Assessment of Hydrogeological Potential and Aquifer Protective Capacity of Odeda, Southwestern Nigeria

In ensuring livelihood and sustainability across the world, groundwater resources play a major and concrete role. Consumption of groundwater as a feasible source of drinking, domestic, industrial and agricultural needs has proven to be not only safer but also more economical than surface water, as it is commonly unpolluted and available. In recent years, investigation of groundwater sources has become a burning issue and a major concern as groundwater basins are being rapidly stressed due to population explosion, high level of urbanisation, industrialisation and other human activities. Presently, the percentage increase in water usage on a global scale has exceeded twice that of the population [1].

Pores and fractured rock formations in the sub-surface are usually hosts of groundwater. In the basement terrain, groundwater occurs within the overlying unconsolidated material derived directly from weathering of rocks and fractured/faulted bedrock, while in the sedimentary terrain, groundwater occurs within the porous and permeable layer of the saturated zone in the sub-surface [2, 3]. Over the years, groundwater exploration has been carried out using geophysical methods, which include electrical resistivity surveying, electromagnetic techniques and seismic methods, to obtain accurate information about the sub-surface settings, such as aquifer’s nature, type and depth of materials (consolidated or unconsolidated), depth of weathered or fractured zone, depth to groundwater, depth to bedrock and salt intrusions into groundwater [4].

Aquifers in the Precambarian basement complex are vulnerable to surface or near-surface contaminants as they commonly occur at shallow depths. Hence, successful exploration of groundwater in a basement terrain requires proper understanding of the hydrogeological characteristics of the aquifer units in relation to their susceptibility to environmental pollution and assessment of their protective capacity [3, 5]. One of the most effective ways of evaluating an environment without interfering with the hydrogeological system is through geophysical studies [6]. Over the years, geophysical survey using the vertical electrical sounding (VES) method has been applied in groundwater exploration within the basement complex rocks in Nigeria [7, 8, 9, 10].

VES using the Schlumberger array method was carried out at 30 different stations in the study area with the aim to determine the geoelectric parameters, such as resistivities and thicknesses of the sub-surface layers and their hydrogeological properties. This study was also aimed at evaluating the groundwater potential of the area, establishing the aquifer protective capacity of the overlying formations, especially its isolation from contamination, and recommending suitable points for groundwater positioning.

The study area, as shown in Figure 1, is located between latitudes 7°10’N and 7°12’N and between longitudes 3°23’E and 3°28’E. The study area is characterised by tropical climate with distinct wet and dry seasons. The annual rainfall ranges from 1400 mm to 1500 mm; the mean temperature is 30°C and varies from 25.7°C in July to 30.2°C in February [11]. The study area is underlain by Precambarian basement rocks (Figure 2), which are innately characterised by near-negligible permeability and low porosity. These rocks, according to a previous paper [12], were acknowledged to belong to the youngest of the three major provinces of the West African Craton. These rocks are of Precambarian age to early Palaeozoic age, which extends from the Northeastern part of Ogun State and dips towards the coast [13].

Figure 1

Figure 2

The electrical resistivity survey method using the VES method was carried out in the study area. The resistivity data were acquired using Campus Ohmega Terrameter. Thirty VES points were positioned in the study area using Schlumberger electrode configuration with half-current electrode separation (AB/2) ranging from 1 m to 132 m. The apparent resistivity values were obtained as the product of the resistance read from the resistivity meter and its corresponding geometric factor (K) for each electrode separation. The apparent resistivity data were then plotted against AB/2 on a bi-logarithm graph as sounding curves. The plotted sounding curves were interpreted manually by partial curve matching using different master curves [15]. The geoelectric parameters from the partial curve matching served as the input model for computer-assisted iteration using WINRESIST.

The values of the longitudinal unit conductance of the overburden rock units in the study area serve as the basis for the characterisation of its aquifer protective capacity. The longitudinal unit conductance gives a measure of the impermeability of the confining clay layer, which has low resistivity and low hydraulic conductivity. The protective capacity of the overburden layers in a particular area is proportional to the longitudinal unit conductance [16]. The longitudinal layer conductance (S) of the overburden at each VES station was obtained as shown in Equation (1) [17].

where h_i is the layer thickness, ρ_i is the layer resistivity, while the number of layers from the surface to the top of the aquifer varies in the range i = 1, …, n. The results of the longitudinal unit conductance were used to classify the areas into good, moderate, weak and poor aquifer protective capacity (Table 1). This was done using the classification given by Oladapo and Akintorinwa [18].

