ABSTRACT
Geo-electrical resistivity techniques are increasingly being applied in addressing a wide range of hydrological, environmental, and geotechnical problems. This is due to their effectiveness in near-surface characterization.
Seven (7) vertical electrical soundings (VES) was integrated with 2-D geo-electrical resistivity in three locations and three traverses to characterize the near-surface and sub- surface and also delineate the underlying aquifer properties in the sedimentary terrain. The geophysical survey was conducted as part of preliminary studies to evaluate the possibility of groundwater resource contamination in Ogba-Iyo, Ijoko, South-western Nigeria.
The Vertical electrical sounding (VES) and two-dimensional (2-D), direct-current ERT methods employed identified resistivities less than 10 ohm-meters, deep in the subsurface, along the 7 VES points and the 2-D resistivity profile lines within the site. Data with resistivity values of below 10 ohm-meters were considered to be anomalies and were interpreted as possible areas of salt contamination/intrusion. Data from the resistivity lines indicate deeper subsurface salt contamination. The two 2-D profile lines located towards the north of the study area, yielded data indicating no salt contamination at a depth of 16m-25m, while the saltwater contamination occurs at a depth greater than 30m. Data from resistivity profile line 1, trending South-East/North-West across the site, identified an area of possible salt contamination at the subsurface, approximately 35 in depth. Resistivity line 2, trending south to north across the study area also indicated an area of possible salt contamination at the subsurface, approximately 25 in depth.
A high-yield relatively unconfined sandy aquifer with a saltwater intrusion, overlain by a low-yield clayey sand aquitard was delineated. Overlying the aquitard is a very resistive and thick layer that is predominantly composed of dry lateric clay.
TABLE OF CONTENTS
Title
Page……………………………………………………………………………………………………………………… i
Certification………………………………………………………………………………………………………………….. ii
Dedication……………………………………………………………………………………………………………………. iii
Ackmowledgements………………………………………………………………………………………………………. iv
Abstract………………………………………………………………………………………………………………………… v
Table Of
Contents……………………………………………………………………………………………………….. vi
List of
Figures……………………………………………………………………………………………………………. vii
List of
Tables…………………………………………………………………………………………………………….. viii
CHAPTER ONE: INTRODUCTION…………………………………………………………………………….. 1
- GENERAL ATATEMENT………………………………………………………………………………………………………………………. 1
- SCOPE OF THE STUDY………………………………………………………………………………………… 4
- CLIMATE AND VEGETATION OF THE STUDY AREA………………………………………….. 5
- PREVIOUS WORK DONE… 8
CHAPTER TWO: LITERATURE REVIEW……………………………………………………………….. 11
- ENVINRONMENTAL POLUTION……………………………………………………………………….. 11
- WATER
POLUTION…………………………………………………………………………………………. 12
STRATIGRAPHY……………………………………………………………………………. 19
CHAPTER THREE: RESEARCH METHODOLOGY………………………………………………… 23
- ELECTRICAL RESISTIVITY………………………………………………………………………………. 23
- ONE DIMENSIONAL SURVEY………………………………………………………………………… 25
EQUIPMENT………………………………………………………………………………………. 31
- 3.3.2
PROCEDURE………………………………………………………………………………………………… 32
CHAPTER FOUR: RESULTS INTERPRETATION AND DISCUSSION……………………… 34
- DATA PRESENTATION AND INTERPRETATION………………………………………………… 34
- RESULT OF THE 2-D ERT SURVEY 37
ONE………………………………………………………………………………………….. 38
- TRAVERSE
TWO…………………………………………………………………………………………. 39
CHAPTER FIVE: CONCLUSION AND RECOMMENDATION…………………………………. 51
5.1 CONCLUSIONS………………………………………………………………………………………………….. 51
5.2
RECOMMENDATION………………………………………………………………………………………… 52
REFERENCES………………………………………………………………………………………………………….. 53
APPENDIX…………………………………………………………………………………..
.57
LIST OF FIGURES
Fig. 1.1: Map extract showing accessibility and topography of study area………………………………. 6
Fig. 1.2: Drainage map of the study area…………………………………………………………………………….. 7
Fig. 2.1: Geological map of the study area., showing VES points and 2-D profile lines………….. 21
Fig. 2.2: Geological map of the Nigerian part of the Eastern Dahomey Embayment. Modified after Gebhardt et al, 2010…………………………………………………………………………………………… 22
Fig 3.2: Schlumberger Array………………………………………………………………………………………….. 30
Fig. 3.3: Wenner Array………………………………………………………………………………………. 31
Fig. 3.1: Sequence of measurements to build up a pseudo-section employing Werner array 27
Fig 4.1: Showing Geo-electric section relating to VES 1-
3……………………………………..42
Fig 4.2: Showing Geo-electric section relating to VES 1-
7……………………………………..43
Fig. 4.3: Diagram showing resistivity value ranges of some common earth’s materials. Modified after Palacky, 2006……………………………………………………………………………………………………….. 46
Fig. 4.4a: Inverted resistivity profile line 1; showing salt intrusion…………………………………….. 48
Fig. 4.4b: Inverted resistivity profile line 2; revealing salt intruding a fresh water aquifer 49
LIST OF TABLES
Table 3.1: Typical electrode configurations commonly used……………………………………………….. 27
Table 4.1 Electrical resistivity result, showing interpreted Geo-electric parameters 35
Table 4.2: Curve types generated and the corresponding……………………………………. number of layers 36
Table 4.3: Calculated average value of parameters in relation to subsurface lithology…………… 41
- GENERAL STATEMENT
CHAPTER ONE INTRODUCTION
In many developed and developing countries there is not only a heavy reliance on ground water as a primary drinking supply but also as a supply of water for both agriculture and industrial use. The reliance on groundwater is such that it is necessary to ensure that there are significant quantities of water and that the water is of a high quality. The use of geophysics for both groundwater resource mapping and for water quality evaluations has increased dramatically over the last 10 years as a result of the rapid advances in microprocessors and associated numerical modeling solutions.
GEOPHYSICAL INVESTIGATION OF SALT-POLLUTED GROUNDWATER IN OGBA-IYO/BLESSED COMMUNITY, IJOKO, SOUTHWESTERN NIGERIA
