COMPLETE SCHOOL PROJECT TOPICS & MATERIALS :
CHAPTERS: Chapter 1-5
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ASSESSMENT OF SEAWATER INTRUSION USING GEOPHYSICAL AND GEOCHEMICAL TECHNIQUES IN LAGOS STATE.
CHAPTER ONE
INTRODUCTION
1.1 Background to the Study
Saltwater encroachment in coastal fresh water aquifers is a major problem of groundwater quality and resource sustainability in low-altitude coastal regions of the world. With ongoing urbanisation, population growth and sea-level rise due to climate change, coastal aquifers are increasingly being threatened by seawater that can make groundwater unsuitable for human and agricultural use (Bear et al., 1999). Lagos State is the commercial capital of Nigeria and one of the most densely populated urban centres in Africa, which is very susceptible to seawater intrusion because of its low-lying relief, proximity to the Atlantic Ocean and Lagos lagoon and unprecedented pressure on its coastal aquifer systems by an estimated population of greater than 15 million.
Quaternary alluvial and coastal plain sand aquifers ( collectively referred to as the Lagos coastal plain sands) are the predominant aquifers supporting fresh water in Lagos State’s metropolitan region. However, these aquifers are open to the nearby Lagos Lagoon, Bight of Benin and many tidal creeks which permeate the metropolitan area. The delicate freshwater/saline water balance of these coastal aquifers is highly susceptible to fluctuations in groundwater pumping and sea-level. Increasing groundwater abstractions causes a decline in piezometric levels (head) in the freshwater zone, which reverses the normal hydraulic gradient between the fresh and saltwater zone and results in inland and downward migration of saline water into production zones (Ayolabi et al., 2013).
Mapping the extent and magnitude of seawater intrusion requires the use of a number of different investigative techniques to complement the inherent shortcomings of each approach. Electrical resistivity geophysical methods, such as Vertical Electrical Sounding (VES) and two-dimensional Electrical Resistivity Tomography (ERT) methods, are effective for detecting the subsurface spatial distribution of salt water-related resistivity changes and determining the freshwater-saltwater interface (Amidu & Olayinka, 2006). Such non-destructive techniques offer non-invasive, cost-effective means of providing spatial coverage of the subsurface and are consequently favoured for use in coastal groundwater studies. In a recent study, the Apapa-Ajegunle region of Lagos experienced a combination of VES and ERT methods with geostatistical analysis to conclusively prove that seawater has intruded into the shallow coastal aquifers due to unregulated groundwater extraction and urban development over the past few decades (Ayolabi et al., 2026).
Geochemical methods are used to supplement geophysical surveys by confirming the presence of salinity in boreholes and wells, and to identify the geochemical processes that can explain observed changes in water quality. Good examples of geochemical indicators include chloride concentrations, electrical conductivity, Na/Cl ratios, and hydrochemical facies classifications, which are commonly used to distinguish between mixing processes between two end-members of fresh and seawater and zones of active seawater intrusion versus natural saline water (Gimenez-Forcada et al., 2010). A geophysical-geochemical approach has been shown to be the best approach for estimating the status of seawater intrusion in Lagos (Ayolabi et al., 2013; Ojo et al., 2022).
1.2 Statement of the Problem
Lagos State is currently facing a water emergency. The state of pipe-borne water supplies in the city is such that it cannot keep up with the ever-growing population. As a consequence, unregulated groundwater extraction by private boreholes and hand-dug wells has grown out of control especially in the peripheral and low-income neighbourhoods of the city. It is thought to have significantly depleted groundwater levels across the city, causing the conditions for seawater intrusion to arise. Although seawater intrusion is acknowledged as a problem, there is no detailed and current map of the spatial distribution of seawater intrusion in Lagos State, nor does anyone know how rapidly the seawater front is progressing.
The impacts of unregulated seawater intrusion in Lagos are far-reaching and profound. Salinised groundwaters risk human health by exposing people to more saline water (with a higher concentration of chloride and sodium). The salinisation of the shallow aquifers is affecting peri-urban farming. Damage to buildings caused by the corrosive action of salty groundwater is also known to occur. Lack of scientific investigation of the present situation and the dynamics of seawater intrusion in Lagos will make it impossible to inform strategies to mitigate seawater intrusion and ensure the sustainable management of coastal groundwater.
1.3 Objectives of the Study
General Objective
The general objective of this study is to assess the extent and geochemical character of seawater intrusion in selected coastal areas of Lagos State using integrated geophysical and geochemical techniques.
Specific Objectives
The specific objectives of this study are:
(i) Conduct Vertical Electrical Sounding (VES) and Electrical Resistivity Tomography (ERT) surveys to map subsurface resistivity distribution and delineate the freshwater–saltwater interface.
(ii) Collect and analyse groundwater and surface water samples for physicochemical parameters and major ion chemistry to assess the degree of saline contamination.
(iii) Determine the spatial distribution and magnitude of seawater intrusion using geochemical indices including the chloride–bicarbonate ratio, seawater mixing index, and hydrochemical facies classification.
(iv) Integrate geophysical and geochemical data to develop a spatial model of seawater intrusion in the study area.
(v) Evaluate the implications of the findings for groundwater resource management and protection of coastal aquifers in Lagos State.
1.4 Research Questions
The following research questions guide the study:
- What is the current spatial extent of seawater intrusion in selected coastal areas of Lagos State?