Olayinka [19] opined that in identifying areas of favourable aquifers within a basement terrain, the resistivity of the basement cannot be exclusively relied upon; hence, one has to consider the basement’s reflection coefficient in effectively evaluating groundwater potential in the study area. The degree of fracturing of the underlying basement is shown by the reflection coefficient [3]. The reflection coefficients (r) of the study area were calculated using Equation (2), as given by Bhattacharya and Patra [20] and Loke [21].

where ρn is the layer resistivity of the nth layer and ρ(n⁻ 1) is the layer resistivity overlying the nth layer.

In a basement terrain, groundwater yield can be grouped into high, medium and low depending on the overburden thickness and/or reflection coefficient (Table 2), as stated by Bayewu et al. [3]. The highest groundwater yield is found in areas where thick overburden overlies the fractured zone [18].

The summary of the geoelectric parameters and inferred lithologies in the study area is presented in Table 3. The curve types obtained after partial curve matching ranges from the three-layer H type (66.7%), A type (3.3%) and K type (3.3%), through the four-layer KH type (23.3%), to the five-layer curve HKH type (3.3%). The predominant H type curve recorded in the study area further affirmed the findings of Oloruntola and Adeyemi [9], who recorded >72% of H curve type in the basement geological terrain at Abeokuta. Figures 3 and 4 show the typical iterated curves generated from the field measurements. The geoelectric interpretations revealed three to five layers, as follows: topsoil (23–700 Ωm); the weathered layer, which is composed of clay/sandy clay and clayey sand/sand (4–790 Ωm); underlying this layer are the fractured layer (93–437 Ωm) and the fresh basement (532–2106 Ωm). The aquifer unit in the study area is basically found in the fractured layer, the yield being dependent on the amount of clay content.

Figure 3

Figure 4

The resistivities and thicknesses of the underground layers were used to compute the longitudinal unit conductance (S) of the layers in the study area. Table 4 shows the calculated longitudinal unit conductance in mhos and the protective capacity rating for the study area. The longitudinal unit conductance values of the overburden materials in the study area ranged from 0.049720 to 1.452000 mhos. It can be observed that the protective capacity in the study area reveals poor, weak, moderate and good capacity rating. Four VES stations have poor protective capacity, six shows weak protective capacity, 17 show moderate protective capacity, while three show good protective capacity rating, with 33% of the study area falling within the poor/weak overburden protective capacity. About 57% falls within the moderate range, while 10% falls within good overburden protective capacity. The longitudinal unit conductance map of the study area in Figure 5 shows that the northeastern and northwestern parts of the study area are characterised by moderate-to-good protective capacity, and this signifies that there is a little or no infiltration due to precipitation.

Figure 5

Figures 6 and 7 show the reflection coefficient and overburden thickness map of the study area. The reflection coefficient in the study area varies from 0.24 to 1.00. The groundwater prospects in the study area are categorised into high, medium and low potentials. In this study, zones where the overburden thickness is >13 m and the reflection coefficient is <0.8 are considered as zones with high groundwater potential, while zones with overburden thickness <13 m and reflection coefficient <0.8 are considered as zones having very low groundwater potential.

Figure 6

Figure 7

Generally, about 33% of the study area has high groundwater potential, which is restricted mostly to areas underlain by porphyritic granite and porphyroblastic gneiss, as established by a previous paper [22], while 23% of the study area has medium groundwater potential; moreover, 43% of the area has low groundwater potential. This result invariably indicates the significance of detailed groundwater survey and exploration in the study area for locating areas where successful boreholes can be sited.

The geoelectric investigation of the study area has revealed three to five subsurface geoelectric layers: top soil, weathered basement and fresh basement rocks. The fractured layer constitutes the sole aquifer unit in the study area. The protective capacity in the study area is more of the moderate type and is therefore not exposed to pollution. About 33% of the study area falls within the high rated groundwater potential zone, while the remaining 67% constituted the medium/low groundwater potential zone. Hence, the groundwater potential rating of the area is considered generally as medium/low. Therefore, areas for locating groundwater should be narrowed to zones of moderate/good groundwater protective capacity.