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INTEGRATION OF RESISTIVITY METHODS AND SINGLEPOINT RESISTANCE LOG DATA IN EVALUATION OF AQUIFER VULNERABILITY AND GROUNDWATER QUALITY IN WESTERN NIGER DELTA BY AWETO KIZITO EJIRO PG/Ph.D/05/39488 THESIS SUBMITTED TO THE DEPARTMENT OF GEOLOGY FACULTY OF PHYSICAL SCIENCES UNIVERSITY OF NIGERIA NSUKKA, IN FULFILLMENT OF THE REQUIREMENT FOR THE DEGREE OF DOCTOR OF PHILOSOPHY AUGUST, 2014 i CERTIFICATION AWETO, Kizito Ejiro, a postgraduate student in the Department of Geology, University of Nigeria, Nsukka, with registration number PG/PhD/05/39488 has satisfactorily completed the requirements for the research work for the Degree of Doctor of Philosophy in Geology (Applied Geophysics). The work embodied in this research is original and has never been submitted in whole or in part for the award of any other diploma or degree in this or any other University. --------------------------------Dr L.I. Mamah (Supervisor) --------------------------------Prof. O.P. Umeji (Head of Department) ii DEDICATION This thesis is dedicated to God Almighty and to memory of my late father, Mr. Anthony Digun Aweto. iii ACKNOWLEDGEMENTS I wish to express my profound gratitude to God Almighty who has kept me by His grace, for bringing me this far in my academic pursuit and for the successful completion of this work. I sincerely appreciate the indispensable role my supervisor Dr. L.I. Mamah played; not only for diligently and painstakingly guiding me through this work but also for many useful suggestions and constructive criticism as well as thoroughly reading through the completed manuscript. I will forever be grateful and indebted to him. I am grateful to the Head and all lecturers of the Department of Geology, University of Nigeria, Nsukka, for their guidance and encouragement throughout the duration of my programme and for positive criticisms during my seminar presentations. I also wish to express my gratitude to all others who contributed to the success of this work, beginning with Dr. E.A. Atakpo who was with me at some stage during the field work and Dr. I. A. Akpoborie for his advice and contribution. I am very much indebted to Mr. E. Ogbemudia (Director, Geology and Hydrogeology Department, Ministry of Water Resources, Asaba, Delta State) for providing the terrameter used for this research. My gratitude also goes to Mr. D. Okpughwu of Ministry of Water Resources, Asaba who helped in no small measure during the field work. My appreciation also goes to the communities where this research was carried out. I wish to extend my greatest thanks to Egbeleku community for their hospitality and the amiable Secretary-General of the community. iv Lastly, I have to thank the members of my family, my lovely wife Juliet and the four best daughters in the world: Jessica, Joan, Gabrielle and Juanita; my mother, Mrs. T. I. Aweto; my eldest sister, Mrs. E. Ogwe; Prof. A.O. Aweto of the Department of Geography, University of Ibadan; Mrs. P.E. Ebe; Mrs. C. Akrake; Arch. and Mrs. H.O. Aghwadoma; Dr. (Mrs) P.O. Aweto of Bexley College, London; Mrs. J.E. Amagun; Mr. D. E. J. Aweto and Mr. D.E. H. Aweto. I will also like to express my gratitude to my pastors, Pastor Seun David, Pastor Felix Achoja of Redeemed Christian Church of God and friends too numerous to mention who prayed and kept encouraging me throughout the duration of this programme. God bless you all. v TABLE OF CONTENTS Contents Title page Page -- -- -- -- -- -- -- -- -- i Certification -- -- -- -- -- -- -- -- -- ii Dedication -- -- -- -- -- -- -- -- iii Acknowledgements -- -- -- -- -- -- -- -- iv Table of Content -- -- -- -- -- -- -- -- vi List of Figures -- -- -- -- -- -- -- -- ix List of Tables -- -- -- -- -- -- -- -- xiv Abstract -- -- -- -- -- -- -- -- xvi -- -- CHAPTER ONE: 1.0 INTRODUCTION -- -- -- -- -- -- -- 1 1.1 Background of the study -- -- -- -- -- -- 1 1.2 Statement of problem -- -- -- -- -- -- 5 1.3 Location of the Study Area -- -- -- -- -- -- 7 1.4 Aim and objectives -- -- -- -- -- -- -- 7 1.5 Literature review -- -- -- -- -- -- 9 -- CHAPTER TWO: 2.0 GEOLOGY AND HYDROGEOLOGY -- -- -- 13 2.1 Geology of the Study Area 2.1.1 Structures -- -- -- -- -- 13 -- -- -- -- -- -- -- -- 19 2.2 Local Geology -- -- -- -- -- -- -- 20 2.3 Hydrogeology -- -- -- -- -- -- -- 22 CHAPTER THREE: 3.0 MATERIALS AND METHOD -- -- -- -- -- 25 3.1 Geoelectric Method -- -- -- -- -- 25 3.2 Electrical Resistivity Imaging -- -- -- -- -- 28 3.3 Azimuthal Resistivity Sounding -- -- -- -- -- 32 -- vi 3.4 Resistivity Logging Method -- -- -- -- -- 35 3.5 Data Acquisition -- -- -- -- -- 39 4.0 DATA ACQUISITION AND PROCESSING -- -- -- 51 4.1 Determination of Geoelectric Parameters -- -- -- --- 51 4.2 Isoresistivity -- -- -- -- -- --- 51 4.3 Determination of protective capacity -- -- -- -- 52 4.4 Electrical Resistivity Imaging profiles -- -- -- -- 52 4.5 Azimuthal Resistivity Sounding polar diagrams -- -- -- 53 4.6 Determination of Water Quality 4.7 Groundwater head contour maps -- -- CHAPTER FOUR: -- -- -- -- -- -- -- 54 -- -- -- -- 56 -- -- -- -- 57 CHAPTER FIVE: 5.0 RESULTS 5.1 Geoelectric Parameters and Delineation of Geologic Sequence in -- the Study Area -- -- -- -- 57 5.2 Identification of aquifer units, depth and lateral extent -- -- 87 5.3 Determination of areas prone to contamination -- -- -- 109 5.4 Inverted electrical resistivity imaging profiles -- --- -- 144 5.5 Water quality -- -- -- 178 5.6 Determination of groundwater flow direction -- -- -- 208 -- -- -- -- -- -- -- -- -- CHAPTER SIX: 6.0 DISCUSSION -- -- -- -- --- -- -- -- 225 6.1 Ughoton -- -- -- -- -- -- -- -- 225 6.2 Ekakpamre -- -- -- -- -- -- -- -- 231 6.3 Uvwiamuge -- -- -- -- --- -- -- -- 234 6.4 Egbeleku -- -- -- -- --- -- -- -- 239 6.5 Otor – Jeremi -- -- -- -- --- -- -- -- 242 6.6 Burutu -- -- -- -- --- -- -- -- 245 6.7 Sapele -- -- -- -- --- -- -- -- 255 vii CHAPTER SEVEN: CONCLUSION AND RECOMMENDATIONS -- 258 7.1 Conclusion -- -- -- -- -- -- -- -- 258 7.2 Recommendations -- -- -- -- -- -- -- 262 REFERENCES -- -- -- -- -- -- 265 -- viii LIST OF FIGURES Figure Page 1. Map of study area --- -- -- -- -- -- -- 8 2. Cross- section through the Niger Delta (after Burke, 1972) -- 15 3. Depobelt map of Niger Delta (After Doust and Omastola, 1990) 4. Four-electrode resistivity array showing current (solid) 18 and n equipotential (dashed) lines. (Modified after Benson et al. 1984) 26 5. Dipole-dipole electrode array for 2D resistivity measurement. 30 6. Geometry of a DC resistivity traverse using 41 electrodes at 5 m Separations 7. -- -- -- -- -- -- -- 31 Layout of azimuthal resistivity sounding rotated 45 degrees clockwise and successive resistivities are measured -- -- -- 34 8. The general form of electrode configuration in resistivity logging -- 36 9. Conventional single-point resistance log (Keys, 1990) -- -- 38 10. Data acquisition map of Ughoton -- -- -- -- 42 11. Data acquisition map of Ekakpamre -- -- -- -- 43 12 Data acquisition map of Uvwiamuge -- -- -- -- 44 13 Data acquisition map of Egbeleku -- -- -- -- 45 14 Data acquisition map of Otor-Jeremi -- -- -- -- 46 15 Data acquisition map of Burutu -- -- -- -- -- 47 16 Data acquisition map of Sapele -- -- -- -- -- 48 17 Computer generated model data curve for Ughoton VES 6 -- 57 18 Computer generated model data curves for Ughoton VES 12 and 18 19 Computer generated model data curves for Ekakpamre VES 24 and 25 20 -- -- -- -- -- -- 58 -- 59 -- 60 -- 61 Computer generated model data curves for Uvwiamuge VES 40 and 43 21 -- -- -- -- -- -- -- -- -- Computer generated model data curves for Egbeleku VES 47 and 50 -- -- -- -ix -- -- -- 22 Computer generated model data curves for Otor-Jeremi VES 76 and 77 -- -- -- -- -- 62 23 Computer generated model data curves for Burutu VES 84 and 98 63 24 Computer generated model data curves for Sapele VES 105 and 140 64 25 Geologic section for Ughoton along SW-NE traverse -- -- 89 26 Geologic section for Ughoton along NW-SE traverse -- -- 90 27 Geologic section for Ekakpamre along NW-SE traverse -- -- 91 28 Geologic section for Ekakpamre along S-NE traverse -- -- 92 29 Geologic section for Uvwiamuge along N-W traverse -- -- 94 30 Geologic section for Uvwiamuge along NW-NE traverse -- -- 95 31 Geoelectric section for Egbeleku along NW-N traverse -- -- 98 32 Geoelectric section for Egbeleku along N-SE traverse -- 99 33 Geoelectric section for Otor-Jeremi along NW-SE traverse - 100 34 Geologic section for Otor-Jeremi along S-N traverse -- -- 101 35 Geologic section for Burutu along W-E traverse -- -- -- 104 36 Geologic section for Burutu along N-S traverse -- -- -- 105 37 Geologic section for Sapele along NNE-SE traverse -- -- 107 38 Geologic section for Sapele along W-E traverse -- -- -- 108 39 Isoresistivity map of Ughoton at 5m depth -- -- -- -- 110 40 Isoresistivity map of Ughoton at 5m, 10m and 20m depth -- 111 41 Isoresistivity map of Ekakpamre at 5m depth -- 113 42 Isoresistivity map of Ekakpamre at 5m, 10m and 20m depth -- 114 43 Isoresistivity map of Uvwiamuge at 5m depth -- 116 44 Isoresistivity map of Uvwiamuge at 5m, 10m and 20m depth -- 117 45 Isoresistivity map of Egbeleku at 5m depth -- -- 119 46 Isoresistivity map of Egbeleku at 5m, 20m, 30m and 40m depth -- 120 47 Isoresistivity map of Otor-Jeremi at 5m depth -- 122 48 Isoresistivity map of Otor-Jeremi at 5m, 10m and 20m depth -- 123 49 Isoresistivity map of Burutu at 5m depth -- 126 x -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 50 Isoresistivity map of Burutu at 5m, 20m, 40m and 60m depth -- 127 51 Isoresistivity map of Sapele at 5m depth -- -- 128 52 Isoresistivity map of Sapele at 5m, 10m and 20m depth -- -- 129 53 Longitudinal unit conductance map of Ughoton -- -- -- 133 54 Longitudinal unit conductance map of Ekakpamre -- -- -- 134 55 Longitudinal unit conductance map of Uvwiamuge -- -- 137 56 Longitudinal unit conductance map of Egbeleku -- -- -- 138 57 Longitudinal unit conductance map of Otor-Jeremi -- -- 140 58 Longitudinal unit conductance map of Burutu -- -- -- 141 59 Longitudinal unit conductance map of Sapele -- -- -- 143 60 2D resistivity structure and pseudosection for profile 1 in Ughoton 146 61 2D resistivity structure and pseudosection for profile 2 in Ughoton 147 62 2D resistivity structure and pseudosection for profile 3 in Ughoton 148 63 2D resistivity structure and pseudosection for profile 4 in Ughoton 149 64 2D resistivity structure and pseudosection for profile 1 in Otor-Jeremi 65 -- -- -- -- -- -- -- -- -- 153 -- -- 154 2D resistivity structure and pseudosection for profile 2 in Otor-Jeremi -- -- -- -- -- -- 66 2D resistivity structure and pseudosection for profile 3 in Otor-Jeremi 155 67 2D resistivity structure and pseudosection for profile 4 in Otor-Jeremi 157 68 2D resistivity structure and pseudosection for profile 5 in Otor-Jeremi 158 69 2D resistivity structure and pseudosection for profile 6 in Otor-Jeremi 159 70 2D resistivity structure and pseudosection for profile 1 in Burutu 163 71 2D resistivity structure and pseudosection for profile 2 in Burutu 164 72 2D resistivity structure and pseudosection for profile 3 in Burutu 165 73 2D resistivity structure and pseudosection for profile 4 in Burutu 166 74 2D resistivity structure and pseudosection for profile 5 in Burutu 167 75 2D resistivity structure and pseudosection for profile 1 in Sapele 171 76 2D resistivity structure and pseudosection for profile 2 in Sapele 172 xi 77 2D resistivity structure and pseudosection for profile 3 in Sapele 173 78 2D resistivity structure and pseudosection for profile 4 in Sapele 175 79 2D resistivity structure and pseudosection for profile 5 in Sapele 176 80 2D resistivity structure and pseudosection for profile 6 in Sapele 177 81 Single point resistance log from Ughoton -- -- -- -- 179 82 Single point resistance log from Ekakpamre - -- -- 180 83 Single point resistance log from Uvwiamuge -- -- -- 181 84 Single point resistance log from Egbeleku -- -- -- -- 182 85 Single point resistance log from Otor-Jeremi Motor Park -- -- 187 86 Single point resistance log from Otor-Jeremi Market -- -- 188 87 Single point resistance log from Burutu -- -- 189 -- -- 88 Single point resistance log from Sapele (Urhiapele Primary School) 190 89 Single point resistance log from Sapele (Headwork) -- -- 191 90 Bar chart showing variation of TDS with depth at Ughoton -- 199 91 Bar chart showing variation of TDS with depth at Ekakpamre -- 200 92 Bar chart showing variation of TDS with depth at Uvwiamuge -- 201 93 Bar chart showing variation of TDS with depth at Egbeleku -- 202 -- 203 94 Bar chart showing variation of TDS with depth at Otor-Jeremi Motor Park -- -- -- -- -- -- -- 95 Bar chart showing variation of TDS with depth at Otor-Jeremi Market 204 96 Bar chart showing variation of TDS with depth at Burutu -- -- 205 -- 206 97 Bar chart showing variation of TDS with depth at Sapele (Urhiapele Primary School). -- -- -- -- 98 Bar chart showing variation of TDS with depth at Sapele (Headwork) 207 99 Polar diagram of apparent resistivity at Ughoton against azimuth at AB/2 of 25, 50, 75 and 100 m -- -- -- -- -- 211 100 Polar diagram of apparent resistivity at Ekakpamre against azimuth at AB/2 of 25, 50, 75 and 100 m -- xii -- -- -- -- 212 101 Polar diagram of apparent resistivity at Uvwiamuge against azimuth at AB/2 of 25, 50, 75 and 100 m -- -- -- -- 213 102 Polar diagram of apparent resistivity at Egbeleku against azimuth at AB/2 of 25, 50, 75 and 100 m -- -- -- -- -- 214 103 Polar diagram of apparent resistivity at Otor-Jeremi against azimuth at AB/2 of 25, 50, 75 and 100 m -- -- -- -- -- 215 104 Polar diagram of apparent resistivity at Burutu against azimuth at AB/2 of 25, 50, 75 and 100 m -- -- -- -- -- 216 105 Polar diagram of apparent resistivity at Sapele against azimuth at AB/2 of 25, 50, 75 and 100 m -- -- -- -- -- 106 Groundwater head contour map showing flow direction at Ughoton 217 218 107 Groundwater head contour map showing flow direction at Ekakpamre 219 108 Groundwater head contour map showing flow direction at Uvwiamuge -- -- -- -- -- -- -- -- 109 Groundwater head contour map showing flow direction at Egbeleku 220 221 110 Groundwater head contour map showing flow direction at Otor-Jeremi -- -- -- -- -- 222 111 Groundwater head contour map showing flow direction at Burutu 223 112 Groundwater head contour map showing flow direction at Sapele 224 xiii -- -- -- LIST OF TABLES Table Page 1. Stratigraphic sequqnce of Niger Delta -- -- -- 14 2. Surficial deposits of Western Niger Delta -- -- -- -- 22 3. Geoelectric parameters, lithology, longitudinal conductance and the protective capacity at Ughoton ---- -- 65 Geoelectric parameters, lithology, longitudinal conductance and the protective capacity at Ekakpamre --- -- 68 Geoelectric parameters, lithology, longitudinal conductance and the protective capacity at Uvwiamuge --- -- 70 Geoelectric parameters, lithology, longitudinal conductance and the protective capacity at Egbeleku. ---- -- 72 Geoelectric parameters, lithology, longitudinal conductance and the protective capacity at Otor-Jeremi --- -- 74 8. Geoelectric parameters and their interpretation at Burutu -- 77 9. Geoelectric parameters, lithology, longitudinal conductance and the protective capacity at Sapele. ---- -- 82 10. Resistivity of water and sediments at Burutu -- -- -- 103 11. Single-point Resistance Log Data at Ughoton -- -- -- 183 12. Single-point Resistance Log Data at Ekakpamre -- -- -- 184 13. Single-point Resistance Log Data at Uvwiamuge -- -- 185 14. Single-point Resistance Log Data at Egbeleku -- -- -- 186 15. Single-point Resistance Log Data at Otor-Jeremi Motor Park -- 192 16. Single-point Resistance Log Data at Otor-Jeremi market -- 193 17. Single-point Resistance Log Data at Burutu -- -- -- 194 18. Single-point Resistance Log Data at Sapele (Urhiapele Primary School) --- -- -- -- 195 19. Single-point Resistance Log Data at Sapele (headworks) -- 196 20. Coefficient of Anisotropy -- -- 210 21. Water quality from dug wells and Ughoton River based on chloride content ------- -- 227 4. 5. 6. 7. -- xiv -- -- -- 22. Heavy metals in surface water in Warri in mg/L -- 23. 24. 25. 26. -- 233 Summary of heavy metals of soils and decomposed wastes -- 236 Groundwater Physical and chemical properties, Ughelli West engineered dumpsite ------ -- 236 Heavy metals and hydrocarbon content in groundwater, Ughelli West engineered dumpsite ---- -- 236 Geochemical Analysis Results of Water Samples from Burutu -- 248 xv -- ABSTRACT Resistivity investigation was carried out in seven (7) communities of the Western Niger Delta: Ughoton, Ekakpamre, Uvwiamuge, Egbeleku, Otor-Jeremi, Burutu and Sapele which include one hundred and forty (140) vertical electrical soundings (VES) utilizing the Schlumberger configuration, twenty one (21) electrical resistivity imaging (ERI), seven (7) azimuthal resistivity soundings (ARS) and nine (9) single-point resistance logging was carried. The study was aimed at delineating the subsurface layers and hence, the aquifer unit(s) and to evaluate the protective capacity of the overburden above the aquifer and to assess the groundwater quality in the aquifer. Results of the resistivity survey indicated the presence of three to four geologic layers with resistivity values of 25.0 – 1152.0 Ωm, 3.8 – 1390.9 Ωm, 14.8 – 1402.4 Ωm and 35.0 – 1200.0 Ωm. The subsurface lithology comprised of fine through medium grained sand to coarse grained intercalated in most cases with clay, sandy clay, and clayey sand. The aquifer which exist at an average depth of 10.0 – 25.0 m is between 10.5 m to more than 75.3 m thick is unconfined at the surface and semi-confined and confined at depths. The longitudinal conductance maps delineated areas with poor (< 0.1mho), weak (0.1 – 0.19 mho), moderate (0.2 – 0.69 mho) and good (0.7 – 4.9 mho) protective capacity. The protective capacity of the overburden above the aquifer in these communities is mostly poor or weak except at Egbeleku and Burutu where it is moderate to good as a result of thick clays and sandy clays (exceeding 20 m in thickness) protecting the aquifer. Electrical resistivity imaging (ERI) profiles showed that the groundwater in the uppermost parts of the aquifers were poor in quality and beyond potable limit as per the standard set by World Health Organization, United State Environmental Protection Agency and Standard Organization of Nigeria. The concentration of TDS evaluated from single-point resistance (SPR) logs ranged from 518.00 – 92,219.01 ppm at depths of 8.0 m to about 30.0 m for upper coastal sands and 3350.08 – 474,074.07 ppm at depths of 8.0 m to 54.0 m for lower coastal sands at Burutu. This indicated that the quality of water in the uppermost parts of the aquifer is poor because of TDS concentration above the lower limits of general acceptability of 500 ppm. The groundwater in the aquifer at Burutu which lies by the Forcados estuary has suffered salinization at locations close to the Forcados River. Salinization of groundwater was however not noticed at Ughoton which also lies within the Brackish Mangrove Swamps (BMS). The estimated values of coefficient of anisotropy varied between 1.20 – 1.73. The estimate values of coefficient of anisotropy are generally found to increase in magnitude with depth indicating increasing grain size with depth. The radial polar diagrams and groundwater gradient contour maps showed the direction of groundwater flow and establishing the dominant direction to be in NW – SE. xvi CHAPTER ONE 1.0 INTRODUCTION 1.1 Background to the Study Groundwater is the water that fills all pores and openings within the zone of saturation. Exploration for groundwater in sedimentary environments involves locating formations that possess appropriate porosity and permeability. While the location of permeable clean sands that are capable of yielding useful quantities of water to wells is important, the quality of water yielded is also crucial. Within the last decade or so, the continuous increases in population and industrial growth in the Niger Delta persistently has caused great demand for water, generation of huge waste matters and unending land-use problems. The cheap mode and indiscriminate disposal of huge refuse waste matters being generated domestically and industrially has, over the years been reducing the potential sources of utilizable water for the growing population. Both surface water and groundwater are exposed to the danger of pollution by leachate effluent from even discriminately dumped wastes. Effluent from the refuse dumpsite and landfill will either infiltrate the refuse or run off over as land flow. During the vertical percolation process (with rain water) the water leaches both organic and inorganic constituents from refuse. The leachate becomes part of the groundwater flow system immediately they reach the water table. The extent of pollution is greater in high rainfall area than less humid and arid areas (Al-Yaqout and 1 Hamoda, 2003). Where the land surface is flat, groundwater table shallow, the soil porous and permeable like that of the Niger Delta thus permitting quick infitration, the polluting effluents are capable of escaping into the subsurface to contaminate the potable water in the aquifer and create a pollution plume that can extend for several hundreds of meters (Keswic et al., 1982). The contaminant plume is capable of persisting in groundwater environment several years after the source must have been eliminated, as is the case with the Canadian Air force Base sanitary landfill site at Borden, Ontario (Sykes et al., 1982). Indiscriminate dumping of wastes has affected the quality of surface and groundwater. Consequently, much of the emphasis in groundwater investigation has shifted from problems of its supply to consideration of groundwater quality (Freeze and Cherry, 1979; Matias et al., 1994). It has now been recognized that measures for groundwater protection should focus on the best possible safety provisions for future waste disposal sites (USEPA, 1997). This should also include investigations to assess the potential; of groundwater pollution from existing sites (Meju, 2000). The Niger Delta is endowed with rich groundwater resources in several aquifers, but unfortunately, the public water supply by State Water Agency is inadequate and unable to satisfy the demanded quantities (Akpoborie et al., 2000) and consumers must make alternative arrangements. These arrangements in most cases consist of hand dug wells or relatively cheaper shallow boreholes that are constructed with the aid of augers operated manually. These boreholes are usually 2 slightly deeper than the dug wells but also exploit the shallow aquifers that are the most susceptible to contamination from various sources. The Forcados estuary is a major discharge source that directs water into the ocean. Because it faces the high energy dynamic environment of the Atlantic Ocean, the estuary is characterized by longshore current activity accentuated by tidal currents which are numerous on the coastline. The maximum measured velocities of tides are as much as 280 cm/s (Delta State Government, 1998), while maximum velocities in the inshore tidal channels are as much as 180 cm/s (NEDECO, 1961). These tides results in a heavy salt water wedge, which at high tides pushes far inland. The implication of this for water supply is that for coastal communities, though these rivers carry freshwater from their sources, on getting to the coast and around the estuary and further inland they are very saline and brackish during the high tide. The sand that occupies the lower deltaic plain is the source of water for the communities that occupy the coast. Groundwater occurs in the sand lenses which are interbedded with silts and clays. Unfortunately geophysical survey (Oteri, 1990) around Ugulaha has shown that the groundwater is saline to brackish. Freshwater is apparently limited to perhaps only the first 0 – 2 m or less from the surface. The peculiar geographic locations of some communities in the study area within the Brackish Mangrove Swamp (BMS) make the shallow aquifer unsafe for potable water extraction. Etu-Efeotor and Odigi (1983) observed that the groundwater problem in the area includes salinity, bacteriological contamination and the presence of some undesirable ion. 3 Various studies (Ejechi, et al., 2007; Akpoborie, et al., 2000; Abimbola, et al., 2002; Olobaniyi et al., 2007) have indicated that the quality of groundwater in the Western Niger Delta especially at shallow depths has been compromised by contaminants from a variety of sources. Rim-Rukeh et al. (2007) have indeed suggested that the problem is not limited to the west but widespread over the delta, hence, exploration for groundwater should not be restricted only to location of suitable aquifers using surface investigation techniques to provide data interpretable in terms of aquifer depths, thicknesses, continuity, areal extent and structures but also the aspect of groundwater quality. A complete appraisal of available water resources is often best accomplished when the aspects of water quality are included. This is because in a planned water supply system, quality constraints and requirements dictate the source of water allocated to various stages. The quality of any water resource is its suitability for the intended uses. The quality of any water resource depends on the physical, chemical and biological characteristics of the water which in turn depends on the geology of the area and impacts of human activities (Ezeigbo, 1989). Pollution of a water resource occurs when the quality of the water is degraded or its usefulness is impacted as a result of the presence of certain substances (Davis and Cornwell, 1998) 4 1.2 Statement of Problem One of the commonest ways of waste disposal in the Niger Delta is by open dumping. The primary environmental consequence of these indiscriminate dumping of waste in open dump is the generation of leachates due to decomposition of the waste materials. The leachates are subsequently released into the groundwater by infiltration and this poses serious environmental problems including health hazard (Tijani et al, 2002; Samanjara and Banadara, 2003). This is especially the case in developing countries. In Nigeria, for example, the estimated 508,000 tons of domestic solid wastes generated by 1980 has been more than doubled in the following years on the account of increase in population (Filani and Abumere, 1987). These waste materials are commonly indiscriminately deposited in dumpsites without appropriate protective measures (e.g clay and/or plastic liners). Thus rainfall entering the dumpsites or insitu fluids may percolate through the dumpsites into the groundwater zone leading to contamination of groundwater. The most common approach to monitor the leachate emanating from a landfill/dumpsite is to drill a series of monitoring wells around the landfill/dumpsite that penetrate into either the vadose and saturated zones or both. However, these wells are expensive to drill and maintain especially in a developing country. Because the monitoring wells are commonly located randomly because of budgetary constraints (Shemang, et al., 2011), these wells provide only point source information and leachate plumes tend to migrate along 5 preferential pathways and even closely spaced monitoring well may miss some of the contaminants (Zume et al., 2006). Therefore electrical resistivity techniques are useful in defining the boundaries of dumps/landfills and direction of contaminant plumes. Some of the communities comprising the study area are oil producing and hence, there is a possibility of environmental consequences of hydrocarbon pollution as some of the area have experienced oil pollution at one point in time or the other. Oil spills have degraded most agricultural land and groundwater and can occur due to a variety of reasons (Ozumba et al., 1999), including blowouts due to over-pressure, equipment failure, operational errors, corrosion, sabotage (vandalisation of pipelines), flowline replacement, flowstation upgrades, tank rehabilitation and natural phenomena such as heavy rainfall, flooding, falling of trees and lightening. Also the proximity of some communities in the study area to creeks could or may have led to contamination of groundwater by salt water intrusion associated with salt water influx and and high iron from the mangrove swamps. Geological and hydrogeological studies in this areas show an elevation of less than 10m above sea level and occurrence of shallow clean sand deposits (Akpokodje and Etu-Efeotor, 1987; Olobaniyi and Owoyemi, 2006) hence, the aquifers are easily vulnerable to contamination in the event of pollution as a result its shallowness. Geophysical studies for groundwater investigation in the western 6 Niger Delta area have also reached aquifers at depths of less than 10 m (EtuEfeotor and Michalski, 1989). 