EVALUATION OF A REGOLITH AQUIFER IN A TYPICAL BASEMENT

COMPLEX TERRAIN; A CASE STUDY OF IKOLE EKITI

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GEOPHYSICAL INVESTIGATION FOR GROUNDWATER POTENTIAL

EVALUATION OF A REGOLITH AQUIFER IN A TYPICAL BASEMENT

COMPLEX TERRAIN; A CASE STUDY OF IKOLE EKITI

TABLE OF CONTENTS

Title page Page

Certification ii

Dedication iii

Acknowledgements iv

Table of Contents v

List of Figures viii

List of Table x

Abstract xi

CHAPTER ONE: INTRODUCTION

1.1 General Statement 1

1.2 Description of the study area 2

1.2.1 Location and accessibility of the study area 2

1.2.2 Relief, climate and vegetation 4 1.2.3 Drainage pattern of the study area 4

1.3 Aim and objectives 4

1.4 Scope of the project work 5

1.5 Previous work 5

1.6 Expected contribution to knowledge 8

CHAPTER TWO: LITERATURE REVIEW

2.1 Regional geology of the study area 9

2.1.1 Migmatite-Gneiss-Quartzite Complex 11

2.1.2 Slightly Migmatized to Non-Migmatized Meta-Sedimentary

and Metaigneous rocks 11

2.1.3 Older Granites (Pan African Granitoids) 11

2.1.4 Younger Granites 12

2.2 Geology of the study area 12

2.3 Hydrogeology of the study area 15

2.3.1 Groundwater 15

2.4 Principle of Electrical Resistivity Method 16

2.4.1 Factors Affecting Resistivity of Earth Materials 16

2.4.2 Basic Theory of Electrical Resistivity Method 17

2.4.3 Generalized Apparent Resistivity Equation 24

2.4.4 Electrode Array or Configuration 29

2.4.4.1 Schlumberger Electrode Array 29

2.5 Field Techniques 33

2.6 Data Presentation 34

2.7 Data Interpretation 35

CHAPTER THREE: MATERIALS AND METHODOLOGY

3.1 Materials 36

3.2 Methodology 37

CHAPTER FOUR: RESULTS AND DISCUSSION

4.1 Preamble 40

4.2 Groundwater Potential Evaluation 48

4.2.1 Overburden thickness isopachmap 48

4.2.2 Weathered layer thickness map 50

4.2.3 Weathered layer resistivity map 52

4.2.4 Bedrock relief map 54

4.2.5 Groundwater potential map 56

CHAPTER FIVE: CONCLUSION AND RECOMMENDATION

5.1 Conclusion 58

5.2 Recommendation 58

References 59

LIST OF FIGURE

Fig. 1.1: Location Map of the study areashowing the distribution of VES points 3

Fig. 2.1:Geological sketch map of Nigeria showing the major geological components;

The BasementComplex, and Sedimentary Basins. 10

Fig. 2.2: Geological map of Ekiti State showing the study area. 14

Fig. 2.3: Schematic Diagram of the Flow of Current through a Cylindrical Model. 17

Fig. 2.4: Spherical Body of Radius ‘r’ 20

Fig. 2.5: Current Source on aHemispherical Surface 22

Fig. 2.6: A Simple 2-Electrodes Array System 24

Fig. 2.7: Typical Electrical Resistivity Array 26

Fig. 2.8: Typical Schlumberger Electrode Configuration 30

Fig. 3.1: Map showing the distribution of VES points 39

Fig. 4.1: Typical VES representative model curves. 45

Fig. 4.2: Pie chart showing the frequency of curve types obtained from the study area 46

Fig. 4.3: Map showing the distribution of the curve types 47

Fig. 4.4: Overburden thickness isopach map 49

Fig. 4.5: Weathered layer isothickness map 51

Fig. 4.6: Weathered layer isoresistivity map 53

Fig. 4.7: Bedrock relief map 55

Fig. 4.8: Groundwater potential map 57

LIST OF TABLES

Table 4.1: Summary of the layer geoelectric parameters and lithologic interpretation. 41

Table 4.2:Classification of the resistivity sounding curves. 44

ABSTRACT

A geophysical investigation involving the electrical resistivity method was carried out at IkoleEkiti of South Western, Nigeria with the main aim of investigating the area for groundwater potential of the regolith aquifer.

The objectives are to identify geological structures and aquifers favorable to groundwater accumulation andalso input geoelectric layers parameters into a software to produce maps of the subsurface in order todetermine the groundwater potential of the regolith aquifer.

Thirty (30) Vertical Electrical Soundings (VES) using the Schlumberger array with maximum electrode separation AB/2 of 100m was carried out with ABEM SAS-300 Resistivity Meter. The VES data were presented as sounding curves and interpreted quantitatively through the method of partial curve matching and 1-D computer assisted forward modelling. The sounding curves show three layers tofour layers earth models. The three layer curve are characterized by H and A type which represents altogether about 54% of the curve types in the study while the four layer models are characterized by KH, KQ, QH and AH which altogether covers about 46% of the curve type in the study area. The overburden was assumed to include the topsoil, upper and lower saprolite, saprock, and weathered basement.

