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Article

Geoelectrical Characterization of Coastal Aquifers in Agbado-Ijaye, Lagos, Southwestern Nigeria; Implications for Groundwater Resources Sustainability

by
Kehinde D. Oyeyemi
1,*,
Joyce Abuka-Joshua
1,
Oluwatosin J. Rotimi
2,
Bastien Dieppois
3,
Modreck Gomo
4,
Abayomi A. Olaojo
5,
Philips O. Falae
6 and
Mohamed Metwaly
7
1
Applied Geophysics Programme, Department of Physics, Covenant University, Ota 112211, Nigeria
2
Department of Petroleum Engineering, Covenant University, Ota 112211, Nigeria
3
Center for Agroecology, Water and Resilience, Coventry University, Coventry CV1 2TU, UK
4
Faculty of Natural and Agricultural Sciences, Institute for Groundwater Studies, University of the Free State, Bloemfontein 9301, South Africa
5
Department of Earth Sciences, Ajayi Crowther University, Oke-Ebo 211271, Oyo, Nigeria
6
Department of Geology, Afe Babalola University, Ado-Ekiti 360101, Nigeria
7
Department of Archaeology, College of Tourism and Archaeology, King Saud University, Riyadh 11421, Saudi Arabia
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(4), 3538; https://0-doi-org.brum.beds.ac.uk/10.3390/su15043538
Submission received: 13 November 2022 / Revised: 8 February 2023 / Accepted: 9 February 2023 / Published: 14 February 2023
(This article belongs to the Section Sustainable Water Management)
Browse Figures
Versions Notes

Abstract

:
Water is a natural resource; its availability depends on climatic and geological conditions, and it is invariably controlled by human activities. Agbado-Ijaye lies within a coastal area, where local communities have been facing incessant water shortages, especially during the dry season. This study investigated the groundwater-bearing geological unit(s) using hydrogeophysical techniques in the coastal environment. The electrical resistivity technique, involving vertical electrical sounding (VES) and two-dimensional (2D) electrical resistivity imaging via Wenner array electrode configuration, was used to characterize the geoelectric distribution. Twenty VES stations were investigated and current electrodes (AB/2 m) spacing expanded from 1–200 m; four 2D electrical resistivity imaging traverses having a length of 200 m each and interelectrode spacing of 10 m (level 1) to 60 m (level 6) was adopted. Four geoelectric units were delineated, namely: topsoil (15–251 Ωm), clayey (28–100 Ωm), clayey sand (125–190 Ωm) and sandy (205–876 Ωm) with thicknesses ranging from 0.7–1.3 m, 4.1–19.0 m, 2.6–15.6 m and undefined depth, respectively. The 2D imaging sections also detected similar geoelectric layers, corroborating the VES-derived sections. The inverted sections delineated two different aquifers: the shallower low-yield aquifer comprising sandy clay/clayey sand units with a maximum depth of about 5.5 m. This layer is adjudged to be the continental plain sand of the Benin Formation. The deeper high-yield aquifer with a maximum depth of 30.4 m is a beach sand unit that belongs to the Tertiary Alluvium of the Dahomey Basin. The study showed that hydrogeophysical investigation is vital in exploring, developing, and managing coastal groundwater resources.