1.3 Location of the Study Area The study area covers seven communities: Ughoton, Ekakpamre and Uvwiamuge, Egbeleku, Otor-Jeremi, Burutu and Sapele in Delta State. These areas lie within longitude 5o30'E, 5o56'E and latitude 5o20' N, 5o55'N (Figure 1). The average elevation in these areas is about 10 m above sea level. 1.4 Aim and objectives The aim of this study is to investigate the aquifer systems in order to provide information about the subsurface layers of the area using geophysical tools. Hence the study covered the following objectives. i) Determination of the geoelectric parameters and delineation of geologic sequence of the study area. ii) Identification of aquifer units, depth and lateral extent. iii) Determination of the overburden protective capacity iv) Determination of areas prone to contamination and areas already contaminated. v) Determination of groundwater quality in the study area. vi) Determination of groundwater flow/contaminant plume direction. 7 Figure 1: Map of Study Area 8 1.5 Literature Review The increase in recent years in underground sources of water has led to a need for more intensive studies of the geometry and properties of aquifer. Geophysics has played a useful part in such investigations for many years and improvements in instruments and the development of better methods is resulting in a widening of its application. It is still used mainly to determine the structure but there is a considerable interest in the possibilities of estimating aquifer properties such as permeability and porosity from the measurement of geophysical properties. Over the past twenty years, geophysics has been successfully used globally to tackle a growing number of environmental challenges. Presently, its uses include locating pollution spots, predicting direction of flow of pollutants to monitoring underground fuel tank to defining areas of potential contamination. Most environmental geophysics literature involves the upper 300m of the subsurface (Steeples, 2001). Some of geophysical tools used to investigate the near surface environment include, Induced Polarization, electrical resistivity, electromagnetic, seismic, geothermal, radioactivity, gravity, ground penetrating radar (GPR) and geophysical borehole logging techniques (Sharma, 1997; Reynolds, 1997; Lowrie, 1997). DC methods date from the early part of this century (Ward, 1980) with application in groundwater dating from the late 1930s (Lee, 1936; Sayre and Stephenson, 1937; Swartz, 1937). The method is also used in environmental studies because it can clarify the subsurface structure, delineate contaminated 9 zones of groundwater and is inexpensive (Mazak et al., 1987; Balia et al., 2003). The resistivity techniques have applications in groundwater exploration (Van Overmeeren, 1989; Olorunfemi et al., 2001). Recently, resistivity imaging survey has been used to map groundwater contamination and it is widely used for environmental surveys (Griffiths and Barker, 1993). Olayinka and Olayiwola (2001) used integrated geoelectric imaging and hydrochemical methods to delineate limits of polluted surface and groundwater at a landfill site in Ibadan area of southwestern Nigeria. The results delineated polluted surface and groundwater with the limits of pollution clearly indicated. It has also been successful used in engineering studies and dam site investigation (Ako, 1976). Henriet (1976) conducted resistivity surveys in karstic limestones and determined the storage coefficient for aquifer and also showed that the combination of layer resistivity and thickness in the Dar Zarrouk parameters (longitudinal conductance, S) and (Transverse resistance, T) may be of use in aquifer protection studies. The protective capacity is considered to be proportional to the longitudinal conductance in mhos. Oladapo et al., 2004, using the geoelectric method in a study of the Ondo State housing corporation estate, modified the longitudinal conductance/protective capacity rating as > 0.10 (poor), 0.10 – 0.19 (weak), 0.20 – 0.69 (moderate) and 0.70 – 4.90 (good). Acworth and Griffiths (1985) demonstrated that resistivity imaging has application in hydrogeology in basement areas and mapping of strongly faulted areas even where the weathered layer is so deep and laterally variable such that 10 other electrical and electromagnetic methods prove ineffective. It has been proved to be useful for mapping saline intrusion into an aquifer (Barker, 1990; Mooney, 1980; Patra and Bhattacharya, 1967). It can also be used to map rock quality for quarrying purposes and where tunneling is required (Dahlin et al., 1996). Kosinski and Kelly (1981) established useful relations between hydraulic and electrical properties for unconfined aquifer by using the VES method while Mbonu et al. (1991) used Schlumberger vertical electrical sounding (VES) to determine aquifer characteristics, in Umuahia, Nigeria. The aquifer transmissivity were also calculated for the geoelectric model. Most resistivity logging techniques have been in use by the petroleum industry where holes being logged are usually deep and filled with drilling mud or saline water. Many of these techniques are not suitable, or must be adapted, for use in freshwater aquifers, which are the focus of near surface hydrogeological investigations where they are primarily used to determine water quality. Schlumberger (1972) observed that in fresher, more resistive waters, as resistivity of formation water rises and as the grain size of the sand decreases, the value of the formation factor decreases. This relationship is attributed to a “ greater proportionate influence of the surface conductance of the grains in fresher water” . Recently, Kwader (1985) has applied this relationship and correlated the formation factor with permeability of sand and carbonate aquifers in the Southeastern coastal plain of the United States. Pryor (1956) developed a technique for estimating TDS from long and short normal resistivity curves. 11 Poole (1989) used geophysical logs to estimate water quality of basal Pennsylvanian sandstones, Southwestern Illinois and found out that TDS increases away from outcrop area of the Pennsylvanian sandstones. Changes in TDS concentration with distance from the outcrop area are an expression of water residence time within the aquifer 12 CHAPTER TWO 2.0 GEOLOGY AND HYDROGEOLOGY 2.1 Geology of the Study Area The Niger Delta basin is situated on the continental margin of the Gulf of Guinea in West Africa between latitudes 30 and 60N and longitudes 50 and 80E. (Reijers, 1996). The evolution of the Niger Delta basin can be attributed to the evolution of the Benue Trough, a failed arm of the triple junction in the Cretaceous that led to the separation of South America from Africa (Reijers, 1996). The geology of the Niger Delta has been described by Allen (1965), Burke et al. (1972), Short and Stauble (1967), Murat (1970) and Kulke (1995). The Cenozoic section of the Niger Delta basin (Table 1 and Figure 2) is represented by a strongly diachronous sequence divided into three lithofacies units (Short and Stauble, 1967) representing prograding depositional facies that are distinguished mostly on the basis of sand-shale ratios. 13 Table 1: Stratigraphic sequqnce of Niger Delta SUBSURFACE SURFACE OUTCROPS YOUNGEST OLDEST YOUNGEST OLDEST KNOWN KNOWN KNOWN AGE KNOWN AGE AGE RECENT BENIN AGE OLIGOCENE PLEISTOCENE FORMATION RECENT AGBADA BENIN MIOCENE FORMATION EOCENE FORMATION MIOCENE EOCENE OGWASHI- OLIGOCENE ASABA FORMATION EOCENE AMEKI FORMATION RECENT AKATA EOCENE EOCENE IMO SHALE FORMATION FORMATION (After Short and Stauble, 1967) 14 PALEOCENE Figure 2: Cross- section through the Niger Delta (after Burke, 1972) 15 The Niger Delta basin shows an overall upward and updip transition from marine prodelta shales (Akata Formation) through an alternating sand/shale paralic interval (Agbada Formation) to continental sands (Benin Formation). The Akata Formation which lies at the base of the delta is of marine origin and is composed of thick shale sequence (potential source rock), tubidite sand (potential reservoirs in deep water) and minor amounts of clay and silts. The Akata Formation and open marine facies (prodelta) unit formed during lowstands beginning in the Paleocene and through the Recent when terrestrial organic matter and clays were transported to deep water area is characterized by low energy condition and oxygen deficiency (Stacher, 1995). It is estimated that the formation is between 600 – 6000 m thick (Short and Stauble, 1967). The formation overlies the entire delta and is typically over-pressured. Turbidity currents likely deposited deep water sands within the upper Akata Formation during development of the delta (Burke, 1972). The overlying Agbada Formation deposition began in the Eocene and continues into the Recent. This formation is a deltaic facies (clinoform) unit made up of an alternating paralic sequence of sandstone and shale. It consists of an upper predominately sandy unit with minor shale intercalations and a lower shale unit which is thicker than the upper sandy unit. This paralic sequence (sandstoneshale) is in response to eustatic change in sea level during sedimentation. The thickness of this formation is between 300 – 4500 m (Short and Stauble, 1967) and serves as the major petroleum bearing unit. 16 The Agbada Formation is overlain by the Benin Formation which is dominantly a fluvial facies unit made up of 90 % sand and sandstone with clay intercalation. In the subsurface it is of Oligocene age in the north and becoming progressively younger (Recent) southward. It is coarse grained, gravely, locally fine-grained, poorly sorted, subangular to well rounded and bears lignite streaks and wood fragments (Asseez, 1989). This stratigraphic unit is up to 2000 m (Avbovbo, 1978) and thickest in the central area of the Delta. The total sedimentary sequence of the Niger Delta was deposited in a series of megasedimentary belts (or depobelts) in time and space representing the successive phases of the delta growth (Doust and Omatsola, 1990) which are distinguished by their ages (Figure 3). During major progradational phases, the delta top deposits advanced over the Agbada Formation (Evamy et al., 1978). The interplay of subsidence and supply rates resulted in deposition of discrete depobelts. When further, crustal subsidence of the basin could no longer be accommodated, the focus of sediment deposition shifted seaward, forming new depobelts. The delta can be divided into seven depobelts viz: Northern Delta (Late Eocene – Early Miocene) Greater Ughelli (Oligocene - Early Miocene) Central Swamp I (Early – Middle Miocene) Central Swamp II (Middle Miocene) Coastal Swamp I (Middle – Late Miocene) 17 DEPOBELT MAP OF NIGER DELTA Figure 3: Depobelt map of Niger Delta (After Doust and Omastola, 1990) 18 Coastal Swamp II (Middle – Late Miocene) Offshore (Late Miocene) Each depobelt is bounded to the north by a major structure – bounding fault and to the south by a change in regional dip of the delta by a counter-regional fault (Evamy et al., 1978). Within the depobelts, both the Akata and Agbada Formations are thought to have served, to varying degrees, as oil and gas source rocks (Weber and Daukoru, 1975; Ekweozor and Okoye, 1980; LambertAikhionbare, 1981). 2.1.1 Structures The influence of basement tectonics on the structural evolution of the Niger Delta was largely limited to movement along the Atlantic Ocean fracture zones, which extend beneath the Delta and determined the initial locus of the proto-Niger Delta in the Benue Trough (Stoneley, 1966; Burke et al., 1972; Weber and Daukoru, 1975)). As the Delta advanced onto the thinned continental crust, continuous subsidence and thinning of the basement created more space for the thick sedimentary pile of the prograding Cenozoic Niger Delta. Growth faults triggered by a pene-contemporaneous deformation of deltaic sediments are the dominant structural features in the Niger Delta. Weber and Daukoru (1975) explained the term growth fault as the name derives from the fact that after their formation the fault remained active and thereby allow a foster sedimentation in the down throw relative to the up throw back. Their origin and mode of formation may be due to gravitational slumping in under compacted marine clays. The 19 growth faults appear crescent-shaped with the concave side toward the downthrow block which tends to rotate along an axis roughly parallel to the fault. According to Merki (1970) the underlying marine Akata shale is undercompacted and over-pressured as evidenced from wireline logs and pressure indicators, the clays contain free water and their bulk density is lower than that of the overlying sands thus leading to the compaction of the shales of the Benin and Agbada Formations. This differential loading (sedimentation rates) creates gravitational instability and the mobile clays react to this by lateral and upward flowage. Associated with growth faults are rollover anticlines which develop by downward movement along the concave fault planes thus causing rotation of the down thrown layers. Growth faults and related rollover structures are the dominant hydrocarbon traps in the Niger Delta (Reijers, 1996). The diapiric structures on the continental slope of the delta (Mascle et al., 1973) and the apparent overthrust near the foot of the slope (Beck, 1972) are likely due to the mass flowage of abnormally pressured Akata Formation under the gravitational load of the delta (Dailly, 1976). Toe thrusting of the delta from lateral flow and extrusion of the Akata prodelta shales during growth faulting and related extension, account for the diapiric structures on the continental slope of the Niger Delta in the front of the prograding depocentre with parallic sediments 2.2 Local Geology The present day expression of the surficial deposits in the Niger Delta (Table 2) is a result of three depositional environments: continental, transitional 20 and marine. The continental environment comprises of the alluvial environments including the braided-stream and meander belt systems are described as the Upper Deltaic Plain. The deposits of the Quaternary Upper Deltaic Plain overlie and mask the Benin Formation (Oomkens, 1974). These continental deposits are 40 – 150 m thick, comprising of rapidly alternating sequences of sand and silt/clay, with the latter becoming increasingly prominent seawards (Etu-Efeotor and Akpokodje, 1990). They were thought to have been laid down during Quaternary interglacial marine transgression (Oomkens, 1974; Durotoye, 1989). Amajor (1991) has shown that they are an admixture of fluvial/tidal channel, tidal flats and mangrove swamp deposits. The sands are micaceous and feldspathic, subrounded to angular in texture and constitute good aquifers. The transitional environment comprises the brackish-water swamps described as the Lower Deltaic Plain. This is made up of mangrove swamps and marshes and includes the coastal area with its beaches, barrier bars and lagoons. The sediments in this environment are finer-grained than in the continental environment. The marine environment includes the submarine part of the delta and fringe with its fine sand, silt and clay. This environment grades laterally into the holomarine environment which is not affected by deltaic activity. 21 Table 2: Surficial deposits of Western Niger Delta Age Deposits Characteristics Late Pleistocene – Early Sombreiro – Warri Dry land with abundant Holocene Deltaic Plain swamp zones Holocene – Recent Lower Deltaic Plain Mangrove estuaries, swamps, beaches and bars 2.3 Hydrogeology Groundwater in the Niger Delta is contained in mainly very thick and extensive sand and gravel aquifer. Three main zones have been differentiated and these are a northern zone consisting of shallow aquifers of predominantly continental deposits, a transition zone of intermixing marine and continental materials and a coastal zone of predominantly marine deposits (Etu-Efeotor and Odigi, 1983; Amajor, 1989; Etu-Efeotor and Akpokodje, 1990). Akpokodje et al., 1990 have summarized the hydrostatigraphic units of the Niger Delta as five well defined aquifers. The first aquifer occurs under phreatic conditions between depths of 0 – 45 m. It supplies water to small private and commercial boreholes and is the most extensively exploited causing water table decline, pollution and saline water intrusion. Most aquifers in this study are within these depths. The second and third aquifers (45 – 130 m and 130 – 212 m 22 deep, respectively) are semi confined and are usually penetrated by medium sized industrial, community and municipal boreholes. The fourth aquifer is 212 – 300 m deep and is tapped by few large scale deep boreholes for municipal and industrial water schemes. The fifth aquifer is more than 300 m depth. Majority of boreholes usually penetrated only the first and second aquifers. The aquifers vary from unconfined to confined conditions at depths. They are separated by highly discontinuous layers of clays giving a picture of a complex, non-uniform, discontinuous and heterogeneous aquifer system. Aquifers in the northern zone are composed of river loads coming from the hinter land and are encountered at shallow depths of about 60 m. In the transition zone, two types of swamp lands are observed – the Brackish Mangrove Swamps (BMS) and the freshwater mangrove swamps (FWS). The BMS are associated with tidal inlets and are more prominent in areas where estuaries penetrated. The BMS show very strong evidences of marine conditions. Thicker lenses of marine clays are encountered and saline conditions well noticed. They are unprotected from dynamic activities hence there is an intermixing of continental and marine sediments resulting in a very complex aquifer system, this makes it possible for marine conditions to penetrate further inland creating a very complex transition zone. Farther inland freshwater persists more within the front of the delta where dense network of streams and rivers combine to empty into the sea. The aquifers are shallow consisting predominantly sand and gravel materials with clay intercalations becoming more prominent than within the northern zone. 23 The annual rainfall in the Niger Delta is high and varies from 500 mm per annum at the coasts to about 300 mm at the northern part (Etu-Efeotor and Odigi, 1983). Evapo-transpiration is 1000 mm, leaving an effective rainfall of 2000mm. Of this effective rainfall, 750 mm (37 % of effective rainfall) recharges the aquifers while the remaining 1250 mm (65 % of effective rainfall) flows directly into surface waters (Akpokodje et al., 1996). This recharge is 75 % of the total precipitation. The hydraulic conductivity varies from 0.04 – 60 mld, transmissivity ranges from 59 – 6050 mld2, storage coefficient varies from 10-6 – 1.5x10-1 and borehole yield is very good with production rates of about 20,010 l/h which indicates potentially productive aquifers (Etu-Efeotor and Odigi, 1983; Odigi, 1989; Amadi and Amadi, 1990). 24 CHAPTER THREE 3.0 MATERIALS AND METHODS 3.1 Geoelectric Method The direct current (DC) also called “ galvanic” electrical resistivity method measures the resistance to flow of electricity in subsurface materials. DC methods involve the placement of electrodes, called current electrodes on the surface for injection of current into the ground. The current stimulates a potential response between two other electrodes, called potential electrodes that are measured by a voltmeter (Figure 4). Resistivity (measured in ohm-meters) can be calculated from the geometry and spacing of the electrodes, the current injected and the voltage response. The theory of resistivity and its application to groundwater and environmental studies have been discussed by many authors (Dobrin and Savit, 1988; Telford et al., 1990; Kearey and Brooks, 2002). Starting from ohms law, the potential distribution (V) about an electrode carrying a current I has been derived as V= I� 2� 1 r ……………………… (1) Where r is the radius of a hemisphere and ρ is the resistivity. 25 Figure 4: Four-electrode resistivity array showing current (solid) and n equipotential (dashed) lines. (Modified after Benson et al. 1984). 26 Consider an arrangement consisting of a pair of current electrodes and a pair of potential electrodes (Figure 4). The current electrodes A and B act as source and sink, respectively. Where r1 is distance between AB r2 is distance between BM r3 is distance between AN r4 is distance between BN The potential at C is Vc = �I �1 1 � � � � (2) 2� � �r1 r2 � Similarly, the resultant potential at D is VD = �I �1 1 � � � � (3) 2� � �r3 r4 � The potential difference measured by a voltmeter connected between C and D is ∆V = V c – VD - - - - - (4) ∆V = �I ��1 1 � �1 1 �� � � �� � � � �� (5) 2� �� �r1 r2 � �r3 r4 �� All quantities in equation 5 can be measured at the ground surface except the resistivity, which is given by 27 V ��1 I ��r1 � � 2� �� � 1� � r2 � � �1 1 �� � �r r � �� (6) � 3 4 �� ρa = G x R ------------(7) G = 2� (8) �1 1 � �1 1 � � � �r r � � � � � � 1 2 � �r3 r4 � The resistivity obtained from equation 7 for inhomogeneous subsurface is known as the apparent resistivity ρa. The resistivity picture is converted into a geologic picture when we have some knowledge of typical resistivity values for different types of subsurface materials and the geology of the area surveyed (Telford et al., 1990). 3.2 Electrical Resistivity Imaging The physics of 2D resistivity are no different than those of 1D resistivity as initially investigated by Conrad Schlumberger in France and Frank Wenner in the United States in 1917 and 1918 (Burger, 1992). In the 2D case it is assumed that the resistivity of the ground varies only in the vertical and is horizontal direction along the profile (Janik and Krummel, 2006). There is no resistivity variation perpendicular to it (strike direction). Electrical resistivity imaging (2D resistivity surveys) has played an increasingly important role in the last few years. The advantages of 2D measurements are their high vertical and lateral resolution along the profile and comparatively low costs due to computer-driven data acquisition. 28 Electrical resistivity imaging (ERI) can be done using Wenner, Schlumberger and dipole-dipole arrays. The dipole-dipole array which offers the best depth of investigation has the disadvantage of a comparatively low signal to noise ratio. Experience, confirmed by theoretical studies (Roy and Apparao, 1971; Edwards, 1977; Barker, 1989) suggests that the dipole-dipole array has the best resolution with regard to the detection of single objects. At regions with high electrical noise, other configurations like Schlumberger or Wenner arrays should be used to increase the signal strength (Janik and Krummel, 2006). Electrical resistivity imaging was accomplished using four electrode systems employing the dipole-dipole array as shown in Figure 5. The apparent resistivity values were calculated from the field resistance values using the equation. ρa ………………. (9) To obtain a good 2D picture of the sub-surface, the coverage of measurements must be 2D as well. Figure 6 shows a possible sequence of measurements for the dipole-dipole array. The spacing between the current electrode pair C2 – C1 is given as “ a” which is the same as the distance between the potential electrode pairs P1 – P2. This array has another factor marked as n. this is the ratio of the distance between C1 and P1 electrodes to the C2 – C1 (or P1 – P2) dipole separation “ a” . For survey with this array the “ a” spacing is initially 29 kept fixed and “ n” factor is increased from 1 to 2 to 3 until up to about 6 in order to increase the depth of investigation (Loke, 1999). Figure 5: Dipole-dipole electrode array for 2D resistivity measurement. 30 Figure 6: Geometry of a DC resistivity traverse using 41 electrodes at 5 m separations. Electrode locations are marked by the arrows. Apparent resistivity measurements are assigned to depths to calculate a pseudosection. For example, the resistivity measurement provided using electrodes 1, 2, 3 and 4 is assigned to location ‘ M ’ at 5 m pseudodepth. Note that this is not 5 m below the ground surface. (Ingham et al., 2006). 31 For the first measurement electrode number 1, 2, 3 and 4 are used. Electrode 1 is used as the second current electrode C2, electrode 2 as the first current electrode C1, electrode 3 as the first potential electrode P1 and electrode 4 as the second potential electrode P2. For the second measurement, electrodes number 3, 4, and 5 are used for C1, P1 and P2 respectively while maintaining electrode 1 at C2, for the third measurement, electrodes number 4, 5 and 6 are used for C1, P1 and P2 maintaining electrode 1 at C2, for the fourth measurement electrodes number 5, 6, 7 are used for C1, P1 and P2 maintaining electrode 1 at C2. After four successive measurements (n = 4) or more depending on the depth of investigation the second current electrode (C2) was then moved to electrode number 2 and while maintaining C2 at electrode number 2, the whole process is repeated 4 time. This is then repeated down the line of electrode until electrodes 38, 39, 40 and 41 are used for the last measurements. 3.3 Azimuthal Resistivity Sounding Azimuthal resistivity survey is a modified resistivity survey in which the magnitude and direction of electrical anisotropy can be determined. Different authors have shown the usefulness of Azimuthal Resistivity Survey (ARS) in determining the principal direction of electrical anisotropy (Lenonard-Mayer, 1984; Taylor and Fleming, 1988; Ritzi and Andolsek, 1992; Skjernaa and Jorgensen, 1993; Odoh, 2010). The first aim of resistivity survey is the study of inhomogeneities. Frequently in practice the effect of anisotropy is displayed together with that of layering or inhomogeneities. It complicates data 32 interpretation within the framework of anisotropic models, and distorts results of interpretation in the framework of layered or inhomogeneous media (Shevnin and Modin, 1989). Ignoring of anisotropy results in wrong data interpretation, at the same time anisotropy studying can give valuable geological information. That means that anisotropy itself and the mutual influence of anisotropy and inhomogeneities needs to be studied. Typically, any observed change in apparent resistivity with azimuth is interpreted as invocative of anisotropy. It is often assumed that the principal direction of hydraulically conductive fracture may be inferred from the measured electrical anisotropy (apparent resistivity as a function of azimuth and depth), since both current flow and groundwater are channeled through fractures in the rock. The direction of flow of contaminant in groundwater is very important and it is possible to determine the direction of contaminant flow from electrical resistivities measured in azimuths. Since the potential for groundwater resources as well as for contaminant transport are governed by pores/fractures of the rock the electric current flow has different magnitudes in difference directions (Ibe and Njoku, 1999). When apparent resistivities in different directions are plotted as radii (Figure 7) they generate anisotropy figures, which is an ellipse in the simple case of parallel fractures with the long axis parallel to the strike of the fractures (Mamah and Ekine, 1989; Okorumeh and Olayinka, 1998). 