The weathered basement is the aquifer type delineated for the area. Groundwater potential was evaluated from the maps (i.e. overburden thickness, weathered layer thickness, weathered layer resistivity,and bedrock relief maps) revealing that the Northeastern, Eastern and Southeastern parts of the study area are the mostpromising region for borehole development. However, the western region of the study area can also be considered as fair forborehole development.

CHAPTER ONE

INTRODUCTION

1.1 General Statement

The science of geophysics applies the principles ofphysics to the study of the Earthapplicable in the delineation or mapping of subsurface features arising from local variation in the measured physical properties of specific target relative of its host. Thus, the measurements taken during geophysical investigation are influenced by the internal distribution of physical properties of underlying rocks(Kearyet al.,2002).

Geophysical investigation is the process of selecting an area of geologic interest and delineate the physical parameter of the object involved. The acquisition of data is fundamental to geophysical investigations and ‘real’ data is only acquired in the field.

Without real data no true practical conclusions can be made about a targeted causative. Geophysical investigation is found relevant in groundwater exploration, mining, engineering site investigation and environmental impact assessment.

Water,remains one of the vital elements in life and it is very much important to human existence. It is one natural resource that is not only essential for the survival of mankind but also for the survival of the natural environment. The availability of water has played a key role in the development of all civilizations. Indeed, especially in the ancient times, water scarcity prevented the development of settlements. Social welfare and economic development may also be hampered in theabsence of reliable water supplies.The rapid increase in population of the study area owing tourbanization has led to an increased pressure on underground water which is the major water resource in the area (Alabiet al., 2016).

The geoelectrical resistivity method has been successfully employed in the delineation of subsurface geological sequence, geological structures/features of interest, aquifer units, types and depth extent in almost all geological terrains (Oladapoet al., 2004; Akoet al., 2005). This is because of the significant resistivity contrasts that exist between different earth materials (Olorunfemiet al., 1993).

The Vertical Electrical Soundings (VES) has proved very popular with groundwater studies due to simplicity of the technique. Using this method, depth and thickness of various subsurface layers and their water yielding capabilities can be inferred. Therefore, evaluation of groundwater potential was done in order to know the groundwater yielding capabilities or groundwater conditions of the study area. In basement complex, unweathered basement rocks contain negligible groundwater. Significant aquifers however, develop within the weathered overburden and fractured bedrock. This research is particular to know feasibility of potable water (i.e. to know the promising areas for groundwater prospects) within the study area.

However, the groundwater conditions of an area is properly understood, it could be used as an effective tool in the planning of reliable water borehole in such area (Sunmonuet al., 2012).

1.2 Description of the Study Area

1.2.1 Location and Accessibility of the Study Area

The study area is located in IkoleEkiti of EkitiState, South western, Nigeria. The study area lies within 7^o45̍52.1̎N – 7^o48̍54.5̎ N and 5^o28̍05.3̎E– 5^o33̍08.5̎E(Fig 1.1). The area is accessible through various footpaths or pathways.

Figure 1.1: LocationMap of the study area

1.2.2 Relief, Climate and Vegetation

The relief are of very rough hills to isolated hills and low lands. The study area falls within the tropical rain forest of southwestern Nigeriawith two distinct seasons which are the rainy season (April–October) and the dry season (November–March). Temperature ranges between 21°C and 28°C while the mean humidity is over 70%. The south westerly wind and the northeast trade winds blow in the rainy and dry (Harmattan) seasons respectively. Tropical forest exists in the south, while savannah occupies the northern peripheries. The mean annual rainfall is about 1800 mm. (http://ekitistate.gov.ng/about-ekiti/overview/).

The vegetation of the study area is greatly influenced by climate and relief of the area. It is the evergreen forest type which comprises of palm trees, timbers and grass. Human activities such as farming and hunting are prevalent in the study area. Industries thriving in the study area are Agriculture and Lumbering which include Timber/Saw mills.

1.2.3 Drainage Pattern of the Study Area

The most common type of drainage pattern in this area is dendritic type with undulating topography, where the small tributaries is joining to the main river. The study area is one of the most fertile, and with high degree of accessibility to itapaji dam with enormous minihydroelectric power potential, as well as water supply opportunities for irrigation and townships. Even Oye river nearby flows into River Ele and provides substantial alluvial deposits in the study area plains for year round agriculture. Ero dam in Moba is also nearby.

1.3 Aim and Objectives

The aim of the study was to determine the groundwater potential of the regolith aquifer in the study area. The objectives of the study include:

i. Carry out reconnaissance survey of the study area and also acquire geographical coordinates of the study area in order to generate the base map. ii. Acquisition and interpretation of the Vertical Electrical Sounding data for structure and subsurface geologic sequence delineation respectively.

iii. From(ii), identify geological structures and aquifers favorable to groundwater accumulation and iv. From (iii), geoelectric layers parameters was inputinto a software to produce maps of the subsurface in order todetermine the groundwater potential of the regolith aquifer.

