Seventeen (17) vertical electrical sounding (VES), employing the Schlumberger electrode array, were conducted using SAS 1000 Terrameter and other accessories (four rolls of connecting cables, two rolls of tapes, compass, hammer, a pair of crocodile clips, cutlass etc.) in the study area ( Figure 1 ). The maximum current electrode separation ranged between 300 - 450 m. The choice of using the Schlumberger array was based on the fact that it is more efficient in differentiating subsurface lateral and vertical variations in resistivity as well as the fact that its field operation is simpler and faster when compared with other geophysical methods. The SAS 1000 Terrameter has an in built 12 V battery that ensured adequate energy to be supplied to the subsurface thereby giving a reliable signal as well as good quality data. It also has an in-built digital display unit. The geoelectric survey was carried out with all the four electrodes placed in a straight line in such a way that the distance between the two potential electrodes were kept at a metre (1 m) apart, while the other two current electrodes at two metres (2 m) apart. Consequently, an electrical field within the subsurface was established, generated and determined through the pair of current electrodes and the pair of potential electrodes, connected to the SAS 1000 Terrameter. The process was repeated by increasing the current electrode spacing symmetrically from the central position, while the resultant increase in potential difference of the subsurface was recorded. An increase was made on the potential electrode spacing when a decreasing potential difference was observed on the instrument and the survey continued. During the survey, precaution was taken in line with the works of [26] and [6] that “the distance between the potential electrodes never exceeds one fifth of the distance between the current electrodes”. All the VES data were curve matched, using standard master curves to enable the subsurface layer parameters (resistivity, thickness and depth) to be determined. This was done by plotting the apparent resistivity of the subsurface against half of the current electrode distance for each VES data. Thereafter, the data were subjected to iteration in Win Resist Software, as used by [27] , [28] . Finally, the results from the iteration enabled the determination of the true subsurface layer parameters (resistivity, thickness, depth).

A well was drilled to a depth of 130 m and well cuttings were collected at 3 m intervals and analyzed in the study area with the aim of evaluating the subsurface lithology with respect to depth. Subsequently, geophysical well logging comprising spontaneous logging [SP] ( Figure 3(a) ) and resistivity logging ( Figure 3(b) ) were conducted using SAS 1000 and its accessories (SAS 200 logging tool, a logging probe and a calibrated cable) by lowering the probe into the well ( Figure 3(c) ) at 3 m intervals. The result obtained from the logging exercise was used to evaluate the electrical conductivity as well as the total dissolved solid of the groundwater.

Conventionally, pumping test was conducted in two separate wells drilled to evaluate the aquifer parameters in the study area. The two wells (one referred to as test well while the other designated as observation well) were located 2 m apart. A 1 KW submersible pump was installed into the test well and connected to a 2.5 KW generator. Water was pumped from the well at a constant rate of 0.018 m³/min and after some interval, the depth of water in the observation well was determined through the use of a sensible probe which was attached to a cable fixed to a water detector. The data obtained from the drawdown (difference in the water level at a given time from the water level before the commencement of pumping) of the well was plotted against time of pumping on a semi-logarithm graph. The graph sheet was then used to determine the drawdown per log cycle of time (∆s) and the horizontal intercept (t₀) and the data obtained were then substituted into the [29] straight line equations to evaluate aquifer hydraulic parameters (transmissivity T, Storativity S, Specific Capacity Sc, and Hydraulic Conductivity K). Applying the [29] formula, as used by [6] - [8] as follows:

$\text{Transmissivity}\left(T\right)=\frac{2.25Q}{4\pi \Delta s}$(1)

$\text{Storativity}\left(S\right)=\frac{2.25T{t}_0}{r^2}$(2)

$\text{Specific Capacity}\left(Sc\right)=\frac{Q}{\Delta s}$(3)

$\text{Hydraulic Conductivity}K=\frac{T}{b}$(4)

where Q is the rate of discharge (m³/s),

∆s is the slope (m),

t₀ is the time pumping started (m),

r is the radial distance from test well (m),

b is aquifer thickness (m).

The result of the lithologic log from the drilled well and the well logging is presented in Figure 3 . Figure 3 shows a thick lateritic soil material as the first layer. This thick lateritic crust/layer is composed essentially of reddish lateritic top soil, clayey sand, sandy clay and fine sand. The thickness of this lateritic layer is about 36 m. This first layer is underlain by the second layer composed of reddish brown fine to medium sand of 8 m thick (36 - 44 m). The second layer is underlain by the third layer which is composed of grayish, medium grain sand, stretching from a depth of 44 - 75 m. This represents the first aquifer in the area. The third layer is underlain by another sandy clayey unit from a depth of 75 m to 88 m. Underling the fourth layer is the fifth layer, which consists of medium to coarse sand, extending from 88 m to 108 m depth. This formation is greyish in colour and represents the second aquiferous region. The fifth layer is underlain by the sixth layer which is made up of greyish to whitish coarse sand/gravels, extending to 108 m to 128 m depth where the drilling terminated.