33 Figure 7: Layout of azimuthal resistivity sounding rotated 45 degrees clockwise and successive resistivities are measured (Modified after Carpenter et al., 1991) 34 3.4.1 Resistivity Logging Method Utilizing borehole logging tools can provide valuable geologic information that is often not obtainable by traditional surface geophysical methods. Geophysical logging is the measurement of various physical properties by the way of sensors lowered into a well or borehole. In normal resistivity logging, the apparent resistivity of the formation is measured in ohm-meters. The logging tool applies a constant current across two electrodes while measuring the potential between two other electrodes (Figure 8). The volume of investigation is a sphere whose diameter is equal to twice the potential electrode spacing’ s, which are typically either 16 or 64 inches. However, the shape and volume of investigation change depending on the resistivity of the formation (Kearey and Brooks, 2002). The apparent resistivity has to be corrected for borehole diameter, drilling mud invasion and formation bed thickness to obtain true resistivity. The general equation for computing apparent resisitivity, ρa for any downhole electrode configuration is ρa = 4��V ----------------- (10) �� 1 1 � � 1 1 �� � � � ��� � � � �� ��C1P1 C2 P2 � �C1P2 C2 P2 �� 35 Figure 8: The general form of electrode configuration in resistivity logging 36 Where C1, C2 are the current electrode P1, P2 the potential electrodes between where there is a potential difference ∆V, and I is the current flowing in the circuit. Different electrode configurations are used to give information on different zones around the boreholes. Switching devices allows the connection of different set of electrode so that several types of resistivity logs can be measured during a single passage of the sonde. The region energized by any particular electrode configuration can be estimated by considering the equipotential surfaces on which the potential electrode lie (Kearey and Brooks, 2002). In a homogenous medium, the potential difference between the electrodes reflects the current density and resistivity in that region. The same potential difference would be obtained no matter what the position of the potential electrode pair. The zone energized is consequently the region between the equipotential surfaces on which the potential electrodes lie 3.4.1 Single-point Resistance Log The single-point resistance (SPR) log has been one of the most widely used in non-petroleum logging in the past; it is still useful in spite of the increased application of more sophisticated techniques. SPR logs cannot be used for quantitative interpretation, but they are excellent for lithologic information. 37 Figure 9: Conventional single-point resistance log (Keys, 1990) 38 The conventional single-point resistance (SPR) log which measure the resistance in ohm between an electrode as it is lowered down a well and an electrode at the land surface (Keys, 1990) as shown in Figure 9. The resistance of any medium depends not only on its composition, but also on the cross sectional area and length of path through that medium. Because no provision exists for determining the length or cross sectional area of the travel path of the current, the measurement is not an intrinsic characteristic of the material between the electrodes (Wightman et al., 2003). Single-point resistance logs are used at contaminated sites as they are useful tools for identifying changes in lithology and water quality (Keys, 1990). The resistance measured from SPR log relates current and voltage from ohms law as follow: R= ………………. (11) The apparent resistivity ρa in inhomogenous medium is determined by ρa ……………… (12) Where G is geometric array factor. 3.5 Data Acquisition 3.5.1 Geoelectric Data The ABEM signal averaging system Terrameter (4000) model was used for the resistivity data acquisition. This instrument is very portable and has high to noise ratio with an in-built booster for greater depth of penetration. The 39 Schlumberger array was used with maximum current-electrode separations (AB) of 600 meters. The Schlumberger array which is most preferred because of its sensitivity to surface in homogeneities (Sharma, 1997) was adopted for the data acquisition. The resistivity measurements are normally made by injecting current into the ground through two current electrodes (C1 and C2) and measuring the resulting voltage difference at two potential electrodes (P1 and P2). The depth of investigation is a function of the electrode spacing and is generally about 20 % to 40 % of the outer electrode spacing, depending on the resistivity of the earth. Greater depth of investigation is achieved by increasing outer (current) electrode spacing. A total of one hundred and forty (140) vertical electrical soundings were carried out in the seven communities as follows: Twenty (20) VES stations were occupied in Ughoton (Figure 10), ten (10) VES stations were occupied in Ekakpamre (Figure 11) and fifteen (15) VES stations were occupied in Uvwiamuge (Figure 12), fifteen (15) at Egbeleku (Figure 13), twenty (20) at Otor-Jeremi (Figure 14), twenty (20) at Burutu (Figure 15) and thirty five (35) at Sapele (Figure 16). The available knowledge of the local geology guided the planning and subsequent interpretation of the resistivity data. 40 3.5.2 Electrical Resistivity Imaging Data The 2D resistivity imaging technique using the dipole – dipole array method was adopted for the survey with the aid of the SAS 4000 Terrameter. This array has been, and is still, widely used in resistivity/IP surveys because of the low E.M. coupling between the current and potential circuits. The arrangement of the electrodes is shown in Figures 5 and 6. The spacing between the current electrodes pair, C2 – C1, is given as “ a” which is the same as the distance between the potential electrodes pair P1 – P2. This array has another factor marked as “ n” in Figure 5. This is the ratio of the distance between the C1 and P1 electrodes to the C2 – C1 (or P1 – P2) dipole separation “ a” . For surveys with this array, the “ a” spacing is initially kept fixed and the “ n” factor is increased from 1 to 2 to 3 until up to about 6 in order to increase the depth of investigation (Loke, 1999). A dipole spacing of a = 6, a = 8, a = 10 and n = 4 were used for the profiles. The stored data in the Terrameter was transferred to a computer for processing and inversion using the DIPPROf inversion software. The inversion of the field resistivity data was carried out with the aim of delineating the subsurface geologic sequences present in the study area. Twenty one (21) electrical resistivity imaging was done using dipole – dipole array. 41 42 43 44 45 46 47 48 3.5.3 Azimuthal Resistivity Sounding Data Nine (9) Azimuthal Resistivity Sounding (ARS) were carried out at different locations within the study area using an ABEM SAS 4000 Terrameter using Schlumberger electrode configuration expanded about a center point. The current electrode spacing (AB) having a maximum spread of 200 m and maximum potential electrode spacing (MN) of 200 m were rotated about a center point at each location and measurements were made in 450 increments (i.e. 00, 450, 900, 1350) which are N – S, NE – SW, E – W and SE – NW directions. The apparent resistivity measured along different azimuths for a given AB/2 separations at each location were plotted along their corresponding azimuths. Lines of the resistivity of the same value along different azimuths were joined together, thus resulting in a polygon. A set of such polygons obtained corresponding to different AB/2 separations is known as a polar diagram or anisotropy polygon. For an isotropic homogenous formation, this polygon will assume a circular shape where coefficient of anisotropy equals one, because its resistivity is independent of direction. Any deviation from a circle to an ellipse is indicative of anisotropic nature of the rock formation (Mallik et al., 1983; Busby, 2000). 3.5.4 Single-point Resistance Log Data The conventional single-point resistance (SPR) log which measures the resistance in ohm-m between an electrode as it is lowered down a well and an electrode at the land surface (Keys, 1990) was employed for this study. The 49 ABEM signal averaging system Terrameter (4000) model was also used for the resistivity logging of wells. The wells were logged in each community from the top at a minimum depth of 8.0 m to a maximum depth of 130.0 m at the bottom. 50 CHAPTER FOUR 4.0 DATA ANALYSIS AND RESULTS 4.1. Determination of Geoelectric Parameters The data acquired on the field were plotted on a bi-log graph paper with the apparent resistivity (ρa) values on the ordinate and the current electrode spacing (AB/2) on the abscissa. The resulting curves were then subjected to partial curve matching (Koefoed, 1979), engaging master curves and auxiliary point charts (Orellana and Mooney, 1966). The results obtained were then modeled with the computer software, winResist Version 1.0 based on the work of Vander Velpen, 2004, which reduced the interpretation error to acceptable levels (Barker, 1989). The contrast in electrical resistivity existing between lithological sequences in the subsurface (Dodds and Ivic, 1998, Lashkarripour, 2003) was used in the delineation of geoelectric layers and identification of aquifers materials (Deming, 2002). The resistivity and depths values obtained were used to produce geologic section in the various communities. 4.2 Isoresistivity maps Using SURFER 2002, terrain and surface modeling software, the resistivity values were used to generate isoresistivity maps at depths of 5.0 m, 10.0 m, 20.0 m, 40.0 m and 60.0 m. 51 4.3 Determination of Protective Capacity First order geoelectric parameters (apparent resistivity and thickness of layers above the aquifers) were used in deriving the longitudinal unit conductance (S) which is a second order geoeletric parameter or Dar Zarrouk parameter (Maillet, 1947). The total longitudinal unit conductance is given by Where n is the number of layers from the surface to the top of the aquifer varies from 1 to n. The longitudinal unit conductance values obtained from equation 13 was used for calculating the protective capacity of the subsurface layer as presented in Tables 3 – 9. 4.4 Electrical Resistivity Imaging Profiles The raw data obtained for this study comprising of measured apparent resistivity, were processed using the computer programme DIPROfwin. Interpretation of a 2D set of data requires a 2D model of the subsurface. The 2D model used by the programme divides the surface into a number of rectangular blocks. The programme then determines the resistivity of the rectangular block resistivity) which agrees with the actual measurement. A finite-difference forward modeling subroutine was used to calculate the apparent resistivity values. A nonlinear least-squares optimization technique (Loke and Barker, 1996) was used for 52 the inversion routine. The objective function minimized by the inversion is based on DeGroot-Hedlin and Constable (1990) with the starting model being the average apparent resistivity values for the respective data set. The optimization method adjusts the resistivity of the model blocks and iteratively tries to reduce the difference between the calculated and measured apparent resistivity values. A measure of the difference is expressed as RMS (root mean square) error. The model at the iteration after which the RMS error does not change significantly is usually considered the “ best” model. 4.5 Azimuthal Resistivity Sounding Polar Diagrams The apparent resistivity values obtained from the azimuthal soundings were plotted as functions of direction to produce resistivity polar diagrams. The figures are elliptical and are diagnostic of anisotropy and inhomogeneous medium. A pattern of anisotropy (Keller and Frischknecht, 1966) was calculated from the resistivity polar diagrams obtained. The anisotropy was calculated using the equation: …………….. (14) (Habberjam, 1975) Where ρt is the transverse resistivity and ρL the longitudinal resistivity. The length of the major axis of the ellipse is equivalent to the numerical value of the transverse resistivity ρt while the length of the minor axis of the ellipse is equivalent to the numerical value of the longitudinal resistivity ρL. Hence, the coefficient of anisotropy is defined as the ratio of transverse resistivity 53 to the longitudinal resistivity. To minimize the possible effect of overburden, the ARS data were analysed by plotting the apparent resistivities against azimuths at AB/2 of 20, 25, 75 and 100 m on azimuthal polar diagrams, 4.6 Determination of Water Quality Electrical resistivity provides information about the fluid that is in the pore spaces within the rock matrix in oil and water wells. Because electrical resistivity is controlled by ion flow in liquids, electrical resistivity logs can provide information on water quality. Total dissolved solids (TDS) will be used in specifying the quality of groundwater in this study. TDS can be measured in terms of electrical conductance. Within an aquifer, the resistivity of the formation water or pore water ρw can be related to the bulk resistivity ρb by Archie’ s law for sandy materials (Archie, 1942). ρb = C ρw � −mS−n …………… (15) where ρw is the resistivity of the water in the aquifer and the other parameters (porosity �, degree of saturation S, assumed to be 1 below the water table, and constants C, m and n with typical values of 0.5 ≤ C ≤ 2.5, 1.3 ≤ m ≤ 2.5, and n~2) F = C � -mS-n ……………… (16) Where F is the formation factor related to porosity in the following empirical equation (Winsauer et al., 1952). 54 F �a / �m …………………........... (17) Where a, is a constant taken as 0.62 for soft deposits (Repsold, 1989). � is the porosity (0.34 for Sombreiro-Warri Deltaic Plain deposits) and m, a cementation index taken as 2.15 for soft deposits (Repsold, 1989). Combining equations (15) and (16) and dividing both sides by ρw we have F ………………………... (18) From equations (18) the resistivity of the pore water or formation water ρw is obtained by dividing the bulk or formation resistivity ρb by the formation factor. The resistivity of the formation or pore water ρw can then be used for estimating the total dissolved solids (TDS) in ppm of the formation water using the expression below: TDS = Kc x EC …………… (19) (Lloyd and Heathcote, 1985) Kc is the correlation factor which varies between 0.55 and 0.8. The values obtained from the laboratory analysis of TDS values was correlated with the one inferred from resistivity method. Based on this correlation, 0.64 empirical factor was used. TDS = 0.64 x EC …………….. (20) Where, EC is electrical conductivity of the formation in micromhos/centimeter (µS/cm). Equation 20 can also be written as 55 TDS = 0.64 X 10,000 ………………. (21) ρw 4.7 Groundwater Head Contour Maps The dynamic water level (DWL) in each of the dug wells were measured with an electronic water level indicator. An Ertec model GPS instrument was used to determine well head coordinates. Averaged elevation readings from three GPS instruments at each site were used with the sparsely distributed benchmarks established for oil exploration activities were used to approximate the elevation at each well location. The head at each well was found by subtracting the dynamic water level from elevation. Groundwater head contour map was produced using SURFER 2002, terrain and surface modeling software 56 CHAPTER FIVE 5.0 RESULTS 5.1 Geoelectric Parameters and Delineation of Geologic Sequence in the Study Area Qualitative interpretation results of the computer generated model data curves show three to five geologic layers. Some selected examples of the 140 modelled curves are shown in Figures 17, 18, 19, 20, 21, 22, 23, and 24 while Tables 3, 4, 5, 6, 7, 8 and 9 shows the interpreted layer resisitivities, layer thicknesses and inferred lithologies. Figure 17: Computer generated model data curve for Ughoton VES 6 57 Figure 18: Computer generated model data curves for Ughoton VES 12 and 18 58 Figure 19: Computer generated model data curves for Ekakpamre VES 24 and 25 59 Figure 20: Computer generated model data curves for Uvwiamuge VES 40 and 43 60 Figure 21: Computer generated model data curves for Egbeleku VES 47 and 50 61 Figure 22: Computer generated model data curves for Otor-Jeremi VES 76 and 79 62 Figure 23: Computer generated model data curves for Burutu VES 84 and 98 63 Figure 24: Computer generated model data curves for Sapele VES 105 and 140 64 Table 3: Geoelectric parameters, lithology, longitudinal conductance and the protective capacity at Ughoton. VES 1 2 3 4 5 6 Layer Resistivity Thickness Lithology 1 2 3 4 5 6 1 2 3 4 5 1 2 3 4 5 6 1 2 3 4 5 6 230.0 536.7 930.0 433.3 853.3 375.0 602 933.3 700.2 503.3 350.1 421.3 280.1 1698.5 323.1 590.0 321.4 32.1 181.3 31.2 111.4 361.0 93.3 1.2 2.3 16.4 15.0 14.3 1 2 3 4 5 1 2 3 4 5 50.0 61.1 151.7 700.0 469.8 502.0 277.5 92.0 496.8 750.0 1.0 2.8 10.3 8.5 Top soil Sand Sand Sand Sand Sand Top soil Sand Sand Sand Sand Top soil Sand Sand Sand Sand Sand Top soil Clayey sand Clay Clayey sand Sand Sand 1.0 1.9 5.6 18.0 1.1 3.3 3.1 12.0 30.0 1.0 0.2 1.6 6.3 15.4 Top soil Clay Sand Sand Sand Top soil Sand Clay Sand Sand 1.5 11.0 20.2 15.1 65 Longitudinal conductance 0.0052 0.0042 0.0346 0.005 Protective capacity 0.03 Poor 0.002 0.002 0.008 0.036 0.05 Poor 0.0026 0.0118 0.0018 0.0371 0.051 0.05 Poor 0.0312 0.0011 0.0513 0.0566 0.043 0.14 Weak 0.02 0.0458 0.0679 0.012 0.14 Weak 0.003 0.0396 0.2196 0.0304 0.26 Moderate VES 7 8 9 10 11 12 13 Layer 1 2 3 4 5 6 1 2 3 4 5 1 2 3 4 5 6 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 Resistivity Thickness Lithology 189.4 1693.0 525.3 405.7 370.5 183.5 975.5 613.9 418.2 638.1 561.0 178.4 781.7 719.4 967.4 370.1 217.9 466.5 195.0 1402.4 428.1 237.8 762.2 1296.7 212.9 461.5 364.1 999.8 1533.7 529.6 632 455.4 681.7 1066.6 275.6 350.9 255.6 1.4 2.9 4.1 17.6 37.1 Top soil Sand Sand Sand Sand Sand Top soil Sand Sand Sand Sand Top soil Sand Sand Sand Sand Sand Top soil Clayey sand Sand Sand Sand Top soil Sand Sand Sand Sand Top soil Sand Sand Sand Sand Top soil Sand Sand Sand Sand 1.2 3.8 5.7 29.1 1.1 0.6 3.7 11.3 32.7 1.3 3.8 5.8 37.1 1.1 0.9 6.0 18.7 1.2 1.5 9.2 19.2 1.0 1.3 8.1 51.4 66 Longitudinal conductance 0.0074 0.0015 0.008 0.0434 0.100 Protective capacity 0.06 Poor 0.0012 0.0062 0.0014 0.0456 0.01 Poor 0.0062 0.0008 0.0051 0.0117 0.0088 0.02 Poor 0.0028 0.0195 0.0041 0.087 0.03 Poor 0.0014 0.0007 0.0282 0.0405 0.03 Poor 0.0012 0.001 0.0174 0.0303 0.02 Poor 0.0015 0.0012 0.0294 0.1464 0.03 Poor Table 3 continued VES Layer Resistivity Thickness Lithology 14 15 16 17 18 19 20 1 2 3 4 5 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 5 6 1 2 3 4 804.0 1518.6 444.3 461.3 702.7 146.3 52.0 323.7 976.3 28.1 77.9 210.6 536.2 1022.4 1072.6 802.0 543.1 554.3 1355.1 714.4 542.0 762.4 1536.7 842.0 658.0 196.7 198.9 986.8 1138.5 557.0 487.2 1.0 2.6 12.0 46.0 Top soil Sand Sand Sand Sand Top soil Clay Sand Sand Top soil Clay Sand Sand Top soil Sand Sand Sand Top soil Sand Sand Sand Top soil Sand Sand Sand Sand Sand Top soil Sand Sand Sand 0.9 3.5 43.3 1.9 4.0 32.0 2.3 10.4 13.4 1.2 5.0 60.5 1.1 2.0 8.8 24.2 31.6 1.8 7.3 10.5 67 Longitudinal conductance 0.0012 0.0017 0.027 0.099 Protective capacity 0.03 Poor 0.0062 0.0673 0.1338 0.21 Moderate 0.0676 0.0513 0.1519 0.27 Moderate 0.0022 0.0097 0.0167 0.03 Poor 0.0022 0.0037 0.0847 0.09 Poor 0.0014 0.0013 0.006 0.0368 0.1607 0.05 Poor 0.0018 0.0064 0.019 0.03 Poor Table 4: Geoelectric parameters, lithology, longitudinal conductance and the protective capacity at Ekakpamre VES 21 22 23 24 25 26 27 28 Layer 1 2 3 4 5 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 5 1 2 3 4 5 1 2 3 4 Resistivity Thickness Lithology 713.0 806.0 491.0 370.0 895.0 138.0 184.0 498.0 521.0 103.0 392.0 905.0 390.0 450.0 361.0 488.0 389.0 474.8 742.5 834.0 633.9 401.0 1001.0 511.0 483.0 610.0 164.0 150.0 510.0 605.0 153.0 43.0 119.0 341.0 402.0 1.1 2.1 14.3 20.1 Top soil Sand Sand Sand Sand Top soil Clayey sand Sand Sand Top soil Sand Sand Sand Top soil Sand Sand Sand Top soil Sand Sand Sand Top soil Sand Sand Sand Sand Top soil Clayey sand Sand Sand Clayey Sand Top soil Clayey sand Sand Sand 1.5 5.2 19 0.6 6.1 16.2 0.8 4.0 22.6 0.8 2.1 24.6 0.7 6.2 2.9 19 1.1 3.9 9.1 10.5 0.8 5.0 20.8 68 Longitudinal conductance 0.00154 0.00261 0.0291 0.0543 Protective capacity 0.03 Poor 0.0109 0.0283 0.0382 0.11 Weak 0.00583 0.0156 0.0179 0.04 Poor 0.00178 0.011 0.0463 0.06 Poor 0.00168 0.00283 0.0295 0.03 Poor 0.00175 0.00619 0.00568 0.0393 0.05 Poor 0.0067 0.026 0.0178 0.0174 0.10 Weak 0.0186 0.042 0.061 0.10 Weak Table 4 continued VES Layer Resistivity Thickness Lithology 29 30 1 2 3 4 1 2 3 4 57.0 136.0 598.0 495.0 129.0 168.0 506.0 331.0 1.1 5.2 32.6 Top soil Clayey sand Sand Sand Top soil Clayey sand Sand Sand 0.7 4.2 27.1 69 Longitudinal conductance 0.0193 0.038 0.0545 Protective capacity 0.11 Weak 0.00543 0.025 0.0536 0.0329 0.10 Weak Table 5: Geoelectric parameters, lithology, longitudinal conductance and the protective capacity at Uvwiamuge VES 31 32 33 34 35 36 37 38 Layer 1 2 3 4 1 2 3 4 5 1 2 3 4 5 1 2 3 4 1 2 3 4 5 1 2 3 4 5 1 2 3 4 1 2 3 4 5 Resistivity Thickness Lithology 222.0 338.0 927.0 850.0 1102.0 1051.0 982.0 719.0 175.0 930.0 671.0 532.0 300.0 21.0 121.0 160.0 694.0 395.0 600.0 725.0 550.0 371.0 929.0 660.0 220.0 706.0 378.0 65.0 840.0 151.0 671.0 33.0 918.0 201.0 231.0 465.0 151.0 1.3 4.2 25.8 Top soil Sand Sand Sand Top soil Sand Sand Sand Sand Top soil Sand Sand Sand Clay Top soil Clayey sand Sand Sand Top soil Sand Sand Sand Sand Top Soil Sand Sand Sand Clay Top soil Clayey sand Sand Clay Top soil Sand Sand Sand Clayey sand 1.0 1.9 11 17.5 1.0 1.5 25.6 42.7 1.1 3.0 2.0 1.0 1.8 13 19.1 1.2 3.2 23 75.3 0.5 3.0 11.2 1.5 0.6 17.1 56.9 70 Longitudinal conductance 0.0059 0.0124 0.0278 Protective capacity 0.05 Poor 0.0009 0.0018 0.0112 0.024 0.04 Poor 0.001 0.002 0.0481 0.142 0.05 Poor 0.009 0.0188 0.0029 0.03 Poor 0.0017 0.0025 0.024 0.051 0.03 Poor 0.002 0.015 0.033 0.199 0.05 Poor 0.0006 0.0199 0.0167 0.04 Poor 0.0016 0.003 0.074 0.122 0.07 Poor Table 5 continued VES Layer Resistivity Thickness Lithology 39 40 41 42 43 44 45 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 831.0 155.0 1340.0 530.0 209.0 120.0 90.0 850.0 877.0 248.0 80.0 25.0 420.0 951.0 840.0 211.0 896.0 980.0 724.0 190.0 388.0 580.0 420.0 103.0 59.0 540.0 340.0 338.0 130.0 663.0 522.0 0.8 2.2 15 13 Top soil Clayey sand Sand Sand Sand Top soil Clay Sand Sand Sand Top soil Clay Sand Sand Sand Top soil Sand Sand Sand Top Soil Sand Sand Sand Top soil Clay Sand Sand Top soil Clayey sand Sand Sand 1.2 3.7 12.5 12 0.5 1.3 4.1 20 1.0 3.0 34 1.3 3.0 28.3 0.7 3.3 35.8 1.3 4.0 35 71 Longitudinal conductance 0.001 0.014 0.011 0.025 Protective capacity 0.03 Poor 0.01 0.04 0.0147 0.0137 0.10 Weak 0.0063 0.052 0.0098 0.021 0.10 Weak 0.0047 0.0033 0.0347 0.01 Poor 0.0068 0.0077 0.0488 0.08 Poor 0.0068 0.0559 0.0663 0.13 Poor 0.0038 0.0308 0.0528 0.10 Weak Table 6: Geoelectric parameters, lithology, longitudinal conductance and the protective capacity at Egbeleku. VES Layer Resistivity Thickness Lithology Longitudinal Protective conductance capacity 46 1 1496.1 4.1 To soil 0.0027 0.70 2 182.8 3.2 Clayey sand 0.0175 Moderate 3 25.7 17.0 Clay 0.660 4 495.9 Sand 47 1 273.7 2.2 To soil 0.008 0.96 2 82.7 5.9 Clay 0.071 Good 3 21.6 19.0 Clay 0.880 4 833.5 Sand 48 1 1340.0 1.2 To soil 0.0009 0.74 2 62.0 2.1 Clay 0.0339 Good 3 47.0 33 Clay 0.70 4 350.0 Sand 49 1 211.0 2.2 Top soil 0.0104 0.20 2 77.0 1.3 Clay 0.0169 Moderate 3 203.0 36 Sand 0.1773 4 381.0 Sand 50 1 858.9 4.3 Top soil 0.005 0.28 2 413.3 4.3 Sand 0.0104 Moderate 3 156.0 41.6 Clayey sand 0.2666 4 589.0 Sand 51 1 407.9 1.2 Top soil 0.0029 0.13 2 115.8 7.2 Sandy clay 0.0622 Weak 3 303.0 19.7 Sand 0.0650 4 1004.4 Sand 52 1 991.0 2.5 Top soil 0.0025 0.32 2 309.0 3.6 Sand 0.0117 Moderate 3 125.0 38.2 Sandy clay 0.3056 4 880.0 Sand 53 1 1125.0 2.7 Top soil 0.0024 0.75 2 350.0 1.5 Sand 0.0043 Good 3 55.0 40.8 Clay 0.7418 4 580.0 Sand 54 1 625.0 1.8 Top soil 0.0029 1.18 2 121.0 3.6 Sandy clay 0.0298 Good 3 36.0 41.3 Clay 1.147 4 320.0 Sand 72 Table 6 continued VES Layer Resistivity 55 56 57 58 59 60 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 1 2 3 4 1 2 3 4 250.0 110.0 57.0 315.0 219.0 667.0 145.0 1200.0 620.0 477.0 129.0 964.0 467.0 134.0 1100.0 261.3 65.2 30.8 438.0 165.0 117.0 16.0 1015.0 Thickness Lithology 1.9 2.0 33.2 Top soil Sandy clay Clay Sand Top soil Sand Clayey sand Sand Top soil Sand Clayey sand Sand Top soil Clayey sand Sand Top soil Clay Clay Sand Top soil Sandy clay Clay Sand 1.9 2.0 44 2.2 1.1 35.2 1.9 30.2 3.3 6.3 30.0 1.4 3.2 11 73 Longitudinal conductance 0.0076 0.0182 0.5825 Protective capacity 0.61 Moderate 0.0087 0.0030 0.3034 0.32 Moderate 0.0014 0.0023 0.2730 0.30 Moderate 0.0041 0.2254 0.23 Moderate 0.0126 0.0966 0.974 1.08 Good 0.0085 0.0274 0.6875 0.72 Good Table 7: Geoelectric parameters, lithology, longitudinal conductance and the protective capacity at Otor-Jeremi VES 61 62 63 64 65 66 67 68 69 Layer 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 Resistivity Thickness Lithology 25.9 32.3 390.2 881.7 67.6 311.1 536.2 700.0 162.0 592.0 615.0 823.0 228.3 419.8 735.5 945.0 108.6 316.0 655.0 987.0 131.2 176.1 560.0 221.0 304.0 673.0 403.0 339.0 146.0 509.0 361.0 542.0 383.9 392.4 272.9 211.2 0.6 1.6 18.1 Top soil Clay Sand Sand Top soil Sand Sand Sand Top soil Sand Sand Sand Top soil Sand Sand Sand Top soil Sand Sand Sand Top soil Sand Sand Sand Top soil Sand Sand Sand Top soil Sand Sand Sand Top soil Sand Sand Sand 1.1 4.6 12.4 0.7 10.2 18.4 0.6 1.9 30.7 1.2 6.7 18.4 1.5 1.8 34.1 1.2 8.4 10.1 1.2 8.4 18.0 0.6 2.8 8.7 74 Longitudinal Protective conductance capacity 0.0232 0.12 0.0495 Weak 0.0464 0.0163 0.0148 0.0231 0.05 Poor 0.0043 0.0172 0.0299 0.05 Poor 0.0026 0.0045 0.0417 0.05 Poor 0.0110 0.0212 0.0281 0.06 Poor 0.0114 0.0102 0.0609 0.08 Poor 0.0039 0.0125 0.0251 0.04 Poor 0.0082 0.0165 0.0499 0.07 Poor 0.0015 0.0031 0.0319 0.04 Poor Table 7 continued VES Layer 70 71 72 73 74 75 76 77 Resistivity Thickness Lithology 1 2 410.0 146.0 0.9 0.4 3 4 5 1 2 3 4 1 2 3 4 5 1 2 3 4 1 2 3 4 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 938.0 789.0 428.0 205.0 648.0 832.0 474.0 390.0 659.0 967.0 625.0 851.0 154.8 416.0 651.2 928.7 477.0 583.0 964.2 481.3 85.7 298.6 331.4 773.5 917.6 29.0 84.3 55.9 358.0 985.0 206.4 672.0 432.9 370.8 204.2 13.6 30.5 Top soil Clayey sand Sand Sand Sand Top soil Sand Sand Sand Top soil Sand Sand Sand Sand Top soil Sand Sand Sand Top soil Sand Sand Sand Top soil Sand Sand Sand Sand Top soil Clay Clay Sand Sand Top soil Sand Sand Sand Sand 1.0 6.8 10.5 1.4 3.8 13.6 24.2 0.8 4.1 16.4 1.3 7.1 32.1 1.3 5.8 10.6 19.4 0.7 1.5 11.6 9. 