Hydrogeological assessment and aquifer potential of a coastal area, South Nigeria: insights from VES surveys and spatial analysis

Groundwater is the water found in the subsurface pore space of soil and rocks and flows within aquifers below the water table. It is also found in saturated zones beneath the land surface (Olofinlade et al. 2018; George 2020). All known forms of life depend on a translucent, flavorless, and odorless chemical compound known as water. It is essential for numerous biological, physical, and chemical processes and makes up over 71% of the Earth's surface (Agbasi et al. 2019). For hydrological management and planning, as well as for defining sustainable groundwater resource development, a thorough understanding of hydrogeological features and groundwater flow mechanisms in aquifers is necessary (Chia-chi & Hsin-fu 2008; Abdulrazzaq et al. 2020a; Ptitsyn & Matyugina 2022; Doro et al. 2023).

Given the rising global demand for water and increasing environmental concerns, water conservation and sustainable management are essential. Preserving water resources requires actions such as efficient water usage, wastewater treatment and reuse, watershed protection, and education on water conservation techniques (Ngene et al. 2019; Odjegba et al. 2020). Water is an extraordinary material that is essential to life and has a significant effect on the ecosystems of the globe. To ensure a sustainable future for people and the environment, it is essential to understand its qualities, availability, and ethical use (Umoren et al. 2017; Odjegba et al. 2020).

Groundwater has numerous uses and applications in various sectors, as it is essential for drinking, sanitation, agriculture, industry, energy production, recreation, and transportation. It is also used for generating hydroelectric power and as a coolant in many industrial processes (Aladeboyeje et al. 2021). Ice, liquid water, and gaseous water vapor are the three different states in which water can exist. These states are affected by temperature and pressure, and they undergo phase transitions including melting (from solid to liquid) and evaporation (from liquid to gas) (Dalin et al. 2019; Smith et al. 2020).

The hydrogeological investigation is needed for its understanding; hydrogeological investigation methods include drilling, geophysical surveys like electrical resistivity tomography and ground-penetrating radar, hydrochemical analysis, numerical modeling, and remote sensing. These techniques help understand groundwater distribution, movement, and quality (Akingboye & Osazuwa 2021; Ghiglieri et al. 2021; Jimoh et al. 2023). Drilling and monitoring wells directly sample groundwater, while geophysical surveys characterize subsurface properties. The hydrochemical analysis assesses water chemistry, and numerical modeling simulates groundwater flow and contaminant transport (Abdulrazzaq et al. 2020b; Joshua et al. 2023). Remote sensing, such as satellite imagery, provides insights into surface water–groundwater interactions. Integrating these methods enables comprehensive assessments, aiding in resource management, environmental protection, and hazard mitigation (Adewumi et al. 2023; Okoli et al. 2024).

The study of coastal hydrogeological dynamics is a complex field that requires innovative and effective methods for data collection and analysis. One such method is the use of hydrogeo-electrical analysis, specifically the vertical electrical sounding (VES) geoelectrical method (Chiemela et al. 2019; Ekanem et al. 2021). This technique combines the principles of geophysics and hydrogeology to provide a comprehensive understanding of subsurface water movement and its interaction with the geological framework. This method helps identify and map aquifers, determine their depth and thickness, and assess groundwater quality. Being widely used in hydrogeological studies, VES provides valuable data for sustainable groundwater management and resource assessment, aiding in efficient water extraction and conservation strategies (Akaolisa et al. 2022a).

Coastal regions are particularly vulnerable to changes in hydrogeological dynamics due to their proximity to the ocean. Factors such as tidal fluctuations, sea-level rise, and climate change can significantly alter the hydrogeological dynamics of these areas, leading to issues like a saltwater intrusion, changes in groundwater levels, and alterations in the overall water quality (George & Thomas 2024; Udosen et al. 2024). The VES method offers a non-invasive, cost-effective approach to monitoring changes, delivering crucial data that support sustainable water management practices.

This study aimed to apply the VES method in hydrogeo-electrical analysis to investigate coastal hydrogeological dynamics. The objectives are threefold: first, to develop a robust methodology for conducting VES in coastal regions; second, to use this methodology to investigate the impacts of various environmental factors on coastal hydrogeological dynamics; and finally, to use the findings to inform sustainable water management practices in coastal regions. Through this study, we hope to contribute to the broader understanding of coastal hydrogeology and help mitigate the impacts of environmental change on these critical ecosystems.

