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Geophysical and Geochemical Investigation of Saline Water Intrusion along the Coastal Region
Geophysical and geochemical investigations were carried out in the study area to decipher subsurface geologic formation and assessing seawater intrusion.
Electrical resistivity tomographic surveys carried out in the watershed-indicated low resistivity formation in the upstream area due to the presence of thick marine clays up to thickness of 20-25 m from the surface.
Secondly, the lowering of resistivity may be due to the encroachment of seawater in to freshwater zones and infiltration during tidal fluctuation through mainly the Pikaleru drain, and to some extent rarely through Kannvaram and Vasalatippa drains in the downstream area.
Groundwater quality analyses were made for major ions revealed brackish nature of groundwater water at shallow depth.
The in situ salinity of groundwater is around 5,000 mg/l and there is no groundwater withdrawal for irrigation or drinking purpose in this area except Cairn energy pumping wells which is using for inject brackish water into the oil wells for easy exploration of oil.
Chemical analyses of groundwater samples have indicated the range of salt concentrations and correlation of geophysical and borehole litholog data in the study area predicting seawater-contaminated zones and influence of in situ salinity in the upstream of study area.
The article suggested further studies and research work that can lead to sustainable exploitation/use and management of groundwater resources in coastal areas.
TABLE OF CONTENTS
Table of Contents
List of Tables
List of Figures
CHAPTER 1: INTRODUCTION
1.1 Statement of the Problem
1.2 Location of the Survey Area
1.3 Geology of the Study Area
1.4 Aims and Objectives of the Study
1.6 Scope of the Study
CHAPTER 2: LITERATURE REVIEW
2.2 Movement of Fresh and Salt Water in the Coastal Aquifers
2.3 The Ghyben-Herzberg Relation
2.4 Chemical Constituents of Groundwater
2.5 Literature Review of the Application of Geophysical methods in Saline Water Contaminations Studies
2.6 General Characteristics of Aquifers
2.7 Theoretical Background of Electrical Resistivity Techniques
2.8 Potential difference between electrodes of Earth Surface
2.9 Resistivity Values of Earth Materials (minerals and rocks)
2.10 Concept of Real and Apparent Resistivity
2.11 Electrode Configuration
2.11.1 Wenner Electrode Configuration
2.11.2 Schlumberger Configuration
2.11.3 Double-Dipole Electrode Configuration
2.11.4 Pole-Pole Configuration
2.12 Interpretation of Electrical Resistivity Tomography (ERT) Data
2.13 Assessment Parameters in Geochemical Analysis for Salt water Intrusion
CHAPTER 3: MATERIALS AND METHODS
3.1 Reconnaissance and Pre-survey Activities
3.2 Organisation of the field Crew
3.3 The Resistivity Meter
3.3.1 ERT Investigation
3.4 Data Analysis
3.4.1 ERT Data Analysis
3.4.2 Physicochemical Data Acquisition and Analysis
CHAPTER 4: RESULTS AND DISCUSSION
4.1 Interpretation of Electrical Resistivity Tomography Results (ERT)
4.2 Interpretation of Physicochemical results
4.2.1 Physiochemical Assessment of the Groundwater
126.96.36.199 Electrical Conductivity
188.8.131.52 Total Dissolved Solids
184.108.40.206 Total Alkalinity
220.127.116.11 Sodium (Na+)
18.104.22.168 Magnesium (mg2+) and Calcium (Ca2+)
22.214.171.124 Sulphate (SO42-)
126.96.36.199 Phosphate or Phosphorous (PO42+)
188.8.131.52 Chloride (Cl)
184.108.40.206 Scatter Plots of Anions and Cations
4.2.2 Discussion of the Results
220.127.116.11 Discussion of the ERT Results
18.104.22.168 Discuss of the Physicochemical Behaviours of Water Samples
22.214.171.124 Discussion of Result of the Scatter Plots
CHAPTER 5: CONCLUSION AND CONCLUSION
Contamination of freshwater bodies caused by Saltwater Intrusion (SI) is a global issue, affecting water quality, vegetation and soil conditions along coastal lines.
Nigeria has a coastline that is 1000km long boarding some states in the South with the Atlantic Ocean.
There are Lagos, Ogun, Ondo, Delta, Bayelsa, Rivers, Cross Rivers and Akwa Ibom States (Oyeyemi, Aizebeokhai and Oladunjoye, 2015), saline intrusion into the coastal aquifers in these region has become a major concern (Batayneh, 2006) because it contribute the readily pollutants in freshwater.