1. Introduction

Groundwater is an important resource for many cities worldwide, and Lagos, Nigeria, is no exception. However, the city is facing several challenges in managing and sustainably utilizing its groundwater resources. The Lagos metropolis has significantly benefited from rapid industrialization from the early 90s, which has led to the geometrical progression of population growth [1]. The consequent increase in the demand for water resources due to uncontrollable urban migration has caused heavy reliance on groundwater resources for domestic and industrial uses because industrial effluents often severely pollute surface water. Overdrafting groundwater in the coastal area is responsible for seawater intruding into the aquifer system [2]. Lagos is a cosmopolitan city and rural-urban migration is responsible for the population explosion, and Agbado-Ijaye is not left out of this social order. The economic activities (domestic and industrial) have led to the overextraction of groundwater. This has led to the lowering of the water table and a decline in groundwater quality due to pollution from saltwater intrusion and industrial effluents. Groundwater is an essential water source [3,4] and it is crucial to sustainable development by providing low-cost and high-quality water supplies. However, there is a lack of regulations to manage groundwater extraction and usage in Lagos state, Nigeria [5]. Additionally, there is a lack of enforcement of existing regulations prohibiting the contamination of the resource with industrial effluents [5,6]. Previous studies have shown that the long-term groundwater trends in Lagos have been declining [2,7,8] Water demand in Lagos has been reported to be approximately 724 million gallons per day, whereas 317 million gallons are being supplied, while the shortfall is 407 million gallons [2]. Then, over-reliance on groundwater as the possible alternative is accountable for the excessive withdrawal of this resource. About 20% of the world’s freshwater supply is from groundwater, which constitutes about 0.61% of the entire world’s water, including the oceans and permanent ice [9]. The role of groundwater and household water boreholes for domestic water provision in Lagos, Nigeria, and the entire Sub-Saharan Africa has been adjudged to be crucial to good sanitation, good health, and well-being [10,11]. They, therefore, advocated for good policies and regulations to ensure the sustenance of this natural resource. Groundwater sustainability has been defined as the development and use of groundwater resources so that they can be maintained for an indefinite period without causing unacceptable environmental, economic, or social consequences [12]. Therefore, the assessment, development and management of groundwater resources require adequate knowledge of subsurface aquifer systems. The aquifer’s depth, geometry, petrophysical and hydraulic properties are crucial to groundwater yield and productivity. The electrical resistivity method for groundwater exploration has proven reliable in delineating such properties [13,14,15,16,17]. The technique has been successfully employed in delineating subsurface geological sequences, geological structures of interest, aquifer units, types and depth extent in almost all geological terrains [13,14,15,16,17].
Subsurface geoelectric sequences around the coastal environment using the electrical resistivity technique via vertical electrical sounding (VES) combined lithologic log [18]. The quality of the aquifer was established based on resistivity. One of the major problems encountered by the inhabitants of the coastal area is the nonavailability of fresh potable water [18]. The incursion of saline water into the aquifer contaminates the freshwater and renders it unfit for human consumption [18,19]. Integrated geoelectrical resistivity and geochemical techniques was employed to assess the risks of shallow aquifers to saltwater intrusion in the coastal belts of Lagos [19]. The groundwater quality in Matruh was examined using the physical property [20]. VES was instituted at 18 locations; this aided in delineating the geoelectric layer and establishing the fractured limestone sequence at depth. The approach used was adequate for demarcating geoelectric parameters and hydrogeological conditions. Electrical resistivity, induced polarization (IP), borehole logging and hydrochemical examination were employed in studying some coastal communities in Nigeria [2]. The depicted geoelectric layers were corroborated with the borehole logs and changes in the physicochemical properties around these communities were mapped out by the resistivity and IP contrast. The aquifer of a lateritic unit in Southwestern India was investigated using VES and chemical assessment of the water [21]. The aquifer within the porous soft lateritic unit has resistivity values less than 1400 Ωm and values ≤200 Ωm are tagged as groundwater-bearing zones [21]. The hydrochemical test suggests slightly acidic water originating from Ca-Mg-Cl and Na-Cl facies types [21]. The electrical resistivity technique is effective in groundwater–saltwater pollution studies within coastal environments due to its ability to differentiate between saline and freshwater based on its resistivity contrast [18,19,20,21,22].
The electrical resistivity data can delineate geological formations, such as aquifers, because of the contrasting electrical properties of the aquifer and that of the host rocks. Thus, resistivity surveys are often used to search for groundwater in porous media. Porous media are often related to sedimentary rocks such as clean sands and gravels, which have high porosity and make suitable aquifers. When saturated with freshwater, they can easily be differentiated from higher resistivity (poor aquifers), impermeable clays and marls [13,14,15,16,17]. As an essential tool in groundwater exploration, the surficial geophysical investigation is cost-effective in borehole construction by locating the target aquifer before drilling [23]. The intrusions of salt water into the coastal aquifer of the Lekki-Peninsula area of Lagos was delineated using the VES and borehole logs [24]. Twenty-five (25) VES data were used to map out freshwater aquifers in the Gbagada area of Lagos [25]. Geoelectrical resistivity and water level data from 30 hand-dug wells were used to monitor the rise in water levels and the impact on building structures and foundations [26]. The geoelectrical resistivity data were used to map out the groundwater level within the shallow aquifer in those areas.
Vertical electrical sounding (VES) data have been integrated with 2D electrical resistivity tomography to study groundwater exploration and contamination in sedimentary [14,17,19,24,27,28,29,30] and crystalline basement terrains within Southwestern Nigeria [31,32,33,34,35]. Within the Ota community in Southwestern Nigeria, resistivity soundings data were integrated with 2D ERT to reveal the geometry and characteristics of the sedimentary aquifer [27,28]. A time-lapse 2D ERT were conducted to observe the seasonal changes in groundwater quality and assessment around Olushosun dumpsite Lagos [29]. Also, VES and ERT were combined to assess the distribution of bitumen contaminants and their hazardous effects on groundwater around the Imakun-Omi community in the Eastern Dahomey basin of Southwestern Nigeria [30]. Within the crystalline basement terrain in Southwestern Nigeria, vertical electrical soundings were integrated with multiple-gradient array acquired 2D geoelectrical resistivity imaging to delineate the basement aquifer within Iberokodo, Abeokuta, Southwestern Nigeria [31]. The research revealed the nature and spatial variability of weathered and fractured zones within the subsurface. Vertical electrical soundings and 2D geoelectrical imaging were conducted for groundwater exploration within the complex basement terrain of the Basiri community, Ado-Ekiti, Southwestern Nigeria [32,33]. They delineated the saprolite, saprock and fresh basement in the area. The delineated weathered and fractured zones within the saprock are essential to groundwater accumulation. Integrated electrical resistivity imaging with the hydrochemical method were used to study the groundwater contamination around a dumpsite in Ibadan, Southwestern Nigeria [34]. Groundwater within the upper saprolite aquifers of the crystalline basement terrain is often poor in quality and prone to contamination [33,34]. To further understand the local flow and transport processes within crystalline basement aquifers, a hydrogeophysical research observatory site with four test wells was established in the University of Ibadan campus, Southwestern Nigeria [35]. Their preliminary studies have shown the conceptual model and hydraulic heterogeneities of the groundwater aquifer at the site [35]. VES with remote sensing and GIS techniques were integrated to map and assess the level of saltwater contamination in shallow aquifers around the Lagos lagoons [36,37]. Their studies showed that groundwater around the Lagos lagoons’ upstream part is better for irrigation than the downstream. The scarcity, contaminations and overexploitation of groundwater, groundwater contaminations, declination of water level and other related challenges are being faced all over Lagos state, Nigeria. This study directly responds to the groundwater problems encountered by the inhabitants of the Agbado-Ijaye area of Lagos. To get around this problem, there is a need to decipher the geologic sections and establish the probable groundwater-bearing units. To date, there has been less documented or reported work on the groundwater potential of this community. This study aims to utilize the electrical resistivity method to assess, explore and delineate the aquifer systems within the study area.