Sandstones, shales, and clay deposits make up sedimentary rock layers that make up the region's geology. Rivers and marine activities have deposited these sediments, resulting in a complicated underlying geology. Oil and gas deposits are thought to have formed in the area because of the long-term encasement and maturation of organic-rich sediments (George et al. 2020). The region also possesses features of the sedimentary basin formed by the accumulation of sediments carried by rivers over millions of years. These sediments consist of sand, silt, clay, and organic matter that have undergone compaction and diagenesis, forming various rock formations (George et al. 2020). The majority of the sedimentary rocks in the Eket Local Government Area are shales, sandstones, and clay, which are frequently connected to petroleum reservoirs.

The Niger Delta region, including Eket, is also marked by a network of creeks, swamps, and rivers, resulting in a complex and dynamic ecosystem. The local hydrology and geology contribute to the rich biodiversity of the area, supporting diverse flora and fauna.

The research area's sediment composition ranges from coarse to fine grains and is poorly sorted, indicating size non-uniformity. These sediments are thick and include clay, silt, and sandstones, forming separate aquifers of varying thickness. The upper sandy aquifer is composed of alluvial elements such as gravels, fine– to medium-grained carbonaceous sand, and unconsolidated sands that are deposited by water action, creating a porous layer for groundwater storage and transmission.

VES was used in determining the flow of current along the subsurface in the study area by using a Schlumberger array electrode configuration with an electric spacing of 600 m. The subsurface data were collected using a resistivity meter (ABEM SAS 1000 Terrameter). Five VES locations were established within the research region, as shown in Figure 1(c) and detailed in Table 1. Each VES point was precisely located using a geographic positioning system. The acquired data from the field were smoothened using Excel software to minimize the noise and were imported to WINRESIST software for analysis.

The collected data, encompassing five VES points, provide a robust representation of the coastal subsurface conditions within the densely populated Eket Local Government Area. The strategic selection of VES locations ensures diverse and comprehensive coverage, capturing variations in sediment composition, aquifer characteristics, and hydrogeological dynamics. This sampling approach, combined with advanced data-processing techniques, ensures that the collected data adequately represent the coastal hydrogeological environment of the study area.

The apparent resistivity readings were plotted against AB/2, further computer iterations were performed using the model created by this software, and the geoelectrical parameters were determined. The raw data collected in the field for performing VES surveys were highly loud and difficult to analyze correctly. Based on the collected data, the WINRESIST automatically estimates an initial resistivity model using algorithms and mathematical methodologies. WINRESIST is a sophisticated software tool designed for interpreting resistivity data collected from VES surveys. It employs advanced algorithms and mathematical methodologies to automatically estimate initial resistivity models based on the collected data. It uses an iterative least-squares inversion method to improve the initial resistivity model and makes it more indicative of the underlying structure, which defines the initial resistivity model for depicting the subsurface structure. This iterative process helps minimize the misfit between the calculated model response and the observed data, gradually improving the accuracy of the resistivity model. This software facilitates accurate and efficient analysis of subsurface conditions, enabling users to gain insights into sediment composition, aquifer characteristics, and hydrogeological dynamics. By providing detailed resistivity profiles, WINRESIST supports robust decision-making in hydrogeological investigations and environmental assessments.

To improve the resistivity model's accuracy and make align with subsurface geological data, core samples close to the VES points were obtained for further laboratory analysis (George et al. 2021; Umoh et al. 2022). The results from the software provide information on the depths, thicknesses, and apparent resistivity that are used in determining other hydrogeological parameters.

These metrics aid in the comprehension of an aquifer's electrical characteristics, which are linked to its hydrogeological properties. When interpreting data from VES, a geophysical technique is used for subsurface research; the Dar-Zarrouk parameters are frequently employed. In VES, potential differences are evaluated at different depths by passing a current through the ground (Ifeanyichukwu et al. 2021). Hydrogeologists can determine the characteristics of subsurface layers, including aquifers, by examining electrical responses.