Thus, the understanding of saline intrusion is essential for the management of coastal water resources (Kalpan, Saha and Chakroborty, 2001).
Groundwater has long served as a source of drinking water and it is still very important today. The development of groundwater has provided great socio-economic benefits to humanity.
As groundwater is isolated from the surface, most people take it for granted that groundwater should be relatively pure and free from pollutants.
Although most groundwater are still of high quality, at some locations, it is becoming increasingly difficult to maintain the purity of groundwater.
One of the major sources of pollution of groundwater is by saltwater intrusion (SI). Other sources of pollutant include seepage from underground storage tanks, oilwells, septic tanks, landfills and agriculture leaching (Olufemi, Utieyin and Adeboyo, 2010).
Coastal groundwater aquifers experiencing saltwater intrusion are characterized by continuous declined in groundwater levels precipitated by over stressing.
Decline in hydraulic gradient in groundwater aquifer may lead to reverse in groundwater flow direction (Ohwoghere, Akpoborie and Akpokodje, 2014).
Saltwater intrusion is commonly associated with aquifers located in coastal regions of the word and responsible for freshwater resource depletion and availability (Werner, Mark, Post, Alexander, Chunhul, Behzad, Craig and Barry, 2013).
The stressing of aquifers in coastal region is usually influenced by increase in freshwater demand by urbanization, industries and intense withdrawal for irrigation.
Following the rapid degradation of coastal aquifers, there is need to take actions on proper management and prevention of saltwater intrusion to ensure a sustainable source of groundwater for the future.
As groundwater withdrawals reduces the level of groundwater in storage and discharge to streams, wetlands and coastal estuaries, there is serious need for optimal management for sustainability in order to simulate and predict the response of the aquifer systems to anticipated increased future levels of groundwater in the coastal area of Akwa Ibom State.
The problem of saltwater intrusion was recognized as early as 1854 on Long Island, New York (Freeze and Cherry, 1979), thus predating many other types of drinking water contamination issue in the news.
The ecosystems in the coastal areas are sensitive to the salinity and nutrient concentrations.
In recent years, scientists, coastal managers and public decision makers have recognized that many environmental issues related to coastal ecosystem-red tide, fish kills, loss of sea grass habitats and destruction of coral reefs can be attributed to the introduction of excess nutrients (Nitrogen and prosperous) from freshwater discharge (National Research Council, 2000).
Groundwater being a source of freshwater to some coastal waters and its role in delivering excess nutrient to coastal ecosystem is of increasing concern because of widespread of nutrient, contaminants of shallow groundwater (U.S. Geological Survey, 1999).
In Southeastern Nigeria, Oteri (1988) delineated the depth to the top of the freshwater sands underlying the saline water sands to vary from 77 to 947m below ground level.
Choudhury, Saha and Chakraborty (2001), reported that saline water intrusions into coastal aquifers have resulted in acute environmental problems in the past.
They further conform that the extent of saline water intrusion is influenced by the nature of geological formations present, hydraulic gradient, rate of withdrawal of groundwater and its recharge.
Climate change has significant impact on sea, level rise and a threat to groundwater availability.
Global sea level raise will change both the groundwater recharge and discharge components of the hydrological cycles, which in turn affect the availability and distribution of freshwater at both spatial and temporal scale.
Global sea level rise causes saltwater intrusion in coastal aquifer by lengthening the freshwater and saltwater interface (mixing zones) in the landward of the coast (Hwang, Shin, Park and Lee, 2004).
Freshwater is considered to be less dense than saltwater and as such float on top. Therefore, saline water is located below water discharge from higher attitude in coastal areas.
The boundary between saltwater and freshwater is not distinct, the zone of dispersion or saltwater interface in brackish in colour with saltwater and freshwater mixing.
It therefore implies that salinity will increase with depth where both freshwater and saltwater occur.
The increase in salinity will yield consequent decrease in electrical resistivity of water and resistivity varies with depth within groundwater well in the coastal aquifer. These variations could then be interpreted by methods capable of defecting differences in salinity.
Hence, within the purview of this research work, geophysical technique involving 2D Electrical Resistivity Tomography (ERT) using wenner array was incorporated with geochemical evaluation method to study the extent of saline water intrusion and its environmental impacts in the study area.
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