Geo-electrostratigraphic assessment of aquifer potential, protectivity, and pliable level of vulnerability within a coastal milieu

The exponential growth in the human population and the environmentally hazardous anthropogenic activities carried out in the quest for sustenance have increased the susceptibility of groundwater resources to contamination. Consequently, groundwater potentiality and aquifer protectivity studies are a crucial part of conserving the hydrological system (Edet 2004; Eke et al. 2015; Oke et al. 2018). Regular monitoring of vulnerable aquifer systems is necessary to ensure optimal groundwater abstraction procedures that will reduce the rate of contaminant percolation into the aquifer. The need to monitor an aquifer's potentiality and susceptibility to contamination is critical, given that contaminated groundwater resources are more expensive and difficult to restore compared to surface water resources like streams and lakes.

Multiple approaches have been developed for analyzing groundwater susceptibility to contamination (Foster et al. 2002). These approaches have drawbacks and benefits; hence no single method can be considered optimal for all kinds of geological situations. An example of an approach developed to evaluate groundwater potentiality and susceptibility to contamination is the use of the Dar-Zarrouk parameters which employ quantitative geophysical data to generate measures of longitudinal conductance and transverse resistance. Another approach to evaluate groundwater susceptibility is the Aquifer Vulnerability Index (AVI) methodology. Yet another technique to evaluate aquifer susceptibility is the GOD methodology which employs groundwater occurrence (G), overlying lithology (O) and depth to aquifer (D) in its evaluation, hence the acronym GOD. The GOD method uses parameters obtained from hydro-geophysical information in the determination of aquifer vulnerability to contaminants. There is also the DRASTIC methodology which uses the following parameters in the determination of aquifer vulnerability: depth to groundwater (D), net groundwater recharge (R), aquifer media (A), soil media (S), topography (T), influence of vadose zone (I), and hydraulic conductivity (C), hence the acronym DRASTIC. These aquifer vulnerability methods (AVI, GOD, and DRASTIC), though efficient, are slightly subjective. The Dar-Zarrouk parameters (longitudinal conductance and transverse resistance), on the other hand, are quantitative measures obtained from vertical electrical sounding (VES) data and are not subjective (Oladapo et al. 2004; Huan et al. 2012; Saha & Alam 2014; Udosen 2022; Udosen et al. 2023, 2024c).

The study area, located in Southern Nigeria, is experiencing extremely fast rates of industrialization and urbanization due to the siting of key industries and major Federal and State educational institutions in the region. This rapid rate of development has led to overexploitation of available surface and groundwater resources leading to contamination. Despite a good groundwater rate in the region resulting from annual average rainfall of approximately 1,500–2,400 mm, most boreholes sited in the region are unproductive or contaminated. This is a result of inadequate planning and a dearth of expert geophysical investigations to sort through the region's complex geology. Contamination sources within the region include leached septic wastes, open waste disposal systems, oil spills, and domestic/industrial waste inappropriately disposed on the earth's surface. This challenge of contaminants infiltrating potable groundwater resources has given rise to apprehensions about the consequential health impact on humans and animals ingesting the contaminated water.

Taking the aforementioned into consideration, a comprehensive geophysical evaluation of the coastal environment was necessary to enhance comprehension of the geo-hydraulic characteristics of the aquifer and its vulnerability (Ibuot & Obiora 2021). This was done in an effort to lessen the likelihood of groundwater exploration/conservation-related problems in the region (Okoroh & Ibuot 2022). Aquifer vulnerability problems pose a far greater threat to this area as residents employ unconsolidated tube wells to abstract groundwater from unconsolidated geo-materials. An effective method for assessing groundwater potentiality and aquifer protectivity/vulnerability is electrical resistivity surveying (George et al. 2024). This technique has been applied in a wide range of hydro-geophysical investigations (Dassargues 1997; Yadav & Abolfazli 1998; Lashkaripour 2003; Singh 2005; Batte et al. 2010; Majumdar & Das 2011; Sikandar & Christen 2012; Ojuri & Bankole 2013; Rai et al. 2013; Obiora et al. 2016; Oloruntola et al. 2017; Udosen & Potthast 2018; Udosen & George 2018a, 2018b; Ibuot et al. 2021; Asfahani 2023; Asfahani et al. 2023; Opara et al. 2023). The use of geo-electrical methods, especially in conjunction with borehole lithological information, would help determine optimal locations for water projects (George et al. 2022; Udosen et al. 2024a, 2024b).