The formation factor is an essential tool in the interpretation of geophysical surveys and the characterization of reservoirs. It can provide valuable insights into the hydrogeological dynamics of an area, including the movement and distribution of groundwater, the presence of hydrocarbons, and the potential for geohazards such as landslides or earthquakes (Abdulrazzaq et al. 2020b).

The range of the particle density is from 2.60 to 2.75 g/cm³; the porosity that represents the percentage of pores or void spaces in the material is computed using an average constant value of 2.65 g/cm³, and the relation is used to calculate the bulk density as shown in Equation (6). The same core samples from the study area were used to determine the hydraulic conductivity.

Hydraulic conductivity represents the ability of a porous medium, such as soil or rock, to transmit water. It is often described as the ease with which water can flow through the subsurface material under a hydraulic gradient. Hydraulic conductivity depends on factors such as the size, shape, and interconnectedness of pore spaces within the geological formation. Materials with high hydraulic conductivity, such as coarse sands and gravels, facilitate rapid groundwater movement, while fine-grained materials like clay typically exhibit lower hydraulic conductivity, impeding groundwater flow.

Saturated thickness refers to the vertical extent of an aquifer that is completely filled with water. It represents the portion of an aquifer where all available pore spaces are saturated with groundwater. Saturated thickness can vary spatially and temporally based on factors such as recharge rates, pumping activities, and geological heterogeneity. In regions with significant groundwater recharge or where aquifers are replenished rapidly, the saturated thickness may be greater compared with areas experiencing prolonged drought or extensive groundwater extraction.

Higher transmissivity values mean that water moves more easily through aquifers, while lower values mean that there are fewer flow paths or less hydraulic conductivity in the subsurface formation. Transmissivity is utilized to assess the potential yield of aquifers, predict groundwater flow patterns, design effective well systems, and evaluate the impacts of human activities on groundwater resources.

Table 2 presents a detailed summary of the aquifer characteristics obtained from VES surveys conducted at five specific points (EK1–EK5) within the area of our investigation. The Figure 3 graph provides a visual representation of the electrical characteristics detected during the surveys, which helps in interpreting and analyzing the underground structures and hydrogeological aspects.

These data clearly indicate that the identified layers share properties that are identical to those of aquifer formations. The observed resistivity values, along with the determined thickness and depth measurements, suggest the presence of probable underground structures that could contain aquifers. These findings provide important information for future hydrogeological investigations and the management of groundwater resources. The aquifer properties derived from the VES surveys provide valuable insights into the subsurface characteristics of the study area (Ibuot et al. 2022; Akiang et al. 2024). The apparent resistivity values suggest the presence of distinct layers with varying degrees of resistivity.

The resistivity of aquifers is affected by the composition and characteristics of the underlying rock formations. Aquifers generally have higher resistivities than other types of geological materials. The aquifer resistivities are estimated by analyzing VES data and modeling while taking into consideration the geological properties of the location. As a result, the resistivity of aquifers does not have a specific range.

EK4 exhibits the highest apparent resistivity at 3,688.70 Ωm, suggesting highly resistive subsurface materials, likely indicating the presence of consolidated sediments or rocks (George et al. 2022). In contrast, EK2 has the lowest apparent resistivity of 970.50 Ωm, indicating more conductive materials, possibly due to a higher moisture content or the presence of clay (Umoh et al. 2022).

Thickness and depth also vary, with EK2 having the greatest thickness (247.70 m) and depth (250.20 m), highlighting a substantial subsurface layer. Conversely, EK1 has the thinnest layer (72.20 m) and shallowest depth (87.10 m). These differences reflect the heterogeneous nature of the coastal hydrogeological environment, influencing groundwater storage and movement (Thomas et al. 2020).

Aquifer depth refers to the vertical distance between the saturation zone located below the water table and the aeration zones situated above it. During precipitation episodes, water seeps into the underground through permeable pathways in geological strata and soil, directed by gravity toward the saturation zone (Osório & Moura 2021). Variations in rainfall levels affect the location of the water table, which rises when there is more water flowing into it and recedes when water is extracted or depleted (Okoli et al. 2024). Figure 5(b) provides a clear representation of the varying depths of aquifers in the research area. The depths range from 87.10 m at EK1 to 250.20 m at EK2. The arrangement of differences in aquifer thickness and depth aligns, highlighting the presence of geographical heterogeneity and impacting the movement of groundwater.