3.3 9.4 12.3 14.7 75 Longitudinal Protectiv conductance Capacity 0.0022 0.05 0.0027 Poor 0.0145 0.0386 0.0049 0.0105 0.0126 0.03 Poor 0.0036 0.0058 0.0141 0.0387 0.06 Poor 0.0052 0.0099 0.0252 0.04 Poor 0.0027 0.0122 0.0333 0.05 Poor 0.0152 0.0194 0.0320 0.0251 0.09 Poor 0.0241 0.0178 0.2075 0.0271 0.28 Moderate 0.0150 0.014 0.0262 0.06 Poor Table 7 continued VES Layer 78 79 80 Resistivity Thickness Lithology 1 2 3 4 1 2 3 4 1 2 73.6 487 20.1 374 236.0 383.0 597.0 865.0 177.0 109.0 1.1 12.8 1.7 3 4 217.0 554.0 15.2 Top soil Sand Clay Sand Top soil Sand Sand Sand Top soil Sandy clay Sand Sand 1.5 7.3 13.2 0.9 4.8 76 Longitudinal Protective conductance capacity 0.0149 0.13 0.0263 Weak 0.0846 0.0064 0.0191 0.0221 0.05 Poor 0.0051 0.0440 0.12 Weak 0.0700 Table 8: Geoelectric parameters and their interpretation at Burutu VES 81 82 83 84 Layer Resistivity Thickness Depth 1 2 3 4 5 6 1 2 3 150.0 3.8 7.5 20.7 56.3 90.6 39.2 5.3 11.4 1.3 5.3 25.6 15.8 14.8 1.3 7.1 32.7 48.5 63.3 2.4 4.2 8.7 2.4 6.6 15.3 4 5 6 1 2 3 4 1 2 29.1 34.9 226.0 181.4 26.1 45.0 57.7 210.2 6.1 15.1 28.5 30.4 58.9 1.1 5.4 10.4 1.1 6.5 16.9 1.5 9.2 1.5 10.7 3 4 5 17.9 56.4 394.9 27.0 14.0 37.7 51.7 Lithology/Interpretation Top soil Porous sand, sat. clay/SBW Sat. sand, sandy clay/BW Clayey sand/PQFW Sand, minor clay/IFW Sand/GQFW Top soil Sat. sand, sandy clay/BW Sat. sand, sandy clay/VPQFW Clayey sand/PQFW Sand, minor clay/IFW Sand/VGQFW Top soil Clayey sand/PQFW Sand, minor clay/IFW Sand, minor clay/IFW Top soil Sat. sand, sandy clay/VPQFW Clayey sand/PQFW Sand, minor clay/IFW Sand/VGQFW 77 Longitudinal Protective conductance capacity 0.0087 1.40 1.3947 Good 0.0612 0.7925 0.7632 1.62 Good 0.0061 0.2068 0.21 Moderate 0.0071 1.5082 3.02 Good 1.5084 Table 8 Continued VES Layer 85 86 87 88 Resistivity Thickness 1 2 3 4 1 2 277.4 17.1 31.6 83.3 332.0 14.4 3 4 1 2 Depth 1.8 8.1 10.2 1.8 9.9 20.1 1.4 8.6 1.4 10.0 58.2 69.1 362.1 59.7 13.8 23.8 1.9 5.5 1.9 7.4 3 4 1 2 3 1228 616.5 255.0 4.6 12.9 2.8 10.2 1.1 10.4 16.0 1.1 11.5 27.5 4 5 6 28.6 43.4 343.6 5.0 16.5 32.5 49.0 Lithology/Interpretation Top soil Clayey sand/PQFW Sand, minor clay/IFW Sand/GQFW Top soil Sat. sand, sandy clay/VPQFW Sand, minor clay/IFW Sand, minor clay/IFW Top soil Sand, minor clay/IFW Sand/VGQFW Sand/VGQFW Top soil Sat. sand, sandy clay/BW Sat. sand, sandy clay/VPQFW Clayey sand/PQFW Sand, minor clay/IFW Sand/VGQFW 78 Longitudinal Protective conductance capacity 0.0065 0.46 0.4525 Moderate 0.0042 0.5972 0.60 Moderate 0.0052 0.0912 0.0228 0.119 Weak 0.0043 2.3636 1.2403 3.78 Good 0.1748 Table 8 Continued VES Layer 89 90 91 92 93 Resistivity Thickness Depth 1 2 258.4 7.8 1.3 4.9 1.3 6.2 3 4 5 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 21.6 34.3 68.1 249.2 16.8 49.6 43.5 312.0 20.2 64.8 57.5 616.0 56.8 40.1 54.7 441.0 49.8 47.9 38.1 3.8 5.6 10.0 15.6 1.8 4.1 6.3 1.8 5.9 12.2 1.6 3.1 9.2 1.6 4.7 13.9 1.3 10.2 13.1 1.3 11.5 24.6 1.1 12.4 6.2 1.1 13.5 19.7 Lithology/Interpretation Top soil Sat. sand, sandy clay/VPQFW Clayey sand/PQFW Sand, minor clay/IFW Sand/VGQFW Top soil Clayey sand/PQFW Sand, minor clay/IFW Sand, minor clay/IFW Top soil Clayey sand/PQFW Sand, minor clay/IFW Sand, minor clay/IFW Top soil Sand, minor clay/IFW Sand, minor clay/IFW Sand, minor clay/IFW Top soil Sand, minor clay/IFW Sand, minor clay/IFW Sand, minor clay/IFW 79 Longitudinal Protective conductance capacity 0.0050 0.81 0.6282 Good 0.1759 0.0072 0.2440 0.1270 0.38 Moderate 0.0051 0.1535 0.16 Moderate 0.0021 0.1796 0.18 Weak 0.0025 0.2450 0.25 Moderate Table 8 Continued VES Layer 94 95 96 97 98 1 2 3 4 1 2 3 4 1 2 3 4 5 1 2 3 4 5 1 2 3 4 Resistivity Thickness 280.0 35.7 59.1 35.0 1152.0 79.4 95.0 872.0 231.2 18.8 64.1 32.1 53.7 208.0 25.8 33.9 62.0 895.3 214.0 129.7 382.0 911.0 Depth 1.7 8.1 11.6 1,7 9.8 21.4 1.6 13.3 15.2 1.6 14.9 30.1 1.1 5.7 11.6 18.7 1.1 6.8 18.4 37.1 1.5 3.8 10.4 20.1 1.5 5.3 25.7 35.8 1.6 3.5 15.0 1.6 5.1 20.1 Lithology/Interpretation Top soil Sand, minor clay/IFW Sand, minor clay/IFW Sand, minor clay/IFW Top soil Sand/GQFW Sand/GQFW Sand/VGQFW Top soil Clayey sand/PQFW Sand, minor clay/IFW Sand, minor clay/IFW Sand/VGQFW Top soil Clayey sand/PQFW Clayey sand/PQFW Sand, minor clay/IFW Sand/VGQFW Top soil Sand/VGQFW Sand/VGQFW Sand/VGQFW 80 Longitudinal Protective conductance capacity 0.0061 0.23 0.2268 Moderate 0.00139 0.2239 0.23 Moderate 0.0048 0.3032 0.1810 0.49 Moderate 0.0072 0.1473 0.15 Weak 0.0075 0.0270 0.03 Poor Table 8 Continued VES Layer 99 100 1 2 3 4 1 2 3 4 Resistivity Thickness 135.0 31.2 14.8 48.3 87.1 35.7 55.1 1072.9 Depth 1.3 6.1 17.2 1.3 7.4 24.6 1.4 5.5 19.4 1.4 6.9 26.3 Lithology/Interpretation Top soil Clayey sand/PQFW Clayey sand/PQFW Clayey sand/PQFW Top soil Sand, minor clay/IFW Sand, minor clay/IFW Sand/VGQFW Key Sat. Clay Saturated Clay Sat. Sand Saturated Sand SBW Salty Brackish Water BW Brackish Water VPQFW Very Poor Quality Fresh Water PQFW Poor Quality Fresh Water GQFW Good Quality Fresh Water VGQFW Very Good Quality Fresh Water 81 Longitudinal Protective conductance capacity 0.0096 0.21 0.1955 Moderate 0.0161 0.1541 0.17 Weak Table 9: Geoelectric parameters, lithology, longitudinal conductance and the protective capacity at Sapele. VES Layer Resistivity Thickness Lithology Longitudinal Protective conductance capacity 101 1 242.0 1.1 Top soil 0.0045 0.10 2 684.0 5.2 Laterite 0.0076 Weak 3 272.0 12.9 Sand 0.0474 4 948.0 Sand 102 1 331.0 0.9 Top soil 0.0027 0.04 2 714.0 10.5 Laterite 0.0147 Poor 3 192.0 4.6 Sand 0.0240 4 652.0 Sand 103 1 1129.0 1.4 Top soil 0.00125 0.10 2 636.0 4.3 Sand 0.00676 Weak 3 390.0 31.4 Sand 0.08051 4 236.0 Sand 104 1 550.0 1.2 Top soil 0.0022 0.10 2 935.0 3.2 Laterite 0.0034 Weak 3 187.0 5.3 Clayey 0.0283 sand 4 411.0 19.2 Sand 0.0467 5 987.0 Sand 105 1 108.2 1.3 Top soil 0.0120 0.13 2 416.9 7.3 Sand 0.0175 Weak 3 207.5 9.0 Sand 0.0434 4 398.1 10.2 Sand 5 878.4 Sand 106 1 142.4 2.1 Top soil 0.0147 0.06 2 1082.8 12.5 Sand 0.0115 Poor 3 814.5 25.1 Sand 0.0298 4 361.4 Sand 107 1 140.1 2.5 Top soil 0.0178 0.10 2 135.6 10.8 Clayey 0.0796 Weak sand 3 458.6 35.1 Sand 4 539.2 Sand 108 1 300.0 1.0 Top soil 0.0033 0.09 2 45.0 2.7 Clay 0.06 Poor 3 601.4 14.2 Sand 0.0236 4 375.0 Sand 82 Table 9 continued VES Layer Resistivity Thickness Lithology Longitudinal conductance 109 1 138.0 2.1 Top soil 0.0152 2 715.8 4.2 Laterite 0.0059 3 598.1 12.8 Sand 0.00214 4 283.0 56.7 Sand 5 958.6 Sand 110 1 166.2 1.4 Top soil 0.0084 2 113.6 2.3 Clayey 0.0202 sand 3 962.6 33.7 Sand 0.035 4 259.6 Sand 111 1 233.0 1.1 Top soil 0.0047 2 155.0 3.8 Clayey 0.0245 sand 3 988.0 37.5 Sand 4 1065.0 Sand 112 1 112.0 1.2 Top soil 0.0107 2 575.0 7.3 Sand 0.0127 3 381.0 9.1 Sand 0.024 4 651.0 10.2 Sand 5 907.0 Sand 113 1 51.0 1.2 Top soil 0.024 2 82.0 6.5 Clay 0.079 3 600.0 20.5 Sand 0.030 4 515.0 6.5 Sand 5 967.0 Sand 114 1 1700.0 1.0 Top soil 0.0006 2 1390.9 2.5 Sand 0.0018 3 833.3 4.0 Laterite 0.0048 4 516.7 17.0 Sand 5 1671.4 Sand 115 1 554.8 1.1 Top sand 0.002 2 321.8 7.3 Sand 0.0227 3 410.5 10.6 Sand 0.0026 4 205.4 Sand 116 1 185.6 1.7 Top soil 0.009 2 106.7 3.4 Sandy 0.032 clay 3 468.0 13.7 Sand 0.029 4 652.0 Sand 83 Protective capacity 0.04 Poor 0.06 Poor 0.03 Poor 0.05 Poor 0.13 Weak 0.01 Poor 0.05 Poor 0.07 Poor Table 9 continued VES Layer Resistivity Thickness Lithology Longitudinal conductance 117 1 179.4 1.5 Top soil 0.0084 2 265.1 4.7 Sand 0.0177 3 431.3 10.1 Sand 0.023 4 834.8 Sand 118 1 473.9 1.0 Top soil 0.002 2 575.6 4.6 Sand 0.008 3 669.0 28.4 Sand 0.043 4 472.0 Sand 119 1 986.8 1.8 Top soil 0.0009 2 835.0 7.3 Laterite 0.0087 3 557.0 10.5 Sand 0.0189 4 487.2 Sand 120 1 517.0 1.5 Top soil 0.0029 2 441.0 6.7 Sand 0.0152 3 920.0 20.1 Sand 0.0220 4 652.0 Sand 121 1 600.0 1.2 Top soil 0.002 2 315.0 15.3 Sand 0.0486 3 775.0 12.3 Sand 0.0159 4 1251.0 Sand 122 1 148.9 2.3 Top soil 0.0154 2 563.0 19.2 Sand 0.034 3 495.0 14.2 Sand 4 599.8 Sand 123 1 82.5 1.1 Top soil 0.013 2 363.0 1.5 Sand 0.004 3 525.0 12.1 Sand 0.023 4 819.0 Sand 124 1 63.0 1.2 Top soil 0.019 2 311.0 6.4 Sand 0.021 3 521.0 30.1 Sand 0.058 4 968.0 Sand 125 1 326.0 1.1 Top soil 0.0034 2 497.0 7.2 Sand 0.0145 3 619.0 6.9 Sand 0.0111 4 908.0 Sand 84 Protective capacity 0.05 Poor 0.05 Poor 0.03 Poor 0.04 Poor 0.07 Poor 0.05 Poor 0.04 Poor 0.09 Poor 0.03 Poor Table 9 continued VES Layer Resistivity Thickness Lithology Longitudinal conductance 126 1 142.0 1.1 Top soil 0.008 2 320.0 5.8 Sand 0.018 3 462.0 50.1 Sand 0.108 4 862.0 Sand 127 1 122.0 1.5 Top soil 0.0123 2 351.0 5.8 Sand 0.0165 3 286.0 37 Sand 0.1294 4 912.0 Sand 128 1 152.0 1.3 Top soil 0.009 2 208.0 8.1 Sand 0.039 3 409.0 42 Sand 0.103 4 241.0 Sand 129 1 300.0 1.2 Top soil 0.004 2 722.0 8.2 Laterite 0.0114 3 513.0 34 Sand 0.066 4 906.0 Sand 130 1 64.0 1.2 Top soil 0.0188 2 831.0 10.3 Laterite 0.0124 3 355.0 38 Sand 0.107 4 420.0 Sand 131 1 93.0 1.8 Top soil 0.0194 2 311.0 5.3 Sand 0.017 3 544.0 40 Sand 0.074 4 408.0 Sand 132 1 191.0 1.1 Top soil 0.0058 2 46.0 3.7 Clay 0.08 3 788.0 20.1 Sand 0.0255 4 326.0 24 Sand 5 515.0 Sand 133 1 215.0 1.0 Top soil 0.0047 2 785.0 5.2 Laterite 0.0066 3 553.0 21.8 Sand 0.039 4 982.0 19.6 Sand 5 847.0 Sand 134 1 243.0 1.8 Top soil 0.0074 2 657.0 5.2 Laterite 0.0079 3 425.0 28 Sand 0.066 4 809.0 32 Sand 5 411 Sand 85 Protective capacity 0.13 Weak 0.16 Weak 0.151 Weak 0.10 Weak 0.14 Weak 0.11 Weak 0.11 Weak 0.05 Poor 0.10 Weak Table 9 continued VES Layer Resistivity Thickness Lithology Longitudinal conductance 135 1 157.0 1.5 Top soil 0.0096 2 693.0 4.3 Laterite 0.0062 3 722.0 47 Sand 0.0650 4 421.0 Sand 136 1 529.0 0.8 Top soil 0.0015 2 1077.0 5.2 Sand 0.0048 3 444.0 38.8 Sand 0.0874 4 663.0 Sand 137 1 46.5 1.0 Top soil 0.0215 2 697.5 4.6 Laterite 0.0066 3 777.0 28.4 Sand 0.0365 4 910.0 Sand 138 1 508.0 1.1 Top soil 0.0022 2 564.5 4.6 Sand 0.0081 3 957.7 26.1 Sand 0.0273 4 735..0 Sand 139 1 165.5 1.2 Top soil 0.0073 2 111.0 3.1 Sandy 0.0279 clay 3 261.0 30.6 Sand 0.1172 4 936.0 Sand 140 1 44.5.0 1.3 Top soil 0.029 2 227.8 7.9 Sand 0.035 3 460.8 41.7 Sand 0.091 4 270.0 Sand 86 Protective capacity 0.10 Weak 0.09 Poor 0.10 Weak 0.04 Poor 0.15 Weak 0.16 Weak 5.2 Identification of Aquifer Units, Depth and Lateral Extent Geologic sections were produced from the interpreted layer resistivity values, layer thickness and inferred lithology of the study area. Geologic sequences at Ughoton The qualitative interpretation of the sounding curves at Ughoton shows the following curve types: AA, KQ, HA, HK, QH, AAA, HAK, HKH, KHK, QHA, QHK, AAKH, AKHK, AKQQ, HKHK, KHAK and KQQH. The geologic section for Ughoton (Figures 25 and 26) indicates four geologic layers made up of top soil; clay, sandy clay, clayey sand and sand. The resistivity of the first layer ranges from 50.1 – 1022.4 Ωm with thickness varying between 0.9 – 5.5 m. The second layer is composed of clay, sandy clay, clayey sand (at VES 4, 5, 10, 15 and 16) having resistivity varying between 31.2 – 195.0 Ωm with thickness between 1.6 – 6.3 m and sand at the remaining locations with resistivity varying between 277.5 – 1693.0 Ωm having a thickness of 1.5 – 18.7 m. The third layer comprising of sand has resistivity values ranging from 210.6 – 1402.4 Ωm and thickness of 5.8 – 72.2 m corresponds to the aquifer, depth to this aquifer is between 8.0 – 20.0 m. This lgayer is however clayey at VES 6 with resistivity value of 92.0 Ωm and thickness of 20.2 m. The resistivity of the fourth layer ranges from 93.3 – 976.3 Ωm and diagnostic of clay (at VES 4) and sand at other VES locations. The exact thickness of this layer could not be determined as the electrode current terminated within this layer. However, inference from VES 10 87 and 19 shows the possibility that the sand horizon exceeds 30.0 m in thickness. The sand of the fourth layer also forms part of the aquifer. Geologic sequences at Ekakpamre The qualitative interpretation of the sounding curves at Ekakpamre shows the following curve types: AA, HK, KHK, AAK, KQH and HKH. Four distinct geologic layers which include the top soil; sandy clay, clayey sand and sand were delineated at Ekakpamre (Figures 27 and 28). The first layer which represents the top soil has resistivity values ranging from 43.0 – 713.0Ωm and thickness varying between 0.6 – 1.5m. The second layer is sandy clay, clayey sand (at VES 22, 27, 28, 29 and 30) and sand (at VES 21, 23, 24, 25 and 26) with resistivity values ranging from 119.0 – 1001.0 Ωm and thickness varying between 2.1 – 16.4m. The third layer comprising of sand has resistivity values ranging from 341.0 – 905.0 Ωm and with a thickness of 16.2 – 32.6m corresponds to the aquifer, the depth to this aquifer is between 7.0 – 18.0 m A fourth layer made up of sand with resistivity values ranging from 153.0 – 895.0 Ωm. The exact thickness of this layer could not be determined as the electrode current terminated within this layer. 88 Figure 25: Geologic section for Ughoton along SW-NE traverse 89 Figure 26: Geologic section for Ughoton along NW-SE traverse 90 Figure 27: Geologic section for Ekakpamre along NW-SE traverse 91 Figure 28: Geologic section for Ekakpamre along S-NE traverse 92 Geologic sequences at Uvwiamuge The qualitative interpretation of the sounding curves at Uvwiamuge shows the following curve types: AK, HK, KQH, HKQ and HAA. The geologic section for Uvwiamuge (Figures 29 and 30) shows three to four geologic layers made up of top soil; clay, clayey sand and sand. The resistivity of the first layer, range from 25.0 – 1102.0 Ωm and thickness varying between 0.5 – 2.9 m. The second layer is made up of sand at VES 31, 32, 33, 35, 36, 38, 39, 42 and 43 has resistivity values ranging between 160.0 – 982.0 Ωm and a thickness of 3.0 – 27.4 m. This second layer with resistivity values of 25.0 – 90.0 Ωm and a thickness of 1.3 – 3.7 m is clay at VES 40, 41 and 44; clayey sand at VES 34, 37 and 45 having resistivity values of 130.0 – 160.0 Ωm and a thickness of 3.0 – 4.0 m and sand at other locations. A third layer comprising of sand having resistivity values ranging from 300.0 – 1340.0Ωm and thickness of 2.0 – 75.3 m constitutes the aquifer, the depth to the aquifer is between 6.0 – 19.0 m. The fourth layer which is mostly sandy (at VES 31, 34, 35, 39, 40, 41, 42, 43, 44 and 45) has resistivity values ranging between 151.0 – 929.0 Ωm and diagnostic of clay with resistivity values of 21.0 – 65.0 Ωm (at VES 33, 36 and 37). The thickness of this horizon could not be determined as the electrode current terminated within this horizon. However, an inference from VES 39 indicates that the sand exceeds 13.0 m in thickness and also constitutes the aquifer. 93 Figure 29: Geologic section for Uvwiamuge along N-W traverse 94 Figure 30: Geologic section for Uvwiamuge along NW-NE traverse 95 Geologic sequences at Egbeleku The qualitative interpretation of the sounding curves shows that the following 2 curve types KQ and QH exist at Egbeleku. The geologic section (Figures 31 and 32) shows four distinct geologic layers made up of top soil, sandy clay/clayey sand, clay and sand. The resistivity of the first layer is variable and it varies between 211.0 – 858.9 Ωm while its thickness ranges from 1.2 – 4.3 m. This layer is underlain by a second layer made up of clay/clayey sand and having resistivity value ranging between 62.0 – 182.0 Ωm. Its thickness varies from 1.3 – 7.2 m. However, at VES 50, 52, 53, 56 and 59, this layer is sandy with resistivity values ranging from 309.0 – 667.0 Ωm with a thickness of 1.1 – 4.3 m. The third layer is constituted by clays has resistivity value ranging from 16.0 – 82.7 Ωm, while its thickness varies from 11.0 – 41.6 m. The clays are found almost everywhere except at VES 52 and 57 where the lithology is sandy clay of between 35.2 – 38.2 m thick, clayey sand of about 44.0 m thick at VES 56 and sand at VES 49 and 51 with thickness between 19.7 – 36.0 m. The resistivity of the sandy clay and clayey sand lenses ranges from 125.0 – 145.0 Ωm. The third layer is underlain by a fourth layer, this layer with resistivity values varying between 315.0 – 1200.0 Ωm comprising of sand constitutes the aquiferous unit in this area. The thickness of this layer could not be determined as the electrode current terminated within this layer. The third layer (at VES 49 and 51) and fourth layer constitute the aquifer in Egbeleku; the depth to the aquifer ranges between 7.0 – 47.0 m. 96 Geologic sequences at Otor-Jeremi The qualitative interpretation of the sounding curves at Otor-Jeremi shows that four geologic layers (Figures 33 and 34) made up of top soil, clay/clayey sand and sand exist. The resistivity of the first layer which is the top soil varies between 25.9 – 477.0 Ωm while its thickness ranges from 0.6 – 3.1 m. This layer is underlain by a second layer made up of clay and sand and having resistivity value ranging between 62.0 – 182.0 Ωm, its thickness varies from 0.4 – 12.8 m. The third layer with resistivity values ranging from 20.1 – 361.0 Ωm with thickness varing from 8.7 – 34.1 m is mostly sandy except VES 78 where the lithology of this layer is clayey sand. The third layer is underlain by a fourth layer comprising of sand having resistivity values ranging from 211.2 – 987.1 Ωm. The thickness of this layer could not be determined as the electrode current terminated within this layer, but inference from VES 80 and 74 indicates that this layer is between 9.7 m and over 30.5 m thick. The third and fourth layer constitutes the aquiferous unit in this area; the depth to the aquifer is between 6.0 – 13.0 m. 97 Figure 31: Geoelectric section for Egbeleku along NW-N traverse 98 Figure 32: Geoelctric section for Egbeleku along N-SE traverse 99 100 101 Geologic sequences at Burutu The qualitative interpretations of the resistivity sounding curves at Burutu show the following curve types: QH, HK, HAA and HAAA. Zohdy et. al., 1993 presented a useful account of resistivity varaiation as a function of salinity and water quality for Oxnard Plain, California. A modified form of their interpretation is presented in Table 10 The geologic sections (Figures 35 and 36) shows four geologic layers made up of the following lithologies namely: top soil, saturated clay/sandy clay/sand, clayey sand and sand. The first layer has resistivity values of 39.2 –1152.0 Ωm with thickness of 1.1 – 2.4 m, this represent the top soil. The second layer has resistivity values varying from 3.8 – 59.7 Ωm and thickness of 3.1 – 36.2 m. This is interpreted as saturated clay, sandy clay and porous sand formation, the water in this layer is saturated with 4.8 % salty brackish water; 23.8 % brackish water; 4.7 % very poor quality fresh water; 38.1 % poor quality fresh water; 23.8 % intermediate fresh water and 4.8 % good quality fresh water. The third layer has resistivity values of about 14.8 – 122.8 m with thickness between 2.8 – 43.6 m, this represent clayey sand and sand bearing 20.8 % poor quality fresh; 66.6 % intermediate fresh water; 4.3 % good quality fresh water and 8.3 % very good quality fresh water. The fourth layer is sand bearing 5.0 % poor quality fresh water, 45.0 % intermediate fresh water; 15.0 % good quality fresh water, 35.0 % very good quality fresh water and has resistivity values of 35.0 – 1072.9 Ωm. The thickness of this layer could not be determined as the electrode current terminated within this layer. 102 Table 10: Resistivity of water and sediments at Burutu Resistivity (Ωm) Inferred sediments Interpretation 2.0 – 4.5 Porous sand/Saturated clay Salty brackish water 4.5 – 10 Saturated sand/sandy clay Brackish water 10 – 15.0 Saturated sand/Sandy clay Very poor quality freshwater 15.0 – 30.0 Clayey sand Poor quality freshwater 30.0 – 70.0 Sand, minor clay Intermediate freshwater 70.0 - 100.0 Sand, no clay Good quality freshwater Over 100.0 Coarse sand Very good quality freshwater (Modified after Zohdy et. al., 1993) 103 104 105 Geologic sequences at Sapele The qualitative interpretation of the sounding curves of Sapele shows the following curve types: AA, KH, AKH, KHK, KHA, HAK, KQ, QH, and AKQ The geologic section (Figures 37 and 38) indicates four to five layers which include the topsoil; clay, sandy clay, clayey sand, laterite and sand. The top soil has resistivity values ranging from 43 – 1129.0 Ωm and thickness varying from 0.8 – 4.4 m. The second layer is composed of clay at VES 108, 113 and 132; sandy clay at VES 139; clayey sand at VES 107, 110, 111 and 116; laterite at VES 101, 102, 109, 114, 119, 129, 130, 133, 134, 135 and 137. This layer is sand at other VES locations, the resistivity values of this layer ranges from 45 – 1390.9 Ωm and thickness of between 1.5 – 19.2 m. A third layer having resistivity values ranging from 192.6 – 988.0 Ωm with thickness of between 9.0 – 50.1 m and an underlying fourth layer with resistivity values ranging between 205.5 – 982.0 Ωm made up of medium to coarse and gravelly sand constitutes the aquiferous unit. The depth to this aquifer is between 6.0 – 25.0 m, the exact thickness of the fourth layer could not be determined as the electrode current terminated within this layer but however except at VES 109 and 113 where the thickness was 56.7 m and 6.5 m respectively. Inference from lithologic log shows the possibility of thickness of more than 80 m. A fifth layer which also forms part of the aquifer with resistivity values ranging between 411.0 – 958.6 Ωm was also delineated. 106 Figure 37: Geoelectric section for Sapele along NNE-SE traverse 107 Figure 38: Geoelectric section for Sapele along W-E traverse 108 5.3 Determination of Areas Prone to Contamination 5.3.1 Isoresistivity Maps To determine, the resistivity variations of the subsurface at various depths, isoresistivity maps were generated. The colours indicate the various lithologies and their resistivity range. The blue colour having resistivity values ranging from 1 – 100 Ωm indicates areas with clay lithology, the yellow colour having resistivity values ranging from 106.7 – 195.0 Ωm indicates area with sandy clay and clayey sand lithology, the green colour having resistivity values ranging from 657.0 – 835.0 Ωm indicates area underlain by laterites while the red colour having resistivity values ranging from 203.0 – 1698.0 Ωm represent areas with sandy lithology. Isoresistivity Map of Ughoton The Isoresistivity map generated for Ughoton at 5 m is shown in Figure 39. The area with blue colour (VES 15 and 16) representing about 10.0 % of Ughoton is underlain by clay; the area with yellow colour (VES 4, 5 and 10) representing 15.0 % of Ughoton is underlain by sand clay and clayey sand while the remaining 75.0 % (red colour) is underlain by sand. 109 VES 3 VES 8 VES 1 VES 2 5.615 VES 7 VES 20 VES 15 VES 16 VES 4 5.61 VES 17 BH VES 6 VES 5 VES 12 VES 9 VES 11 VES 18 5.605 VES 19 VES 13 VES 14 5.655 5.66 5.665 5.67 5.675 VES 10 5.68 LEGEND CLAY 1 - 100 ohm-m SANDY CLAY/CLAYEY SAND 111.4 - 195 ohm-m Figure 39: Isoresistivity map of Ughoton at 5 m depth 110 SAND 210 - 1698.5 ohm-m 5m 10m 20m LEGEND CLAY 1 - 100 ohm-m SANDY CLAY/CLAYEY SAND 111.4 - 195 ohm-m SAND 210 - 1698.5 ohm-m Figure 40: Isoresistivity map of Ughoton at 5 m, 10 m and 20 m depth 111 The isoresistivity map of Ughoton at depths of 5.0 m, 10.0 m and 20.0 m is shown in Figure 40. As the depth increases the lithology changes mainly to sand; at 10.0 m only 5.0 % of the area around VES 5 is underlain by clayey sand while the remaining 95.0 % is sandy while at 20.0 m, the lithology is mainly sand except around VES 6 where the lithology is clay. Isoresistivity Map of Ekakpamre The isoresistivity map of Ekakpamre at 5m depth is shown in Figure 41. About 45 % of Ekakpamre at 5.0 m depth is underlain by clayey sand (yellow colour) around VES 22, 27, 28, 29 and 30. The remaining 55.0 % around VES 21, 23, 24, 25 and 26 is underlain by sand (red colour). The isoresistivity map of Ekakpamre at 5.0 m, 10.0 m and 20.0 m is shown in Figure 42. It can be observed from the map that as depth increased to 10.0 and 20.0 m, the lithology changed to 100.0 % sand 112 VES 21 VES 22 VES 24 VES 23 5.525 VES 27 5.52 VES 30 VES 29 VES 28 VES 25 VES 26 5.885 5.89 5.895 5.9 LEGEND SANDY CLAY/CLAYEY SAND 119 - 184 ohm-m SAND 341 - 1001 ohm-m Figure 41: Isoresistivity map of Ekakpamre at 5 m depth 113 5.905 5m 10m 20m LEGEND SANDY CLAY/CLAYEY SAND 119 - 184 ohm-m SAND 341 - 1001 ohm-m Figure 42: Isoresistivity map of Ekakpamre at 5 m, 10 m and 20 m depth 114 Isoresistivity Map of Uvwiamuge The isoresisitivity map of Uvwiamuge at 5 m depth is shown in Figure 43. The area with blue colour (about 5.0 % of the community) at VES 44 is underlain clay. The yellow colour (30.0 % of the community) at VES 34, 37, 40, 45 and around the dumpsite is underlain by clayey sand, while the remaining 65.0 % of the community is underlain by sand (red colour). The isoresistivity map of Uvwiamuge at depths of 5.0 m, 10.0 m and 20.0 m is shown in Figure 44. As the depth increased to 10.0 m and 20.0 m, the lithology changed to 100.0 % sand. 115 LEGEND CLAY 1 - 100 ohm-m CLAYEY SAND 130 - 160 ohm-m SAND 231 - 1340 ohm-m Figure 43: Isoresistivity map of Uvwiamuge at 5 m depth 116 10m 20m LEGEND CLAY 1 -100 ohm-m CLAYEY SAND 130 - 160 ohm-m SAND 231 - 1340 ohm-m Figure 44: Isoresistivity map of Uvwiamuge at 5 m, 10 m and 20 m depth 117 Isoresistivity Map of Egbeleku The isoresistivity map of Egbeleku at 5.0 m depth is shown in Figure 45. The area with blue colour (33.3 % of Egbeleku) is underlain by clay at VES 47, 48, 53, 55 and 59). A large portion (46.7 %) of Egbeleku at 5.0 m depth is underlain by sandy clay and clayey sand (yellow colour) around VES 46, 51, 54, 56, 57, 58 and 60. The remaining areas (20.0 %) around VES 49, 50 and 52 is underlain by sand (red colour). Figure 46, shows the isoresistivity map of Egbeleku at 5.0 m, 20.0 m, 30.0 and 40.0 m. As depth increased to 20.0 m, the lithology changed mainly to clay underlying about 46.7 % of the community; another 33.3 % is underlain by sandy clay and clayey sand while the remaining 20.0 % is sandy At 30.0 m, the clays underlying areas around VES 48, 53, 55 and 59 at 20m still persist up to 30.0 m depth except at VES 46 where the lithology is sand thus reducing areas underlain by clay at 30.0 m to 33.3 % and increasing sand proportion to about 40.0 %, the remaining 26.7 % of the area is underlain by sandy clay and clayey sand. At 40.0 m, the areas covered by sand have increased to about 73.5 % except around VES 53, 54 where it is clay and VES 50 and 56 where the lithology is sandy clay and clayey sand. 118 VES 46 VES 48 VES 56 VES 47 VES 59 5.698 VES 49 VES 50 VES 55 VES 60 5.696 VES 58 VES 51 VES 54 5.694 5.786 5.788 VES 52 VES 53 VES 57 5.79 5.792 5.794 5.796 5.798 LEGEND CLAY 1 - 100 ohm-m SANDY CLAY/CLAYEY SAND 115.8 - 182.8 ohm-m Figure 45: Isoresistivity map of Egbeleku at 5 m depth 119 SAND 203 - 1100 ohm-m 5.8 5m 20m 30m 40m LEGEND CLAY 1 - 100 ohm-m SANDY CLAY/CLAYEY SAND 115.8 - 182.8 ohm-m SAND 203 - 1100 ohm-m Figure 46: Isoresistivity map of Egbeleku at 5 m, 20 m, 30 m and 40 m depth 120 Isoresistivity Map of Otor-Jeremi The isoresistivity map of Otor-Jeremi at 5 m depth is shown in Figure 47. The area with blue colour representing 5.0 % of Otor-Jeremi is underlain by clay at VES 76. Another 5.0 % of Otor-Jeremi at 5.0 m depth is underlain by sandy clay (yellow colour) around VES 80. The remaining 90.0 % is underlain by sand (red colour). Figure 48, shows the isoresistivity map of Ekakpamre at 5.0 m, 10.0 m and 20.0 m. However as depth increased to 10.0 and 20.0 m, the lithology changed mainly to sand except at VES 76 where the clays persist up to 10.0 m. Isoresistivity Map of Burutu The Isoresistivity map generated for Burutu at 5.0 m is shown in Figure 49. The walnut colour (VES 81) in the northwestern part depicts areas with porous sand/saturated clay and contain salty brackish water, the brown colour (VES 82, 83, 84, 88 and 89) in the northwestern, northern and northeastern parts represents area with saturated sand and sandy clay bearing brackish water; the red colour (VES 86) in central part indicates saturated sand/ sandy clay bearing very poor quality fresh water; the orange colour (VES 85, 87, 90, 91, 96 and 97) in the northwestern, northern, northeastern, western and southwestern parts are areas with clayey sand containing poor quality fresh water; the peach colour (VES 92, 93, 94, 99 and 100) in the northeastern parts represents sand with minor clay containing intermediate fresh water while the remaining areas (blue and sky blue colours) in the southwestern parts (VES 95 and 98) are underlain by sand bearing good to very good quality fresh water respectively. 121 VES 61 5.45 VES 64 5.448 VES 62 VES 73 VES 63 VES 65 5.446 VES 66 VES 72 VES 74 VES 71 5.444 VES 80 VES 67 VES 75 5.442 VES 68 VES 70 5.44 VES 69 VES 76 VES 79 5.438 5.436 VES 78 5.434 VES 77 5.432 5.864 5.866 5.868 5.87 5.872 5.874 5.876 5.878 5.88 5.882 5.884 LEGEND CLAY 1 -100 ohm-m SANDY CLAY/CLAYEY SAND 109 - 146 ohm-m Figure 47: Isoresistivity map of Otor-Jeremi at 5 m depth 122 SAND 217 - 987 ohm-m 5m 10m 20m LEGEND CLAY 1 -100 ohm-m SANDY CLAY/CLAYEY SAND 109 - 146 ohm-m SAND 217 - 987 ohm-m Figure 48: Isoresistivity map of Otor-Jeremi at 5 m, 10 m and 20 m depth 123 At 20.0 m (Figure 50), the clay and sandy clay horizons were gradually thinning out and becoming sandy. Around the northwestern part, the area is underlain by saturated sand and sandy clay bearing brackish water (brown colour) around VES 81; in the southern (VES 99) and areas in southeastern and northwestern parts are saturated sand and sandy clay bearing very poor quality fresh water (red colour). The orange colour around VES 82, 84 and 88 in the northwestern; northern, areas in the southeastern and southern parts are areas with clayey sand containing poor quality fresh water. While in the northwestern, northeastern, eastern, southern and southwestern parts, the area is underlain by sand with minor clay bearing intermediate fresh water (peach colour) around VES 83, 85, 87, 89, 90, 91, 92, 93, 94, 96, 97 and 100. The remaining part is underlain by sand bearing good fresh water quality to very good quality fresh water (blue and sky blue colours) around VES 87, VES 86 and 98. At 40.0 m (Figure 50), the northwestern (VES 81) and southeastern part, the area is underlain by clayey sand bearing poor quality fresh water (red colour) while in the northwestern, northeastern southeastern and southern parts are sands with minor clay bearing intermediate fresh water (orange colour) representing 65.0 % around VES 82, 83, 84, 87, 88, 90, 91, 92, 93, 94, 96 and 99. The remaining parts in a belt trending north - southwest is underlain by sand bearing good quality fresh water (blue colour) around VES 85 and very good quality fresh water (sky blue colour) around VES 86, 95, 97, 98 and 100. However, at 60.0 m (Figure 50) about 60.0 % of the area is underlain by sand 124 bearing good quality fresh water (blue colour) around VES 81 and 85 and very good quality fresh water (sky blue colour) around VES 82, 83, 84, 86, 88, 95, 96, 97, 98 and 100 except around VES 87, 89, 90, 91, 92, 93, 94 and 99 (40.0 % of the area) where the lithology is sand with minor clay bearing intermediate fresh water. Isoresistivity Map of Sapele Figure 51 shows the isoresistivity map of Sapele at 5.0 m depth. The area around VES 113 and 132 representing 5.0 % of the metropolis (blue colour) is underlain by clay while 10.0 % of the area (yellow colour) is underlain by sandy clay and clayey sand around VES 107, 111, 116 and 139. The area with green colour around VES 101, 102, 103, 109, 114, 119, 129, 130, 133, 134, 135 and 137 representing 30.0 % of Sapele is underlain by laterites. The remaining areas around VES 104, 105, 106, 108, 110, 112, 115, 117, 118, 120, 121, 122, 123, 124, 125, 126, 127, 128, 131, 136, 138 and 140 representing 55.0 % of Sapele is underlain by sand (red colour). Figure 52, shows the isoresistivity map of Sapele at 5.0 m, 10.0 m and 20.0 m. As depth increased to 10.0 m, the lithology changed mainly to sand except around VES 107 where the sandy clay persisted up to 10.0 m; while at 20m the lithology is 100.0 % sand 125 LEGEND POROUS SAND/SATURATED CLAY/SANDY CLAY SALTY BRACKISH WATER 2.0 - 4.5 ohm-m SAND,MINOR CLAY INTERMEDIATE FRESH WATER 30 - 70 ohm-m SATURATED SAND/ SANDY CLAY BRACKISH WATER 4.5 - 10 ohm-m SATURATED SAND/ SANDY CLAY VERY POOR QUALITY FRESH WATER 10 - 15 ohm-m SAND GOOD QUALITY FRESH WATER 70 - 100 ohm-m Figure 49: Isoresistivity map of Burutu at 5 m depth 126 CLAYEY SAND POOR QUALITY FRESH WATER 15.