This study was conducted in the urban area of Ikot Ekpene and its surrounding areas. Ikot Ekpene, sometimes referred to as the Raffia City is located in the northwestern region of Akwa Ibom State in southern Nigeria (Fig. 1). It is positioned between latitudes 5.072°–5.140° N and longitudes 7.390°–7.458° E.

It covers an approximate area of 25 square kilometres. The region exhibits a gently rolling terrain, with elevated areas in the north and a gradual descent towards the southwest. The highest point above sea level in the area is approximately 102 meters in the northern region. In comparison, the lowest point is around 54 meters in the southern region (Fig. 2). The arrows depicted in Fig. 2 illustrate the trajectory of surface water (rain) movement, predominantly directed towards the ravine situated in the southwestern region of the land. The placement of the dumpsites in the ravine may have been required due to the direction of surface water flow, as depicted in Fig. 2. The region experiences extensive drainage due to the presence of inland coastal water. The vegetation in the studied area exhibits characteristics typical of a rainforest ecosystem. The area experiences a high average relative humidity of 83% and receives a significant amount of precipitation, with an annual average of 250 mm [22].

Ikot Ekpene is located in an area with a humid tropical climate. This climate is characterized by two different seasons: the wet or rainy season, which occurs from March to October, and the dry season, which occurs from November to April [22]. Geographically, Ikot Ekpene is situated in the Niger Delta region, which is positioned right on the Gulf of Guinea on the Atlantic Ocean in Southern Nigeria. The predominant geological composition of the area consists of Coastal Plain Sands (CPS), specifically belonging to the Benin Evaluation of Protectivity of Aquifer… 124

Formation [23]. The Formation at the highest point of the Niger Delta consists of sands and gravels that are not well sorted, with a range of grain sizes from fine to coarse [23], [24]. The Agbada Formation, located in the Niger Delta region, is a significant hydrocarbon-bearing unit that underlies the Benin Formation [24], [25]. The primary hydrogeological units in the research region are the CPS, which are characterized by the presence of clay and sandy clay in certain localities [26]. At various areas in the area, a multi-aquifer system is formed by the accumulation of alternating sand and clay layers [2], [27], [28]. The Agbada Formation is situated below the Akata Formation, which forms the base of the Delta [24], [25]. The schematic diagram in Fig. 3 illustrates the broad stratigraphy of the Niger Delta, which includes the research region.

This study utilized two resistivity measuring approaches to sample the resistivity distribution in the subsurface. The IGIS signal enhancement resistivity meter SSP-MP-ATS and its accessories were employed for this purpose. The two techniques used are Vertical Electrical Sounding (VES) and Electric Resistivity Tomography (ERT). Both methods involve the placement of two electrodes, referred to as the current electrodes (A and B), on the surface of the earth to introduce an electric current into the subsurface. Additionally, two corresponding potential electrodes (M and N) are positioned on the surface to measure the resulting potential difference. The resistivity meter was linked to all four electrodes arranged in a line. The meter's output LED display indicated the ratio of the potential difference to current, which represents the apparent resistance of the earth layers that the electric current passed through. The extent of penetration is directly proportional to the increase in the distance between the current electrodes [2], [3], [16]. The VES approach employed the Schlumberger electrode configuration, whereas the ERT technique utilized the Wenner electrode configuration. The Schlumberger electrode arrangement entails incrementally increasing the distance between the current electrodes AB and, on occasion, the distance between the potential electrodes MN around the central part of the array in order to collect samples from deeper layers. The study found that the distance AB varied between 2 and 400 meters, whereas the distance MN varied between 0.5 and 20 meters. The electrode spacing in all the occupied sounding locations was set to ensure that AB is more than or equal to 5 MN, as per the assumption of potential gradient [30], [31]. The Wenner arrangement, on the other hand, requires the employment of a consistent distance between electrodes, which can be progressively raised to sample deeper layers at regular intervals. In the study for the ERT scenario, the initial electrode spacing was kept constant at 5 m. The spacing was then increased in increments of 5 m until a total spread length of 105 m was reached, following the methodology described in [3].

The purpose of this investigation was to examine any horizontal differences in the electrical resistivity of the shallow layers in order to compare them with the VES results for the corresponding shallow layers. A total of 20 VESs and 10 ERTs were conducted in the study region. The spatial arrangement of these sounding stations, which was limited by the road network and physical infrastructure, is illustrated in Fig. 1. Several soundings were conducted near existing boreholes that had lithological data available in the vicinity. This was done to assist in the computer interpretation of the gathered data. The lithological data provided limitations on the identification of the lithological and hydrogeological units in the area. Fig. 4 displays the arrangement of the field for collecting data using the VES approach in the research region.