Table 3 presents key hydrogeological and aquifer potential parameters for the study area. Values include permeability (mD), porosity, formation factor, hydraulic conductivity (m/day), and transmissivity (m²/day) obtained from VES data. Variations across points highlight spatial heterogeneity in groundwater properties, which is crucial for assessing groundwater flow, storage, and extraction potential in hydrogeological studies.

Permeability refers to the ability of the subsurface material to transmit fluids, such as water, through its pore spaces. In the context of groundwater aquifers, permeability dictates how easily water can flow through the geological formations that compose the aquifer. High permeability indicates that the aquifer material has abundant interconnected pore spaces, allowing water to move more freely (Iserhien-Emekeme et al. 2020). Conversely, low permeability suggests that the aquifer material has fewer interconnected pores, restricting the movement of groundwater. Higher permeability values suggest that groundwater may be more accessible and easier to extract, while lower permeability values may indicate challenges in groundwater extraction and potential limitations in groundwater flow (Ekwok et al. 2020).

Conversely, areas with lower permeability values, such as EK1 (2.38 mD) and EK5 (1.86 mD), may exhibit reduced groundwater flow rates and storage capacities. Aquifer materials with lower permeability may restrict the movement of groundwater and limit the availability of groundwater resources for extraction. Such areas may be more vulnerable to groundwater depletion and may require careful management to prevent overexploitation and maintain a sustainable water supply (Akinwumiju 2020).

Porosity, a fundamental parameter in hydrogeology, refers to the volume percentage of void spaces, or pores, within a geological formation or sediment that can hold water. Porosity is influenced by various factors, including the size, shape, sorting, and packing arrangement of grains composing the aquifer material. Coarser-grained materials like sand and gravel tend to have higher porosities due to their larger pore spaces, while finer-grained materials like clay typically exhibit lower porosities (Agbasi et al. 2020).

The porosity values provided in Table 3 (EK1–EK5) offer important insights into the capacity of aquifer materials to store groundwater. Porosity directly influences the storage capacity of an aquifer system. Higher porosity values, such as those observed at EK2 (0.43) and EK5 (0.43), suggest that a larger proportion of the subsurface material consists of pore spaces capable of storing groundwater. Aquifers with higher porosity can potentially store more water and may have greater groundwater reserves available for extraction.

Aquifers with higher porosity values are generally more effective in storing and transmitting infiltrated water from precipitation or surface runoff. Areas (e.g., EK5) with higher porosity may experience enhanced groundwater recharge rates, as water can percolate more readily through the porous subsurface material and replenish groundwater resources.

Variations in porosity values among the VES point, increasing from the eastern part of the study area toward the western part of the study area, indicate spatial heterogeneity in the aquifer properties. The differences in porosity may be attributed to variations in lithology, sediment composition, depositional environment, and geological processes across the study area (Laabidi et al. 2020). While higher porosity values indicate greater storage capacity, they also imply increased vulnerability to contamination as in the case of EK2 that is a high settlement area. Aquifers with higher porosity may be more susceptible to contamination from surface pollutants or anthropogenic activities due to the interconnected nature of the pore spaces as in the case of EK5, which is within a close distance of a water body as shown in Figure 6(b). Therefore, these areas with higher porosity values may require greater attention to groundwater quality protection and pollution prevention measures.

VES location with higher formation factor values (EK4 with 4.66) has lower porosity (0.39) and reduced groundwater storage capacity compared with points with lower formation factor values (EK2 with 3.84) has higher porosity (0.439) as shown in Table 3. Understanding formation factor variations is crucial for evaluating groundwater flow rates, storage capacities, and the overall hydrogeological potential of the aquifer system. The formation factor is closely related to the electrical resistivity of the aquifer material (Laouini et al. 2016; Agbasi et al. 2019). Higher formation factor values indicate higher resistivity, suggesting that the aquifer material is more resistive to the flow of electrical currents as shown in Tables 2 and 3.

