Delineation of Groundwater Potentials and Aquifer Vulnerability Assessment in Uyo, Akwa Ibom State, Using the Vertical Electrical Sounding (VES) Technique

ABSTRACT

The spatial variation of surface water quality and quantity is as a result of seasonal variation. The
variability of rainfall has made groundwater resources the sustainable means of water supply for domestic,
agricultural and industrial usages. Hence, the need to assess the groundwater potentials, delineate them into
their bearing capacity and conserve them for optimization and sustainable quality water supply. However,
problems associated with extremely low yield of borehole, abortive boreholes, total failure of wells and
pollution of the well are as a result of choosing locations arbitrarily without conducting proper geophysical
survey of the area and also evaluating the protective capacity of the well, thus resulting to wastage of
resources, time and capital. In this study, the research was aimed at delineating the aquiferous units and
assessing the vulnerability rate of the aquifers in Uyo, Akwa Ibom State, Nigeria by determining their
depths, thicknesses, resistivities and the potential depth at which boreholes could be drilled at various
locations within uyo using the Vertical Electrical Sounding (VES) alongside the Schlumberger array.
Seventeen (17) sounding locations were probed. The data was interpreted using the conventional partial
curve matching and computer aided iteration techniques. The VES curve types identified in the study area
includes K, A and H. About 52.94% of all the sounding curves are of the K type while the remaining
47.05% belongs to the two curves as indicated within the study area. Results showed three to six geo-
electric layers comprising of the top soil, clayey sand, sand, Sandstone, Sandstone/Gravel and saturated
sand were delineated within the study area. The aquifer depth in the study area varies between 42m and
160m with an average depth of 103.4m. The aquifer thickness varies between 33m to 153m. Lithologic log
from boreholes and a dug well located near three of the sounding stations at Uyo revealed that some of the
geologic units were either suppressed or merged into a single geo-electric unit probably due to similarities
in electrical resistivity and slight variation in the depth to water table of the VES and the well and the
boreholes. It was observed that hydraulic conductivity obtained ranges from 0.0556cm/s to 0.4545cm/s
while the transmissivity ranges from 2.5754cm2
/s to 38.6325cm2
/s. The protective capacity of the aquifer
ranges between 0.045667mhos to 0.06281mhos. From the study, it showed that the study area has
sufficient groundwater resource potentials but for adequate groundwater infrastructural development, it is
recommended that boreholes should be drilled at a depth 42-160m depending on the location. Also, the
study reveals that the protective capacity of the aquifer is poor thus making the aquifer within the study
area vulnerable to pollution, as such protective measures should be taken to safeguard the aquifer from
exceeding the allowable contaminant rate for proper utilization.

TABLE OF CONTENTS
Title Page i
Certification ii
Approval Page iii
Dedication iv
Acknowledgement v
Table of Contents vi
List of Tables xi
List of Figures xiii
Abstract xvi
CHAPTER ONE
INTRODUCTION
1.1 Background of Study 1
1.2 Statement of Problem 3
1.3 Aim and Objectives of the Study 4
1.4 Scope of Study 5
1.5 Significance of the Study 5
CHAPTER TWO
LITERATURE REVIEW
2.1 Origin and Occurrence of Groundwater

2.2 Underground water and aquifers 6
2.3 Geological Factors Governing the Occurrence of Groundwater 8
2.3.1 Porosity of the soil 8
2.3.2 Permeability and transmissibility 9
2.4 Groundwater Seepage 9
2.4.1 Artificial groundwater recharge 10
2.5 Concepts of Geophysical Technique 11
2.5.1 Electromagnetic method (EM) 12
2.5.2 Seismic refraction method 14
2.6 Electrical Resistivity Method 15
2.6.1 Theory of electrical resistivity 20
2.6.2 Apparent resistivity 22
2.6.3 Types of resistivity survey 23
2.6.3.1 Vertical electrical resistivity sounding 23
2.6.3.2 Electrical resistivity profiling 23
2.6.4 Limitations of electrical resistivity method 24
2.6.5 Advantages of resistivity methods 24
2.6.6 Disadvantages of resistivity survey 25
2.7 Electrode Configuration 25
2.7.1 Schlumberger array 27
2.7.1.1 Advantages of schlumberger array 29