2. Location and Geologic Setting

The study area lies within the residential area of the Agbado-Ijaye area of Lagos mainland. The area is between latitudes 06 ° 4 0 ′ 3 2 ″ – 06 ° 4 0 ′ 4 1 ″ N and longitudes 003 ° 1 7 ′ 2 6 ″ – 003 ° 1 7 ′ 4 2 ″ E (Figure 1), with an elevation between 61 and 64 m above sea level. The area is a gently sloping, low-lying terrain in the humid tropical region [29]. Southwestern Nigeria, where the study area is located, has two major climatic seasons: the dry season from November to March and the rainy (or wet) season between April and October, with a slight break in August [38]. Within coastal regions, such as Lagos, occasional rainfall within the dry season is often witnessed. On average, Agbado-Ijaye receives mean annual precipitation amounts greater than 2000 mm, serving as a significant source of groundwater recharge in the area [38].
The cretaceous stratigraphy obtained from the outcrop and borehole records reveals that the subsurface in the study area (Figure 2) [39] is composed of the Abeokuta Group, subdivided into three formational units, namely Ise, Afowo and Araromi [40]. The Abeokuta Group is located directly above the base of the basement complex and Ewekoro, Akinbo, Oshosun, Ilaro, and Benin Formations overlie it. The Abeokuta Group consists mainly of poorly sorted ferrous grit, siltstone and mudstone with shale-clay layers [41,42,43]. The Akinbo Formation is the upper part of the shale overlaid by the Ewekoro limestone, which may be of the lowest Eocene period. The top of the formation is characterized by pure white, rough sand, and a little clay. The Akinbo Formation slowly moves into vast mud and is overlaid by the Oshosun Formation, typically Marine and Eocene. The Akinbo Formation is overlaid by the Oshosun Formation, which consists of the Eocene shale. The Ilaro Formation comprises a series of coarse sandy estuarine, deltaic, and continental beds. The Ilaro Formation exhibits dramatic lateral shifts. Overlying the Ilaro Formation is the Benin Formation, primarily coastal plain sands and Tertiary alluvial deposits.

3. Materials and Methods

3.1. Data Acquisition and Field Procedures

A total of 20 vertical electrical soundings (VES) were acquired using an ABEM resistivity meter (SAS 4000) with a Schlumberger electrode configuration (Figure 3). The current electrode (AB/2 m) adopted for this investigation ranged from 1.0 to a maximum of 200 m. It was used due to the unavailability of space in the residential area. The spread was sufficient for the effective depth of investigation anticipated [44]. Precautions were taken to minimize electrode positioning errors as the surveys were conducted manually. A minimum stack of three and a maximum of six VES data was used for the VE-derived geoelectric section. In addition, we acquired four 200 m long 2D electrical resistivity tomography (ERT) traverses (T1–T4) with a Wenner array configuration and a minimum electrode spacing of 10 m (Figure 1). Several works have attested to the efficacy of the Wenner array to detect vertical distributions of subsurface resistivities, thereby being suitable for groundwater exploration in the sedimentary environment [45,46,47,48,49]. Level 1 involves interelectrode spacing of 10 m and the electrodes were moved collinearly by 10 m until the end of the traverse. At level 2, interelectrode spacing of 20 m was adopted and the movement was also 10 m. At levels 3, 4, 5 and 6, the interelectrode spacing of 30 m, 40 m, 50 m and 60 m were engaged, respectively, and the electrodes were moved by 10 m. Thus, the data were acquired up to a data level 6, with a maximum interelectrode spacing of 60 m on each profile. The length of each profile is 200 m, giving a total of 41 electrode positions and 183 data points obtained for each traverse (T1–T4). The effective depth ( Z E ) range, according to previous work [44], is approximately given as Equation (1):
Z E = 0.190 × AB

3.2. Data Processing and Inversion

The measured apparent resistivity data were processed manually by plotting the apparent resistivity values against the half-current electrode spacing (AB/2) or half the spread length, at each station on the bi-logarithmic graph (Figure 3). We employed a partial-curve-matching approach to obtain geoelectric parameters, such as several geoelectric layers and their respective resistivities and thicknesses, which were used as the initial models for the VES data. Subsequently, the resistivity data were iteratively processed and inverted using WinResist software. The iterative process attempted to improve the goodness of fit between the measured and computed data. The 2D ERT data were processed and inverted using the DIPRO software to produce the inversed resistivity model of the subsurface. The DIPRO inversion code uses a smoothness-constrained least-squares optimization technique to obtain the 2D distribution of the resistivity in the subsurface [50]. The algorithm subdivides the subsurface into several small rectangular blocks using a finite element model iteratively.