Analysis of the well log (resistivity log) indicates that the resistivity of the subsurface increases downwards from 10 m to 36 m depth, representing the lateritic sand layer. At 36 m depth the value of the resistivity log decreases slightly from the formation above it, indicative of a fine to medium sandy formation. At a depth of 44 m, a steady increase in the resistivity log was observed. This observation supports the fact that the subsurface material is sandy in nature. This increase in resistivity value of the log is suggestive of an increase in grain size of subsurface material as well as increase in hydraulic conductivity of the aquifer with depth. The spontaneous potential (SP) log on the other hand showed a low value of –60 mV to an increase of about 20 mV, within 10 m to 36 m depth. This was followed by a steady increase throughout the depth of the well. This stability in the resistivity values as well as increase in the SP value downwards shows that the underlying formation would be better source of groundwater for domestic and other purposes than the overlying layers.

The groundwater parameters (electrical conductivity and total dissolved solids) were evaluated in the field from the result of the electrical and SP logs. The result presented ( Table 1 and Figure 3 ) revealed an average value of 15 μs/cm for electrical conductivity (EC) while an average of 112 mg/l was obtained for the total dissolved solid (TDS). These values, when compared with the Standard Organization of Nigeria [30] permissible limit for drinking water are lower than the 1000 μs/cm value set for EC 500 mg/l set for TDS. These low values (EC and TDS) indicate that the groundwater is free from pollutants, as well as a fresh to moderately mineralized subsurface which is suggestive of a groundwater that is suitable and adequate for domestic and other purposes ( Figure 4 ).

Seventeen (17) VES data obtained from the area was curve matched and later subjected to computer iteration, interpretation using Win Resist software. The representative model field curves obtained from the iteration of the VES are presented in Figures 5(a)-(c) . Table 2 shows the geoelectric parameters obtained from the geophysical interpretation. The result revealed a four to six geoelectric layers that consist of lateritic top soil, fine grained sand, sandy clay/clayey sand, medium grained sand, sandy gravel and coarse/gravelly sand. The table indicates a four layered subsurface in VES 1, 2, 3, 4, 5, 11, 13, 14, 15, 16, 17; a five layered subsurface in VES 6 and 7, while it is six layered subsurface in VES 8, 9, 10 and 12. Figure 6 shows the borehole log in comparison with the geoelectric section of some selected VES stations in the area. A close examination shows a good correlation between the borehole log and the geoelectric data. The parameters used in generating the geoelectric sections include the layer’s thicknesses and the resistivity values. The geoelectric section gives a two-dimensional insight into the structural disposition and subsurface geologic sequence of the area.

The first layer consists of lateritic topsoil/sand with resistivity range from 64 - 1835 Ω·m and thickness range from 0.5 - 1.9 m. The second layer consists of lateritic sand/fine grained sand with resistivity range from 266 - 2558 Ω·m and thickness range from 2.4 - 31.8 m. The third layer comprises fine to coarse gravelly sand with resistivity range from 126 - 27,742 Ω·m and thickness range from 4.9 - 52.3 m. The fourth layer comprises of medium to gravelly sand with resistivity range from 976 - 15,166 Ω·m and thickness range from 9.3 - 50.4 m. The fifth layer consists of course to gravelly sand with resistivity range from 1215 - 23,543 Ω·m and thickness range of 34.4 - 50.0 m. The sixth layer is coarse to gravelly sand with resistivity range of 1130 - 3540 Ω·m. The current electrode separation terminated in this layer; thus, the exact thickness could not be ascertained. This phenomenon was also observed in the fourth layer of VES 1, 2, 3, 4, 5, 11, 13, 14, 15, 16, 17 and the fifth layers of VES 6 and 7. The VES data analysis revealed that groundwater can be sourced from 18.4 - 76.6 m depths from the fourth, fifth and sixth layers respectively but the fifth and sixth layers would provide more prolific aquifers than that of the fourth layer. This assertion is correlated with the borehole log, which gives a good correlation between the two results.