- 30 ohm-m SAND VERY GOOD QUALITY FRESH WATER > 100 ohm-m 5m 20 m 40 m 60 m LEGEND POROUS SAND/SATURATED CLAY/SANDY CLAY SALTY BRACKISH WATER 2.0 - 4.5 ohm-m SAND,MINOR CLAY INTERMEDIATE FRESH WATER 30 - 70 ohm-m SATURATED SAND/ SANDY CLAY BRACKISH WATER 4.5 - 10 ohm-m SATURATED SAND/ SANDY CLAY VERY POOR QUALITY FRESH WATER 10 - 15 ohm-m SAND GOOD QUALITY FRESH WATER 70 - 100 ohm-m CLAYEY SAND POOR QUALITY FRESH WATER 15.- 30 ohm-m SAND VERY GOOD QUALITY FRESH WATER > 100 ohm-m Figure 50: Isoresistivity map of Burutu at 5 m, 20 m, 40 m and 60 m depth 127 VES 134 5.9VES 140 VES 135 VES 133 5.89 VES 139 5.88 VES 138 VES 136 VES 137 5.87 VES 123 VES 122 VES 124 VES 119 VES 125 VES 117 VES 130 VES 120 VES 118 VES 116 VES 129 VES 121VES 114 VES 115 VES 128 VES 127 VES 132 VES 131 5.86 VES 102 VES 103 VES 105 VES 112 VES 111 VES 113 5.85 VES 110 VES 107 VES 106 VES 109 VES 108 5.62 VES 101 VES 126 5.63 5.64 5.65 5.66 5.67 5.68 5.69 5.7 5.71 LEGEND CLAY 1 - 100 ohm-m LATERITE 650 - 840 ohm-m SAND 207 - 988 ohm-m SANDY CLAY/CLAYEY SAND 106.5 - 187 ohm-m Figure 51: Isoresistivity map of Sapele at 5 m depth 128 VES 104 5.72 5m 10m 20m LEGEND CLAY 1 - 100 ohm-m LATERITE 650 - 840 ohm-m SANDY CLAY/CLAYEY SAND 106.5 - 187 ohm-m SAND 207 - 988 ohm-m Figure 52: Isoresistivity map of Sapele at 5 m, 10 m and 20 m depth 129 5.3.2 Evaluation of Protective Capacity from Longitudinal Unit Conductance Map. The longitudinal unit conductance values in Tables 3, 4, 5, 6, 7, 8 and 9 calculated from equation 13 were used to generate maps which were used to evaluate the protective capacity of the study area. The combination of thickness and resistivity into single variables, the Dar Zarrouk parameters (Maillet, 1947); can be used as a base for the evaluation of properties such as aquifer transmissivity and protection of ground water resources (Henriet, 1975). Aquifer transmissivity is defined in the hydrogeology, as the product of its hydraulic conductivity for the thickness of the layer. As the hydraulic conductivity is directly proportional to the resistivity (Kelly, 1977) and the product of the resistivity for its thickness, it is defined as being the transverse resistance (T), on a purely empirical basis and it can be admitted that the transmissivity of an aquifer is directly proportional to its transverse resistance (Henriet, 1975; Ward, 1990). Clay layer corresponds with low resistivities and low hydraulic conductivities, and vice versa, hence, the protective capacity of the overburden could be considered as being proportional to the ratio of thickness to resistivity i.e longitudinal conductance (S). The modified longitudinal conductance/protective capacity rating approach of Henriet, 1976; Oladapo et al., 2004 was adopted for this study. The area with yellow colour having longitudinal conductance of 0.10 – 0.19 mho indicates area with weak protective capacity. The area with blue colour having 130 longitudinal conductance of 0.20 – 0.69 mho depicts area with moderate protective capacity while the area with green colour having longitudinal conductance of 0.70 – 4.90 mho are areas with good protective capacity. Longitudinal Unit Conductance Map of Ughoton The overburden protective capacity of Ughoton as indicated by the longitudinal conductance map is shown in Figure 53. The aquifer in the poor and weak protective areas is vulnerable to contamination. The following VES locations; 1, 2, 3, 7, 8, 9, 10, 11, 12, 13, 14, 17, 18, 19 and 20 (60.0 % of the community) are the area with poor protective capacity. This coincides with areas where porous sand overlies the aquifer which is at depths of between 8.0 – 20.0 m. The following VES locations; 4 and 5 (about 25.0 % of the community) lay zones with weak protective capacity, the aquifer in this area at depths of between 9.0 – 14 m is overlain by clays of 1.6 – 2.8 m thick, the risk of contamination is lesser than the area with poor protective capacity. The central portion of the community (15.0 % of the community) around VES 6, 15 and 16 lies moderately protected zones. The aquifer in this area which exists at depths of between 8.0 – 13.0 m is protected by clays of 4.0 – 20.2 m thick, hence not vulnerable to contamination. The protective capacity of overburden at VES 4 and 5 is weak while at VES 6, 15 and 16 moderate. Studies have shown that minimum design requirement for the protection of aquifer by suitable aquiclude such as clays should be a thickness of between 0.6 – 1.5 m and in-situ permeability of less than 131 1 x 10-9 m/s (Allen, 2001; Verwiel, et al., 2001). It is expected that the clays of 1.6 – 20.2 m thick at these locations should have good protective capacity and give adequate protection for the aquifer beneath. Longitudinal unit conductance map of Ekakpamre. The longitudinal conductance map of Ekakpamre (Figure 54) shows that the aquifer in 70.0 % of the area around VES 21, 23, 24, 25, 26 and 30 is poorly protected and coincides with area where the aquifer is overlain by porous sand. This indicates that the aquifer here that exists at depths of 7.0 – 18.0 m is not protected and could be vulnerable to surface and near surface contamination. The remaining 30.0 % of the community around VES 22, 27, 28 and 29 is weakly protected. This coincides with areas where the aquifer that exists at depths of 6.0 – 14.0 m is overlain by clayey sand with thickness of between 3.9 – 5.2 m; as a result the risk of contamination is probable less than in the area with poor protective capacity. The clayey sand of between 3.9 – 5.2 m around VES 22, 27, 28 and 29 only provided weak protection for the aquifer beneath. This is probably due to the high percentage of sand thereby leading to increase hydraulic conductivity and creating path ways for pollutants. 132 VES 3 VES 8 VES 1 VES 2 5.615 VES 7 VES 20 VES 15 VES 16 VES 4 VES 17 5.61 VES 6 VES 5 VES 12 VES 9 VES 11 VES 18 5.605 VES 19 VES 13 VES 14 5.655 5.66 5.665 5.67 5.675 LEGEND Poor Protective Capacity Zone < 0.10 mho Weak Protective Capacity Zone 0.10 - 0.19 mho Moderate Protective Capacity Zone 0.20 - 0.69 mho Figure 53: Longitudual unit conductance map of Ughoton 133 VES 10 5.68 . VES 21 VES 22 VES 24 VES 23 5.525 VES 27 5.52 VES 30 VES 29 VES 28 VES 25 VES 26 5.885 5.89 5.895 5.9 LEGEND Poor Protective Capacity Zone < 0.10 mho Weak Protective Capacity Zone 0.10 - 0.19 mho Figure 54: Longitudual unit conductance map of Ekakpamre 134 5.905 Longitudinal unit conductance map of Uvwiamuge In Uvwiamuge, the longitudinal conductance map (Figure 55) shows that. 73.0 % of the community around VES 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 42 and 43 has poor protective capacity. The aquifer in these area occur at depths of 7.0 – 19.0 m stand the probable risk of contamination as they are overlain by porous sands. The remaining 27.0 % around VES 41, 44, 45 and around the dumpsite have a weak protective capacity; the aquifer in this area exists at 6.0 – 10.0 m and is given some protection by clayey sand of 1.3 – 8.0 m thick. Longitudinal Unit Conductance Map of Egbeleku The overburden protective capacity of Egbeleku as indicated by the longitudinal conductance map is shown in Figure 56. About 6.7 % of the area around VES 51 has weak protective capacity; this coincides with area where sandy clay overburden of 7.2 m overlies the aquifer. The aquifer in this area which is at a depth of 8.4 m is vulnerable to contamination. Areas around VES 46, 49, 50, 52, 55, 56, 57 and 58 (53.3 % of the community) have moderate protective capacity while the remaining 40 % around VES 47, 48, 53, 54, 59 and 60 fall within areas with good protective capacity. The aquifer in these areas exists at an average depth of 22.5 m is protected from contaminated fluids by the overlying layers of clay and sandy clay with thickness of 1.3 – 44.0 m. Groundwater is given protection by protective geologic barriers having sufficient thickness also called protective layers (Mundel et al., 2003) and low hydraulic 135 conductivity. Silts and clays are suitable protective layers and when they are found above an aquifer they constitute a protective cover (Lenkey et al., 2005). The protective capacity of overburden at VES 49 and 55 is moderate. The thickness of the clays above the aquifer at these locations is between 1.3 – 33.2 m thick and exceeds the minimum thickness requirement of 0.6 – 1.5 m for geologic barriers and as a result is expected to give the aquifer beneath a good protective measure. Longitudinal Unit Conductance Map of Otor-Jeremi The overburden protective capacity of Otor-Jeremi as indicated by the longitudinal conductance map is shown in Figure 57. Areas with poor protective capacity around VES 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 77 and 79 makes up 80 % of the community. The aquifer in these areas is at depths of 6.0 – 13.0 m and is overlain by porous sand as a result; the aquifer here is highly vulnerable to contamination. The southeastern and eastern portion of the community around VES 78, 80 and VES 61 in the northwestern part (representing 15.0 % of the community) are weakly protected zones. This coincides with areas where clays and clayey sand of 1.6 – 4.8 m overlie the aquifer which exists at depths of 6.0 – 14.0 m. The probable risk of contamination is less than the area with poor protective capacity. However, around VES 76 (about 5.0 % of the community) lie in areas with moderate protective capacity. The aquifer here exists at depths of 14.0 m is not vulnerable to contamination from surface and near surface sources as it is protected by clays of 13.1 m. 136 LEGEND Poor Protective Capacity Zone < 0.10 mho Weak Protective Capacity Zone 0.10 - 0.19 mho Figure 55: Longitudual unit conductance map of Uvwiamuge 137 VES 46 VES 48 VES 56 VES 47 5.698 VES 59 VES 49 VES 50 VES 55 VES 60 5.696 VES 58 VES 51 VES 54 5.694 5.786 5.788 5.79 VES 52 VES 53 VES 57 5.792 5.794 5.796 5.798 LEGEND Weak Protective Capacity Zone 0.10 - 0.19 mho Moderate Protective Capacity Zone 0.20 - 0.69 mho Good Protective Capacity Zone 0.70 - 4.90 mho Figure 56: Longitudinal unit conductance map of Egbeleku 138 5.8 The protective capacity at VES 61, 76 and 78 is weak, moderate and weak respectively. Overburden clays at these VES locations is between 1.6 – 13.1 m thick is expected to give the aquifer beneath good protection. Clay – rich layers near the surface are potentially suitable as geologic barriers with the ability to attenuate contaminants. Longitudinal Unit Conductance Map of Burutu The overburden protective capacity of Burutu as indicated by the longitudinal conductance map is shown in Figure 58. The areas around VES 98 in the southwest; southeast and northeast have poor protective capacity; hence the aquifer in this area may be vulnerable to contamination. The following areas (VES 92, 97 and 100) in northeast, south and southwest have weak protective capacity; location VES 83, 85, 86, 87, 90, 91, 93, 94, 95, 96 and 99 trending in a west-northeast belt falls within areas with moderate protective capacity while areas around VES 81, 82, 84, 88 and 89 in the northern and northwestern part of Burutu (parallel to the Forcados River) have good protective capacity. The aquifers within the good protective capacity zones in the north and northwestern parts is close to the tide influenced Forcados River but are protected from infiltrating saline water by thick or extensive aquiclude that exclude further penetration of saline water. 139 VES 61 5.45 VES 64 5.448 VES 62 VES 73 VES 63 VES 65 5.446 VES 66 VES 72 VES 74 VES 71 5.444 VES 80 VES 67 VES 75 5.442 VES 68 VES 70 5.44 VES 69 VES 76 VES 79 5.438 5.436 VES 78 5.434 VES 77 5.432 5.864 5.866 5.868 5.87 5.872 5.874 5.876 5.878 5.88 5.882 5.884 LEGEND Poor Protective Capacity Zone < 0.10 Weak Protective Capacity Zone 0.10 - 0.19 mho Moderate Protective Capacity Zone 0.20 - 0.69 mho Figure 57: Longitudinal unit conductance map of Otor-Jeremi 140 VES 88 VES 90 VES 89 VES 91 5.355 VES 81 VES 82 VES 84 VES 86 VES 92 VES 83 VES 85 VES 87 5.35 VES 96 VES 94 VES 95 5.345VES 97 VES 93 VES 98 VES 99 VES 100 5.505 5.51 5.515 5.52 5.525 LEGEND Poor Protective Capacity Zone < 0.1 mho Weak Protective Capacity Zone 0.10 - 0.19 mho Moderate Protective Capacity Zone 0.20 - 0.69 mho Figure 58: Longitudinal unit conductance map of Burutu 141 Good Protective Capacity Zone 0.70 - 4.90 mho Longitudinal Unit Conductance Map of Sapele The overburden protective capacity of Sapele as indicated by the longitudinal conductance map is shown in Figure 59. The areas with poor protective capacity constitute about 58.0 % of the metropolis and located around VES 102, 103, 106, 108, 109, 110, 111, 112, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 133, 136, 137 and 138. The aquifer in these areas which exists at depths of 6.0 – 18.0 m is overlain by sand and laterites of 1.3 – 2.5 m thick. This represents zones of probable risk of contamination. The remaining 42.0 % of the metropolis around VES 101, 104, 105, 107, 113 126, 127, 128, 130, 131, 132, 134, 135, 137, 139 and 140 lies weakly protected zones. The aquifer in these areas exists at depths of between 7.0 – 25.0 m is overlain by clays, clayey sand and laterites having thickness of 3.1 – 10.8 m. The probable risk of contamination of the aquifer in these areas is less than in the area with poor protective capacity. The protective capacity at VES 101, 104, 105, 107, 113 126, 127, 128, 130, 131, 132, 134, 135, 137, 139 and 140 is weak despite having laterite overburden of 3.1 – 10.8 m thick above the aquifer. At VES 113 and 132 where there is a clay overburden of 6.5 m and 3.7 m respectively above the aquifer respectively, the protective capacity is weak. Laterites and clays are aquitards with low hydraulic conductivity that can prevent percolation of water and so is expected to give the aquifer beneath good protection. A low hydraulic conductivity layer with thickness of 0.6 – 1.5 m will help minimize contaminant 142 migration out of a unit. Thicker clay and laterite layers provide additional time to minimize leachate migration prior to the clay and laterite becoming saturated. VES 134 5.9VES 140 VES 135 VES 133 5.89 VES 139 5.88 VES 138 VES 136 VES 137 5.87 VES 123 VES 122 VES 124 VES 119 VES 125 VES 117 VES 130 VES 120 VES 118 VES 116 VES 129 VES 121VES 114 VES 115 VES 128 VES 127 VES 132 VES 131 5.86 VES 102 VES 103 VES 105 VES 112 VES 111 VES 113 5.85 VES 110 VES 107 VES 106 VES 109 VES 108 5.62 VES 101 VES 126 5.63 5.64 5.65 5.66 5.67 5.68 5.69 5.7 5.71 LEGEND Poor Protective Capacity Zone < 0.10 mho Weak Protective Capacity Zone 0.10 - 0.19 mho Figure 59: Longitudinal unit conductance map of Sapele 143 VES 104 5.72 5.4 Inverted Electrical Resistivity Imaging Profiles The results of the 2D resistivity inversion in the study area are shown in Figures 60 – 80. ERI Profile 1 (Ughoton) The inverted resistivity model (Figure 60) shows a variation of resistivity distribution with low resistivity values ranging from 86.4 – 199.0 Ωm in the first layer mapped out in green colour. This represents the top soil made up of sandy material with some clay. The resistivity increased from 244.0 Ωm in the second layer to 1487.0 Ωm in the fourth layer. This is indicative of sandy lithology of increasing grain size with depth as scaling colour changed from yellow-brownred. Following Archie’ s law, increasing porosity leads to decreasing electrical resistivity. However, it must be kept in mind that porosity of coarse material is in general, lower than porosity of finer material, although the effective porosity and permeability is higher for coarse material than for fine material (TNO 1976). ERI Profile 2 (Ughoton) The 2D electrical resistivity structure of this profile (Figure 61) indicate a first and second layer with resistivity varying between 69.7 – 120.0 Ωm (deep blue/blue colour) and resistivity varying between 125.0 – 293.0 Ωm (green colour). This is interpreted as clay/silty clay (deep blue/blue colour) abd fine sand (green colour). These clays/silty clays were also delineated by the vertical electrical sound at location 4 and 5 (close to this profile) where clays and silty clays of between 2.0 – 6.0 m exist. The third and fourth layer is characterized by 144 sand. The increase in resistivity from 311.0 – 1299.0 Ωm (changing scaling colour from yellow – brown – red – purple) is indicative of increasing grain size with depth. ERI Profile 3 (Ughoton) The result of electrical resistivity imaging at this profile (Figure 62) shows a top two layers with resistivity values ranging between 89.7 – 169.0 Ωm. This is indicative of sandy lithology, the somewhat low resistivity of 89.7 – 91.0 Ωm may be due to clays near the bank of the Ughoton River. The third and fourth layers are sands characterized by a coarsening downward sequence; this is evidenced by increase in resistivity from 240.0 – 2182.0 Ωm. ERI Profile 4 (Ughoton) The inverted model of this profile (Figure 63) indicate sandy lithology with increase with increasing grain size from top to bottom as resistivity values increased from 132.0 Ωm in the first layer to 2084.0 Ωm in the fourth layer. 145 Figure 60: 2D resistivity structure and pseudosection for profile 1 in Ughoton 146 Figure 61: 2D resistivity structure and pseudosection for profile 2 in Ughoton 147 Figure 62: 2D resistivity structure and pseudosection for profile 3 in Ughoton 148 Figure 63: 2D resistivity structure and pseudosection for profile 4 in Ughoton 149 ERI Profile 1 (Otor-Jeremi) This profile was done at a dumpsite along Okwagbe road (Figure 14). The inverted resistivity model as shown in Figure 64 indicate the leachate effect of dumpsite as mapped out in deep blue colour. The low resistivity values varying between 3.78 – 21.0 Ωm shows that the subsurface may have been contaminated by leachate of decomposing wastes from the surface to depths of between 3.0 – 15.0 m. The shape of the profile also indicates downward migration of the contaminant plume. Other zones contaminated by ionized fluid with less amount of leachate are mapped out in yellow colour. Below this low resistivity zone is an underlying zone of increasing resistivity (157.0 – 638.0 Ωm) indicated by red colour between lateral distances 30.0 – 45.0 and 57.0 – 80.0 m. The high resistivity values indicate that these zones found at depths of 4.0 m on the east and 8.0 m at the central part of the profile may have not been contaminated. ERI Profile 2 (Otor-Jeremi) This profile is located 10.0 m from profile 1. The resistivity image (Figure 65) shows that the top is characterized by a contaminated zone (deep blue colour) with high amount of leachate on the western part of the profile with resistivity values varying between 2.48 – 15.8 Ωm. This zone is found at a depth of 5.0 m within lateral distance to 10.0 – 55.0 m. Another isolated contaminated zone around the eastern part was delineated between lateral distances 60.0 – 80.0 m from the surface to 15.0 m. The zone mapped out in yellow colour with resistivity 150 values less than 74.1 Ωm may have been contaminated with ionized fluid with lesser amount of leachate. Within lateral distance 10.0 – 54.0 m and from depths of 5.0 – 15.0 m lies a zone of higher resistivities (122.0 – 1046.0 Ωm). The high resistivity values show that this zone may have not been contaminated. ERI Profile 3 (Otor-Jeremi) Figure is the 2D profile carried out at 900.0 m from the dumpsite. The inverted resistivity model show that the high resistivity values (112.0 – 2966.0 Ωm) is interpreted as sand that has not been contaminated. The low resistivities at the top are possibly due to sand with minor clayey materials. ERI Profile 4 (Otor-Jeremi) This profile was carried out in at the dumpsite located in Otor-Jeremi market. The inverted resistivity model (Figure 67) shows an isolated zone of low resistivity (deep blue colour) with resistivity values varying between 1.96 – 25.3 Ωm from the surface up to 12.0 m within lateral distance 30.0 – 70.0 m. Two other zones of low resistivity (15.6 – 43.7 Ωm), one on the west from the surface to 15.0 m and one in the east up to 6.0 m from the surface were also delineated. These zones represent migrating contaminant plume with high amount of leachate. The areas mapped in yellow may have been contaminated to a lesser extent by ionized fluids. Zones that imply that the subsurface and groundwater may have not been contaminated were however delineated in the eastern part at the surface (brown and red colour) up to 25.0 m from the surface within lateral distance 62 – 73 m, in the west from 2.5 – 11.0 m within lateral distance 10.0 – 151 18.0 m and at the central part at depths between 2.5 – 15.0 m within lateral distance 30.0 – 60.0 m. These zones were characterized by high resistivity values (205.0 – 1651.0 Ωm). ERI Profile 5 (Otor-Jeremi) This profile was done 8.0 m away from profile 4. The resistivity structure (Figure 68) shows a zone of contamination by leachate from decomposing wastes from the surface up to 15.0 m within lateral distance 40.0 – 55.0 m. The resistivity within this zone varies between 1.91 – 26.9 Ωm and is mapped out in deep blue colour. Two other zones of very low resistivity were mapped out in isolation, one on the west between lateral distances 11.0 – 24.0 m and extends up to 2.5 m from the surface. The other zone is on the extreme east of the profile within lateral distance 76.0 – 80.0 m and extends up to 4.0 m from the surface. The resistivity values of these zones vary between 7.76 – 27.3Ωm. The zones mapped in yellow may have been contaminated to a lesser extent by ionized fluids with less amount of leachate. The shape of the resistivity structure indicates downward and lateral movement of the contaminant plume. On the extreme western part of the profile from depths of 4.0 – 15.0 m within lateral distance 10.0 – 26.0 m and in the east from the surface to 15.0 m within lateral distance 55.0 – 78.0 m lies zones of higher resistivity values (122.0 – 1150.0 Ωm) mapped in brown and red colours. The high resistivities in these zones indicate that the subsurface and indeed groundwater here may not contaminated. 152 Figure 64: 2D resistivity structure and pseudosection for profile 1 in OtorJeremi 153 Figure 65: 2D resistivity structure and pseudosection for profile 2 in OtorJeremi 154 Figure 66: 2D resistivity structure and pseudosection for profile 3 in OtorJeremi 155 ERI profile 6 (Otor-Jeremi) This profile was carried out at 1000.0 m away from the market. The inverted resistivity model (Figure 69) shows that the natural soil conditions is returning as the topmost and lower most part of this profile show that the sands have not been contaminated. The low resistivity value of the top layer may be due to the presence of clayey regoliths and not contaminants. 156 Figure 67: 2D resistivity structure and pseudosection for profile 4 in OtorJeremi 157 Figure 68: 2D resistivity structure and pseudosection for profile 5 in OtorJeremi 158 Figure 69: 2D resistivity structure and pseudosection profile 6 in OtorJeremi 159 ERI profile (Burutu) Figures 70 – 74 shows the five 2D electrical images obtained in the study area related with depth and true resistivity of the subsurface investigated. Two distinctive zones in the upper and lower parts respectively have been isolated: the low resistive zone (red colour) and the anomalous high resistive zone (blue colour). ERI Profile 1 (Burutu) The inverted Electrical Resistivity Imaging profile 1, shown in Figure 70 is a six layer model, the low resistivity zones from the surface to the fourth layer at depths ranging from 0.0 – 40.0 m within lateral distance 40.0 – 90.0 m; from the surface to the six layer at depths ranging from 0.0 – 60.0 m within lateral distance 0.0 – 40.0 m and from the surface to the six layer at depths ranging from 0.0 – 75.0 m within lateral distance 90.0 – 180.0 m having resistivity values of 4.0 – 68.8 Ωm could be associated with sandy/clayey layers filled with water of poor quality. The thickness of the low resistivity layer was observed to be thicker towards river side, and these values reduced are maybe due to saturated strata for a subsurface river water flow zone. Within the fifth and sixth layers lies a high resistivity zone (blue colour) within lateral distance 90.0 – 180.0 m. This isolated high resistivity zone with values ranging from 77.6 – 165.0 Ωm at depths ranging from 40.0 – 75.0 m reflect saturated sand with water of good quality. 160 ERI Profile 2 (Burutu) As shown in Figure 71, the image presents a lateral variation in resistivity distribution, with low electrical resistivity layers characterized by sandy/clayey layers filled with water of poor quality towards NE within lateral distance 0.0 – 80.0 m from the surface to about 22.0 m and in the NW within lateral distance 100.0 – 120.0 m; 120.0 – 170.0 from the surface to between 35.0 m and 50.0 m. This zone with resistivity varying between 2.55 – 65.9 Ωm is conspicuously present at the top of the entire profile. Lower resistivity zones were observed to be thick (up to 50.0 m) possibly because of proximity to the Forcados River. Two zones of high resistivity with values varying between 90.6 – 547.0 Ωm were delineated Ωm in the NE and NW within lateral distances 40.0 – 90.0 m; 120.0 – 160.0 m occur at depths ranging between 22.0 – 50.0 m and 35.0 – 50.0 m. This implies that the groundwater in these zones may have not been contaminated. ERI Profile 3 (Burutu) Figure 71 shows the inverse model resistivity structure of ERI profile 3, the first three layers shows low resistivity values of the range 2.28 – 62.2 Ωm within lateral distances 0.0 – 90.0 m; 120.0 – 160.0 m (in the west and east of structure respectively). The depth of occurrence is from the surface to 19.0 m in the south and up to 21.0 m in the north and is interpreted as sandy/clayey formations saturated with water of poor quality. A zone saturated with water of poor quality up to the fourth layer from the surface to about 30.0 m occupies the central portion of the profile within lateral distance 90.0 – 110.0 m. Within the 161 fourth layer beginning at a depth of 18.0 – 20.0 m are two isolated zones of high resistivity values varying between 75.5 – 215.0 Ωm in the south within lateral distance 40.0 – 90.0 m and in the north within lateral distance 120.0 – 160.0 m representing sandy formation saturated water of good quality ERI Profile 4 (Burutu) ERI profile 4 as shown in Figure 73 is a four layer model, within the first three layers lies a low resistivity zones from the surface to about between 22.0 – 30.0 m within lateral distances 0.0 – 180.0 m. The resistivity values of these layers vary between 4.03 – 68.8 Ωm and could be associated with sandy/clayey layers filled with water of poor quality. Below beginning from the third layer into the fourth layer at depths spanning between 22.0 – 50.0 m is zone of high resistivity (blue colour) with resistivities varying between 71.5 – 221.0 Ωm. This implies that the groundwater in this zone may be of good quality. 