4. Results

4.1. Vertical Electrical Soundings

The representatives of inverse model resistivity curves obtained from the computer iteration of the resistivity soundings are presented in Figure 4 and Figure 5. The geoelectric parameters for the VESs (T1-V1 to T1-V5) revealed three to four geoelectric layers, which vary from topsoil, clayey sand, clayey, and sandy horizons (Table 1). The corresponding geoelectric section constructed is presented in Figure 6. The first layer represents the topsoil with resistivity and thickness values that range between 88.2–251.4 Ωm and 0.8–1.2 m, respectively. From T1-V1 to T1-V3, the second identified layer denotes a clayey sand unit with resistivity and thickness values ranging between 132.2–176.4 Ωm and 2.6–15.6 m, respectively. While the second geoelectric layer in T1-V4 and T1-V5 is diagnostic of the clayey unit with resistivity and thickness values that range between 65.1–79.1 Ωm and 15.5–18.9 m, respectively.
Table 2 shows the geoelectric parameters and delineated layers for VESs (6–10) and the corresponding geoelectric section constructed is presented in Figure 7. The delineated geoelectric layers are about three to four, which vary from topsoil, clayey, clayey sand, and sandy horizons. The topsoil has resistivity and thickness values that vary between 57.7–225.0 Ωm and 0.7–0.9 m, respectively. The second layer across VES 6, VES 8 and VES 9 denote clayey to clayey sand units with resistivity values ranging from 70.2–124.9 Ωm and a layer thickness of 2.3–16.9 m. Also, the second identified layer in VES 7 indicates a sandy layer with resistivity and layer thickness values of 205.1 Ωm and 2.3 m, respectively. The second geoelectric layer beneath VES 10 depicts a clayey unit with resistivity and layer thickness values of 84.8 Ωm and 4.1 m, respectively. The third stratum in VES 6 is the sandy horizon with a resistivity value of 519.6 Ωm. Still, its layer thickness could not be determined because the current terminated on this horizon. Also, the third geoelectric layer beneath VESs 7 and 8 depicts clayey to clayey sand formations with resistivity and layer thickness values that range between 71.3–189.7 Ωm and 12.2–12.5 m, respectively. Meanwhile, the third horizon identified in VES 9 and VES 10 is indicative of a clayey unit with resistivity and layer thickness values that range between 75.2–99.7 Ωm and 11.5–25.4 m, respectively. The last identified layer in VES 7, VES 9 and VES 10 is interpreted to be a sandy horizon with resistivity values ranging from 307.7–875.8 Ωm. However, the delineated last layer in VES 8 is interpreted to be the clayey unit with a resistivity value of 49.7 Ωm.
The geoelectric parameters of the delineated layers on traverse T3 consist of T3-V11, T3-V12, T3-V13, T3-V14 and T3-V15 (Table 3). The corresponding geoelectric section constructed is presented in Figure 8. Table 3 reveals three to four delineated geoelectric layers: topsoil, clayey, clayey sand and sandy. The topsoil delineated across all the VESs has resistivity and thickness values that range between 34.7–187.7 Ωm and 0.7–1.1 m, respectively. The second delineated layer in T3-V11 is interpreted to be clayey with resistivity and thickness values of 67.1 Ωm and 11.5 m, respectively. Also, the second geoelectric layer delineated by T3-V12 and T3-V15 is clayey with resistivity and thickness values ranging between 42.0–48.6 Ωm and 8.0–12.1 m, respectively. Also, T3-V13 and T3-V14 have a second geoelectric layer indicative of clayey with resistivity and layer thickness values that range between 27.6–27.8 Ωm and 4.8–16.2 m, respectively. The third stratum beneath T3-V11, T3-V12, T3-V14 and T3-V15 is clayey sand to sandy with a resistivity value that ranges between 143.7–252.5 Ωm. However, the layer thickness could not be determined as this layer is the last delineated geoelectric horizon. However, the delineated third geoelectric layer by T3-V13 is interpreted to be clayey with resistivity and layer thickness values of 72.0 Ωm and 19.4 m. The fourth geoelectric layer by T3-V13 depicts a clayey horizon with a resistivity value of 67.3 Ωm.
Table 4 shows the geoelectric parameters of the delineated geoelectric layers across traverse T4, consisting of V16–20. The corresponding geoelectric section constructed is presented in Figure 9. Three to four geoelectric layers were delineated: topsoil, clayey, sandy clay, clayey sand, and sand. Generally, the delineated topsoil has resistivity and thickness values ranging from 15.2–212.7 Ωm and 1.0–1.3 m. The second delineated layer along T4-V16 and T4-V17 is clayey with resistivity and layer thickness values ranging from 38.3–56.5 Ωm and 11.9–16.8 m, respectively. However, the second delineated geoelectric layer by T4-V18 is interpreted as clayey with resistivity and layer thickness values of 40.0 Ωm and 12.5 m, respectively. Also, the delineated second layer at T4-V19 and T4-V20 is clayey to clayey sand with resistivity and layer thickness values of 71.4–160.6 Ωm and 2.8–3.1 m, respectively. The third stratum delineated at T4-V16, T4-V17 and T5-V18 is diagnostic of clayey sand to sandy with resistivity values ranging between 174.1–234.7 Ωm. However, the third delineated layer at T4-V19 and T4-V20 is interpreted as clayey with resistivity values ranging from 21.6–29.7 Ωm and a layer thickness of 10.9–17.2 m. The fourth delineated layer by T4-V19 and T4-V20 is the extended clayey unit with resistivity values ranging from 10.3–18.6 Ωm.