Hydraulic conductivity is a key parameter in hydrogeology that measures the ability of an aquifer material to transmit water. It quantifies the ease with which water can flow through the subsurface under the influence of a hydraulic gradient (Joshua et al. 2023).

Higher hydraulic conductivity values, such as those observed at EK2 (2.78 m/day) and EK3 (2.42 m/day) (see Table 3), suggest that the aquifer material facilitates faster groundwater movement. Areas with higher hydraulic conductivity values generally experience greater groundwater flow rates, enhancing the potential for groundwater recharge and extraction. Locations with higher hydraulic conductivity values typically exhibit higher aquifer productivity, allowing for increased rates of groundwater extraction to meet water supply demands. Aquifers with high hydraulic conductivity can sustainably yield larger volumes of groundwater, supporting various agricultural, industrial, and domestic water uses (El-Rayes et al. 2020).

Variations in hydraulic conductivity values among the VES points reflect spatial heterogeneity in the aquifer properties as shown in Figure 7(b), with higher values in the eastern part of the study area as compared with the western part. Differences in lithology, grain-size distribution, and porosity contribute to variations in hydraulic conductivity across the study area, influencing groundwater flow dynamics and aquifer behavior (Korus et al. 2020).

Transmissivity is a critical parameter in hydrogeology that describes the ability of an aquifer to transmit water through its thickness under a unit hydraulic gradient. It represents the product of hydraulic conductivity and the thickness of the aquifer and is measured in units of volume per unit time per unit width (e.g., m³/day/m) (Udosen et al. 2024).

The potential for groundwater at each VES location was calculated by measuring the aquifer's transmissivity. Table 4 shows the calculated aquifer transmissivity values derived from VES data. Aquifers are classified based on their transmissivity rating as shown in Table 5.

A thorough examination of aquifer features and hydrogeological data obtained by VES surveys provides a valuable understanding of the groundwater dynamics and potential in the research area. This work utilizes resistivity, thickness, depth, and transmissivity measurements to elucidate the regional variability of aquifer properties. This information is crucial for the efficient management and planning of groundwater resources.

The resistivity measurements acquired from the VES surveys reveal the existence of various subsurface layers with different levels of resistance to the flow of electric current. The resistivity values of aquifers at various places, including EK1 (970.50 Ωm) to EK5 (3,688.70 Ωm), suggest the possible existence of formations containing groundwater. An investigation of spatial distribution shows a large variation in resistivity across the research area, highlighting the various characteristics of underground geological formations and their connection to settlement patterns.

Measurements of aquifer thickness and depth enhance our comprehension of groundwater availability and hydrological circumstances. The isopach maps depict the differences in the thickness of the aquifer, showing that sites such as EK2 (250.20 m) have broader aquifers compared with EK1 (87.10 m). Similarly, a thorough investigation reveals variations in the vertical separation between the water table and the saturation zone throughout the research area, which, in turn, affects the dynamics of groundwater migration.

The hydrogeological parameters, such as permeability, porosity, formation factor, hydraulic conductivity, and transmissivity, are essential for understanding the movement, storage, and extraction potential of groundwater. The differences in these characteristics among VES sites highlight the regional diversity in aquifer features, which affect the rates of groundwater recharge, storage capacity, and total hydrogeological potential. Transmissivity assessments categorize aquifers based on their ability to transmit groundwater, which helps in making educated decisions about the utilization and management of water resources. Locations with high transmissivity, such as EK2 (687.68 m²/day), have a considerable capacity to provide water for a large region, but locations with lesser transmissivity, such as EK5 (93.16 m²/day), may only be suitable for meeting smaller-scale local water supply demands.

A combination of VES data and hydrogeological investigations yields a thorough comprehension of the movement and potential of groundwater in the research region. These findings are essential for the sustainable management of groundwater, planning land use, and making decisions to provide water security and conserve resources in the region. It is advisable to continue monitoring and conducting additional research to improve our comprehension of groundwater systems and effectively tackle the changing difficulties related to water resources. Further work should include extended monitoring and advanced modeling to enhance the understanding of groundwater systems and address water resource challenges. Limitations of this study involve potential inaccuracies in resistivity measurements due to heterogeneity in subsurface materials, which may affect data interpretation and conclusions.

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

The authors declare there is no conflict.