The apparent resistivity for each approach used in this investigation was calculated for each station by multiplying the observed resistance with the associated geometric parameters. The geometric factors KS and KW for the Schlumberger and Wenner arrays are determined by Eq. (1) and Eq. (2), respectively.

where AB, MN, and a are current electrode separation, potential electrode separation and Wenner constant electrode separations, respectively.

The VES data involved plotting the apparent resistivity values for each sounding station against half of the current electrode separation (AB/2) on a log-log scale, resulting in the creation of sounding curves. The curves were later smoothed to eliminate noisy signatures, which could potentially cause errors in the computer modeling phase of data interpretation [2], [16], [17]. This was accomplished by calculating the mean of the two apparent resistivity values at crossover distances. Any outliers were removed while ensuring that the major trend of the curve was preserved. The traditional curve matching technique was employed to analyze each of the smoothed field curves in order to get the initial resistivities and thicknesses of the layers [32]. The initial layer indices were utilized as inputs in the computer-aided quantitative interpretation. In this investigation, the interpretation was conducted using a One-Dimensional (1D) least square forward modeling software tool called WINRESIST [33]. The software utilizes the input data to create a theoretical model. Subsequently, it establishes a correlation between the model and the observed field data, resulting in the final 1D resistivity model curves. The adequacy of the fit is quantified using the Root-Mean-Square Error (RMSE) Evaluation of Protectivity of Aquifer… 126

[34], which varied between 2.3% and 3.1%. Fig. 5.a displays a screenshot of the WINRESIST window during the calculation of the theoretical model, while Fig. 5.b shows the matching resist graph display. This inversion method was constrained using data from the three existing boreholes in the area. Fig. 7 displays the final model curves for VES 4, 7, 12, and 20, along with the accompanying drill lithological data. The geoelectric indices obtained from these final model curves are the genuine resistivity, thickness, and depth of each layer, respectively.

In this investigation, the ERT data was inverted in Two-Dimensions (2D) using a computer modeling software tool called RES2DINV [35–37]. The RES2DINV software utilizes the supplied field data to generate a theoretical model and then compares it to the actual field data. In the instance of the VES interpretation, the goodness of fit is quantified using the RMSE, which varied between 9.7% and 45.1%. Fig. 6 displays a momentary view of the RES2DINV window while conducting the 2D data inversion.

In Fig. 6, a)the uppermost panel, displays the measured apparent resistivity pseudosection, b) the middle panel, presents the calculated theoretical model resistivity pseudosection, and c) the lower panel, exhibits the inverse model resistivity pseudosection, which represents the actual resistivity variation of the subsurface. The 2D modelling yielded the accurate resistivity distribution of the shallow subsurface that was penetrated by the current, resulting in the final picture sections. Fig. 7 displays examples of these picture sections for ERTs 3, 5, 7, and 8, with the final VES model curves (VESs 4, 7, 12, and 20), facilitating straightforward comparison and linkage with the drill lithological log. By comparing the lithological data obtained from the boreholes near VESs 4, 7, 12, and 20 with the VES curves depicted in Fig. 7, it is evident that the primary aquifer in the region is located in the third layer. Nevertheless, the aquifer depths predicted from the VES curves do not completely align with the depths determined from the borehole lithological log. This discrepancy can occur because geological sections may not align precisely with geo-electrical sections [38]. The findings of the ERT interpretations exhibit a reasonably strong correlation with the borehole lithological log.

AKWAPOLY, d. Library Avenue. The results of both VES and ERT interpretations correlate fairly well with the borehole lithological log.