4.2. 2D Resistivity Imaging

The results of 2D electrical resistivity imaging are presented in Figure 10, Figure 11, Figure 12 and Figure 13. The inversion models reveal geoelectric layers similar to those observed in the sounding results. The inverted resistivity model for Traverse T1 (Figure 10) shows resistivity values ranging from 11–544 Ωm with a depth of penetration of up to 50 m. Four geoelectric layers are mappable in the inverted model, including topsoil, clayey, sandy, and compacted sand units across the profile. The topsoil is comprised of compacted sand mixed with lateritic clay. The clay units are of two types; wet clay with resistivity values of less than 31 Ωm occurs as lenses at about 10–15 m depth at the central portion of the traverse, while the second type of clay occurs at a deeper depth of about 18 m downwards. A clayey unit with resistivity values of 31–60 Ωm occurs laterally about 100 m between the horizontal distance of 40 m and 140 m across the profile. The clayey unit also occurs between 5 m and 35 m depths, enclosing the wet clay units within the subsurface. This clayey unit forms a perched aquifer in the subsurface.
Figure 11 shows the inversion model for Traverse T2 with resistivity ranges of 7–986 Ωm. The interpretive geoelectric layers include the topsoil comprising reworked sand and clay intercalations. A clayey section with resistivity values of 15–80 Ωm occurs from the surface to about 25 m depth. This unit enclosed some pockets of clay lenses with resistivity values of 7–15 Ωm at about 12–13 m within the electrode positions 40 and 150 across the profile. A third geoelectric layer is a clayey to sandy unit with resistivity values 87–294 Ωm occurring at a depth of 10–20 m from the origin to 75 m along the traverse and a depth of 20–50 m from the horizontal distance of 75 m to 200 m across the profile. A laterally extensive thick compacted sandy unit with resistivity values greater than 300 Ωm occurs at a depth of 20 m downwards from the origin of the traverse line to 75 m.
The inverted resistivity model for Traverse T3 is presented in Figure 12, with resistivity values 9–990 Ωm. The delineated geoelectric layers are interpreted to include topsoil, clayey, sandy, and compacted sandy units. The topsoil consists of the intercalation of sand and clay units that have become compacted due to the human activities in the area. The clayey unit has resistivity values of 30–90 Ωm, with the depth of occurrence ranging from 5 m from the traverse inception to 30 m at the center portion to the 5 m at the end of the profile line. The geoelectric unit envelops several pockets of wet clay units with resistivity values of 9–18 Ωm. The sandy unit occurs at a depth range of 5–15 m from the inception of the profile line to about 75 m, while it occurs down to the depth of 50 m from the horizontal distance of 75 m to 200 m on the traverse. Pockets of clay units appear from a depth of about 15 m to 50 m between 0–75 electrode positions across the profile line. Figure 13 shows the inversion model for traverse T4, with resistivity ranges of 11–990 Ωm. The interpretive geoelectric layers include the topsoil comprising reworked sand and clay intercalations. Pockets of wet clay lenses with resistivity values 11–23 Ωm occur close to the surface and at the profile line’s central portion. There is an occurrence of a clayey layer with resistivity values of 23–100 Ωm with depths of about 5 m at the origin of the traverse, 50 m at the central portion and about 5 m depth at the end of their profile line. A third geoelectric layer was delineated as a clayey sand to sandy unit with resistivity values of 104–321 Ωm, occurring between the depth of 5–10 m from the origin to 75 m along the traverse. This same sand unit occurs again at 140–180 m along the traverse from the depth of 20 m downwards. A laterally extensive thick compacted sandy unit with resistivity values greater than 400 Ωm also occurs at a depth of 10 m downwards from the origin of the traverse line to about 75 m.

