HYDROGEOPHYSICAL STUDY OF THE GROUNDWATER POTENTIAL OF ILARA-MOKIN SOUTHWESTERN NIGERIA

Geoelectric study of the groundwater potential of Ilara-Mokin in Ondo State Southwestern Nigeria was carried out using electrical resistivity (Vertical Electrical Sounding) method with the view to providing adequate information on the different sub-surface geoelectric layers, structural configuration of the concealed basement rocks and the groundwater potential of the study area. Forty-one Schlumberger vertical electrical soundings (VES) were conducted across the study area. The computer assisted VES data interpretation enabled generation of geoelectric curves, sections and overburden thickness map which were used in the delineation of key hydrogeologic features like the topsoil, weathered basement, fractured basement and the fresh basement. Resistivity values range from 21 Ω-m to 798 Ω-m in the topsoil, 14 Ω-m to 209 Ωm in the weathered basement, 51 Ω-m to 209 in the partially weathered/fractured basement and 312 Ω-m to ∞ within the fresh basement. Layer thickness values also vary from 0.3 m to 6.1 m in the topsoil and 0.9 m to 28.6 m in the weathered basement. The depth to the resistive bedrock ranges from 0.3 m to 29.3 m across the study area. The study revealed that greater part of Ilara-Mokin town is underlain by marginally thick overburden thus constituting shallow aquifer units with poor to marginal groundwater potential. Partially weathered/fractured basement were delineated in isolated cases with tendency for low to marginal groundwater yield around central and northeastern areas of the town. The results reasonably provide basic information that is expected to assist in the future development of groundwater resources in Ilara-Mokin.


INTRODUCTION
The study area, Ilara-Mokin in Ondo State, Southwestern Nigeria, is a developing town situated on a basement terrain with inadequate potable water supply.The population of the town is increasing with establishment of a polytechnic in the community and the proximity of a Federal University.The populace depends mainly on groundwater resources for their domestic needs.
The electrical resistivity method is one of the most popular of the several non-invasive geophysical prospecting methods commonly used for groundwater studies (Zohdy, 1969;Flathe, 1970;Verma et al, 1980;De Beer and Blume, 1985;Olayinka and Barker 1990;Yang et al, 1994;Moller et al, 1998;Omosuyi et al, 2003;Ekwe et al, 2006;Oladapo and Akintorinwa, 2007).The electrical resistivity is a physical property of rocks and soils most affected by the presence of water.Variation in groundwater chemistry may also affect the electrical properties (Mota, et al., 2004).However, groundwater investigations in basement complex areas are often more difficult, as wide variations of aquifer parameters characterize the groundwater regime.Spatial variations of these characteristics are attributable to, among other causes, tectonism and degree of weathering of the basement rocks (Barker et al., 2001).The Schlumberger vertical electrical sounding adopted in this study is popular for determining the resistivity variation with depth.Beeson and Jones (1988), Olayinka and Barker (1990), Hazell et al. (1988 and1992), Barker et al. (1992), Carruthers and Smith (1992) and Olayinka and Olorunfemi, (1992) have demonstrated the use of electrical resistivity method for siting wells and boreholes in crystalline basement terrain.
In this study, a hydrogeophysical evaluation of the area around Ilara-Mokin was carried out.The study was undertaken to determine the geoelectric parameters in form of layer resistivities and thicknesses of the superficial/overburden materials overlying the basement bedrock and the structural framework of the bedrock.The geoelectric parameters were used to generate geoelectric sections and maps.

Geology and Geomorphology
Ilara-Mokin is underlain by rocks of the Precambrian Basement Complex of Southwestern Nigeria (Figure 1) (Rahaman, 1976).The lithological units include variably migmatized biotite-hornblendegneiss with intercalated amphibolite.Low lying outcrops of migmatite-gneiss complex are situated on the western and northeastern parts of the town while boulders of amphibolites/charnockite rocks are located in the central and north central areas of the town.The study area is situated on gently undulating terrain with elevation between 336m and 350m above mean sea level.The region which lies within the tropical rainforest of Nigeria is characterized by two distinct seasons (wet from March to October and dry from November to February).The annual mean rainfall is about 1800mm.The annual mean temperature ranges between 24°C and 27°C.

Hydrogeology
Regolith and fractured basement generally occur in a typical basement terrain (Odusanya and Amadi, 1990).In tropical and equatorial regions, weathering processes create superficial layer with varying degree of porosity and permeability.Studies have shown that the unconsolidated overburden could constitute reliable aquifer if significantly thick (Satpathy and Kanungo, 1976, Offodile, 1983: Olorunfemi and Olorunniwo, 1985;Dan-Hassan and Olorunfemi, 1999;Bala and Ike, 2001).Also the concealed basement rocks may contain highly faulted and folded area, incipient joints and fracture systems derived from multiple tectonic events they have experienced.These structures may house abundant groundwater in a typical basement setting.Detection and delineation of such structural features may facilitate the location of groundwater prospect zones (Omosuyi et al., 2003).

