay-dominated zone, while the fractured basement suggests groundwater potential.

For VES 5 (Figure 7), the curve outlines five layers. The first layer has resistivity of 251 with thickness of 0.539 m. The second layer has resistivity of 5.72 Ω.m with thickness of 0.235 m. The third layer has resistivity of 1047 Ω.m with thickness of 1.64 m. The fourth layer has resistivity of 30.7 Ω.m with thickness of 3.77 m, with basement occurring at 11-12 m. The conductive clay layer may act as an impermeable cap that traps within the fractured basement.

For VES 6 (Figure 8), the curve outlines three layers. The first layer has resistivity of 307 Ω.m with thickness of 3.19 m. The second layer has resistivity of 27.9 Ω.m with thickness of 2.22 m, representing a fractured zone, and basement occurs at 6 m. The fractured zone extending into the third layer suggests a groundwater-bearing formation.

For VES 7 (Figure 9), the curve outlines four layers. The first layer has resistivity of 861 Ω.m with thickness of 0.587 m. The second layer has resistivity of 193 Ω.m with thickness of 4.45 m. The third layer has resistivity of 33.3 Ω.m with thickness of 4.35 m, and basement occurs at 9 m. Fractured zones occur in the second and third layers depth between 1 – 9 m, indicating significant groundwater potential.

For VES 8 (Figure 10), the curve outlines three layers. The first layer has resistivity of 304 Ω.m with thickness of 0.386 m. The second layer has resistivity of 38.8 Ω.m with thickness of 5.64 m, and basement occurs at 7 m. The low resistivity of the second Layer suggests a saturated formation indicating possible water accumulation (see VES 8).

In Figure 12, From this location (N8o51’15’’,E7046’48’’), the resistivity values of the first and second layers appear similar, but their depths differentiate them. The interpreted layer from 2.5 to 50 m represents a fractured formation. As shown in Figure 13, at location (N8051’25’’, E7047’25’’), the fractured zone occurs at 41 m depth and extends to 60-70 m, indicating fractured lithology.

From location (N8051’35’’, E7047’23’’) in Figure 14, a thin surface layer of 1.5-2.0 m is observed, with a fractured zone overlain by a hydrothermal vein at of 41-50 m depth.

From location (N8052’01’’, E7047’43’’), as shown in Figure 14, the fractured zone extends from 1.4-50 m with thickness of 0.5-3.4 m.

Pseudo-cross sections

Figure 15 – 26 present the Pseudo-cross sections, which display measured apparent resistivity values in pictorial form and guide for further interpretations. Red coloration represents litho-layers with high resistivity, while blue coloration represents low resistivity zones.

The Pseudo-Section reveals potential fracture zones that vary with depth. Color variations represent layer with similar geo-electrical properties. Red shades correspond to regions of low electrical conductivity such as compact rocks or fresh basement. Blue shades indicate conductive zones associated with fractures or metallic minerals.

High-resistivity zones at shallow depths correspond to compact rock formations, whereas low-resistivity zones correspond to fractured or hydrothermal layers that may indicate groundwater potential or mineralized veins.

Geo-Electrical Section

The first geo-electric layer represents topsoil with resistivity values of 80-849 Ω.m and thickness of 0.9-1.5 m, suggesting Laterite and sandy clay.

The second geo-electric layer consists of saturated laterite and loamy clay with resistivity values of 58 - 944 Ω.m and thickness 0.8-3.5 m, indicating possible shallow groundwater reservoirs.

The third geo-electric layer represents saturated weathered or fractured formations with resistivity values of 59 - 340 Ω.m and thickness of 2.5 - 15.5 m, suggesting the main aquifer zone.

The fourth geo-electric layer represents the fresh basement with resistivity values of 78-639 Ω.m and thickness of 10 - 30 m, indicating deeper groundwater potential.

The fifth layers represents the fresh basement with resistivity values of 201 - 927 Ω.m with infinite depth, composed of hard and compact rock, and generally non-water-bearing.

The study area consists of sedimentary formations overlying crystalline basement rocks. The variations in resistivity are influenced by fracturing and weathering, hydrothermal alteration, and topography or drainage conditions.

Overall, the results suggest promising groundwater potential, where fractured basement layers acts as primary aquifers, while hydrothermal veins may indicate zones of mineralization requiring further exploration.

Physico-chemical and Heavy Metals

BH = Borehole water, W = well water

Physical Parameters

The physical parameters analyzed in the groundwater samples include pH, turbidity, electrical conductivity, and resistivity, and are discussed below.

Hydrogen ion concentration (pH)

pH measures hydrogen ion concentration in water and indicates its acidity or alkalinity. A value of 7 is neutral, values below 7 are acidic, and above 7 are alkaline. The pH (Table 4) of groundwater samples in the study area ranges from 6.06 to 7.25, with a median of 6.58. These values fall within the permissible limits of 6.5 – 8.5 recommended by WHO (2010) and NSDWQ (2007). All samples from locations W1 to W7 (Table 3) are within acceptable limits, except W2, which is slightly acidic. Slight acidity may enhance rock weathering and dissolution, potentially increasing metal concentrations in groundwater.