5. Discussion

The 2D electrical resistivity imaging and resistivity soundings were integrated to characterize the subsurface. Electrical resistivity properties of the subsurface geology depend significantly on permeability, porosity, degree of water saturation, fluid chemical composition, and clay content. The electrical resistivity of a geologic formation decreases as porosity, water saturation, and permeability increase. The presence of clay minerals in a water-bearing rock formation increases the conductivity of the rock formation through the ion exchange process [51]. The delineated geoelectric layers, including topsoil, clayey, compacted and unconsolidated sandy units, were present in all the traverses, as revealed by the 2D resistivity images. The topsoil consists of reworked unconsolidated sand–clay intercalations. The clay unit occurred in two forms: a wet clay unit closer to the surface and formed pockets of clay lenses across the study area. The compacted sandy unit delineated along the profile was extensive. The third delineated geoelectric layer is a mixture of clayey and clayey sand, which appears as clayey sand in some interpretive sounding curves in the study area. The fourth delineated geoelectric layer is an unconsolidated sand unit present across the entire study area. The sand unit is a coastal plain sand, a mixture of alluvium and Benin continental sand Formation.
The initial hydrogeological conceptual models for each survey traverse were developed, showing the two-dimensional distribution of the mapped two aquiferous units in the area (Figure 14, Figure 15, Figure 16 and Figure 17). The interpreted clayey sand and clayey units are the shallow, unconfined low-yield aquifer, or aquitard. This shallow aquiferous unit is believed to have a low yield due to its high clay content, which will reduce its permeability, thereby reducing the fluid flow through the unit. This shallow aquifer (aquitard) seems to occur along all the traverse lines and is believed to have low productivity due to poor transmissivity and low yield [52,53]. The aquifer’s productivity and yield depend on its porosity, permeability, and transmissivity, which are related to the clay, shale, and silt volume within its formation [52,53,54,55]. The results of the sounding curves showed the thicknesses of the shallow, low-yield aquifer to range from 15.5–18.8 m, 11.5–25.4 m, 8.0–19.4 m, 3.1–16.8 m for traverse T1, T2, T3 and T4, respectively. This shallow aquifer comprised sand, silt, and clay units of the continental plain sand of the Benin Formation [39,40] (Figure 2). The 2D resistivity images revealed that this layer is thickened within the central portion of traverses T1–T4, enclosing a clay lens unit and forming a perched aquifer within the study area. This shallow unconfined aquifer may be prone to pollution from industrial effluents and seawater incursions based on its much shallower depth of occurrence [24,25,26,36,37,43,56,57,58,59,60]. The high-yield main aquifer is the delineated unconsolidated sand unit. The resistivity soundings revealed the depth to the top of this coastal plain sand unit as 1.0–20.1 m, 0.7–30.4 m, 12.6 m to 17.1 m and 13.0 m to 18.1 m on traverses T1, T2, T3 and T4, respectively. 2D resistivity images revealed the presence of the high-yield main aquifer across all the traverses. The interpreted deeper main aquifer is the beach sand unit of the Tertiary alluvium deposits within the Dahomey Basin [39,40] (Figure 2). The compacted sand at the base confines the main aquifer, while the clayey sand and clayey unit confine it at the top.
Figure 18 shows the 3D distribution of the geoelectric sections with depth. Along the study area’s northern, central and southern parts, the clay sequence acts as the perched aquifer. At the western end, there is a pocket clayey unit grading into a sandy horizon; the clay horizon stretches from the surface to the investigated depth. The impervious unit (clay) protects the groundwater-bearing unit (sand) by acting as the aquitard; thus, it impedes the direct flow of surficial water percolating into the groundwater system. The groundwater is confined within the geoelectric sequences. Figure 19 revealed the lithologic units from the direct observation of the borehole drilled cuttings. The major lithology is the sandy materials with varying sizes. The lateritic unit (impervious) is the overlying section. Beneath this unit are different sandy horizons permitting the transmission of the groundwater; they are underlain by clayey sand horizons preventing further groundwater infiltration. These impervious units (laterite and clayey sand) confine the groundwater within the study area. Generally, there is an agreement between the deductions obtained from the geoelectric sequence and the borehole lithologic units in establishing that the aquifer unit within the area is confined.
It is recommended that the main aquifer within the area should be the target for efficient groundwater extraction due to its high yield. It is equally recommended that an adequate hydrogeophysical report from the experts should be required before digging any borehole in Lagos, Southwestern Nigeria. This will forestall cases of drilling into the polluted aquifer and indiscriminate drilling of boreholes that can lead to land subsidence and building collapse. Despite the crucial role of groundwater for domestic, industrial and agricultural use and sustainability of the ecosystem, many studies have revealed the empirical relationship between groundwater overexploitation and land subsidence in many urban cities across the world [61,62,63,64,65,66,67,68,69]. There is a need for the Lagos state government to come up with regulation standards for getting approval and licenses for digging boreholes and the volume of water that could be abstracted from these natural resources. Land subsidence, the gradual sinking or settling of the earth’s surface, can result from various causes, including excessive groundwater withdrawal [61,62,63,64,65,66,67,68,69]. Land subsidence has caused severe damage to infrastructure and the environment, making it a serious issue that needs to be prevented by the government of Lagos. One of the most serious impacts of the overexploitation of groundwater on land subsidence is the reduced volume of water within the aquifer, causing it to collapse, leading to subsidence [63,65,67,68,69]. When an aquifer is depleted, it can no longer support the weight of the land above it, causing it to sink or settle. This can damage roads, buildings and other structures, causing erosion, increased flooding and loss of agricultural land. Also, the overexploitation of groundwater can result in soil and water resource salinization [2,6,7,20,24,37]. When too much water is pumped from the aquifer, the water table can drop, bringing saltwater from deeper layers up to the surface [2,24,37]. This can cause soil and water salinization, making the soil and water less productive and affecting agriculture, wildlife, and human health [2,6,7,20,24,37]. To prevent all these hazardous impacts of overexploitation of groundwater in Lagos, it is important for the Lagos state government to implement sustainable groundwater management practices. These include implementing standard regulations for managing groundwater pumping and monitoring the water table across the entire Lagos state. While groundwater continues to be a valuable and indispensable natural resource for human survival, its exploitation should be done in a responsible and sustainable manner to protect the environment and ensure the long-term health and well-being of communities and ecosystems.
The findings of this research serve as a baseline for groundwater resource management within the coastal areas of Lagos. Before developing groundwater resources, it is pertinent to understand the subsurface geology and groundwater potential. The data and the information obtained from this study provide information on the groundwater potential of the coastal areas in Lagos. They are vital for siting both boreholes and hand-dug wells in these typical coastal aquifers. The aquifers’ vulnerability to pollution and the development of protective measures relies largely on understanding aquifer lithology. In this case, the shallow aquifer is typically unconfined and, therefore, vulnerable to surface pollution compared to the deeper aquifer. Due to unconfinement conditions, the shallow aquifer also has great direct recharge potential. Sustainable development and management of groundwater resources require a good understanding of the groundwater potential and the vulnerability of aquifers to pollution and recharge potential, among other factors. While this study is on a local scale, the application of the findings would be wide in many coastal areas that exhibit similar geology.