2.7.1.2 Disadvantages of schlumberger array 29
2.7.2 Other Configurations Using Cases 29
2.8 Theory of Depth of Penetration 30
2.8.1 Interpretation of VES data 31
2.9 Vulnerability of the Groundwater to Pollution 33
2.9.1 Ground water pollutants 33
2.9.2 Aquifer sensitivity 35
2.9.3 Vulnerability assessment 35
2.9.4 The Influence of protective layers 37
2.9.5 How to quantify vulnerability 37
2.10 Use of the Geoelectric Method for Assessment of Groundwater Potentials
and its protective capacity 39
CHAPTER THREE
METHODOLOGY
3.1 Location and Geology of the Study Area 44
3.2 Materials 44
3.3 Methods 45
3.3.1 Instrumentation and data acquisition 48
3.3.2 Field procedure 48
3.4 Assessment of the Aquifer Vulnerability 51

CHAPTER FOUR
RESULTS AND DISCUSSION
4.1 Qualitative Interpretation 52
4.2 Quantitative Interpretation 53
4.2.1 Interpretation of VES 1 Itam Etoi 53
4.2.2 Interpretation of VES 2 Afaha Oku 56
4.2.3 Interpretation of VES 3 Iba Oku 58
4.2.4 Interpretation of VES 4 Afaha Effiat 60
4.2.5 Interpretation of VES 5 Ifa Atai 62
4.2.6 Interpretation of VES 6 Ifa Ikot Okpon 64
4.2.7 Interpretation of VES 7 Nsukara 66
4.2.8 Interpretation of VES 8 Ikot Anyang 68
4.2.9 Interpretation of VES 9 Uniuyo 70
4.2.10 Interpretation of VES 10 Use Offot 72
4.2.11 Interpretation of VES 11 Aka 73
4.2.12 Interpretation of VES 12 Ikot Ntuen 75
4.2.13 Interpretation of VES 13 Obot Ubom 77
4.2.14 Interpretation of VES 14 Nduetong Oku 79
4.2.15 Interpretation of VES 15 Nung Uyo Idoro 81
4.2.16 Interpretation of VES 16 Mbak Etoi 83
4.2.17 Interpretation of VES 17 Itam Itu 86

4.2.18 Correlation of the sounding result with the hydrogeology 87
4.3 Aquifer Parameter 88
4.3.1 Aquifer resistivity 90
4.3.2 Aquifer thickness 90
4.3.3 Coefficient of anisotropy map layer 91
4.3.4 Hydraulic conductivity 92
4.3.5 Transmissivity 93
4.3.6 Final Groundwater Potential Map of the Study Area 95
4.4 Vulnerability Assessment of the Aquifer 97
4.4.1 Protective Capacity 100
4.5 Agricultural Implication and Discussion 102
4.6 Environmental Implication and Discussion 104
CHAPTER FIVE
CONCLUSION AND RECOMMENDATIONS
5.1 Summary 106
5.2 Conclusion 107
5.3 Contribution to Knowledge and Recommendations 109
REFERENCES 111

LIST OF TABLES

Table 2.1 Porosity Values of a few Rock Formation 9
Table 2.2 Typical Values of Electrical Resistivity and Conductivity for Different Earth
Materials 17
Table 2.3: Longitudinal Conductance/Protective Capacity Rating 38
Table 4.1: VES curve types and their various locations 52
Table 4.2: Interpretation for VES 1 54
Table 4.3: Interpretation for VES 2 57
Table 4.4: Interpretation for VES 3 59
Table 4.5: Interpretation for VES 4 61
Table 4.6: Interpretation for VES 5 63
Table 4.7: Interpretation for VES 6 65
Table 4.8: Interpretation for VES 7 67
Table 4.9: Interpretation for VES 8 68
Table 4.10: Interpretation for VES 9 70
Table 4.11: Interpretation for VES 10 72
Table 4.12: Interpretation for VES 11 74
Table 4.13: Interpretation for VES 12 76
Table 4.14: Interpretation for VES 13 78
Table 4.15: Interpretation for VES 14 80
Table 4.16: Interpretation for VES 15 82

Table4.17: Interpretation for VES 16 84

Table 4.18: Interpretation for VES 17 86
Table 4.19 Aquifer parameters estimated from VES data 89
Table 4.20: Protective capacity of the Aquifer estimated from VES data 98