MATERIALS AND METHOD OF STUDY
Field data acquisition was undertaken along existing road paths using ABEM SAS 1000 Terrameter.Forty-one (41) vertical electrical sounding (VES) locations were occupied within the town (Fig. 2) utilizing the schlumberger electrode configuration.Electrode spacing (AB/2) was varied from 1.0 m to 100 m.The VES data were utilized in generating field curves.The curves were interpreted using partial curve matching technique.The geoelectric parameters obtained from manual interpretation of each VES curve were refined using the software algorithm RESIST version 1.0 (Vander Velpen, 1988)

RESULTS AND DISCUSSION
The results of the study are presented as sounding curves, geoelectric sections and overburden thickness map.The curve types obtained from the study area are the H, A, K, HK, HA, QH and HKH type.The HKH curve type in a typical basement environment in many instances, indicates fracturing and presents favourable zones for groundwater abstraction (Oladapo et al., 2004).The A-type curves are characterized by a steady rise in resistivity profile from topsoil to the bedrock while the intermediate layer in the H-type is commonly water saturated and is often characterized by low resistivity, high porosity, low specific yield and low permeability (Jones, 1985;Olayinka and Olorunfemi, 1992).The typical VES curves obtained from the study are presented in Figure 3 The first order parameters obtained in form of layer thicknesses and resistivity values from the interpretation of the field curves (see Table 1 The second layer (weathered basement) resistivity values vary from 14 Ω-m to 181 Ω-m.The electrical resistivity values of this layer are controlled by the degree of water saturation (Odusanya and Amadi, 1990) and also degree of shaliness.Significant saturation may induce reduction in resistivity.The thickness of the second layer varies from 1.4 m to 12.9 m.
The fresh basement resistivity varies from 361 Ωm to ∞. VES 13 exhibits the thickest overburden (13.3 m) within the section.However the weathered materials are characterized by low resistivity (44 Ω-m) indicative of clay.Thus, poor hydrogeoelectric characteristics exist along the section.The section along profile C-D which is 470 m long (Figure 5) comprises VES 18, 10, 9, 8 and 24 along the NNW-SSE azimuth.Topsoil resistivity values along the section range from 61 Ω-m to 798 Ω-m with thickness varying between 0.3 m and 1.6 m.The section along profile E-F is presented in Figure 6.This section comprises VES 7, 24, 30, 12, 15, 38 and 37 along SW-NE azimuth.Topsoil resistivity values in the E-F section range from 28 Ω-m to 154 Ω-m while layer thickness ranges from 0.4 m to 1.6 m.
The second layer constitutes the weathered basement and is characterized by resistivity values ranging from 39 Ω-m to 363 Ω-m.The thickness of the weathered layer varies from 2.4 m to 16.7 m.The fresh basement has resistivity values ranging from 360 Ω-m to ∞.The weathered materials underlying E-F section are fairly thin and characterized by low resistivity values that may not facilitate sufficient groundwater accumulation.
The section along profile G-H which is presented in Figure 7

Groundwater Potential Evaluation
The groundwater potential of Ilara-Mokin was evaluated based on the geoelectric sections (Figures 4 to 7 Fig. 1: Geological map of Akure environ showing the study area. ) were utilized in generating geoelectric sections and overburden thickness map.The geoelectric sections taken along four directions (A-B, C-D, E-F and G-H in Figure 2) showed that four subsurface geologic layers consisting of topsoil, weathered basement, fractured basement and fresh basement underlie Ilara-Mokin.The section along profile A-B is about 1300 m long and connects VES 20, 18, 11, 13, 28 and 27 along NW-SE azimuth (Fig. 4).The topsoil resistivity values range from 27 Ω-m to 798 Ω-m.The low resistivity (<100 Ω-m) is characteristic of clay while the fairly high resistivity values (>100 Ω-m) may typify sandy clay, clayey sand and laterite.Topsoil thickness ranges from 0.4 m to 1.0 m along this section.

Fig. 4 :
Fig. 4: 2-D Geoelectric section in across profile A-B comprises VES 40, 4, 5, 7 and 21 and is oriented along NW-SE azimuth.The topsoil resistivity values in the section range from 62 Ω-m to 287 Ω-m with layer thickness ranging between 0.5 m and 1.3 m.The second layer which is the weathered basement has resistivity values varying from 42 Ω-m to 193 Ω-m.The thickness of the weathered layer varies between 0 m at VES 40 and 13.8 m at VES 5. Fracture bedrock columns were observed at VES 7 and VES 21.The fractured basement resistivity values range from 50 Ω-m to 209 Ωm while the fresh basement has resistivity values in excess of 366 Ω-m.The combination of weathered and fractured basement beneath VES 7 and 21 in this section constitute relatively deep but clayey aquifer units that may enhance groundwater accumulation.

Fig. 6 :
Fig. 6: 2-D Geoelectric section in across profile E-F ) and overburden thickness map (Figure8) generated from the characteristic geoelectric parameters, obtained from the VES interpretation results.The map as hatched presents local groundwater prospect of the town classified into poor and marginal potentials.The map shows that poor groundwater potential zones (areas of thin overburden) are ubiquitous in the town (blank pattern areas of figure8).The works of Okhueand Olorunfemi, (1991)  and Bala and Ike, (2001) indicated that areas underlain by thick and sandy (relatively permeable) overburden materials are high groundwater potential zones.Such high groundwater potential zones are however absent in Ilara-Mokin.
A k u re /B e n in C it y HYDROGEOPHYSICAL STUDY OF THE GROUNDWATER POTENTIAL OF ILARA-MOKIN 197 Fig. 2: Layout map of Ilara-Mokin showing VES locations

Table 1 :
Summary of Results obtained from Ilara-mokin Geoelectric Study VES No