6. Conclusions

This research focuses on applying geoelectrical resistivity imaging to assess the groundwater potential of the coastal aquifer. Resistivity soundings were integrated with 2D electrical resistivity imaging to evaluate the subsurface layered media and their corresponding geoelectric parameters, such as average electrical resistivities and thicknesses. The delineated geoelectric layers include topsoil, clayey, compacted, and unconsolidated sand. Two aquiferous units were mapped in the area due to their clay content. The first is a shallow, low-yield aquifer, the delineated clayey/clayey sand unit. The shallow aquifer unit is the continental plain sand of the Benin Formation. The electrical resistivity values and depth to the top of this aquiferous zone are 65–190 Ωm and 0.8−5.5 m, respectively, which is considered clayey/clayey sand. However, this shallow aquifer is prone to several pollutions from industrial effluents and seawater intrusion. The second aquifer is clay-free, unconsolidated beach sand, adjudged to be of the Tertiary alluvium deposits. The electrical resistivity values and depth to the top of this main aquifer system are 176–520 Ωm and 0.7–30.4 m. The lateral variability and distribution of the geoelectrical layers are revealed in 2D geoelectrical resistivity images. The study has confirmed that the results of 2D electrical resistivity imaging complement those of the electrical soundings in groundwater resource evaluation within a coastal environment. Pumping tests are recommended to evaluate the hydraulic parameters of the two delineated aquifer units in the area. The geoelectrical resistivity technique can resolve groundwater-related challenges involving identifying groundwater-bearing units and establishing the nature of the aquifer unit. It has also revealed that the perched aquifer and the main aquifer unit could be delineated using the geophysical technique, which in turn aids the siting of either tube wells or boreholes for groundwater extraction. The findings of this research are essential for groundwater resource sustainability in Lagos, Nigeria. Since understanding subsurface geology is essential for successful groundwater resource extraction, there is a need for a viable policy restricting unprofessional borehole drillers with little-to-no knowledge of earth sciences in Lagos. Also, there is a need to implement a policy regulating groundwater abstraction in Lagos to ensure its management and sustainability.

Author Contributions

Conceptualization, K.D.O., M.M. and M.G.; methodology, K.D.O., O.J.R., P.O.F. and J.A.-J.; investigation, K.D.O., O.J.R., J.A.-J., A.A.O., B.D., P.O.F. and M.G.; writing—original draft preparation, K.D.O.; writing—review and editing, M.M., A.A.O., B.D. and M.G.; funding acquisition, M.M. and K.D.O. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the Research Supporting Project number (RSP2023R89), King Saud University, Riyadh, Saudi Arabia.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data is available upon request from the corresponding author.