The interpretation of the VES data reveals that the research area consists of 3-4 geoelectric layers within the maximum current electrode spread used. The findings are succinctly presented in Table 1. The initial stratum, identified as the diverse surface soil, exhibits resistivity values ranging from 157.3 to 1278.2 Ωm and has a thickness ranging from 0.6 to 19.2 m. This layer spans the entirety of the research region. The observed substantial fluctuation in resistivity in this layer may be attributed to the artificial composition of the layer and the ongoing bioturbating activities inside it [40]. The analysis discovered a second layer at a depth of 0.6–19.2 m with a resistivity range of 31.8–2648.10 Ωm. This layer was characterized as sandy clay in certain spots and fine/graded sand in other regions. The observed fluctuation in resistivity in this layer can be attributed to the varying particle sizes of the geomaterials present. These grain sizes are representative of the CPS found in the Niger Delta [23], [24]. In the studied region, the third geoelectric layer is found at a depth of 9.0–86.6 m and has a resistivity range of 214.4–2839.0 Ωm. This layer represents the hydrogeological units (aquifer) that are economically important for groundwater extraction in the area. It consists mostly of fine/coarse and gravelly sands, as indicated by the borehole lithological log. The aquifer layer at specific sites (VESs 1, 2, 4, 5, 8, 9, 13, 15, and 19) has high resistivity values over 1500 Ωm. This phenomenon can be attributed to the presence of gravelly sand, which is confirmed by the borehole lithological log. This phenomenon can be attributed to the fact that the resistivity of the second layer is comparatively lower than that of the aquifer layer at certain sites, as seen in Table 1.

The aquifers are often unconfined, as shown by the absence of impermeable confining layers observed in the VES results. This finding aligns with the geological information obtained from the boreholes. The stacked ID pictures in Fig. 9 display the spatial distribution of resistivity for the first three layers found in the research region.

Fig. 9 Stacked image maps of layer 1, 2 and 3 resistivities. Layer 3 is the major exploitable aquifer in the study area.

The three layers in the southern half of the study region have rather high resistivity values, as depicted in Fig. 9. In certain areas of the research site, the final geoelectric layer had resistivity values ranging from 45.0 to 865.4 Ωm. In some sites (VESs 1, 4, 5, 8, 18, and 20), this layer was characterized as fine sand, whereas in other locations (VESs 11, 12, and 17), it was interpreted as sandy clay. The thickness and depth of the layer could not be established since the current did not reach the bottom of the layer at a maximum current electrode separation of 400 m in this study.

The ERT interpretation yields the electrical resistivity fluctuations of the near-surface layers. Two geoelectric layers were detected within the electrode spacing limit used, as indicated by the results given in Fig. 7. The resistivity values for the sample ERT stations depicted in Fig. 7 exhibit an upward trend from the uppermost layer to the lower layers for ERTs 7 and 8, whereas the opposite is observed for ERTs 3 and 5. The results demonstrate a strong correlation with both the VES data and the borehole lithology information for the various locations, as depicted in Fig. 7.

This study utilized three secondary geoelectric indices to evaluate the level of protection provided by the hydrogeological units in the study area. The indices mentioned refer to the longitudinal conductance, resistivity reflection, and anisotropy coefficients of the aquifer protective layers (layers 1 and 2). These indices were calculated using the relevant equations provided in Section 3.3. Table 2 provides a summary of the computed results. The longitudinal conductance varies between 0.02 and 0.85 Ω−1, whereas the resistivity RC ranges from -0.84 to 0.85.

Additionally, the anisotropy coefficient extends from 1.00 to 1.48. The thickness of the aquifer overburden varies between 9.0 and 86.6 meters. Fig. 10 displays the spatial arrangement of these factors within the designated study region. Based on the combination of these factors, the image map of Fig. 11 indicated four classes of protectivity.

The classes are presented in Table 3 and align with the classifications of [11], [12] and the ratings of weak, moderate, and good protectivity.

According to Abiola et al. [13], out of the 20 subjects etc. Fig. 11 and Fig. 12 depict the VES stations that were utilized in the study area. Fig. 11 reveals that the hydrogeological units in the study area exhibit a low level of protectivity. Specifically, out of the 20 stations analyzed, 13 stations (65%) were rated as having poor protectivity, two stations (10%) were rated as having weak protectivity, and four stations (20%) were rated as having moderate protectivity. Only one station (5%) received a good protectivity rating. The distribution map in Fig. 11 shows that the aquifers in the northeastern, central, and southeastern parts of the study area have a low level of protection. The remaining areas have a moderate level of protection, except for AKWAPOLY in the southwestern part of the area (VES 10), which has a high level of protection.

Fig. 12 Bar chart showing the aquifer protectivity rating of the VES stations in the study area.