Table 3 shows the summary of the geophysical data interpretation in terms of the aquifer parameters obtained from the VES stations. The aquifer resistivity ranged from 1175 Ω·m (VES 11) to 27,272 Ω·m (VES 14), while the thickness ranged from 9.7 m (VES 14) to 52.3 m (VES 13) with an average of 34.28 m. The transmissivity ranged from 0.81m²/day (VES 14) to 17.30 m²/day (VES 11) with an average of 2.82 m²/day. Also, the longitudinal conductance ranged from 0.001048 Ω⁻¹ (VES 14) to 0.027828 Ω⁻¹ (VES 11). The transverse resistance estimated ranged from 105470.4 Ω·m² (VES 4) to 947150 Ω·m² (VES12). The values for hydraulic conductivity estimated ranged from 0.028 m/day (VES 14) to 0.529 m/day (VES 11), with an average value of 0.1042 m/day. The aquifer depth ranged from 28.8 m (VES 14) to 76.6 m (VES 12). These values agreed with similar VES survey results at Agbor by [6] , where the aquifer depth range of 29 - 82 m was revealed. Abavo is about 13 kilometers from Agbor and belongs to the same geological formation. Figure 7 shows the depth to the aquifer map. It reveals that the northeast, southeast and northwestern parts have the highest aquifer depth while a moderate aquifer depth was revealed in majority of the VES stations in the central part of the study area.

The implications of the result indicate that the average transmissivity value of 2.82 m²/day obtained when compared with the standard for transmissivity values by [31] , implies “a low groundwater potential and smaller withdrawal for local water supply/private consumption”. The values of longitudinal conductance below 0.1 (poor) indicates that the aquifer is unprotected thus, prone to risk of contamination [31] [32] . Also, the high transverse resistance suggests high yield for private consumption/local water supply [33] . The hydraulic conductivity obtained indicates a heterogeneous aquifer material consisting of clay, silt and sand with average/moderate yield.

The result obtained from the investigation (pumping test analysis) in the drilled well is presented in Table 4 . Figure 8 shows the relationship between the water level and time of pumping, which enabled the evaluation of the drawdown. Applying the [29] , with a constant pumping rate (discharge) Q, drawdown per log cycle ∆s as 0.78 m (obtained from the plot), time pumping started t₀ (x-intercept) as 3 minutes, radius of the well r as 2 m and aquifer thickness b of 34.28 m, the aquifer parameters (transmissivity, specific capacity, storativity, hydraulic conductivity) were evaluated as follows:

$\begin{array}{c}\text{Transmissivity}\left(T\right)=\frac{2.25Q}{4\pi \Delta s}=\frac{2.25\times 0.018}{4\times 3.14\times 0.78}\\ =0.0041{\text{m}}^2/\min \\ =5.9{\text{m}}^2/\min \end{array}$

$\begin{array}{c}\text{Specific Capacity}\left(Sc\right)=\frac{Q}{\Delta s}=\frac{0.018}{0.78}\\ =0.023{\text{m}}^2/\min \\ =33.12{\text{m}}^2/\text{day}\end{array}$

$\text{Storativity}\left(Sc\right)=\frac{2.25T{t}_0}{r^2}=\frac{2.25\times 0.004\times 3}{2^2}=0.0069$

$\begin{array}{c}\text{Hydraulic Conductivity}K=\frac{T}{b}=\frac{0.0041}{34.28}\\ =0.0001196\text{m}/\min \\ =0.1722\text{m}/\text{day}\end{array}$

Results of the analysis revealed that the transmissivity is 0.0041 m²/min (5.9 m²/day), specific capacity is 0.023 m²/min (33.12 m²/day), storativity is 0.0069 and hydraulic conductivity is 0.0001196 m/min (0.1722 m/day). The result obtained for transmissivity suggests a low/intermediate transmission rate of groundwater for local water supply to communities, plant etc. The specific capacity of 0.023 m²/min (33.12 m/day) suggests that the aquifer can yield sufficient water for private consumption/local water supply to communities. The storativity (storage coefficient) of 0.0069 indicates that the aquifer is semi-confined and therefore implies that a substantial pressure exists within the aquifer which can withstand pumping for local water supply to communities. Also, this result is in agreement with 0.005 - 0.02 standard set by [34] for semi-confined aquifer. The hydraulic conductivity value of 0.0001196 m/min (0.1722 m/day) obtained indicates a heterogeneous aquifer material consisting of clay, silt and sand with average/moderate yield. Thus, the results for transmissivity, specific capacity and storativity obtained are in agreement with the results obtained by [6] who carried out a similar investigation at Agbor (an area within the same geological formation) and obtained the values of 0.0036 m²/min (5.184 m/day), 0.02 m/min (28.8 m/day) and 0.016 for transmissivity, specific capacity and storativity respectively. Thus, the values obtained for the aquifer parameters from this study indicate that the aquifer can yield sufficient water for local water supply to communities, though with low protective capacity.