Groundwater Potential Evaluation of College of Engineering, Afe Babalola University, Ado-Ekiti, Southwestern Nigeria

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2.2. Climate, Geology and Hydrogeology of The Area

The area is situated within the tropical rain forest region, with a climate characterized by dry and wet seasons. Average annual rainfall in this area is 1300 mm, with average wet days of about 100. The annual temperature varies between 18°C to 34°C.

The study area lies within the basement complex of south-western Nigeria and is made up of; older granite, Migmatite and Charnockites. The overburden is relatively shallow within the study area with average of 11m. The groundwater is found within the overburden and fractured basement while the area is drained by the river Ogbese which flow SW-NE direction.

The basement complex rocks are poor aquifers as they are characterized by low porosity and negligible permeability, resulting from their crystalline nature, thus availability of groundwater resource in such areas can only be attributed to the development of secondary porosity and permeability resulting from weathering and fracturing.

3. Methodology, Data acquisition and Interpretation

A reconnaissance survey of study college was carried out for site familiarization and planning. This was followed with geophysical investigation of the college and environs. Geophysical methods are indirect site investigation techniques and predominantly non-intrusive. Two methods namely Resistivity Sounding and Resistivity Profiling were adopted. Soil resistivity meter (PASI) was used for the geophysical tests. It is highly reliable and reproducible.

Resistivity sounding was adopted in resolving resistivity variation with depth, thus sounding helped in delineating the various subsurface lithological units, hydrogeological significance and the protective capacity or vulnerability of the subsurface layers to possible pollution. Resistivity profiling was adopted in resolving horizontal resistivity variation in this study, hence profiling helped in establishing the lateral continuity of the various subsurface lithological units.

4. Results and Discussions

4.1. Vertical Electrical Sounding

A total of 8 VES locations across 3 traverses were spread over the study area (Figure 1). The processed data were interpreted, resulting curve types were assessed, existing subsurface lithologic units were established, and the geoelectric properties of the various subsurface layers were used in delineating the aquiferous units in the study area. The results are presented in the form of table (Table 1), geoelectric curves (Figure 3) and sections (Figure 4a & Figure 4b).

Five different subsurface lithologic sequences were established namely; lateritic topsoil, sandy-clay, weathered basement, fractured basement and, basement. The curve types range between simple H, K, HA, KH and HK. The topsoil, sandy-clay and weathered basement materials are characterised with relatively low resistivity values while the basement materials are typified with high resistivity values. A summary of the results of interpretation, on which the following findings were hinged, is shown in Table 1.

Figure 2. Geology Map of the Study Area

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Figure 3. Typical Geoelectric curves from data interpretations

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Table 1. Correlation Table

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Tables index

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4.1.1. Geoelectric Units

The geoelectric sections (Figure 4a & Figure 4b) show the variations of resistivity and thickness values of layers within the depth penetrated in the study area. Two traverses were taken along the NW-SE direction and one traverse along N-S direction. Five subsurface layers were revealed: Lateritic Topsoil, Sandy-clay, Weathered basement, Fractured basement and presumed Fresh basement.

Topsoil

The topsoil is relatively thin along these traverses. The average resistivity and thickness values for the topsoil are 178Ωm and 1.0m respectively.

Sandy-clay

Sandy-clay was encountered at shallow depths of 2.0meters on the average in six locations and the average resistivity and thickness values for the Clay are 23Ωm and 7.0m respectively.

Weathered-basement

Weathered-basement was encountered in six locations and the average resistivity and thickness values of the weathered-basement are 231Ωm and 8.0m respectively.

Fractured-basement

Fractured basement was encountered in four locations and the average resistivity and depth values to the top of fractured basement are 294Ωm and 18.0m respectively.

Basement

The basement is the fresh bedrock and is the last layer. It is relatively shallow in the study area, it was encountered in three locations and the average resistivity and depth values to the top of basement are 1000Ωm and 11.0m respectively.

Figure 4a. Geoelectric sections of VES 1, 2, 3 & 4 of college of engr. Abuad

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Figure 4b. Geoelectric sections of VES 5, 6, 7 & 8 of college of engr. Abuad

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Overburden

The overburden in assumed to include all materials above the presumably fresh basement. The depth to the bedrock varies from 1.0 to 26.0m and the average depth to the bedrock is 11.0m (Table 1 and Figure 4a & Figure 4b). Overburden thickness was established in four locations and the average thickness value is 11.0m. The relatively moderate overburden thickness has potentials for groundwater saturation, however, the clayey nature and poor permeability of the materials will pose a challenge for water abstraction.

4.2. Horizontal Profiling

Result of the profiling carried out across one traverse section in NW-SE direction is presented in the section below. The traverse is 200meters long and is located right in front of the college. Four continuous subsurface lithologic units namely; Lateritic topsoil (blue), sandy-clay (green), weathered basement (yellowish-brown) and fresh basement (reddish purple) were established by the profile section. A fractured zone (in between the fresh basement) was also revealed.

The result is presented in form of 2-D resistivity structures (Figure 5). The 2-D resistivity plot revealed a relatively thin overburden and shallow basement across the study area.

Figure 5. 2-D resistivity structures of the Profile

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