- How does the subsurface resistivity distribution, as mapped by VES and ERT surveys, reflect the position of the freshwater–saltwater interface?
- To what extent has seawater intrusion compromised the quality of groundwater in the study area based on geochemical indices?
- What geochemical processes govern the evolution of groundwater chemistry in areas affected by seawater intrusion?
- What management interventions are most appropriate for mitigating seawater intrusion in Lagos coastal aquifers?
1.5 Research Hypotheses
H₀₁: There is no significant difference in chloride concentration between groundwater samples collected from coastal zones and inland zones in Lagos State.
H₁₁: Groundwater samples from coastal zones have significantly higher chloride concentrations than inland zones, indicating active seawater intrusion.
H₀₂: The depth to the freshwater–saltwater interface does not significantly vary with distance from the shoreline in the study area.
H₁₂: The depth to the freshwater–saltwater interface increases significantly with distance from the shoreline, reflecting the progressive displacement of freshwater.
1.6 Significance of the Study
This study is of direct significance to the Lagos State Water Corporation and the Lagos State Ministry of Environment, which are responsible for managing water resources and protecting the urban environment. The spatial maps of seawater intrusion produced by this study will provide actionable intelligence for the regulation of groundwater abstraction and the zoning of areas where borehole drilling should be restricted or controlled.
The study will also benefit urban planners, civil engineers, and the construction sector by identifying areas of corrosive saline groundwater that pose risks to building foundations and underground infrastructure. Public health agencies will benefit from the water quality assessment data, which can be used to update health risk maps for communities relying on potentially contaminated groundwater. The study adds to the regional scientific literature on coastal aquifer vulnerability in the Gulf of Guinea.
1.7 Scope of the Study
This study focuses on selected coastal and lagoonal areas of Lagos State where seawater intrusion is suspected to be most severe. Both shallow alluvial aquifers (less than 30 m depth) and intermediate-depth coastal plain sand aquifers (30–100 m) are within scope. The geophysical survey covers multiple transects perpendicular to and parallel to the coastline. The study is limited to the Lagos metropolitan area and does not extend to other coastal states.
1.8 Limitations of the Study
The study faces logistical challenges associated with field work in the densely urbanised Lagos metropolitan area, including restricted access to private land for geophysical electrode deployment, interference from buried utilities and metallic structures, and noise from traffic and industrial activities that can affect the quality of resistivity measurements.
The spatial coverage of water quality sampling may be constrained by the availability of accessible boreholes and the willingness of landowners to permit sampling. Furthermore, the dynamic and rapidly changing nature of the urban groundwater system means that a single sampling campaign may not fully capture the temporal variability in seawater intrusion extent. Budget constraints may also limit the number of geophysical profiles that can be acquired.
1.9 Definition of Terms
Seawater intrusion: The landward movement of saline water from the sea or a tidal body of water into freshwater coastal aquifers, driven by a reduction in the hydraulic head of the freshwater zone.
Vertical Electrical Sounding (VES): A geophysical technique that measures the variation of electrical resistivity with depth below the surface to characterise subsurface geological layers and identify saline water bodies.
Electrical Resistivity Tomography (ERT): A two-dimensional or three-dimensional geophysical imaging technique that produces a cross-sectional model of subsurface resistivity distribution from a series of multi-electrode measurements.
Chloride–bicarbonate ratio: A geochemical index used to assess the degree of seawater mixing in groundwater; high values indicate saline contamination, while low values indicate fresh groundwater.
Freshwater–saltwater interface: The zone of mixing between freshwater and saline water in a coastal aquifer, the position of which is controlled by the balance between freshwater recharge pressure and the weight of overlying saltwater.
Hydrochemical facies: Groundwater types classified according to their dominant cation and anion composition, used to interpret geochemical evolution and identify mixing processes.
References
Amidu, S. A., & Olayinka, A. I. (2006). Environmental assessments of sewage disposal systems using 2D electrical resistivity imaging and geochemical analysis: A case study from Ibadan, southwestern Nigeria. Environmental & Engineering Geoscience, 12(3), 261–272.
Ayolabi, E. A., Folorunso, A. F., Odukoya, A. M., & Adeniran, A. E. (2013). Mapping saline water intrusion into the coastal aquifer with geophysical and geochemical techniques: the University of Lagos campus case (Nigeria). SpringerPlus, 2(1), 433.
Ayolabi, E. A., Olatunji, A. S., & Adeola, T. D. (2026). Geophysical assessment of seawater intrusion in Apapa-Ajegunle, coastal area of Lagos, Southwestern Nigeria. Scientific Reports, 16, 1–18.
Bear, J., Cheng, A. H. D., Sorek, S., Ouazar, D., & Herrera, I. (1999). Seawater intrusion in coastal aquifers: concepts, methods and practices. Kluwer Academic Publishers.
Gimenez-Forcada, E., Bencini, A., & Pranzini, G. (2010). Hydrogeochemical considerations about the origin of groundwater salinization in some coastal plains of Elba Island, Tuscany, Italy. Environmental Geochemistry and Health, 32(3), 243–257.
Ojo, A. S., Afolabi, O. M., & Olarinoye, T. A. (2022). Joint geophysical and hydrogeochemical assessment of groundwater quality degradation in coastal aquifer systems at a suburb of Kosofe, Lagos, southwest Nigeria. Sustainable Water Resources Management, 8(6), 186.