LIST OF FIGURES
Fig 2.1 Confined, Unconfined Aquifers and Aquitard 8
Fig 2.2 Schematic Diagram of the EM method 13
Fig 2.3 Schematic Representation of Seismic Method for Groundwater Exploration 15
Fig 2.4 Showing DC resistivity measurements 19
Fig2.5 Equipotentials and current lines for current A, B and potentials electrode M, N 19
Fig 2.6 Different types of Electrode Configuration 26
Fig 2.7 Electrode configuration of Wenner and Schlumberger 27
Fig 2.8 Different Types of Penetration 31
Fig 2.9 Classification of curve types 32
Fig 3.1 Map of Akwa Ibom State indicating Uyo 46
Fig 3.2 Geological Map of Akwa Ibom State 47
Fig 3.3 Map of the Uyo indicating the Sounding points 50
Fig. 3.4 Sketch diagram of Schlumberger Configuration 51
Fig.4.1a Resistivity graph of VES 1 54
Fig. 4.1b Geoelectric layers of Itam Etoi 55
Fig.4.2a Resistivity graph of VES 2 56
Fig. 4.2b Geoelectric layers of Afaha Oku 57
Fig.4.3a Resistivity graph of VES 3 58
Fig. 4.3b Geoelectric layers of Iba Oku 59
Fig.4.4a Resistivity graph of VES 4 60

Fig. 4.4b Geoelectric layers of Afaha Effiat 61
Fig.4.5a Resistivity graph of VES 5 62
Fig. 4.5b Geoelectric layers of Ifa Atai 63
Fig.4.6a Resistivity graph of VES 6 64
Fig. 4.6b Geoelectric layers of Ifa Ikot Okpon 65
Fig.4.7a Resistivity graph of VES 7 66
Fig. 4.7b Geoelectric layers of Nsukara 67
Fig.4.8a Resistivity graph of VES 8 68
Fig. 4.8b Geoelectric layers of Ikot Anyang 69
Fig.4.9a Resistivity graph of VES 9 70
Fig. 4.9b Geoelectric layers of Uniuyo 71
Fig.4.10a Resistivity graph of VES 10 72
Fig. 4.10b Geoelectric layers of Use Offot 73
Fig.4.11a Resistivity graph of VES11 74
Fig. 4.11b Geoelectric layers of Aka 75
Fig.4.12a Resistivity graph of VES 12 76
Fig. 4.12b Geoelectric layers of Ikot Ntuen 77
Fig.4.13a Resistivity graph of VES 13 78
Fig. 4.13b Geoelectric layers of Obot Ubom 79
Fig.4.14a Resistivity graph of VES 14 80
Fig. 4.14b Geoelectric layers of Nduetong Oku 81

Fig.4.15a Resistivity graph of VES 15 82
Fig. 4.15b Geoelectric layers of Nung Uyo Idoro 83
Fig.4.16a Resistivity graph of VES 16 84
Fig. 4.16b Geoelectric layers of Mbak Etoi 85
Fig.4.17a Resistivity graph of VES 17 86
Fig. 4.17b Geoelectric layers of Itam Itu 87
Fig 4.18 Resistivity Map of the study area 90
Fig 4.19 Aquifer Thickness Map of the Study Area 91
Fig.4.20 Coefficient of Anisotropy Map of the Study Area 92
Fig 4.21 Hydraulic Conductivity Map of the Study Area 93
Fig.4.22 Transmissivity Map of the Study Area 95
Fig. 4.23 Groundwater Potential Map of the Study Area 96
Fig.4.24 Vulnerability Map of the Study Area 102