The presence of a thick layer of impermeable protective material above the aquifer greatly increases its ability to prevent contamination from reaching the groundwater [4], [13]. Nevertheless, the southern-eastern portion of the study region (namely VES 2, 3, 4, 6, 9) exhibits zones with a substantial thickness of overburden composed of permeable fine/coarse sands. As a result, these layers do not have the capacity to effectively reduce the speed and filter the movement of fluids into the aquifer system, leading to a low protectivity rating. RC values below zero indicate the presence of a second layer composed of impermeable materials with low resistivity, whereas positive RC values suggest the presence of a second layer made up of permeable materials with high resistivity. Similarly, a negative RC indicates a moderate-to-good level of protection, whereas positive values indicate a poor-to-weak level of protection. Resistivity anisotropy occurs when the vertical resistivity is higher than the horizontal resistivity, indicated by an anisotropy coefficient λ greater than 1. When the resistivities in both directions are equal, the value of λ is 1, indicating isotropy. Specifically, resistivity remains constant regardless of the direction. Resistivity anisotropy can occur when there are alternating layers of thin-bedded sand and clay. Clay exhibits a high level of porosity and a low level of permeability, which means that it does not allow fluid to penetrate the subsurface easily. Based on the information provided, areas with negative RC values and an anisotropy coefficient larger than 1 indicate a moderate-to-good level of protectivity, as shown in Fig. 10. Nevertheless, the determining factor is the thickness of the overburden, which clarifies why the aquifer units are classified as having poor to weak protective qualities in specific areas (VES 1, 2, 5, 8, 18, and 20) while having negative RC and seeing anisotropy.

Both VES and ERT modelling results show that the uppermost layer, known as motley topsoil, is typically porous and permeable, with a resistivity range of 157.3–1278.2 Ωm. Typically, utility pipes for water distribution are buried within this layer. The pipes may experience corrosion if the resistivity of the topsoil is not sufficient. Resistivity levels below 10 Ωm are typically linked to highly corrosive layers. Levels ranging from 60 to 180 Ωm are connected with mildly corrosive layers, while values over 180 Ωm indicate non-Evaluation of protectivity… 136

corrosive layers [41–43]. According to this categorization, the motley topsoil is generally not corrosive, except at VES 15 (with a resistivity of 170.7 Ωm) and VES 20 (with a resistivity of 157.3 Ωm), where the layer is somewhat corrosive. Therefore, utility pipes can be safely buried in the diverse topsoil in the research region without any concern for corrosion-related damage.

Fig. 9 illustrates the contrast between the flow directions of surface water (rainwater) and groundwater. Water moves from areas of higher elevation to areas of lower elevation, as described by Mc-Neill [44]. The surface topography map (topmost image map in Fig. 13) indicates that the northern portion of the study region has a higher elevation compared to the rest. This is consistent with the aquifer protectivity rating map shown in Fig. 11, where the ravine area is marked in red and has a low protectivity rating. The water table topography map (bottom image map in Fig. 13) shows a sink in the middle part, with pockets of elevated topography in the southeastern, northeastern, and western regions of the research area. Groundwater predominantly migrates towards the central region, aligning with the direction of surface water flow into the ravine. The direction of groundwater flow is significant since it coincides with the direction of any subsurface pollutant movement in the vicinity. Upon careful analysis of Fig. 13, it becomes apparent that the water table closely resembles the surface topography, albeit in a less pronounced manner. Consequently, any underground pollution plumes will migrate towards the central region of the research area.

This study utilized the geoelectric method, specifically the VES and ERT techniques, to evaluate the protective capacity of hydrogeological units in the area of Ikot Ekpene Urban and its surrounding regions in Southern Nigeria. The VES data were manually interpreted using the partial curve matching technique and quantitatively analyzed using the WINRESIST computer software program. The ERT data were interpreted using the RES2DINV computer software tool. The VES data indicate that the studied area consists of 3–4 geoelectric layers. The predominant composition of these layers consists of sandy materials, including fine, coarse, and gravelly particles. However, in certain areas, there are clay intercalations, as shown by the borehole lithological data that is currently available. The primary hydrogeological formations are found in the third layer, exhibiting resistivity values ranging from 214.4 to 2839.0 Ωm. The thickness of the layers that protect the overburden ranges from 9.0 to 86.6 m. The protectivity of the hydrogeological units in the area was assessed using the longitudinal conductance, resistivity RC, and anisotropy coefficient of the protecting layers. The study region has been categorized into four zones based on the distribution of these secondary geoelectric indices, with each zone assigned a protectivity grade. Out of the aquifer units in the area, 65% have a poor protectivity rating, 10% have a weak rating, 20% have a moderate rating, and 5% have an excellent rating. The water table appears to be a less intense version of the surface topography, indicating that any underground contamination plumes will move towards the central region of the study area. The ERT image sections depict the fluctuations in resistivity at a shallow depth and exhibit a strong correlation with the VES results.