162 Figure 70: 2D resistivity structure and pseudosection for profile 1 in Burutu 163 Figure 71: 2D resistivity structure and pseudosection for profile 2 in Burutu 164 Figure 72: 2D resistivity structure and pseudosection for profile 3 in Burutu 165 Figure 73: 2D resistivity structure and pseudosection for profile 4 in Burutu 166 Figure 74: 2D resistivity structure and pseudosection for profile 5 in Burutu 167 ERI Profile 5 (Burutu) The inversion result of this profile (Figure 74) shows variation of resistivity values from 5.93 – 68.4 Ωm (red colour) within the first three layers from the surface to 20.0 m within lateral distance 0.0 – 100.0 m and from the surface into part of the fourth layer up to 28.0 m depth within lateral distance 100.0 – 120.0 m. The resistivity values within these layers signify clayey formation bearing water of poor quality. Underlying this is a zone of high resistivity (blue colour) with resistivities varying between 83.4 – 331 Ωm that begins from the third layer at depth 20.0 – 40.0 m within lateral distance 0.0 – 100.0 m, this represents sand bearing water of good quality. Within lateral distance 100.0 – 120.0 m lie a deeper interface to water of good quality at depths ranging from about 28m to about 40 m. ERI Profile 1(Sapele) This profile is located at a dumpsite along New Road (Figure 16). The 2D resistivity structure in Figure 75 shows two zones of low resistivity (deep blue colour) with resistivity values varying from 10.7 – 15.2 Ωm in the first layer up to 4.0 m from the surface within lateral distance 70.0 – 80.0 m and 105.0 – 130.0 m. Based on previous studies of waste dumps (Soupious et al., 2007; Cardarelli and Di Filippo, 2003), resistivities of these values may be caused by organic wastes with leachate. Another low resistivity zone (12.6 – 20.0 Ωm) was mapped out in the second, third and fourth layer within lateral distance 52.0 – 92.0 m. This zone is found at depths of between 6.0 – 24.0 m and is interpreted to be contaminant 168 plume with high amount of leachate. The shape of the 2D resistivity indicates the downward migration of the plume. A zone of probable contamination but to a lesser extent by ionized fluid (with low amount of leachate) was mapped out with green colour having resistivity values varying from 24.9 – 57.8 Ωm. There exist two isolated zones (yellow and brown colour) of high resistivity (74.0 – 115.0 Ωm) in the east and west of the section at depth of between 5.0 – 18.0 m. These structures were interpreted as possibly uncontaminated zones. Also, between lateral distances 92.0 – 98.0 m lie an isolated zone (yellow colour) about 4.0 m thick with resistivity value of 109.0 Ωm, this also represent an uncontaminated zone. ERI Profile 2 (Sapele) This profile was carried out 6 m away from profile 1. The result of the inversion of the ERI profile is shown in Figure 76. The resistivity structure shows a small zone of low resistivity (deep blue colour) with resistivity values ranging from 5.87 – 12.0 Ωm with significant depth extent of 4.0 – 22.0 m. These zones are suspected to be leachate impacted zones and are found between lateral distances 25.0 – 45.0 m; 50.0 – 88.0 m and 112.0 – 130.0 m. These areas correlate with the actual location of dumpsite which lies between 20.0 – 138.0 m. The low resistivity values may represent higher amounts of leachate. The zones with slight higher resistivity values ranging from 14.7 – 70.0 Ωm (green colour) may be due to ionized fluids (with low amount of leachate) migrating downward and laterally. Within lateral distance 50.0 – 80.0 m (yellow and brown colours) and 110.0 – 169 138.0 m (yellow and red colours) having high resistivity values between 85.0 – 264.0 Ωm is an indication that the subsurface zones which is mapped between depths of 14.0 – 24.0 m and 4.0 – 24.0 m respectively is an indication that the groundwater in these zones may have not been contaminated by leachate effect. ERI Profile 3 (Sapele) The inverted 2D resistivity image along profile 3 shown in Figure 77 was carried out at 450.0 m from the dumpsite as a control. The high resistivity values that vary between 144.0 – 2587.0 Ωm are indications that this location has not been contaminated. The low resistivity values observed in the first layer (93.7 – 173.0 Ωm) may be due to clay/clayey sand formation. These formations were delineated by the vertical electrical sounding at location 133 which was close to this profile. ERI Profile 4 (Sapele) This profile was carried out at a dumpsite at Reclamation Road (Figure 16). The inversion of the profile data (Figure 78) shows three isolated zones of low resistivity (deep blue colour) with resistivity values varying between 8.35 – 19.4 Ωm located at depths of 3.0 m from the surface within lateral distances 42.0 – 52.0 m, 63.0 – 83.0 m and another zone of low resistivity between lateral distance 30.0 – 58.0 m at depths between 3.0 – 18.0 m. These indicate regions of pollution possibly as a result of infiltrate leachate. The zones depicted by green colour having resistivity values varying between 24.0 – 49.2 Ωm may be due to migration of ionized fluids with less amount of leachate. 170 Figure 75: 2D resistivity structure and pseudosection for profile 1 in Sapele 171 Figure 76: 2D resistivity structure and pseudosection for profile 2 in Sapele 172 Figure 77: 2D resistivity structure and pseudosection profile 3 in Sapele 173 However, from depths of about 6.0 – 24.0 m at lateral distance 66.0 – 82.0 m on the east lies a zone mapped out by red and purple colour. The high resistivity values (307.0 – 550.0 Ωm) indicate that this zone may not have been contaminated. The low resistivity values of 103.0 – 122.0 Ωm (yellow colour) observed above at depth of 5.0 m could likely be due to clayey formation which does not permit infiltration by leachate effect. Another uncontaminated zone (yellow colour) was delineated on the western part at lateral distance 13.0 – 22.0 m and between depths of 6.0 – 12.0 m. ERI Profile 5 This profile was carried out 5.0 m away from profile 4. The 2D resistivity structure obtained from the inversion of the data is shown in Figure 79. Three distinct zones of low resistivity (deep blue colour) of which two were located within the first layer and the third zone within the second and third layer. The resistivity of these zones vary between 1.06 – 2.41Ωm and occur from the surface up to 3 m between lateral distance 32.0 – 48.0 m and 72.0 – 76.0 m while the third zone lie between lateral distance 18.0 – 24.0 m and at depths of between 3.0 – 12.0 m. These zones are suspected to be leachate impacted zones. The zones with resistivity values varying between 4.74 – 68.2 Ωm (green, yellow and brown colour) has been impacted by contaminated plume with highly to slightly conductive leachate up to 18.0 m from the surface on the western part and 6.0 – 10.0 m in the central and eastern part. Underlying these low resistivity zones are zones of high resistivities (>103.0 Ωm) to the west and east of the section (red and purple colour) at depths between 3.0 – 18.0 m. This is interpreted as probable groundwater zone that have not been impacted by leachate effect. 174 Figure 78: 2D resistivity structure and pseudosection profile 4 in Sapele 175 Figure 79: 2D resistivity structure and pseudosection for profile 5 in Sapele 176 Figure 80: 2D resistivity structure and pseudosection for profile 6 in Sapele 177 ERI Profile 6 (Sapele) This profile was carried out as a control for profiles 4 and 5 was done at 800 m away from profile 4. The result of inverted resistivity model (Figure 69) shows that the layers are sands having resistivity values between 151.0 – 3430.0 Ωm. This may have not been contaminated by leachate plume migrating from the dumpsite. The low resistivity in the first layer may be due to clayey regolith and not contaminants. 5.5 Water Quality The results of water quality based on total dissolved solids concentration at Ughoton are shown in Table 11, Figures 81 and 90. TDS concentration at depths between 8.0 – 16.0 m varied from 1104.0 – 6336.0 ppm. Between the depths of 18.0 – 22.0 m, the TDS concentration varied from 613.0 – 818.8 pm while at depths between 24.0 – 44.0 m, the TDS concentration varied from 199.9 – 494.7 ppm. The results of water quality based on total dissolved solids concentration computed from the single point resistance log of the well at Ekakpamre are shown in Table 12, Figures 82 and 91. The result shows that at depths between 8.0 – 12.0 m, the TDS concentration varied between 518.4 – 1935.7 ppm while at depths between 14.0 – 40.0 m, the TDS concentration varied from 36.5 – 406.7 ppm. The results of water quality of the well at Uvwiamuge as shown in Table 13, Figures 83 and 92 showed that at depths between 8.0 – 20.0 m, the TDS concentration varied between 900.9 – 3403.4 ppm while at depth of 22.0 – 40.0 m, the TDS concentration varied from 116.2 – 494.7ppm. 178 Figure 81: Single-point resistance log from Ughoton 179 Figure 82: Single-point resistance log from Ekakpamre 180 Figure 83: Single-point resistance log from Uvwiamuge 181 Figure 84: Single-point resistance log from Egbeleku 182 Table 11: Single-point Resistance Log Data at Ughoton ρb(Ωm) ρw(Ωm) EC(µS/cm) TDS (ppm) 8 5.4545 1.0101 9900 6336.0 10 7.2430 1.3413 7460 4722.0 12 12.0556 2.2325 4480 2867.0 14 20.9315 3.8762 2580 1651.0 16 31.2952 5.7954 1730 1104.0 18 42.2070 7.8161 1280 818.8 20 56.3582 10.4367 960 613.0 22 54.8845 10.1638 980 629.7 24 69.8668 12.9383 770 494.7 26 83.8782 15.5330 640 412.0 28 81.7447 15.1379 660 422.8 30 101.4617 18.7892 530 340.6 32 84.1163 15.5771 640 411.0 34 112.1548 20.7694 480 308.2 36 168.8180 31.2626 320 205.0 38 171.3739 31.7359 315 201.7 40 172.8270 32.0050 310 199.9 42 165.9200 30.7259 330 208.0 44 144.5963 26.7771 370 239.0 Depth (m) 183 Table 12: Single-point Resistance Log Data at Ekakpamre ρb(Ωm) ρw(Ωm) EC(µS/cm) TDS (ppm) 8 17.8535 3.3062 3024.58 1935.7 10 51.7147 9.5768 1044.19 668.3 12 66.6679 12.3459 809.99 518.4 14 84.9798 15.7370 635.44 406.7 16 138.9123 25.7245 388.73 248.8 18 258.2518 47.8244 209.10 133.8 20 266.1417 49.2855 202.90 129.9 22 323.0019 59.4448 168.20 107.7 24 365.2000 67.6296 147.86 94.6 26 348.4102 64.5204 154.99 99.2 28 496.9047 92.0192 108.67 69.6 30 646.0522 119.6393 83.58 53.5 32 693.0873 128.3495 77.91 49.9 34 666.8962 123.4993 80.97 51.8 36 704.8189 130.5220 76.62 49.0 38 627.3364 116.1734 86.08 55.1 40 946.9800 175.3666 57.02 36.5 Depth (m) 184 Table 13: Single-point Resistance Log Data at Uvwiamuge ρb(Ωm) ρw(Ωm) EC(µS/cm) TDS (ppm) 8 35.100 6.5104 1535.99 983.0 10 6.8796 3.7150 2691.70 1722.5 12 0.01861 1.8805 5217.79 3403.4 14 0.04768 4.8179 2075.59 1328.4 16 0.07031 7.1046 1407.54 900.8 20 0.04756 4.8058 2080.83 1331.7 22 0.17006 17.1860 581.87 372.4 24 0.12930 12.9359 773.04 494.7 26 0.37629 38.0198 263.02 168.3 28 0.4369 44.1473 226.51 145.0 30 0.1775 17.9358 557.54 3356.8 32 0.27886 28.1779 354.89 227.1 34 0.40188 40.6087 246.25 157.6 36 0.45189 45.6620 219.00 140.2 38 0.50777 51.3085 194.90 124.7 0.54496 55.0664 181.60 116.2 Depth (m) 40 185 Table 14: Single-point Resistance Log Data at Egbeleku Depth (m) ρb(Ωm) ρw(Ωm) EC (µS/cm) TDS (ppm) 8 7.698800 1.425700 7014.10 4,489.020 10 0.374800 0.069400 144,092.21 92,219.01 12 0.937400 0.173600 57,603.69 36,866.36 14 4.263300 0.789500 12,666.24 8,106.400 16 3.615800 0.669600 14,934.29 9557.9400 18 6.612800 1.224600 8,165.93 5,226.200 20 9.551000 1.768700 5,653.87 3618.5000 22 16.58750 3.071800 3255.42 2083.5000 24 18.99530 3.517600 2842.85 1807.9000 26 20.81050 3.853800 2594.84 1660.7000 28 25.65060 4.750100 2105.22 1347.3400 30 28.20990 5.224100 1914.21 1225.1000 32 35.15410 6.510000 1536.10 983.10000 34 48.32210 8.948500 1117.51 715.20000 36 55.23390 10.22850 977.66 625.70000 38 334.2719 61.90220 161.55 103.39000 40 378.4163 70.07710 142.70 91.330000 42 416.7962 77.18450 129.56 82.920000 44 525.3120 97.28000 102.80 65.790000 46 458.4163 84.89190 117.80 75.390000 48 601.2241 111.3378 89.82 57.480000 50 773.5461 143.2493 69.81 44.680000 186 Figure 85: Single-point resistance log data at Otor-Jeremi Motor Park 187 Figure 86: Single-point resistance log data at Otor-Jeremi Market 188 Figure 87: Single-point resistance log data at Burutu 189 Figure 88: Single-point resistance log data at Sapele (Urhiapele Primary School) 190 Figure 89: Single-point resistance log data from Sapele (Headwork) 191 Table 15: Single-point Resistance Log Data at Otor-Jeremi Motor Park Depth (m) ρb(Ωm) ρw(Ωm) EC (µS/m) TDS (ppm) 8 14.6318 2.7096 3690.58 2362.00 10 11.5695 2.1425 4667.44 2987.17 12 13.0297 2.4129 4144.39 2652.42 14 20.6561 3.8252 2614.24 1673.10 16 15.7966 2.9253 3418.45 2187.81 18 19.2407 3.5631 2806.54 1796.20 20 26.0750 4.8287 2070.95 1325.40 22 25.9513 4.8058 2080.82 1331.73 24 8.5968 1.5920 6281.41 4020.12 26 258.9500 47.9537 208.53 133.46 28 273.0289 50.5609 197.78 126.58 30 301.4918 55.8318 179.11 114.63 32 351.3981 65.0737 153.67 98.35 34 341.1985 63.1849 158.27 101.29 36 358.3947 66.3694 150.67 96.43 38 561.4947 103.9805 96.17 61.55 40 708.0516 131.1207 76.27 48.81 42 683.1634 126.5117 79.04 50.56 44 516.8896 95.7203 104.47 66.88 192 Table 16: Single-point Resistance Log Data at Otor-Jeremi market Depth (m) ρb(Ωm) ρw(Ωm) EC (µS/m) TDS (ppm) 8 13.8407 2.5631 3901.52 2497.02 10 9.5386 1.7664 5661.23 3623.19 12 7.4417 1.3781 7256.37 4644.16 14 16.0466 2.9716 3365.19 2153.73 16 65.0587 12.0479 830.02 531.22 18 77.1827 14.2931 699.64 447.77 20 75.7123 14.0208 713.23 450.44 22 179.6834 33.2747 300.53 192.34 24 180.1850 33.3676 299.69 191.80 26 184.3484 34.1386 292.92 187.47 28 220.2633 40.7895 245.16 156.90 30 249.0990 46.1295 216.78 138.74 32 381.7941 70.7026 141.44 90.52 34 339.2226 62.8190 159.19 101.88 36 286.5196 53.0592 188.47 120.62 38 506.7449 93.8416 106.56 68.20 40 541.1838 100.2192 99.78 63.86 193 Table 17: Single-point Resistance Log Data at Burutu Depth (m) ρb(Ωm) ρw(Ωm) EC (µS/m) TDS (ppm) 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 0.0729 1.8603 1.4834 0.4606 0.6242 0.4655 0.5778 0.7366 0.5492 0.6496 1.8916 0.9353 1.8079 6.1560 2.2453 0.8894 8.6848 10.3162 0.9191 0.9445 5.4081 1.4747 1.5930 14.0681 443.6132 552.8099 1,051.1731 1,217.5920 1,311,0714 1,411.3461 1,263.1473 1,148.5524 0.0135 0.3445 0.2747 0.0853 0.1156 0.0862 0.1070 0.1364 0.1017 0.1203 0.3503 0.1732 0.3348 1.1400 0.4158 0.1647 1.6083 1.9104 0.1702 0.1749 1.0015 0.2731 0.2950 2.6052 82.1506 102.3722 194.6617 225.4800 242.7910 261.3604 233.9162 212.6949 740,740.74 29,027.58 36,403.35 117,233.29 86,505.19 116,009.28 93,457.94 73,313.78 98,328.42 83,125.52 28,546.96 57,736.72 29,868.58 8,771.93 24,050.02 60,716.45 6,217.75 5,234.51 58,754.41 57,175.53 9,985.02 36,616.62 33,898.31 3,838.48 121.73 97.68 51.37 44.35 41.19 38.26 42.75 47.02 474,074.07 18,577.65 23,298.14 75,029.31 55,363.32 74,245.94 59,813.08 46,920.82 62,930.19 53,200.33 18,270.05 36,951.50 19,115.89 5,614.04 15,392.02 38,858.53 3,979.36 3,350.08 37,602.82 36,592.34 6,390.41 23,434.64 21,694.92 2,456.63 77.91 62.52 32.88 28.38 26.36 24.49 27.36 30.09 194 Table 18: Single-point Resistance Log Data at Sapele (Urhiapele Primary School) Depth (m) ρb(Ωm) ρw(Ωm) EC (µS/m) TDS (ppm) 8 51.8400 9.6358 1,037.80 664.19 10 8.7766 1.6253 6,152.71 3937.73 12 21.3840 3.9600 2,525.25 1616.16 14 338.2117 62.6318 159.66 102.18 16 390.7202 72.3556 138.21 88.45 18 219.2400 40.6000 246.31 157.64 20 331.5006 61.3890 162.90 104.25 22 147.3860 27.2937 366.38 234.49 24 185.8599 34.4185 290.54 185.95 26 154.3433 28.5821 349.87 223.92 28 192.9425 35.7301 279.88 179.12 30 34.8116 6.4466 1551.21 992.77 32 88.3300 16.3574 611.34 391.26 34 344.3337 63.7655 156.82 100.37 36 565.1321 104.6541 95.55 61.15 195 Table 19: Single-point Resistance Log Data at Sapele (headworks) Depth (m) 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 ρb(Ωm) ρw(Ωm) 51.1428 25.1717 31.9641 224.1519 76.7147 103.8861 97.1253 167.8184 511.4278 244.5331 431.5200 417.5352 575.3600 654.4695 738.3787 1791.9625 1534.2934 679.2462 373.9840 1844.7089 1342.5067 486.6612 1582.2906 2848.8102 2761.7079 2637.1173 5298.0816 3020.6906 2416.5323 2572.3464 455.4176 1932.2154 3068.5868 2702.9998 9.4709 4.6614 5.9193 41.5096 14.2064 19.2382 17.9862 31.0775 94.7089 45.2839 79.9111 77.3213 106.5481 121.1981 136.7368 331.8449 284.1284 125.7863 69.2563 341.6128 248.6124 90.1224 293.0168 527.5574 511.4274 488.3551 981.1262 559.3881 447.5060 476.3604 84.3366 357.8177 568.2568 500.5555 196 EC (µS/m) 1,055.87 2,145.28 1689.39 240.91 703.91 519.80 555.98 321.78 105.57 220.83 125.14 129.33 93.85 82.51 73.13 30.13 35.20 79.50 144.39 29.27 40.22 110.96 34.13 18.96 19.55 20.48 10.19 17.88 22.35 20.99 118.57 27.95 17.60 19.98 TDS (ppm) 675.75 1372.98 1081.21 154.18 450.50 332.67 355.83 205.94 67.58 141.33 80.09 82.77 60.07 52.81 46.81 19.29 22.53 50.88 92.41 18.73 25.74 71.01 21.84 12.13 12.14 13.11 6.52 11.44 14.30 13.44 75.89 17.89 11.26 12.79 Table 19 continued Depth (m) 76 78 80 82 84 86 88 90 92 94 96 98 100 102 104 106 108 110 112 114 116 118 120 122 124 126 128 130 ρb(Ωm) ρw(Ωm) 1141.1206 9020.0365 632.8960 2661.0654 2095.3048 2929.5465 1534.2934 1448.6705 1859.2148 1343.1469 914.1900 1699.8102 6947.4672 6197.0943 1740.2556 8927.6798 5793.6147 2404.5077 1246.6134 3875.3438 2403.7162 6922.3066 2912.3438 7401.7734 5295.4420 3750.6062 3989.4931 5621.3301 211.3186 1670.3771 117.2030 492.7900 388.0194 542.5086 284.1284 268.2723 344.2990 248.7309 169.2944 314.7797 1286.5680 1147.6100 322.2696 1653.2740 1072.8916 445.2792 230.8543 717.6563 445.1326 1281.9086 539.3229 1370.6987 980.6374 694.5567 738.7950 1040.9810 197 EC (µS/m) 47.32 5.99 85.32 20.29 25.77 18.43 35.19 37.27 29.04 40.20 59.07 31.77 7.77 8.71 31.03 6.05 9.32 22.46 43.32 13.93 22.47 7.80 18.54 7.30 10.20 14.40 13.54 9.61 TDS (ppm) 30.29 3.83 54.61 12.99 16.49 11.80 22.53 23.86 18.59 25.73 37.80 20.33 4.97 5.58 19.86 3.87 5.97 14.37 27.72 8.92 14.38 4.99 11.87 4.67 6.53 9.21 8.66 6.15 The results of water quality based on total dissolved solids and electrical conductivity evaluated from the resistivity log at Egbeleku is shown in Table 14, Figures 84 and 93 shows that at depths of between 8.0 – 36.0 m, the total dissolved solids concentration varies between 625.70 – 92, 219.01 ppm while at depths of 38.0 – 50.0 m, the total dissolved solids concentration varies between 55.68 – 103.39 ppm. The results of water quality based on total dissolved solids and electrical conductivity evaluated from the resistivity log at the Otor-Jeremi motor park (Table 15, Figures 85 and 94) the TDS concentration varies from 1325.40 – 4020.12 ppm between the depths of 8.0 – 24.0 m. However from depths of 26.0 – 40.0 m, the TDS concentration varies from 48.81 – 133.46 ppm. At Otor-Jeremi market (Table 16, Figures 86 and 95) shows that at depths between 8.0 – 16.0 m, the TDS concentrations varies between 531 – 4644.16 ppm. Between depths of 18.0 – 40.0 m, the TDS concentration varies from 63.86 – 450.44 ppm. The water quality results at Burutu (Table 17, Figures 87 and 96) based on Total dissolved solids evaluated from single-point resistivity log indicated that the TDS concentration at depths of between 8.0 – 54.0 m varies between 3,350.08 – 474,074.07 ppm. At depths of between 56.0 – 70.0 m, the TDS concentration ranges from 27.36 – 77.91 ppm. The TDS concentration evaluated from the single-point resistance logging done at Urhiapele Primary School, Sapele (Table 18, Figures 88 and 97) showed TDS variation between 664.19 – 1616.16 ppm at depths of 8.0 – 12.0 m. At depths of between 14.0 – 36.0 m, the TDS concentration varies between 61.15 – 391.26 ppm 198 except at 30.0 m where the TDS concentration is 992.71 ppm. At the Sapele headwork (Table 19, Figures 89 and 98), the TDS concentration varies between 675.75 – 1372.98 ppm at depths of 8.0 – 12.0 m while at depths of 14.0 – 130.0 m, the TDS concentration varies between 3.83 – 450.0 ppm. Figure 90: A bar chart showing variation of TDS with depth at Ughoton 199 Figure 91: A bar chart showing variation of TDS with depth at Ekakpamre 200 Figure 92: A bar chart showing variation of TDS with depth at Uvwiamuge 201 Figure 93: A bar chart showing variation of TDS with depth at Egbeleku 202 Figure 94: A bar chart showing variation of TDS with depth at Otor-Jeremi Motor Park 203 Figure 95: A bar chart showing variation of TDS with depth at Otor-Jeremi Market 204 Figure 96: A bar chart showing variation of TDS with depth at Burutu 205 Figure 97: A bar chart showing variation of TDS with depth at Sapele (Urhiapele Primary School) 206 Figure 98: A bar chart showing variation of TDS with depth at Sapele (Headwork) 207 5.6 Determination of Groundwater Flow Direction The basic analogy between electrical flow and groundwater flow is illustrated in the forms of Ohm’ s law and Darcy’ s law (Cherry and Freeze, 1979; Ajibade et al., 2012). It is possible to determine the direction of groundwater flow from electrical resistivities measured as a function of azimuths (Cherry and Freeze, 1979). When apparent resistivities measured in azimuths are plotted as radii, they generate anisotropy figures which are an ellipse. The major axis of these figures (polar diagrams) coincides with the strike of the fractures/pores, while the true resistivity parallel to the fracture is equivalent to the minor axis of the ellipse (Taylor and Fleming, 1988; Skjernaa, and Jørgensen, 1993). The inferred structural trend from the polar diagram is direction of groundwater flow. Groundwater flow direction was obtained from the azimuthal radial sounding polar diagram. Figures 99 – 105 show polar diagram of apparent resistivity obtained at different azimuths and depths for the respective radial sounding stations. The major axis of the maps represents the transverse resistivity and corresponds to the general strike of the predominant structural feature in the vicinity of each station. Since the study area is not in the basement complex the bedding plane and grains size can be considered as a major factor creating anisotropy, while the minor axis represents the longitudinal resistivity. The polar diagrams of apparent resistivity strongly denotes electrical anisotropy hence an inhomogeneous medium, this is typical of the sedimentological framework/structure of the Benin Formation and its mode of deposition especially 208 with quaternary deposits. The coefficients of anisotropy obtained from the results are as presented in Table 20. The study identified NW – SE trend (Ughoton, Ekakpamre, Uvwiamige, Otor-Jeremi and Sapele) and NE – SW trend (Egbeleku and Burutu) as the electrical anisotropy direction. This trend is also the direction of groundwater flow direction in the study area. Linkages of structural trends encourage movement of groundwater (Okorumeh and Olayinka, 1998) and hence contaminant plume. The coefficient of anisotropy varies between 1.20 – 1.73. The estimate values of coefficient of anisotropy are generally found to increase in magnitude with depth of investigation indicating grain size increasing with depth. To further support the ARS interpretation groundwater hydraulic head data from boreholes and hand-dug wells were used to generate groundwater flow contour maps (Figures 106 – 107). These maps show the various directions of groundwater flow establishing the dorminant directions to be NW – SE at Ughoton, Ekakpamre, Uvwiamuge, Otor-Jeremi, Sapele; NE – SW at Egbeleku and Burutu. 209 Table 20: Coefficient of Anisotropy Location Ughoton AB/2 25 50 75 100 Ekakpamre 25 50 75 100 Uvwiamuge 25 50 75 100 Egbeleku 25 50 75 100 Otor-Jeremi 25 50 75 100 Burutu 25 50 75 100 Sapele 25 50 75 100 ρt 7.80 10.10 15.50 16.30 6.90 8.90 9.60 15.50 5.70 12.40 12.50 16.20 3.25 5.75 11.50 14.00 5.00 6.80 12.50 13.60 4.00 7.40 11.70 13.9 6.90 9.00 10.60 16.30 ρL 3.50 4.30 6.20 8.50 2.65 3.80 4.70 7.00 2.70 4.20 6.70 7.60 1.75 3.15 4.10 5.70 2.80 3.60 5.90 6.60 2.35 3.50 3.90 5.40 3.50 6.20 7.00 8.00 210 λ 1.49 1.53 1.58 1.38 1.61 1.53 1.43 1.49 1.45 1.73 1.37 1.46 1.36 1.35 1.67 1.57 1.34 1.37 1.46 1.44 1.30 1.45 1.73 1.60 1.38 1.20 1.23 1.43 Trends NW-SE NW-SE NW-SE NW-SE NW-SE NW-SE NW-SE NW-SE NW-SE NW-SE NW-SE NW-SE NE-SW N-S NE-SW NE-SW N-S NW-SE NW-SE NW-SE N-S NE-SW NE-SW NE-SW NW-SE NW-SE N-S NW-SE Apparent Resistivit y (ohm-m) 25m 50m 75m 100m Figure 99: Polar diagram for apparent resistivity at Ughoton against azimuth at AB/2 of 25, 50, 75 and 100 m 211 Apparent Resistivity (ohm-m ) 25m 50m 75m 100m Figure 100: Polar diagram for apparent resistivity at Ekakpamre against azimuth at AB/2 of 25, 50, 75 and 100 m 212 Apparent Resistivity (ohm-m ) 25m 50m 75m 100m Figure 101: Polar diagram for apparent resistivity at Uvwiamuge against azimuth at AB/2 of 25, 50, 75 and 100 m 213 Apparent Resistivity (ohm-m ) 25m 50m 75m 100m Figure 102: Polar diagram for apparent resistivity at Egbeleku against azimuth at AB/2 of 25, 50, 75 and 100 m 214 Apparent Resistivity (ohm-m ) 25m 50m 75m 100m Figure 103: Polar diagram for apparent resistivity at Otor-Jeremi against azimuth at AB/2 of 25, 50, 75 and 100 m 215 Apparent Resistivity (ohm-m ) 25m 50m 75m 100m Figure 104: Polar diagram for apparent resistivity at Burutu against azimuth at AB/2 of 25, 50, 75 and 100 m 216 -2000 -1800 -1600 -1400 Apparent Resistivity (ohm-m) -1200 -1000 -800 -600 -400 -200 -200 -400 - 600 -800 -1000 -1200 -1400 -1600 25m 50m 75m 100m -1800 -2000 Figure 105: Polar diagram for apparent resistivity at Sapele against azimuth at AB/2 of 25, 50, 75 and 100 m 217 5.68 N 5.67 5.66 5.65 5.64 5.63 5.62 5.64 5.65 5.66 5.67 Direction of water flow Figure 106: Groundwater head contour map showing flow direction at Ughoton 218 5.545 N 5.54 5.535 5.53 5.525 5.52 5.905 5.91 5.915 5.92 5.925 5.93 Direction of water flow Figure 107: Groundwater head contour map showing flow direction at Ekakpamre 219 5.555 5.55 N 5.545 5.54 5.535 5.53 5.875 5.88 5.885 5.89 5.895 5.9 5.905 Direction of water flow Figure 108: Groundwater head contour map showing flow direction at Uvwiamuge 220 5.7 5.698 N 5.696 5.694 5.692 5.69 5.688 5.686 5.684 5.784 5.786 5.788 5.79 5.792 5.794 5.796 5.798 5.8 Direction of water flow Figure 109: Groundwater head contour map showing flow direction at Egbeleku 221 5.452 5.45 N 5.448 5.446 5.444 5.442 5.44 5.438 5.436 5.434 5.432 5.868 5.87 5.872 5.874 5.876 5.878 5.88 5.882 5.884 Direction of water flow Figure 110: Groundwater head contour map showing flow direction at Otor-Jeremi 222 5.886 5.365 5.36 N 5.355 5.35 5.345 5.34 5.335 5.505 5.51 5.515 5.52 5.525 5.53 Direction of water flow Figure 111: Groundwater head contour map showing flow direction at Burutu 223 5.9 5.89 N 5.88 5.87 5.86 5.85 5.64 5.65 5.66 5.67 5.68 5.69 Direction of water flow Figure 112: Groundwater head contour map showing flow direction at Sapele 224 5.7 CHAPTER SIX 6.0 DISCUSSION 6.1 Ughoton At Ughoton, there are three to four geologic layers made up of top soil, clay/sandy clay/clayey sand and sand. The resistivity of the first layer ranges from 281.0 – 2600 Ωm with thickness varying between 0.9 – 2.0 m. The second layer is composed of clay, sandy clay, clayey sand and sand, the resistivity of this layer ranges from 31.2 – 4252.3 Ωm with a thickness of 3.3 – 36.0 m. The third layer comprising of fine to coarse grained sand has resistivity values ranging from 323.1 – 2833.3 Ωm and thickness of 8.5 – 54.7 m corresponds to the aquifer. The resistivity of the fourth layer ranges from 93.3 – 7217.9 Ωm and diagnostic of clay (at VES 4) and sand (at other VES locations). The exact thickness of this layer could not be determined as the electrode current terminated within this layer. However, inference from VES 19 shows the possibility that the sandy clay horizon exceed 31.6 m in thickness while inference from VES 6 shows the possibility of the sand exceeding 15.1 m in thickness. The sand of the fourth layer constitutes the aquifer at VES 6 and 16. The average depth to the aquifer is 12.0 m; the shallow nature makes the aquifer in Ughoton vulnerable to contamination. The isoresistivity map of Ughoton at 5.0 m (Figure 40) shows the 15 % of the area in the central (VESes 5, 15 and 16) and southeastern parts is underlain by clay, 20.0 % of the area in the northwestern, central and southeastern parts 225 (VESes 3, 6, 10 and 13) is underlain by sandy clay and clayey sand while the remaining 65.0 % is underlain by sand; at 10.0 m (Figure 40), areas covered by clay which act as protective layers have been reduced except at VES 5, 6, 15 and 16. At 20.0 m, the lithology is mainly sand except at VES 15 and 16. The longitudinal conductance map (Figure 53) shows that the protective capacity of the overburden above the aquifer at in 85.0 % Ughoton is weak or poor. However, VES 6 and 16 are located in area having moderate protective capacity. Laterite, silt and clay often constitute protective geologic barriers and when found above an aquifer they constitute its cover (Lenkey et al., 2005). However, an effective groundwater protection is given by protective geologic barriers with sufficient thickness (Mundel et al., 2003) and low hydraulic conductivity leading to high residence time of percolating water (Kirsch, 2006). The result of water quality at Ughoton based on total dissolved solid is shown in Table 9. Total dissolved solid in water is related to the resistivity of water, in water wells, higher resistivity in a saturated zone implies higher quality. As TDS decreases, water quality increases (Turcan, 1966). The levels of TDS in groundwater in Ughoton when compared with the lower limits of 500 ppm set by USEPA (2011) for potable drinking indicates that there is indeed cause for concern at Ughoton. At depth of 8.0 – 22.0 m, the TDS concentration ranged from 613.0 – 6339.0 ppm and this indicates low quality groundwater. The quality of groundwater appeared to be higher at depths of between 24.0 – 44.0 m where TDS 226 concentration is less than 500 ppm. The result indicates that the shallow groundwater has been contaminated; the poor and weak protective capacity of overburden above the aquifer in most part of Ughoton may have enhanced contamination. The presence of heavy metals (Lead and Cadmium) may be one reason why concentration level of TDS is above the secondary maximum contaminant levels (SMCLs) of 500ppm. The levels of occurrence in groundwater of these heavy metals (Table 21) when compared with the maximum contaminant level (MCLs) set by World Health Organization (WHO, 2006) indicate that there is indeed cause for concern at Ughoton. Lead levels exceed the MCLs of 0.01 mg/l at fifty percent (50.0 %) of all locations sampled. Similarly, mean cadmium levels are approximately six times higher at 0.02 mg/l than the MCLs of 0.003 mg/l while iron levels exceed the MCLs of 0.3 mg/l at 29.0 % of all locations sampled. Table 21: Water quality from dug wells and Ughoton River based on chloride content Location River 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Cl-(mg/l) 35 22.0 20.0 23.0 24.0 28.0 26.0 6.50 14.23 34.22 54.50 21.66 12.6 42.10 24.6 Cd+ 0.06 0.01 0.01 0.03 0.02 0.03 0.01 0.00 0.001 0.02 0.07 0.01 0.00 0.04 0.02 Pb2+ 0.04 0.00 0.00 0.01 0.00 0.00 0.00 0.001 0.001 0.04 0.09 0.02 0.01 0.06 0.03 Fe2+ 0.31 0.00 0.00 0.01 0.01 0.00 0.01 0.44 0.77 0.87 1.42 0.06 0.02 0.04 0.02 (Source: Akpoborie and Aweto, 2012). 