The region is an important coastal area, housing key industrial complexes and major State and Federal institutions. Groundwater is the major water resource within the region; hence it was necessary to undertake a comprehensive assessment of groundwater potentiality and its susceptibility to contamination. Incomplete knowledge of exploited aquifers and their geo-hydraulic boundaries and composition underscored the necessity of conducting a scientific mapping of the exploitable aquifer resources in the region. The need to undertake well-thought-out vulnerability mappings of the aquifer other than wildcat drilling would avert the problems of abandoned wells and reduce health risks associated with ingestion of contaminated water (Inim et al. 2020; Joshua et al. 2023; Udosen et al. 2024d). Groundwater has a high vulnerability to subsurface contamination and is more expensive to clean up compared to surface water resources like streams and lakes, hence it was important to ascertain the region's aquifer's potentiality and protectivity.

The aim of this research therefore was to investigate the hydro-geophysical properties of the region's aquifer system, assess the potential of groundwater resources within the study area, and evaluate the aquifer's susceptibility to pollution. To carry out these aims, geo-electrostratigraphic assessments employing vertical electrical soundings and 2D electrical resistivity tomography surveys were undertaken. The geophysical surveys would delineate the different lithological layers, determine conditions of groundwater, locate the aquifer zone, and evaluate the geo-hydraulic characteristics of the region's aquifer system. Results generated will aid in ascertaining aquifer properties precluding the need for new testing boreholes.

The region under investigation is situated within the Benin Formation (Coastal Plain Sands) deposited between the Tertiary and Quaternary periods. Underlying these Coastal Plain Sands is the oil-bearing formation in the Niger Delta (Agbada Formation), and the Akata Formation. The Benin Formation comprises various fluvial and lacustrine deposits, rocks and sediments containing clay, and rock matrixes consisting of sand materials, all interwoven with each other (George et al. 2017). Alluvial ecosystems are another feature of the region. The alluvial units in the region comprise tidal and lagoonal sedimentary sands. These Coastal Plain Sands make up a majority of the region's aquiferous units which comprise coarse, medium, and finely sorted continental and gravelly sands that occasionally intercalate with thin horizons of argillaceous materials and lignite streaks. The shallow shale and argillaceous layers in the region create multi-aquifer systems. The region's vegetation comprises green foliage with short trees and patches of abandoned forest, along with secondary forests made up of shrubs and large trees. The region can be reached by a network of crisscrossing roads as well as a few other smaller road systems. Groundwater potential is moderate to good in the majority of regions where sandy formations are present. Some parts of the region area provide a difficult hydrogeological environment due to the intercalations of clay embedded within the sandy rock formations, leading to low groundwater potentials within such zones. In geologic formations with broken shale zones, groundwater is mostly accessible.

Twenty VES profiles were surveyed with an ABEM SAS 1,000 terrameter and its accessories. The Schlumberger array was employed, and the array employed a maximal current electrode spacing of 400 m. To geo-reference the coordinates of the profile lines, a hand-held GPS system was used. Schlumberger array was employed in VES data acquisition due to its heightened sensitivity to vertical subsurface layers and its wide current penetration depth. The terrain of the study area is relatively flat, and the Schlumberger array is generally effectual in locations with flat terrain. Further, the potential electrodes employed in the Schlumberger array are seldom moved, reducing the influence of near-surface lateral variations on the acquired data. The Schlumberger array employed four electrodes: a pair of current electrodes to inject subsurface current into the ground, and a second pair to measure the potential difference between the current electrodes. The spacing between the current electrodes was increased symmetrically about the array center to enable deeper penetration of current into the subsurface. The current electrode spacing (AB) ranged from 2 to 400 m while the potential electrode spacing (MN) ranged from 0.5 to 20 m. The gradual increment of potential electrode spacing was carried out to enable the generation of functional potential difference values. This is because the potential difference values tended to decrease with increments in current electrode spacing. By gradually increasing the potential electrode spacing, recognizable voltages could be generated.

These measures of apparent resistivity were then evaluated using manual curve matching methods, the aim being to generate initial starting parameters that would later be input into the computerized inversion schemes (Choudhury et al. 2001; Ekanem & Udosen 2023a, 2023b). The manual curves were first plotted on a bi-logarithmic graph with apparent resistivity as ordinate against half the current electrode spacing (AB/2) as abscissa. Smoothing was undertaken on the matching curves, the aim being to remove outliers and generate trendlines that would indicate how subsurface resistivity varied with depth. The presence of outliers would have resulted in high root mean square errors during computer inversion of the data due to a poor fit between the acquired field data and calculated values generated by the inversion software. Hence it was necessary that outliers be removed. Data smoothening was undertaken by removing anomalous data points that did not fit the general trend of the curves and also by taking the mean of measurements obtained at cross-over points. Data generated from these smoothed curves were later input into the least-squares iterative software WINRESIST. The inversion software used the depths and resistivity values generated from the master curves as initial inputs during reconstruction (Joshua et al. 2011; Udosen & Potthast 2019). The reconstruction inversion models generated from WINRESIST provided information on the number of subsurface geo-layers, the thickness of each geo-layer, their depths and their mean resistivity values.