1.4 Scope of the Project Work

The scope of this study involve consultation of previous works of different geoscientists on related topics in scientific and geological journals, texts and goggling online researches on Precambrian Basement Complex, aquifer units and groundwater exploration.

Preliminary study of the area for reconnaissance survey geological and geophysical mapping of the study was carried out. Electrical resistivity data were acquired using the ABEM SAS-300 Resistivity Meter.

The processing and interpretation of the data were done. The VES data were interpreted using partial curve matching technique in terms of layer parameters underneath the sounding positions and the interpretation results are used to generate maps.

1.5 Previous Work

Various works, projects and researches had been carried out to delineate regolith aquifer groundwater potential, particularly in respect to their hydrogeological characteristics using various geophysical methods.

Akanaet al., (2016) carried out an investigation on the assessment of aquifer groundwater potential and its protective capacity in some towns of Yenagoa using the electrical resistivity method. They concluded that the Dar-zarrouk parameter (i.e. longitudinal conductance LC) indicate that the Southern Yenagoa had good to moderate aquifer protective capacity rating, while the Northern area had poor to weak aquifer protective capacity.

Alabiet al., (2016) carried out a geophysical investigation around the University Health Sciences of the Osun State University, Osogbo using the Schlumberger technique of the electrical resistivity method with the aim of evaluating the groundwater potential and access how protected the aquifer in the area could be to surfacepollutants. They concluded from their results that the study area might show good potential for groundwater but the groundwater is not safe.For groundwater development, adequate measure should be made to establish water treatment facility.

Eke et al., (2015) carried out a detailed hydro geophysical study of the aquifers of the Upper Imo River Basin, Southeastern Nigeria which they delineated the aquifers, evaluate their geometric characteristics and to assess their vulnerability of pollution from surface contaminants. Layer parameters interpreted from the VES data together with the available well datawere used to assess the vulnerability of the shallow aquifers using the DRASTIC model. The aquifer vulnerability index assessment revealed that about 55% of the study area falls within the moderate vulnerability zones with DRASTIC index values ranging from 102 to 140. About 30% of the study area have high vulnerability index while the remaining 15% of the study area have low vulnerability index with DRASTIC index values of between 85 and 99.

Faridet al., (2017) carried out a research about site-specific aquifer characteristics, subsurface lithology, and groundwater potentialby conducting 80 vertical electrical sounding surveys (VESs) in Rahim Yar Khan District (RYK), Punjab, Pakistan to distinguish the fresh groundwater aquifer from saline groundwater and to evaluate the aquifer protective capacity (APC) of overburden.

Oloruntolaet al., (2017) conducted a combination of vertical electrical soundings (VES),

2D electrical resistivity imaging (ERI) surveys and borehole logs at Magodo, Government Reserve Area (GRA) Phase 1, Isheri, Southwestern Nigeria, with the aim of delineating the different aquifers present and assessing the groundwater safety in the area. Their result shows that the underlying confined aquifer is well protected from contamination and can be utilized as a source of potable groundwater in the study area. This study therefore enabled the delineation of shallow aquifers, the variation of their thicknesses and presented a basis for safety assessment of groundwater potential zones in the study area.

Oni et al., (2017) carried out groundwater vulnerability assessmentat IgbaraOke Southwestern Nigeria, with a view to classifying the area into vulnerability zones, by applying the electrical resistivity method, using Schlumberger electrode arrays. Geoelectric parameters (layer resistivity and thickness) were determined from the interpreted data. The geoelectric parameters of the overlying layers across the area were used to assess the vulnerability of the underlying aquifers to near-surface contaminants with the aid of vulnerability maps generated. The total longitudinal conductance map shows the north central part of the study area as a weakly protected (0.1–0.19) area, while the northern and southern parts have poor protective capacity (<0.1); this is in agreement with the GOD method which shows the northern part of the study area as less vulnerable (0–0.1) while the southern part has low/moderate (0.1–0.3) vulnerability to contamination. The longitudinal conductance exaggerates the degree of susceptibility to contamination than the GOD and

GLSI models. From the models, vulnerability to contamination can be considered higher at the southern part than the northern part and therefore, sources of contamination like septic tank, refuse dump should be cited far from groundwater development area.

Sunmonuet al., (2012) conducted a vertical electrical sounding method at Oyo State industrial estate Ogbomoso with a view to determining thegroundwater potential of the study area. The geoelectric sections obtained from the sounding curves revealed 3-layer and 4-layer earth models respectively. The models showed the subsurface layers categorized into the topsoil, weathered/clay, fractured layers and the fresh bedrock. The weathered basement and fractured basement are the aquifer types delineated for the area. Groundwater potential evaluated from the maps (i.e. overburden thickness, anisotropic coefficient, weathered layer isothickness, weathered layer isoresistivity, transverse resistance and bedrock relief maps) revealed that the Southern and Eastern parts of the study area are the most promising region for borehole development. However, Northeastern region of the study area can also be considered as fair for borehole development.

The various investigation and studies carried out by the different authors above, provided basic background information on the hydrogeological framework, changes in lithology, electrical properties and nature of the rocks in typical basement environment.