227 The precipitation of lead and the adsorption of lead on organic and inorganic sediment surfaces help to maintain low lead concentration levels in groundwater (Hem, 1985). However much of the lead present in tap water may come from anthropogenic sources, particularly lead solder used in older plumbing systems. The primary source of the iron contamination is geologic. According to Etu-Efeotor (1981), the source of iron in groundwater can be related to the leaching of Fe2+ into the groundwater from iron-bearing minerals such as hematite, limonite and goethite that are abundant within sediments of the deltaic plain sands and the Benin Formation that underlie the affected area. Groundwater in gas flared area tend to have more concentrations of heavy metals such as; lead, iron, cadmium, manganese and copper than non gas flared area as acid rain readily leaches potentially toxic metals from soils, transferring them to groundwater supplies and reduces the capacity of soils to neutralize further additions of acid from these materials into the groundwater system. Such acidity has usually beebn associated with acid rain resulting from gas flare stations (Ekakitie et al., 2000). Elevated lead, chromium and iron in groundwater have been observed in the upper coastal plain deposits by Akpoblorie, 2011; Aweto and Akpoborie, 2011. Similar trend of occurrence of this suite of heavy metals have been reported in shallow groundwater from the Saghara Beach area of the Forcados estuary by the Shell Petroleum Development Company (SPDC, 2005). It thus appears that 228 these heavy metals seem to be confined to the shallow groundwaters that occur in the quaternary upper coastal plain deposits. The azimuthal resistivity sounding polar diagram (Figure 99) and groundwater head contour map (Figure 106) indicates that groundwater flow station direction is northwest towards the dominant and perennial Ughoton River. Groundwaterater recharges the Ughoton River and the effect of tide-related brackish water that supports the mangroves is limited to quick flow in the banks (Akpoborie and Aweto, 2011), which finding explains the very delicate and fragile nature of this Mangrove ecosystem. Furthermore, because the Ughoton River is connected to the numerous distributaries and creeks of the dorminant Escravos River, Forcados River to the west and south and indeed to all the distributaries of the delta (Abam, 1999), it receives recharge from all these all these combined sources. An insight into the quantum of groundwater recharge is provided by Akpoborie (2011) who showed that groundwater contributes up to 85 % and 72 % to the total flow of the Adofi and Ethiope rivers which like the Ughoton river flow on the quaternary deposits of the Western Niger Delta. It is also conceivable that some of the groundwater recharge would be from intermediate or regional groundwater flow systems that occur deeper in the underlying Benin Formation (Toth, 1963). This is consistent with the general groundwater recharge model for rivers explained by Ophori (2007) for the Niger Delta region and has important implications for the entire Mangrove Swamp terrain as well as the entire Niger Delta wetland groundwater system. In contex of 229 the groundwater continuum for wetlands defined by Euliss et. al. (2004), the Ughoton area may tentatively be classified as a discharge wetland with local and intermediate flow systems. The confinement of mangroves to the river banks is thus a result of the daily influx of brackish water from the tidal regime in which brackish water is prevented from recharging groundwater because of the prevailing differential in water table head and river water level. Though Ughoton lies within the brackish mangrove swamps, the groundwater is however not saline. This was supported by the 2D electrical resistivity imaging profiles which has shown that the subsurface has not been intruded by saline water and also supported by Akpoborie and Aweto (2012) who reported that chloride content of all water is unexpectedly low (mean chloride content is 25.24 mg/l). The elevated TDS concentration in the shallow aquifer may also be influenced by groundwater pollution and originated from sources such as domestic wastes disposal, landfills and runoff from urban or agricultural areas which are some human factors that may add dissolved constituents to groundwater. Though VES 6 and 16 are located within areas having moderate protective capacity, the groundwater beneath is significantly protected from surface or near surface source pollution. The groundwater in the aquifer may be polluted via base flow within the canals and by moving groundwater already contaminated flowing SE – NW from areas around VES 9, 10, 11, 12, 13, 14, 17, 18 and 19 that have poor protective capacity. 230 6.2 Ekakpamre Three distinct geologic layers which include the top soil, clayey sand and sand were delineated at Ekakpamre. The first layer which represents the top soil has resistivity values ranging from 43.0 – 713.0 Ωm and thickness varying between 0.6 – 1.5m. The second layer is clayey sand (VES 22, 27, 28, 29 and 30) and sand (VES 21, 23, 24 and 26) with resistivity values ranging from 119.0 – 1001.0 Ωm and thickness varying between 2.8 – 16.3 m. The third layer with resistivity values ranging from 341.0 – 2006.0 Ωm and thickness of 16.2 – 32.6m corresponds to the aquifer and comprises of sand. A fourth layer made up of sands with resistivity values ranging from 331.0 – 895.0 Ωm except at VES 27 where clayey sand was also delineated. The exact thickness of this layer which forms part of the aquifer could not be determined as the electrode current terminated within this layer. The isoresistivity map at 5.0 m (Figures 17 and 18) indicates that the areas around VES 28, 29, 22, 27 and 30 is underlain by clayey sand, the presence of sand increase hydraulic conductivity creating path ways for contaminants to get to the aquifer beneath them. The longitudinal conductance map (Figure 22) shows that 70.0 % of Ekakpamre has poor protective capacity. The aquifer occur at an average depth of 11.0 m, the aquifer is not well protected and prone to contamination from surface and subsurface sources such as refuse dumps and from oil well and pipelines in case there are leakages. 231 The TDS concentration of the shallow groundwater at depths between 8.0 –12.0 m indicates concentration above the maximum contamination limits of 500 ppm set by USEPA (2011), thus the groundwater quality is low. The elevated concentration of 518.4 – 1935.7 ppm may be related to contamination from domestic wastes; mineral enrichment from underlying rocks and sediments as ions dissolve slowly from soil particles, sediments and rocks; precipitation water or river water that recharges the aquifer. But as depth increases to between 14 – 40 m, the TDS concentration ranged from 36.5 – 406.7 ppm. At these depths the groundwater quality appears to be higher and is suitable for domestic and industrial purposes. The Azimuthal resistivity sounding polar diagram (Figure 100) and groundwater head contour map (Figure 107) show that groundwater flow is in northwest direction. This support the fact that Obanegbe River is a losing steam which recharges the aquifer and may be a major source of contamination as it carries a lot of dissolved substances. Losing Streams are streams in which water can move from the stream into the ground if the water table is at a lower elevation than the surface of the stream. A rainfall event, flood condition, or man-made discharge can cause the surface of a stream to rise above the elevation of the water table and force water in the stream into the groundwater system. An active water supply well can locally lower the water table and attract the stream water into the groundwater system. There are some concerns about drilling of wells in 232 Ekakpamre because if the water surface of Obanegbe River rises to an elevation above the bottom of the supply well, there is the possibility of attracting water from the River. Rivers are the major transporting means for different contamination into other resources like groundwater (Karbassi and Ayaz, 2007). The causes of pollution in rivers can be attributed to complex industrial processes, rural/urban effluents and atmospheric precipitation (Stoeppler, 1991). Table 22: Heavy metals in surface water in Warri in mg/L Station Warri River Aladja Water Sediment Cadmium Chromium Copper Lead Manganese at 0.265 0.656 0.032 0.258 0.012 0.067 0.073 0.199 0.067 0.194 Edjere River Water Sediment 0.272 0.444 0.084 0.325 0.036 0.190 0.152 0.120 0.514 1.443 Crawford Creek Water Sediment 0.384 0.499 0.069 0.251 0.009 0.043 0.094 0.101 0.591 1.771 Tori Creek Water Sediment 0.248 0.445 0.044 0.152 0.000 0.073 0.095 0.138 0.483 1.100 0.003 0.05 1.00 0.01 0.20 Maximum Permited SON (2007 ) Source: Egborge, 1996 233 According to Delta State Government Water Supply Master Plan (DTSG 1994), the Ethiope River and Adofi River are the only rivers with clear water which can be used after chlorination. The Ase River, Niger Rivers and other rivers in the state are highly contaminated. Egborge (1996) for example has undertaken a detailed examination of the Warri River with respect to Industrial pollution and the presence of heavy metals. Some results of this study which include other nearby creeks are presented in Table 22. 6. 3 Uvwiamuge The geologic section at Uvwiamuge (Figures 29 and 30) shows three to four geologic horizons. The resistivity of the first layer, range from 80.0 –1102 Ωm and thickness varying between 0.5 – 1.5 m. The second horizon has resistivity values ranging between 25.0 – 1532.0 Ωm and thickness of 0.6 – 27.1 m. This horizon comprises of clay (VES 40, 41 and 44), clayey sand (VES 34, 36, 37, 38, 39 and 45) and sand (VES 31, 32, 33, 35, 42 and 43). The third horizon comprising of sand constitute the aquifer and has resistivity values ranging from 300.0 – 2231.0 Ωm and thickness of 2.0 – 98.3 m. The resistivity of the fourth horizon ranges from 21.0 – 929.0 Ωm diagnostic of clay (VES 33, 36 and 37), clayey sand (VES 32, 38 and 39) and sand (VES 31, 34, 35, 40, 41, 42, 43, 44 and 45). The thickness of this horizon could not be determined as the electrode current terminated within this horizon which constitutes the aquifer at VES 34. 234 The longitudinal conductance map (Figure 55) shows that the overburden materials above the aquifer in 73.0 % of Uvwiamuge community have poor protective capacity while the remaining 27.0 % of the community have weak protective capacity. The clays and clayey sand underlying some areas especially around the dumpsite at depth of 5m (Figure 43) just above the aquifer (average depth of aquifer is 10.0 m) were able to give the aquifer a level of protection. The TDS concentration in groundwater at depths of 8.0 – 20.0 m varied between 900.8 – 1722.5 ppm and this is indicative of contamination of the groundwater at these depths. According to Shell Petroluem Development Company report (SPDC, 2005), the integrity of the engineered liner layer consisting of clay and concrete/bitumen stabilized sands at the dumpsite located at Uvwiamuge had been compromised and intense downward percolations of leachate have been recorded. The areas where intense downward percolation of leachates was recorded often coincided with places where the integrity of the engineered liner layer has been compromised. It is suspected that the weak acids in the leachates have helped to dissolve and disaggregate the calcite particles in the cement. The results of the physico-chemical analysis of soil and groundwater are shown in Tables 23, 24 and 25 The soil heavy metal concentration of Cu, Fe, Pb, Cd, V, Cr, Ni were low. Iron is particularly high due to probably to acid solubilization under the prevailing acidic conditions of the soils (Amajor, 1985; Alloway, 1990). For impacted area, Cr, Cd, Ni, Pb, Cu, Zn, Mn, Ag and Fe were high. Except at same isolated 235 locations, the soil physico-chemical conditions in the unimpacted areas were essentially comparable with those within and around the landfill (impacted) area. This suggests that pollution of the ground by the leachates from the landfill is minimal. Table 23: Summary of heavy metals of soils and decomposed wastes Location Cr Cd Ni Pb Cu Zn Mn Fe THC Waste pit soil 0.024 0.007 0.007 0.308 0.045 0.963 0.730 13.98 0.00 - BTEX PAH <0.0001 <0.0001 0.003 Surrounding 0.013 0.005 0.0017 0.242 0.013 0.308 0.597 7.75 <0.0001 <0.0001 <0.0001 0.19 0.094 0.85 1.69 0.21 20.15 6.67 64.70 0.003 <0.0001 <0.0001 area soil Dumpsite (SOURCE: SPDC, 2005) Table 24: Groundwater Physical and chemical properties, Ughelli West engineered dumpsite Location EC Salinty Ca Mg K Na NO3 SO4 Cl Cell 2 dumpsite 3010 0.15 8.41 3.49 2.86 1.75 1.516 7.64 47.72 0.01 0.45 0.24 0.26 0.25 0.079 0.52 4.83 Well at Uvwiamuge 22 Pri. School (SOURCE: SPDC, 2005) Table 25: Heavy metals and hydrocarbon content in groundwater, Ughelli West engineered dumpsite Location Cr Cell 2 0.066 dumpsite Well at 0.02 Uvwiamuge Cd Ni Pb Cu Zn Mn Fe 0.125 0.28 0.127 0.293 0.58 0.76 14.5 0.00 0.00 0.00 0.00 0.008 0.00 0.045 (SOURCE: SPDC, 2005) 236 The groundwater is very low in nitrate (0.079 ppm) and heavy metals (Fe, Zn, Pb, Cu, Cr and Cd). The values for total hydrocarbon (THC = 0.001-0.003 ppm), poly aromatic hydrocarbons (PAH = < 0.001 – 0.15 ppm) and benzene, toluene, ethylene and xylene (BTEX < 0.001 ppm) in groundwater are extremely low indicating the absence of hydrocarbon contamination. The result of microbiological analysis (SPDC, 2005), show that the population of hydrocarbon degraders ranged from 0 – 1x102 for bacteria and from 0 – 1x101 for fungi. The percentage of hydrocarbon utilizers in the area was (0.01 %), a value that is well below the 10.0 % level (Ekundayo, 1987) permitted by regulators, indicating an environment that is not impacted by hydrocarbon, there is no convincing evidence of chemical or microbiological contamination of the environment (groundwater) by leachates discharged from the Ughelli West Engineered dumpsite. From the above account, it can be concluded that the chemical characteristics of the groundwater from the surficial aquifer reveals that there was no groundwater contamination by the landfill. Despite that the integrity of the liner layer being compromised at several areas leading to the downward migration of leachate into the sub-surface beds. The abundant occurrence of clayey lithology however, limited the downward flow of leachates to depths not exceeding 5.0 m at most locations. The clayey materials also probably acted as filters preventing lateral migration of contaminants (SPDC, 2005). Much of these leachates were attenuated, thus suggesting that the leachates may not have impacted areas that are over 1 km from the dumpsite (SPDC, 2005) even though 237 the dumpsite lie in an area of groundwater discharge as seen from azimuthal resistivity sounding polar diagram (Figure 101) and groundwater head contour map (Figures 108). Though leachates from the dumpsite may not have impacted the groundwater in the residential area, there is however concerns about contamination from other sources. The area affected by a point source such as the dumpsite may be small. However, the presence of many overlapping point sources such indiscriminate disposal of wastes and sewage in the residential area may coalesce to form a very large plume. Also, if the source provides a continuous supply of contaminants, the plume will grow in size as a function of time. The probable source of TDS in groundwater includes human wastes, agricultural wastes and mineral enrichment from underlying sediments and rocks. However at depth zones of 22.0 – 40.0 m, the TDS concentration ranged between 116.2 – 494.7 ppm. TDS for uncontaminated groundwater are generally lower (up to 550 ppm) compared to contaminated groundwater (up to 810 ppm) (Atekwana et al, 2004). Also, for potable drinking water USEPA (2011) proposed 500 ppm as the maximum contamination limits for general acceptability, since the TDS concentration in these zones is less than 500 ppm, the quality of groundwater here appeared to be higher and the water suitable for human consumption 238 6.4 Egbeleku The geologic sections at Egbeleku (Figures 31 and 32) show four distinct geologic layers made up of top soil, sandy clay/clayey sand, clay and sand. The first layer is the top soil of variable composition while the second layer is made up of clayey sand and sand. The third layer constituted by clays has resistivity value ranging from 16.0 – 82.7 Ωm, while its thickness varies from 11.0 – 41.6 m. These clays are found almost everywhere except at some points (VES 52 and 57) where the lithology is sandy clay of about 35.2 – 38.2 m thick, clayey sand (VES 56) of about 44.0 m thick and sand (VES 49 and 51) with thickness between 19.7 and 36.0 m. The resistivity of the sandy clay and clayey sand lenses ranges from 125.0 – 145.0 Ωm. This third layer is underlain by a fourth layer comprising of sand which constitutes the aquiferous unit in this area, the thickness of this layer could not be determined as the electrode current terminated within this layer. The depth to this aquifer which is mostly confined however varies from about 7.0 – 47.9 m. The isoresistivity map (Figures 45 and 46) and longitudinal conductance map (Figure 56) of Egbeleku shows that the overburden lithologies provides good protection for the aquifer. The isoresistivity maps shows that at 5.0 m, about 27.0 % and 46.0 % of Egbeleku is underlain by clay and sandy clay/clayey sand; at 20.0 m, about 50.0 % and 30.0 % is underlain by clay and sandy clay/clayey sand and at 30.0 m, about 30.0 % and 15.0 % is underlain by clay and sandy clay/clayey sand. The aquifer in this area which occurs at depths of between 24.3 239 – 47.9 m is confined by clays sandy clays and clayey sand with thickness ranging from 1.3 – 44.0 m. These layers also play important roles in attenuation of pollutants by filtration and sorption. As groundwater flows through the subsurface it is naturally filtered as groundwater has to squeeze through pore spaces of rock and sediment as it moves through an aquifer (the porosity of such layers makes them good filters for natural purification). Because it takes effort to force water through tiny pores, groundwater loses energy as it flows, leading to a decrease in hydraulic head in the direction of flow (long residence time). Larger pore spaces usually have higher permeability, produce less energy loss, and therefore allow water to move more rapidly for this reason, groundwater can move rapidly over large distances in layers whose pore spaces are large leading to poor natural filtration process. In such case the spread of contaminants can be difficult or impossible to prevent. Clay particles in or overlying an aquifer also can trap dissolved substance or at least slow them down so they do not move as fast as water percolating through the aquifer. Hence, layers of silt and clay can provide natural protection to the aquifer from contaminants. The soils ability to lessen the amount or reduce the severity of groundwater contamination is called soil attenuation. Deep, medium and finetextured soils are the best, whereas coarse-textured materials are the worst in terms of contaminant removal (Good and Madison, 1987). Vertical electrical sounding result shows that the aquifer is deep (> 40.0 m) and made of fine to medium sand. Contaminant attenuation in aquifers depends on water flowing 240 through the aquifer at rates that ensures maximum contact between the percolating water that contains contaminants and the soil particles. The clays, sandy clays and clayey sand with thickness ranging from 1.3 – 44.0 m overlying the aquifer help to protect the aquifer from surface and near surface contamination because their low hydraulic conductivity leads to high residence time of percolating water. During this long percolating period, contaminant degradation can occur by mechanical, physicochemical and microbiological process (Kirsch, 2006). The result of water quality indicates water rising above the confining beds at depths of 8.0 – 36.0 m is not potable due to TDS concentration above the limits set by World Health Organization (WHO, 2006) for potable water, the high TDS values may be due to clays (Aweto, 2013). Ion-exchange processes in clays can increase TDS because, in order to maintain electrical charge balance, two monovalent sodium or potassium ions must enter solution for each divalent ion absorbed. In general, if a ground-water sample has a high TDS level, high concentrations of major constituents will also be present in that sample. Clay minerals can have high cation-exchange capacities and may exert a considerable influence on the proportionate concentration of the different cations in water associated with them (Hem, 1985). The exchange of calcium for sodium results in high sodium levels, and total dissolved solids increase in ground water when calcium ions are exchanged for sodium ions (Freeze and Cherry, 1979). However, 241 potable water can be obtained at depths of greater than 38.0 m where TDS fell below 500 ppm. The proposed sanitary landfill in this area when it becomes operational will have minimal impact on the groundwater because the clay and sandy clay which acts as liners are sufficiently thick to prevent infiltration of leachates into the groundwater. Azimuthal resistivity sounding polar diagram and groundwater head contour map (Figures 102 and 109) shows that groundwater flow direction is SW – NE towards the proposed landfill area. The location of the landfill site in the region of groundwater discharge rather than a recharge region as indicated thus means that the presence of the landfill will have minimal effect on groundwater 6.5 Otor-Jeremi The resistivity sounding curves at Otor-Jeremi show that four distinct geologic layers (Figures 33 and 34) made up of top soil, clay/sandy clay/clayey sand and sand exists. The resistivity of the first layer which is the top soil varies between 25.9 – 477.0 Ωm while its thickness ranges from 0.6 – 3.1 m. This layer is underlain by a second layer made up of clay/sandy clay/clayey sand and sand and having resistivity value ranging between 62.0 – 182.0 Ωm, its thickness varies from 0.4 – 12.8 m. The third layer with resistivity values ranging from 20.1 – 1361.0 Ωm with thickness varying between 8.7 – 34.1 m is mostly sandy except VES 82 where the lithology is clayey sand. The third layer is underlain by a fourth layer comprising of sand having resistivity values ranging from 358.0 – 2225.0 Ωm. The thickness of this layer could not be determined as the electrode 242 current terminated within this layer. But inference from VES 80 and 74 indicates that this layer is between 9.7 m and over 30.5 m thick. The third and fourth geoelectric layers constitutes the aquiferous unit in this area, this aquifer is unconfined and occur at an average depth of 10.0 m Water quality results showed that the quality of groundwater in shallow aquifer between the depths of 8.0 –16.0 m is not potable as TDS concentration of 531.0 – 4644.16 ppm is way above the stipulated standard of 500 ppm. The high TDS values may be due to presence of iron and indiscriminate disposal of wastes. The source of the iron contamination is not quite known, but is suggested to have been emplaced by iron fixing bacteria associated with sedimentary environments of decaying vegetative matter. According to Allen (1965) and Oomkens (1974), the Quaternary glaciation was accompanied by eustatic lowering of the sea-level such that the paleo-strand line was at the present edge of continental shelf. This geologic event would have exposed the sediments and created paleo-soils rich in iron oxides. The subsequent rise in sea level would have incorporated the paleosoils into the geologic record. Electrical resistivity imaging profiles have shown that groundwater in the shallow part of the aquifer have been contaminated by leachates infiltrating into the subsurface and lateral migration by subsurface runoff (inter-flow and base-flow). The isoresistivity and longitudinal resistance maps (Figures 47, 48 and 57) indicate that 85.0 % of this community has a poor protective capacity and as a result the aquifer (groundwater) is vulnerable to contamination. Otor-Jeremi is an 243 oil producing community with lots of well heads and oil pipelines that traverse the community. Undetected leakage of hydrocarbon from the pipelines over a long period of time can contaminate the aquifer especially in areas of probable risk of contamination which coincides with the area with weak protective capacity. Also, the water is shallow water table (average depth of 5.0 m), and in case of leakages within these areas, the hydrocarbon will easily get to the water table and impacts will not only be felt in these area but also affect other areas as the contaminants will flow along the direction of groundwater flow which is in a southeast – northwest direction as indicated by the results of azimuthal resistivity sounding polar diagram and groundwater gradient map (Figures 103 and 110). In the studied area eventual contamination plumes of hydrocarbon was not identified. At the market a borehole was sunk quite close to a wwaste dump, electrical resistivity images have shown contamination up to about 12.0 m while water quality result shows TDS of 4644.61 ppm at 12.0 m which were above the stipulated limit of 500 ppm. Pumping of the well can cause or aggravate groundwater contamination. Well drawdown can increase the slope of water table locally, thus increasing the rate of groundwater flow. Drawdown can even reverse the original slope of the water table; perhaps contaminating wells that were pure before pumping began. This study have shown that the aquifer is mostly medium to coarse grained with negligible clay fraction. As a result the natural filtration process is very poor, 244 leading to high total dissolved solids/contaminants. Nduka et al. (2008) reported poor filtration process in the Niger Delta; they asserted that the high turbidity in groundwater is an indication of poor filtration process. Groundwater has to squeeze through pore spaces of sediment to move through an aquifer, the porosity of such aquifers make them good filters for natural purification. Because it takes effort to force water through tiny pores, groundwater loses energy as it flows, leading to a decrease in hydraulic head in the direction of flow. Larger pore spaces usually have higher permeability, produce less energy loss, and therefore allow water to move more rapidly. For this reason, groundwater can move rapidly over large distances in aquifers whose pore spaces are large. Clay particles and other mineral surfaces in an aquifer also can trap dissolved substances or at least slow them down so they do not move as fast as water percolating through the aquifer. Another reason, for the vulnerability of the aquifer to contamination is the small thickness of the vadose zone. As a result the contaminants (dissolved substances) travel a very short time (short residence time) before getting into the saturated zone giving no adequate time for contaminant attenuation. 