Acknowledgments

The authors would like to thank the Research Supporting Project number (RSP2023R89), King Saud University, Riyadh, Saudi Arabia, for funding this work.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Location of the study area showing the VES points and 2D ERT profiles (T1–T4).
Figure 1. Location of the study area showing the VES points and 2D ERT profiles (T1–T4).
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Figure 2. Geological map of the Nigerian part of the Dahomey Basin [39].
Figure 2. Geological map of the Nigerian part of the Dahomey Basin [39].
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Figure 3. A Schlumberger array.
Figure 3. A Schlumberger array.
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Figure 4. Representative vertical electrical sounding curves across traverse T1 (VESs 1–4). These soundings are conducted along the traverse line of the resistivity image Figure 9.
Figure 4. Representative vertical electrical sounding curves across traverse T1 (VESs 1–4). These soundings are conducted along the traverse line of the resistivity image Figure 9.
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Figure 5. Representative vertical electrical sounding curves across traverse T2 (VESs 5–8). These soundings are conducted along the traverse line of the resistivity images Figure 9 and Figure 10.
Figure 5. Representative vertical electrical sounding curves across traverse T2 (VESs 5–8). These soundings are conducted along the traverse line of the resistivity images Figure 9 and Figure 10.
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Figure 6. VES-derived geoelectric section from VESs (1–5) with resistivity values in Ohm-m.
Figure 6. VES-derived geoelectric section from VESs (1–5) with resistivity values in Ohm-m.
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Figure 7. VES-derived geoelectric section from VESs (6–10) with resistivity values in Ohm-m.
Figure 7. VES-derived geoelectric section from VESs (6–10) with resistivity values in Ohm-m.
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Figure 8. VES-derived geoelectric section from VESs (11–15) with resistivity values in Ohm-m.
Figure 8. VES-derived geoelectric section from VESs (11–15) with resistivity values in Ohm-m.
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Figure 9. VES-derived geoelectric section from VESs (16–20) with resistivity values in Ohm-m.
Figure 9. VES-derived geoelectric section from VESs (16–20) with resistivity values in Ohm-m.
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Figure 10. 2D inverse resistivity model for traverse T1.
Figure 10. 2D inverse resistivity model for traverse T1.
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Figure 11. 2D inverse resistivity model for traverse T2.
Figure 11. 2D inverse resistivity model for traverse T2.
Sustainability 15 03538 g011
Sustainability 15 03538 g012 550
Figure 12. 2D inverse resistivity model for traverse T3.
Figure 12. 2D inverse resistivity model for traverse T3.
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Sustainability 15 03538 g013 550
Figure 13. 2D inverse resistivity model for traverse T4.
Figure 13. 2D inverse resistivity model for traverse T4.
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Sustainability 15 03538 g014 550
Figure 14. 2D initial hydrogeological conceptual model for traverse T1.
Figure 14. 2D initial hydrogeological conceptual model for traverse T1.
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Sustainability 15 03538 g015 550
Figure 15. 2D initial hydrogeological conceptual model for traverse T2.
Figure 15. 2D initial hydrogeological conceptual model for traverse T2.
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Sustainability 15 03538 g016 550
Figure 16. 2D initial hydrogeological conceptual model for traverse T3.
Figure 16. 2D initial hydrogeological conceptual model for traverse T3.
Sustainability 15 03538 g016
Sustainability 15 03538 g017 550
Figure 17. 2D initial hydrogeological conceptual model for traverse T4.
Figure 17. 2D initial hydrogeological conceptual model for traverse T4.
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Sustainability 15 03538 g018 550
Figure 18. 3D lithologic modeling from the VES data.
Figure 18. 3D lithologic modeling from the VES data.
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Sustainability 15 03538 g019 550
Figure 19. Borehole logs showing lithologic units from the study area.
Figure 19. Borehole logs showing lithologic units from the study area.
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Table
Table 1. Geoelectrical parameters of the vertical electrical sounding conducted along traverse T1.
Table 1. Geoelectrical parameters of the vertical electrical sounding conducted along traverse T1.
LocationElevation
(m)		LayersResistivity
(Ωm)		Thickness
(m)		Depth
(m)		Curve TypeLithology
T1-V156 m188.21.01.0ATopsoil
2176.413.514.4Clayey Sand
3398.0--Sandy
T1-V258 m1112.70.80.8ATopsoil
2132.215.616.4Clayey Sand
3245.8--Sandy
T1-V357 m1128.10.90.9KQTopsoil
2154.22.63.5Clayey Sand
347.815.318.8Clayey
441.3--Clayey
T1-V456 m199.31.21.2HTopsoil
265.118.920.1Clayey
3212.2--Sandy
T1-V557 m1251.41.11.1HTopsoil
279.115.516.6Clayey
3388.6--Sandy
Table
Table 2. Geoelectrical parameters of the vertical electrical sounding conducted along traverse T2.
Table 2. Geoelectrical parameters of the vertical electrical sounding conducted along traverse T2.
LocationElevation
(m)		LayersResistivity
(Ωm)		Thickness
(m)		Depth
(m)		Curve TypeLithology
T2-V656 m1135.30.80.8HTopsoil
2124.916.917.7Clayey Sand
3519.6--Sandy
T2-V758 m157.70.70.7KHTopsoil
2205.12.33.0Sandy
3189.712.215.1Clayey Sand
4875.8-- Compacted Sand
T2-V857 m1225.00.90.9QTopsoil
2110.52.33.2Clayey Sand
371.312.515.6Clayey
449.7--Clayey
T2-V958 m1202.60.90.9HATopsoil
270.24.04.9Clayey
375.211.516.4Clayey
4431.0-- Sandy
T2-V1056 m179.90.90.9ATopsoil
284.84.15.0Clayey
399.725.430.4Clayey
4307.7-- Sandy
Table
Table 3. Geoelectrical parameters of the vertical electrical sounding conducted on traverse T3.
Table 3. Geoelectrical parameters of the vertical electrical sounding conducted on traverse T3.
LocationElevation
(m)		LayersResistivity
(Ωm)		Thickness
(m)		Depth
(m)		Curve TypeLithology
T3-V1156 m1187.71.11.1HTopsoil
267.111.512.6Clayey
3215.9--Sandy
T3-V1255 m134.71.01.0ATopsoil
242.08.09.0Clayey
3252.5--Sandy
T3-V1356 m177.60.70.7HKTopsoil
227.84.85.5Clayey
372.019.424.9Clayey
467.3--Clayey
T3-V1457 m191.00.90.9HTopsoil
227.616.217.1Clayey
3143.7--Clayey Sand
T3-V1558 m144.41.01.0ATopsoil
248.612.113.2Clayey
3227.3--Sandy
Table
Table 4. Geoelectrical parameters of the vertical electrical sounding conducted on traverse T4.
Table 4. Geoelectrical parameters of the vertical electrical sounding conducted on traverse T4.
LocationElevation
(m)		LayersResistivity
(Ωm)		Thickness
(m)		Depth
(m)		Curve TypeLithology
T4-V1657 m185.41.01.0QTopsoil
271.43.14.1Clayey
329.717.221.3Clayey
410.3-- Clayey
T4-V1756 m129.41.21.2ATopsoil
256.511.913.0Clayey
3234.7--Sandy
T4-V1857 m1212.71.01.0QTopsoil
2160.62.83.7Clayey Sand
321.610.914.7Clayey
418.6--Clayey
T4-V1956 m1107.31.21.2HTopsoil
240.012.513.7Clayey
3190.8--Clayey Sand
T4-V2058 m115.21.31.3ATopsoil
238.316.818.1Clayey
3174.1--Clayey Sand
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MDPI and ACS Style

Oyeyemi, K.D.; Abuka-Joshua, J.; Rotimi, O.J.; Dieppois, B.; Gomo, M.; Olaojo, A.A.; Falae, P.O.; Metwaly, M. Geoelectrical Characterization of Coastal Aquifers in Agbado-Ijaye, Lagos, Southwestern Nigeria; Implications for Groundwater Resources Sustainability. Sustainability 2023, 15, 3538. https://0-doi-org.brum.beds.ac.uk/10.3390/su15043538

AMA Style

Oyeyemi KD, Abuka-Joshua J, Rotimi OJ, Dieppois B, Gomo M, Olaojo AA, Falae PO, Metwaly M. Geoelectrical Characterization of Coastal Aquifers in Agbado-Ijaye, Lagos, Southwestern Nigeria; Implications for Groundwater Resources Sustainability. Sustainability. 2023; 15(4):3538. https://0-doi-org.brum.beds.ac.uk/10.3390/su15043538

Chicago/Turabian Style

Oyeyemi, Kehinde D., Joyce Abuka-Joshua, Oluwatosin J. Rotimi, Bastien Dieppois, Modreck Gomo, Abayomi A. Olaojo, Philips O. Falae, and Mohamed Metwaly. 2023. "Geoelectrical Characterization of Coastal Aquifers in Agbado-Ijaye, Lagos, Southwestern Nigeria; Implications for Groundwater Resources Sustainability" Sustainability 15, no. 4: 3538. https://0-doi-org.brum.beds.ac.uk/10.3390/su15043538

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