The generated resistivity data were modeled with RES2DINV software by GEOTOMO. The first step prior to inversion was the extermination of anomalous data points, the goal being to generate 2D tomograms with reduced root mean square errors. The RES2DINV program which employed an iterative least-squares technique, aimed at reducing the objective function (Udosen et al. 2011, 2013a, 2013b; Udosen & Potthast 2019), as it tried to match the acquired field data with its theoretical model. A resistivity model which took into account the heterogeneous nature of the earth's surface was finally generated to delineate the lateral resistivity distribution with depth along the profile line. Results obtained from electrical resistivity tomography would be integrated with results obtained from vertical electrical soundings.

From Equation (12), it was seen that an increment in the values of longitudinal conductance from one profile line to the next would mean an increment in geo-layer thickness or a decrement in mean resistivity, or both. An increment in transverse resistance (from Equation (13)), on the other hand, would imply an increment in geo-layer thickness or geo-layer resistivity, or both. Hence, reduced values of transverse resistance were related to low-resistivity formations (e.g., shale, clay, and clayey sand) and shallow basements. Larger values of transverse resistance, on the other hand, were related to resistive subsurface formations (Ejiogu et al. 2019; Nwachukwu et al. 2019). There was also a significantly positive correlation between transverse resistance and aquifer transmissivity. High transverse resistance values were linked with regions having elevated aquifer transmissivity values (Ekwe et al. 2020; Eyankware et al. 2020; Eyankware & Aleke 2021), and vice versa. High aquifer potentiality was therefore indicated by high transverse resistance values.

Hydraulic conductivity values ranged from 0.679 m/day at VES 4 to 24.67 m/day at VES 14 (Table 2). The iso-parametric map indicating the spatial distribution of hydraulic conductivity (m/day) over the study area indicated that the northeastern sections had high hydraulic conductivity values, implying the ability of groundwater to move easily through the pore spaces within the rock matrix in those locations (Figure 14). High groundwater potential is typically indicated by locations with high aquifer hydraulic conductivity, which is linked to greater levels of hydraulic flow and groundwater recharge.

Aquifer storativity ranged from 7 × 10⁻⁵ to 0.0004, with a mean storativity of 0.0002, suggestive of a productive aquifer (Ugada et al. 2014). Locations with extremely low storativity values implied low groundwater potentiality, and vice versa. High values of hydraulic diffusivity would indicate formations with fast rates of transmitting stored fluids during changes in hydraulic conditions within the aquifer. Table 3 indicated high values of hydraulic diffusivity in the region with a mean of 1 × 10⁵ m²/day^, indicating the presence of a productive aquifer.

This work was undertaken to assess aquifer potentiality, protectivity, and pliable level of vulnerability to contamination within a major coastal milieu in Southern Nigeria using geo-electrical technology. Four geo-electric strata were delineated: motley topsoil, sandy clay, fine sand, and coarse sand. The regions with the largest potential for groundwater were sandy geo-materials with aquifer thickness >30 m. Areas with aquifer thickness ranging from 15–30 m were classified as having an average groundwater potential, while areas with aquifer thickness that were <15 m were classified as having low groundwater potential. The overburden's composition, marked by medium to low longitudinal conductance values, provided the underlying aquifer with little protection.

Characterization of aquifer geo-hydraulic parameters yielded the following values for those parameters: mean aquifer resistivity of 554.6 Ωm, mean aquifer conductivity of 0.0004 S/m, mean longitudinal conductance of 0.67 Ω⁻¹, mean transverse resistance of 5,601.7 Ωm², mean hydraulic conductivity of 3.5 m/day, mean transmissivity of 283 m²/day, mean storativity of 0.0002, and mean hydraulic diffusivity of 1 × 10⁵ m²/day. The research region had a range of groundwater potentials, ranging from medium to high. The research findings showed that the northeastern parts of the area had greater chances for groundwater potential resulting from its higher values of transmissivity, transverse resistance, and thickness. Locations with sandy lithologies underlying them were noticeably more prolific than locations with argillaceous formations.

These results have contributed to the knowledge of the region's groundwater potentiality and protectivity. It has also indicated how vulnerable the aquifer system is to contamination. The results have also shown via computation of geo-hydraulic characteristics, that the aquifer potentiality is higher in the northeast. This work will aid in the enactment of policies on groundwater management/conservation in the region. It will also aid in the development of sustainable developmental goals within the region to safeguard declining groundwater resources.

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

The authors declare there is no conflict.