Table 3: Physico-chemical and heavy metals parameters analyzed in the water sample from the study area

Parameter	W1	W2	W3	BH4	W5	W6	BH7	NSDWQ (2007)	WHO (2010)
pH	6.58	6.87	6.51	7.25	7.17	6.11	6.06	6.5-8.5	6.5-8.5
TURBIDITY	0.00	6.00	0.00	5.0	7.0	3.0	5.0	5.0	5.0
EC(μS/cm)	2,040	3,500	3,120	960	1,700	1,380	240	1,000	1,000
Chloride	152.1	207.2	239.6	17.5	80.2	71.9	83.9	250.0	250.0
Nitrate	0.000	4.172	0.117	0.083	0.024	0.037	0.0404	0.2	0.2
Calcium	85.8	71.3	165.1	37,7	72.9	52.9	32.9	-	75.0
Sodium	123	173	147	47	80	79	8	200.0	200.0

Table 4: Statistical comparison of the groundwater parameters in the study area with NSDWQ (2007) and WHO (2010)

Parameter (mg/L)	Min.	Max.	Mean	Median	NSDWQ (2007)	WHO (2010)
pH	6.06	7.25	6..65	6.58	6.5-8.5	6.5-8.5
TURBIDITY	0.0	7.0	3.07	5.0	5.0	5.0
CONDUCTIVITY	240	3.500	1,848	1700	1,000	1,000
Chloride	17.5	239.6	121.8	83.9	250	250
Nitrate	0.000	4.172	0.639	0.0404	0.2	0.2
Calcium	32.9	165.1	63.67	71.3	-	75.0
Sodium	8	173	93	80	200	200

Turbidity

Turbidity indicates the clarity of water and reflects the presence of suspended particles. Turbidity values in this study ranged from 0.0 to 7.0 NTU, with a median concentration of 5.0 NTU. Most samples fall within the 5.0 NTU acceptable limit recommended by NSDWQ (2007) and WHO (2010), except at location W2, where turbidity slightly exceeded the standard, suggesting possible contamination.

Electrical conductivity (EC)

EC reflects the ionic content of groundwater and the level of dissolved minerals. EC values ranged from 240.0 to 3500 µS/cm, with a median of 1700 µS/cm (Table 4). Most samples exceeded the 1000 µS/cm limit recommendation by NSDWQ (2007), except BH4 and BH7. Elevated EC values indicates increased mineralization and possible contamination.

Chemical Parameters

Anions in the Water

The analyzed anions include chloride and nitrite.

Chloride: Chloride concentrations ranged from 17.5 to 239.6 mg/L, with a median of 83.9 mg/L (Table 3). All the samples were below limit of 250.00 mg/L set by NSDWQ (2007) and WHO (2010), indicating no significant chloride contamination.

Nitrite: Nitrite concentration varied between 0.0 to 4.172 mg/L, with a median of 0.0404 mg/L (Table 4). Most samples were below 0.2 mg/L limit recommended by WHO (2010) and NSDWQ (2007), except W2, which showed elevated levels associated with risks such as infant methaemoglobinaemia (blue baby syndrome).

Cations in water

The major cations analyzed include sodium and calcium (Table 3).

Calcium: Calcium concentrations ranged from 32.9 mg/L to 165.1 mg/L, with a median of 71.3 mg/L (Table 4). Locations W1 and W3 exceeded the WHO (2010) guideline of 75 mg/L. Although calcium is nutritionally beneficial, excessive levels may contribute to water hardness.

Sodium: Sodium concentrations ranges from 8.0 to 173.0 mg/L, with a median of 80 mg/L (Table 3). All values were below the permissible 200 mg/L limit recommended by WHO (2010) and NSDWQ (2007).

Summary of the Hydrochemical Quality

Overall groundwater quality shows pH values within acceptable limits, except slight acidity at W2.

EC values (240 – 3500 ) exceed recommended limits at several sites. Turbidity (0 – 7 NTU) slightly exceeds limits at W2 and W5. Chloride concentration (17.5 – 239.6 mg/L) remain within safe limits, while nitrite levels up to 4.172 mg/L exceed standard at W2. Calcium ranges from 32.9 – 165.1 mg/L, exceeding limits at W1 and W3, while sodium (8 – 173 mg/L) remains within levels.

Heavy metals; Pb (0.12 – 0.32 mg/L) exceeding WHO guideline of 0.01 mg/L at 3 wells; Fe was elevated in 2 samples (1.5 – 2.8 mg/L, above 0.3 mg/L).

Integration of VES and Hydrochemistry

Low resistivity (10 – 50 Ωm) correspond with high electrical conductivity (>2000 ) and elevated nitrates, confirming leachate infiltration.

Shallow wells (>20 m) near the dumpsite show both low resistivity and poorer water quality, whereas deeper boreholes (50 – 70 m) display higher resistivity and improved water quality.

Comparative Analysis

These findings aligns with similar studies in Minna Niger State, Nigeria [Amadi et al., 2019], Ibadan [Oke and Oladunjoye, 2021], and Sapele [Okezie and Elijah, 2014], where dumpsite leachate produced low resistivity anomalies and hydrochemical exceedances.

In Garaku, the fractured and weathered basement enhances vertical contaminant migration, increasing groundwater vulnerability.

CONCLUSION

In this study, VES was used to determine subsurface resistivity distribution. The interpretation was based on the established geophysical literature, and restricted to resistivity ranges supported by published studies, and the resistivity values greater than 200 Ω.m were observed in several VES locations which could be due to fresh basement rocks, unweathered crystalline rocks, and mineralized zones. Fresh basement rocks typically exhibit high resistivity which could be due to low porosity, low moisture, and high density. This could typically be between 500 – 10,000 Ω.m (Telford et al., 1990) as unweathered crystalline rocks, Mineralized zones and silicified hydrothermal veins exhibit high resistivity (Kearey et al., 2002; Reynolds, 2011; Parasnis, 1986).

This study delineate subsurface layers and aquifer zones. Low resistivity indicate conductive materials such as clay or contamination zones, while higher resistivity represents weathered/fractured basement. These formations control groundwater occurrence and potential contamination migration within the study area. Elevated nitrate, Pb, and Fe levels at some locations pose potential health risks. Regular geophysical surveys and continuous groundwater monitoring are recommended.

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