CHAPTER ONE
INTRODUCTION
1.1 Background of Study
Water, described as the most abundant natural resource on earth is also a major necessity of life,
for there would be no life on earth without water (Jones et al., 2008). Water, perhaps, is the
greatest asset which nature bestows to all living organisms in their various environments. It is the
major component of all living things. According to Jackson (1985), by weight, the average
human adult male is approximately 55% to 75% water. However, there can be considerable
variation in body water percentage based on a number of factors like age, health, weight and sex.
Most of animal body water is contained in various body fluids. These include intracellular fluid,
extracellular fluid, plasma, interstitial fluid and transcellular fluid. Water is also contained in
inside organs, in gastrointestinal, cerebrospinal, peritoneal and ocular fluids. Adipose tissue
contains about 10% of water, while muscle tissue contains about 70% (John and Bruce, 2002).
Providing plant with the exact water needed is the prerequisite for optimal plant growth. In
plants, the exact quantity of water needed by plants differs from plant to plant; it is also affected
by age of plant, sunshine duration and intensity, specie, wind speed etc.
The major sources of water are the oceans- which accounts for 97.22% of all water on earth, the
rest comprise 2.1% in glaciers, 0.6% fresh water lakes, streams and river 0.6%, ground water
0.1% (Jones et al., 1987). Common sources of water supply therefore include; rain, springs,
streams, lakes, oceans and wells. All these are of different qualities depending on certain
influencing factors to which they have been exposed.
Groundwater has always been considered to be a readily available source of water for domestic,
agricultural and industrial use. In many parts of the world groundwater extracted for a variety of
purposes has made a major contribution to the improvement of the social and economic
circumstances of humans (Biswas, 1998). It can also be seen that fresh water is also found in
lakes, streams, springs but rain water constitute very small percentage of the total natural water
supply. The water located beneath the ground surface in the saturated zone, is usually abstracted
through a well, dug or drilled. The depth of the well, and thus the cost of constructing it, depends on the distance from the surface to the groundwater aquifer, ranging from a few meters to several
hundred (Clark, 1996). The potential yield depends on the size of the aquifer, and to be
sustainable, withdrawal must not exceed the natural rate of recharge (Clark, 2004). The cost of
abstracting groundwater, including the cost of pumping, tends to be higher than that of surface
waters (Ganz, 2003). Being located beneath the ground, water quality and temperature is
relatively constant over time, and turbidity, microbiological contamination and content of
organic matter is usually lower than in surface water. The content of minerals is generally higher
in groundwater, as these substances are dissolved from the rocks and soils in which the water
resides (Akankpo and Igboekwe, 2011; Nwankwo and Igboekwe, 2011).
Groundwater is the safest and most reliable water source, used for domestic, irrigation, industrial,
and municipality purposes. Ground water development for water supply purposes is preferable to
low discharge springs and dug wells because they are found to be adequate in their yield and
resistant to drought while other sources might be completely dried out and never easily recovered
after the prolonged drought time. The occurrence, origin, movement and chemical constituents of
groundwater are dependent on geology/lithology, geomorphology/landforms, drainage density,
rainfall, geological structures/lineaments, slope, land use/land cover and soil of groundwater
regime. It is common to have problem in exploitation of groundwater resource in that
unsuccessful rate of well production encountered is alarming and have cost huge waste of
resources. Improper evaluation of groundwater and site selections is mostly expected to pose
problems (Shiferaw and Abebe, 2004). Since the groundwater occurs not in our sight but deep in
the subsurface, there is no direct method to facilitate observation of water below the surface. Its
presence or absence can only be inferred indirectly by studying the groundwater occurrence and
distribution controlling parameters (MacDonald, et al., 2001).
The availability of quality water resources has always been the primary concern of societies in
semi-arid and arid regions, even in areas of more abundant rainfall, the problem of obtaining
adequate supply of quality water is generally becoming more acute due to ever increasing
population and industrialization. As a result of this, surface water cannot be dependable
throughout the year, hence, the need to look for other alternatives to supplement surface water.
This makes the world depend on the largest available source of quality fresh water which lies
underground. Groundwater can be in sedimentary terrain where it is less difficult to exploit except for its chemical composition; it can also be in the Basement Complex terrain where it can
be a bit difficult to locate especially in areas underlain by crystalline unfractured or unweathered
rocks (Ahilan et al., 2010).
The research for groundwater today has become essential, due to its relative low cost and its
chance of obtaining quality water from the bedrock. In hard rock areas, the geological structure
normally encountered is characterized by the existence of a hard rock basement overlain by a
weathered overburden of variable thickness.
Hydrogeologically, the weathered material, which constitutes the overburden, has high porosity
and contains a significant amount of water, and, at the same time, it presents low permeability
due to its relatively high clay content. The bedrock, on the other hand, is fresh but frequently
fractured, presenting high permeability, but as fractures do not constitute a significant volume of
the rock, fractured basement has a low porosity. For this reason, a good borehole providing long
term high yields, is one which penetrates a large thickness of weathered overburden, which acts
as a reservoir, and one which additionally intersects fractures in the underlying bedrock, where
the fractures provide the rapid transport mechanism (Rai et al., 2005).
Several methods employed in groundwater exploration include electrical resistivity, gravity,
seismic, magnetic, remote sensing, electromagnetic, among others, out of which the resistivity
method is most effective for locating productive well and the Vertical Electrical Sounding (VES)
technique can provide information on the vertical variation in the resistivity of the ground with
depth (Rhett, 2001). Evidence has shown that geophysical methods are the most reliable and the
most accurate means of all surveying method of subsurface structural investigations and rock
variation.