6.6 Burutu The result of the resistivity investigation (VES and ERI) revealed that this area suffers from acute salinization close to the Forcados River and in most places where the water is fresh, it mostly of intermediate to very poor quality. The drainage system of the Niger Delta region comprises a dense network of distributaries, rivers, creeks and estuaries formed by the movement and 245 interactions of the predominant river systems among which are the Niger, Forcados, Nun, Ase, Imo, Warri and Sombrero rivers. The dense river network of creates a condition of delta-wide hydrological continuity. Developments in one part of the delta, such as pollution or oil spills, can readily be felt in other parts. Within the estuarine reach of the stream, saline water may encroach into the underlying freshwater aquifers in a variety of ways, depending on the relative positions of the water table, the river stage and the degree of interconnection between the stream and the aquifer (Abam, 1999). The resistivity investigation shows that resistivity values in the upper layers vary between 2.28 – 9.82 Ωm. In literature, salt water resistivity values below 1.0 Ωm were reported, in fact, seawater has an average resistivity of 0.2 Ωm (Nowroozi, et. al.1999), while the resistivity of a layer saturated by saline/salty brackish water and dissolved solids is in the range of 8.0 to 50.0 Ωm (Bauer, et. al., 2006; Nowroozi, et. al.1999; De Breuk and De Moor, 1969; Zohdy, et. al., 1993). Therefore, based on these values of resistivity of layers saturated by saline/salty brackish water and some dissolved solids reported in the literature, results obtained in this work from resistivity investigations highlight the presence of strata saturated with brackish to saline water. Caution should be exercised in this interpretation, however, since clayey layers are also conductive and may be misinterpreted as saline/brackish water contamination. Hence, geochemical analysis of water samples has been performed to provide subsurface control and validate the resistivity measurements. 246 The results of geochemical analysis shown in Table 26 on the basis of Chloride (6.67 – 500.0 mg/l) indicates that the water is fresh (70.0 %) to salty brackish (30.0 %). However, most of the freshwater is not of good quality due the presence of iron and cadmium at concentrations above the maximum contamination limits (MCLs) of 0.3 mg/l and 0.003 respectively. The result shows that 60.0 % of the groundwater is contaminated with iron while 30.0 % is contaminated with cadmium. Elevated lead, chromium and cadmium in groundwater have been observed in the Sombreiro-Warri Deltaic Plain deposits by Akpoborie et al., 2000; Aweto and Akpoborie, 2012. Similar trends of occurrence of this suite of heavy metals have been reported in shallow groundwater from the Saghara Beach area of the Forcados estuary in separate studies by the Shell Petroleum Development Company (SPDC, 1998) and the Niger Delta Development Commission (NDDC, 2005) from the Iyara area of Warri town. However, at Ogulagha on the south bank of the Forcados estuary groundwater from 300 m deep borehole yields water that is devoid of these heavy metals (Richdrill, 2009). It thus appears that these heavy metals seem to be confined to the shallow groundwater that occurs in the Quaternary (Recent) Sombreiro – Warri Deltaic Plain deposits and Mangrove Swamps that overlie the Benin Formation in the western delta. Unfortunately, in the absence of supplies from public facilities, it is these shallow and more vulnerable aquifers that are easily and cheaply exploited. 247 Table 26: Geochemical Analysis Results of Water Samples from Burutu. S/N 1 2 PARAMETERS UNIT S1 6.44 E/CONDUCTIVITY µs/cm WHO STD 2011 6.5 - 8.5 601 235 1000 300.5 117 500 250 50 500 N/A N/A 50 N/A N/A 0.3 5 0.003 0.01 N/A S3 7.04 1018 1479 569 mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L 509 739.1 500 1.01 125.2 6.40 9.06 135.77 13.52 3.22 0.001 0.0369 <0.001 <0.001 <0.001 400 1.43 76.9 5.11 11.27 160.95 16.03 1.36 <0.001 0.0230 0.009 <0.001 <0.001 284.5 150 0.87 38.10 9.52 10.30 58.68 13.80 0.30 0.486 0.0521 0.011 0.002 <0.001 130.06 2.041 21.05 14.640 8.58 123.42 9.32 3.10 0.554 0.0013 0.013 <0.001 <0.001 300.9 1.745 101.28 21.620 10.92 149.02 12.01 2.01 0.150 0.0324 0.012 0.001 <0.001 PARAMETERS UNIT S6 S7 S8 S9 E/CONDUCTIVITY µs/cm mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L 6.16 1170 585.3 219.93 1.893 45.40 20.740 14.69 181.4 29.6 2.61 3.146 0.015 <0.001 0.001 <0.001 7.13 236 117 50.985 3.190 1.85 14.640 19.91 175.1 36.11 1.05 3.552 <0.001 <0.001 0.010 <0.001 7.01 767 383.5 6.67 0.085 62.1 24.31 22.05 127.83 41.02 1.97 <0.001 0.041 <0.001 <0.001 <0.001 7.31 983 491.5 53.983 1.740 5.30 20.00 20.27 160.03 28.66 2.07 3.267 0.033 <0.001 0.001 <0.001 PH 3 4 5 6 7 8 9 10 11 12 13 14 15 16 S5 6.35 S2 6.71 TDS CHLORIDE NITRATE SULPHATE BICARBONATE MAGNESIUM SODIUM POTASSIUM CALCIUM IRON ZINC CADMIUM LEAD CHROMIUM S4 6.02 S/N 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 PH TDS CHLORIDE NITRATE SULPHATE BICARBONATE MAGNESIUM SODIUM POTASSIUM CALCIUM IRON ZINC CADMIUM LEAD CHROMIUM S10 WHO STD 2011 6.16 1170 585.3 219.93 1.893 45.40 20.740 14.69 181.4 29.6 2.61 3.146 0.015 <0.001 0.001 <0.001 6.5 - 8.5 1000 500 250 50 500 N/A N/A 50 N/A N/A 0.3 5 0.003 0.01 N/A (Source: Akpoborie et al., 2012) S1 – S6 : Hand –dug wells S7 – S9 : Boreholes S10 : Forcados River 248 The zones saturated with salty brackish to brackish water are within the proximity of the Forcados River in the northwestern and northern parts of Burutu along the river bank. Thickness of salty brackish/brackish water saturated layers was found to be greater in the proximity of the coastline. The depth to the fresh water varies from about 49.0 – 63.3 m near the Forcados River and became as shallow as 1.6 m in inland areas. Further away from the river towards the south there were no indication of salty brackish/brackish water saturated zones, the most we find is intermediate to very poor quality water of rapidly mixing fresh and brackish water. This groundwater could be encountered at depths of 1.1 – 32.7 m. Isoresistivity maps (Figures 49 and 50) show the lateral and horizontal extent of salinization and distribution of groundwater of various qualities. The protective capacity map of Burutu (Figure 58) shows that the areas in the northern parts close to the Forcados River have good protective capacity, this has helped to protect the groundwater aquifer to screen off salty brackish/brackish water otherwise the extent of salinization would have been extended to much greater depths than revealed by this study. According to UNEP (2008) report, boreholes drilled near the coast may encounter connate water tapped in rapidlydeposited sequences. Below 60.0 m in the Benin Formation aquifers, however, the impermeable silt, clays and shales tend to ‘ screen off’ the saltwater, so that freshwater becomes available, usually in artesian aquifers (e.g at Escravos, Brass, and Bonny Opobor, where freshwater aquifers have been encountered between 80.0 – 120.0 m) from surface; Oteri, (1988) delineated the depth to the top of the 249 freshwater sands underlying the saline water sands to vary from 77.0 – 947.0 m below the ground. The areas in the south and southeastern parts even though are further inland; there was no indication of finding water of good quality even at depths of 60.0 m (Figure 50). The reason is that these areas are low lying wetlands dominated by marshes, generally at less than about 5.0 m above sea level, and crisscrossed by tidal creeks that divide the swamps into somewhat quasi-rectangular blocks. Marshes are frequently or continually inundated with water (Mitsch and Gossenlink, 1993). Marshes derive most of their water from surface water including rivers, runoff and overbank flooding from tide related sea water that is propagated up river from the estuaries at high tide, however, they receive inputs from groundwater as well (Mitsch and Gossenlink, 1993). Marshes may recharge groundwater by infilteration, depending on soil permeability and wetland size. Recharge is relatively plentiful in marshes and may contribute (up to 20.0 % of volume) to regional groundwater supply (Weller, 1981). Marshes slow the flow of water moving through it and facilitate the settling of suspended solids and pollutants adhering to sediments. The greater the amount of open water present, the more sediment attached pollutants will remain suspended in the water column (Whigham et al., 1988). The fact that they are important sinks for pollutants carried in upland from river areas is evidenced by the presence of intermediate to poor water quality in these areas even at great depth. The water quality result shows that water improved with depths. Electrical conductivity values from single-point log at depths of between 8.0 – 54.0 m were 250 generally high (3,838.48 µS/cm to 740,740.75 µS/cm) indicating that the groundwater is fresh to slightly brackish with a lot dissolved solids as indicated by the TDS values (3,350.08 – 474,074.07 ppm) from the log. Electrical conductivity values from water analysis were moderate to slightly high (235.0 – 1479.0 µS/cm) also indicating that the groundwater is fresh to slightly brackish. Single-point resistance log water quality result shows that fresh water occurs at depths of more than 54.0 m as electrical conductivity and total dissolved solid values dropped to between 38.26 – 121.72 µS/cm and 27.36 – 77.91 ppm respectively. These values are at order of magnitude lesser than WHO and Nigerian Drinking Water Standards. The delta is dissected by a dense network of rivers and creeks, which maintain a delicate but dynamic equilibrium between saline, estuarine and freshwater surface bodies with complex underground extensions. Most estuarine rivers are contaminated during high tide by the influx of sea water upstream to distances which depend on discharge, river bed slope, channel shape, tidal level and fluctuations in wind velocity and direction. The distances contaminated upstream may range from less than a kilometre to tens of kilometres and could be accentuated by subsidence. Comparison of water level records in locations on interconnected river systems shows upstream areas have a lower water level compared to tidal areas, such as Forcados, possibly due to subsidence induced by oil and gas production in the region (Abam, 2001). Two million barrels of crude oil and over 300 million standard cubic feet of gas are abstracted daily from the 251 underlying sedimentary basin. Although no authoritative subsidence measurements have been earned out, subsidence rates ranging from 2.5 – 12.5 cm/year have been quoted by different researchers (Fubara, 1986; Ibe, 1988). The relatively higher water level in some coastal areas indicates a reversed river bottom gradient and implies the presence of a sustained hydraulic head driving sea water towards subsiding inland areas and extending saline contamination to far upstream locations. Tidal fluctuation is accompanied by a cyclic diurnal change in water quality, due to the mixing in continuously varying proportions of original river water with sea water. In the lower reaches of the estuary, the water quality approaches that of sea water and at the farthest point inland that of the primary river water (i.e fresh). Within the tidal reach, the change in water quality depends on the composition of the original river water and sea water and the relative quantities involved in the mixing. A zone of mixing and diffusion migrates up and down the river, the maximum saline contamination occurring during high tide and the minimum during low tide. During the high water stage (August – October), in the rainy season, the tidal effect on quality is minimal, because of the large discharge of freshwater. In the lean months (November – June) when the river stage is at its lowest, the quality is most adversely affected by tides, due to the predominant influence of sea water. The waters of the rivers and creeks in the estuaries are dominated by saline sea water. At the interfaces of these waters with fresh groundwater, some mixing takes place leading to some hydrodynamic 252 adjustments (Abam, 1999). The position of the Salt Water Fresh Water Interface (SWFWI) may not be fixed over time, and it may be a transitional zone of changing salinity rather than a distinct boundary between saline and fresh water. Within the estuarine reach of the stream, saline water may encroach into the underlying freshwater aquifers in a variety of ways, depending on the relative positions of the water table, the river stage and the degree of interconnection between the stream and the aquifer. Adepelumi et al., 2008 in an attempt to demarcate possible areas for groundwater development in the Lekki area of Lagos state, established the inherent presence of saline water in the subsurface of their area of investigation as being trapped during the transgressive, and the regressive movement of the ancient sea during the quaternary times when some sediments were contemporaneously deposited under marine condition. They inferred that the saline water found at a shallow depth (10.0 – 30.0 m) was probably trapped during marine transgression. The possibility of seawater intrusion by the tidal movement of saline seawater was not examined. One of the potential causes of subsurface salinity which does not require too geologically long a time, has been reported by Achari et al. (2005) was the inundation of an entire barrier by the surface influx of seawater. They proferred an explanation for the process that led to groundwater salinization thus: when seawater ingressed over the surface, by waves, it carried along some dissolved salts, which were lodged in the soil. The salts brought by the mighty waves sink 253 into the soil and with the first rains of the year, the absorbed salts leach down to the groundwater aquifer and contaminated it. When the dry summer months advance, evaporation causes the salt to accumulate in the subsurface, pending recharge (by the sea, which brings in more salts anyway). Rainfall recharge pushes the saltwater, further down in an attempt to establish hydrodynamic equilibrium capillary rise in rainless months push the salts up. In all, the soil salinity is significant for a considerably long time and is a continuous process. The azimuthal resistivity sounding and groundwater head contour map results (Figures 104 and 111) show that regional groundwater flow is in southwest-northeast direction indicating a hydraulic gradient towards Forcados River and hence the aquifer recharges the Forcados River. There is however, a local groundwater flow which interchanges with the regional flow. The interaction of groundwater and surface water in river valleys is affected by the interchange of local and regional groundwater flow systems with the rivers and by flooding and evapotranspiration. Rivers/streams receive groundwater flow primarily from local flow systems, which usually have limited extent and are highly variable seasonally. Therefore, it is not unusual for rivers/streams to have gaining or losing reaches that change seasonally (USGS, 1998). In the dry season the waters of the rivers and creeks of the estuaries are dominated by saline sea water, saline water may encroach into the underlying freshwater aquifers in a variety of ways, depending on the relative positions of the water table, the river stage and the degree of interconnection between the river and the aquifer. In 254 addition to the tides, human activities can also affect the progression of saltwater into the estuary. Activities such the dredging of canals for navigation or petroleum exploration (pipeline canals) could give saltwater a direct route inland. Assaulted by natural and anthropogenic disturbances, groundwater quality is deteriorating in this region. In 1996, 62.0 % of estuaries had good water quality (USEPA, 1996). By 2000 only 49.0 % of estuaries had good water quality (USEPA, 2000). This study has shown that between 5.0 – 60.0 m, only about 22.5 % percent of this area has water of good quality but may still require attention because of high iron and cadmium content. 6.7 Sapele The geologic section for Sapele show four to five distinct geoelectric layers made up of the following: the top soil, laterite/clay/sandy clay/clayey sand and sand. The third, fourth and fifth geoelectric layers correspond to the aquiferous unit with resistivity values ranging from 272.0 – 2155.0 Ωm and diagnostic of fine/medium/coarse/gravelly sand. The exact thickness of the aquifer is between 47.0 m (VES 135) and 56.7 m (VES 109). Inference from the borehole lithologic log however shows that possibility of thickness exceeding 80.0 m. Groundwater occurs under water table condition in the aquifer which occurs at an average depth of 14.0 m. Isoresistivity map shows that at 5.0 m depth 2.5 % (southern) and 7.5 % (southern and eastern) part of the area is underlain by clay and clayey sand respectively while at 10.0 m, 97.5 % of the area is underlain by sand and at 20.0 m, 100 % of the area is underlain by sand. The aquifer occurs within this sandy lithology at depths of between 6.0 – 25.0 m. The longitudinal conductance map 255 shows that 22.5 % of the overburden above the aquifer in the area has weak protective capacity while the remaining 77.5 % have poor protective capacity and as a result the aquifer in this area is vulnerable to contamination from surface and near surface sources. The 2D resistivity structure obtained for 2 profiles at each dumpsite (New Road and Reclamation Road) shows distinctive zones of low resistivity values varying between 1.06 – 68.2 Ωm depending on the amount of leachate. The low resistivity zones in the profile are probably highly conductive leachate from contaminated zone within the contamination plume. There have been downward and lateral movements of the leachate into the groundwater to depths of up to 12.0 – 18.0 m and 20.0 – 24.0 m at Reclamation and New Roads respectively. The thickness of this low resistivity leachate-saturated layer suggests substantial groundwater contamination beneath and around the dumpsite. On the average, the areas within profiles at New Road are more contaminated and depth of migration of the contaminant plume is greater than at Reclamation Road probably because of the volume of refuse dumped there and age of the dumpsite. Result of water quality from single-point resistance log confirmed that groundwater up to 12.0 m and probably greater has been contaminated by migrating contaminant plume. TDS values at depths between 8.0 – 12.0 m vary between 664.19 – 3937.73 ppm for the two wells logged. The high amount of TDS (> 664.19 ppm) verifies the fact of groundwater pollution in this area due to waste dumping and other anthropogenic activities. The groundwater between 8.0 256 – 12.0 m in this area is unfit for human concumption due to the existence of high TDS, which is far higher than the amount of 500 ppm set by USEPA (2011) guidelines for drinking water. One of the adverse impacts of dumping of municipal and industrial solid wastes is the production of leachate which can cause significant impairment of groundwater use for domestic water supply as well as surface waters that receive the leachate (Lee and Jones, 1996). At depths of 14.0 m and below, the groundwater appear potable as quality improved due to low amount of TDS (61.150 – 450.50 ppm), which is lower than 500 ppm except at 28.0 m in the well at Urhiapele primary school (TDS of 992.7 ppm). The 2D resistivity image shows uncontaminated zones of groundwater below the contaminated zones characterized by higher resistivity values. The shape of the resistivity structure show downward and lateral migration of contamination plume which was made easier because of the poor and weak protective capacity of the area. Results of Azimuthal resistivity sounding and groundwater head contour map (Figures 105 and 112) shows that groundwater flows direction is in NW – SE direction, this also is the direction of contaminant plume. However, the results (2D resistivity control profiles) reveal that good aquifers can still be obtained as most of the contaminants have been attenuated within 100 m away from the dumpsites. But there may still be concerns for other non-point source of contaminants such as sewage and other smaller dumpsites randomly located within the metropolis. 257 CHAPTER SEVEN CONCLUSION AND RECOMMENDATIONS 7.1 Conclusion This study describes the determination of overburden protective capacity and water quality using resistivity surveys. The survey involves a total of one hundred and forty (140) Schlumberger vertical electrical soundings, twenty one (21) electrical resistivity soundings and nine (9) single-point resistance logs distributed in seven (7) communities comprising the study area. The first order geoelectric parameters obtained from interpretation of field data were utilized in deriving the longitudinal unit conductance (S) which is a second-order geoelectric parameter. The overburden protective capacity in the area was evaluated from the longitudinal unit conductance. The first aquifer system is unconfined, exist at shallow depth (mean depth is 13.0 m) and extremely vulnerable to pollution from surface sources. This aquifer consisting essentially of loose to poorly consolidated sandy material is capped by laterized soil especially in the northern part and lenses of clayey soil. The laterite thins out in the southern direction and has become non-existent from the freshwater swamps to the coastline where it becomes thin or non-existent. The longitudinal conductance maps delineated areas with poor protective capacity (S = < 0.10 mho), weak protective capacity (S = 0.10 – 0.19 mho), moderate protective capacity (S = 0.20 – 0.69 mho) and good protective capacity 258 (S = 0.70 – 4.90 mho) protective capacity. The results show that 52.0 % of the study area has poor protective capacity, 23.0 % has weak protective capacity, 15.0 % has moderate protective capacity and 10.0 % has good protective capacity. The depth to aquifer in areas with poor and weak protective capacities is very shallow (average depth is 6.0 m) and as a result the first aquifer system is highly vulnerable to contamination and many activities of man impact directly on this aquifer. Where the laterized soil cover or clayey overburden is thin or completely absent and the water table is shallow, the risk of contamination is much higher. The aquifer within area with moderate and good protective capacity exists at greater depths (average depth is 23.0 m). Coupled with the good protective capacity of the overburden units above the aquifer and greater depth to aquifer, the area within moderate and good protective capacity zones are not vulnerable to contamination from surface or near surface sources. Because contaminants can be transmitted to the groundwater system by infiltration from the surface, the susceptibility of an aquifer system to contamination from surface sources depends in part on the type of material that forms the surface layer above the aquifer. In general, sandy surficial sediments can easily transmit water from the surface, but providing negligible filtering of contaminants. Clay-rich surficial deposits generally have lower vertical hydraulic conductivity than sand and gravel deposits, thereby limiting the movement of contaminated water. However, the presence of lithological heterogeneities in the overburden (e.g discontinuities such as quartz stringer or fissure) can locally decrease the effectiveness to attenuate 259 contaminants. The differences in basic hydrologic properties of sands and clays make it possible to use surficial geology to estimate the potential for groundwater contamination. This study support the conclusion that clay-rich overburden are geologic barriers and have the capacity to attenuate contaminants. However, there were areas in this study where clays with average thickness of 2.6 – 14.4 m and laterites of between 4.0 – 10.5 m thick overlie the aquifers and yet the protective capacity rating was poor, weak and moderate. It is expected that these geologic barriers should provide a good protection for groundwater in the aquifers beneath. Surface and subsurface resistivity surveys were successfully combined to identify contamination of groundwater by leachate produced from waste dumps and saline water encroachment. Both geophysical methods identified the contaminated zones as low resistivities (higher total dissolved solids). Deterioration of groundwater present in the shallow aquifer was probably enhanced due to poor to weak protective capacities of overburden above aquifer. The aquifer was contaminated by a combination of the following factors: agricultural activities, petroleum leakage and spills, land disposal of solid domestic and industrial wastes on land, sea water encroachment, mining activities, mineral enrichment within the aquifer and spread of urbanization to recharge area. Contamination from waste disposal sites typically forms a "plume" that moves outward and downward into surrounding and underlying aquifers up to about 12.0 m. These plumes may contain dissolved carcinogens such as heavy metals (e.g., lead, mercury, chromium, cadmium, arsenic, etc.), volatile organic 260 compounds (VOCs: benzene, ethylbenzene, toluene, etc.) and less harmful ions (sodium, calcium, iron, sulfate, chloride, etc.). Contaminant plume flow is essentially radial, leading to a contaminant plume emanating outwards in all directions from the dumpsite. Groundwater quality was below potable limit standard set by United State Environmental Protection Agency, World Health Organization and Standard Organization of Nigeria. Results however, from deeper zones indicated that the groundwater is satisfactory. Resistivity surveys conducted in Burutu located within the brackish mangrove swamp around the Forcados estuary showed that the aquifer close to the Forcados River have been saturated by salty brackish water. The result revealed that as depth increases and as one move further inland, salinity reduced and groundwater appeared fresh but heavily laddened with iron and chromium. Vertical salinity gradient developed in parts of Burutu are as a result of penetration of saline water inland through creeks and estuaries. Such salinization applies only where the aquifer is uninterrupted in depth. A thick or extensive aquiclude can exclude further penetration of salt water. Resistivity surveys at Ughoton which also lie in the brackish mangrove swamps shows that the subsurface is devoid of any significant saline water. This is because Ughoton is a discharge wetland. The water table gradient as confirmed by azimuthal resistivity sounding and groundwater head contour map indicates that groundwater flow direction is north-northwest towards the dominant and perennial Ughoton River, 261 and thus prevents brackish water from recharging groundwater because of the prevailing differential in water table head and stream water level. The results of this study provides a gloomy prognosis in the future which include the following: (1) aquifer contamination that already exists will gradually spread; (2) many groundwater aquifers that are not presently known to be contaminated will be identified as being contaminated and the discharge of contaminated groundwater into wetlands, streams and rivers will increase. Through this study, it was confirmed that resistivity methods show promise for application at sites of contamination to evaluate aquifer vulnerability, help to identify the source(s) of contamination, strategically to locate monitoring wells and determine quality of groundwater. 7.2 Recommendations Based on the results of this study the following recommendations are made: 1. Drilling of deeper boreholes into the Benin Formation and far from waste disposal sites. Awareness should be created to discourage inhabitants, water from hand dug wells, which are highly vulnerable to contamination. 2. Good borehole completion to exclude the first aquifer from to the yield of the second aquifer. Poor borehole completion practices may also lead to contamination of the second aquifer system by contributing to its yield of the second aquifer. 262 3. Proper sanitation and wastes disposal to sustain the groundwater quality should be put in place hence; dumpsites should be and must not be located in areas of groundwater discharge. 4. The dumpsite at Uvwiamuge should be relocated to Egbeleku. 5. Pumping rates of wells should be controlled in Ughoton so as not to cause unacceptable drawdown that can cause in decrease in hydraulic head inland. 6. Monitoring of pipelines for corrosion should be done frequently and in the recent of spillage either by sabotage or otherwise remediation measures should be done immediately to prevent the contamination of the aquifer which is vulnerable. 7. The effects of salt water intrusion is strongly observed in some parts of the study area, hence, necessary studies should be commissioned by the Government to provide a lasting solution through effective management strategies. 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