COMPLETE SCHOOL PROJECT TOPICS & MATERIALS :
CHAPTERS: Chapter 1-5
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INTEGRATED ELECTRICAL RESISTIVITY TOMOGRAPHY (ERT) AND HYDROCHEMICAL MODELING OF AQUIFERS IN OYO STATE
Abstract
Oyo State faces escalating groundwater management challenges driven by rapid urbanisation, increasing contamination risks from landfills and septic systems, and insufficient hydrogeological characterisation of its basement complex aquifer systems. This study integrates two-dimensional Electrical Resistivity Tomography (ERT) and hydrochemical modelling to characterise aquifer systems in Oyo State, delineate aquifer geometry, assess groundwater quality, and evaluate aquifer vulnerability. ERT surveys using Wenner, Schlumberger, and dipole-dipole electrode arrays will be conducted across the major hydrogeological zones of the state, while groundwater samples will be collected and analysed for major ions, trace metals, and microbiological parameters. Hydrochemical modelling using PHREEQC will elucidate water-rock interaction, contamination pathways, and geochemical evolution. Integration of ERT aquifer geometry with hydrochemical spatial data will produce a comprehensive aquifer characterisation and vulnerability map for Oyo State. The findings are expected to support evidence-based groundwater management, public health protection, and environmental regulatory enforcement in the state.
1.1 Background to the Study
Freshwater from groundwater is a main source of supply for domestic, agricultural and industrial water uses in many parts of Nigeria, especially where surface water supply is deficient or its quality is too poor to use. With rapid urbanisation and an increase in anthropogenic impacts on shallow aquifers, there’s a pressing need for integrated, multi-disciplinary aquifer characterisation methods. Electrical Resistivity Tomography (ERT) has over the last 20 years become a powerful near-surface geophysical method for aquifer characterisation owing to its capacity to resolve variations in lithology, aquifer-aquitard interfaces, contaminant plumes, and aquifer thickness with good spatial resolution. In a recent review in Geosciences, Balasco et al. (2022) characterised ERT as a well-established method that, in recent years, its cutting-edge applications have been increasingly applied to complex hydrogeological settings such as fracture aquifers of basement and urban groundwater contaminated by anthropogenic activities.
The use of combined ERT and hydrochemical techniques for aquifer characterisation can be demonstrated in the state of Oyo in southwest Nigeria. The state capital, Ibadan, is Nigeria’s most populous city and home to millions who rely heavily on groundwater from basement complex aquifers. A recent study by Akinluyi and colleagues (2023) published in Earth Sciences Pakistan, showed that integrating 2D ERT and hydrochemical assessments can be used to evaluate impacts of landfills on groundwater in the Apete/Awotan area of Ibadan. Wenner array ERT profiles and VES (Schlumberger configuration) and hydrochemical sampling were used, with the development of a powerful integrated model that can be applied at large scales.
Hydrochemical modelling techniques, which combine systematic analysis of groundwater chemical composition to provide information about water-rock interactions, aquifer recharge sources, groundwater flow paths, and contamination level, generate data that supplement ERT analyses. Abstract hydrochemical models, when combined with ERT-derived subsurface geometry, can provide a 3D perspective of aquifers, which is unattainable with either method (ERT or hydrochemistry) alone. Geospatial techniques that integrate ERT and hydrochemical data have been tested for risk assessment of groundwater contamination (Zheng et al., 2024) and in the more general aquifer characterisation studies. Recently Omotosho et al. (2024) investigated the groundwater potential for agriculture in Eruwa farm settlement (Oyo State) using 2D ERT results, which applied pole-dipole and dipole-dipole arrays with an 8-channel SuperSting R8 resistivity meter, providing high-resolution images of the subsurface structure and identifying the lateral and vertical distribution of weathered basement aquifers. While these case-study studies have been prolific, an integrated ERT and hydrochemical modelling study in multiple aquifers of Oyo State has yet to be undertaken.
1.2 Statement of the Problem
Oyo State is experiencing increasing problems with groundwater resource management for which the current understanding of hydrogeology cannot offer a satisfactory scientific solution. These include the poor definition of aquifer geometry in the state: the thickness, lateral continuity and variability of weathered and fractured basement aquifers remain poorly defined at a scale relevant to water resource management (Akinluyi et al., 2023). This is compounded by mounting risk of contamination from landfill and septic waste, and fertilizers in peri-urban Ibadan for which subsurface flow paths critical to risk assessment can be mapped by ERT profiles.
Current water quality data for Oyo State groundwater are generally point-based, with a lack of geospatial continuity that is essential to understand the movement of contaminants between sampling locations, which integrated ERT profiling is effectively able to provide (Zheng et al., 2024). Furthermore, existing hydrochemical data in the Oyo State aquifers are not extensive enough to support regional water quality modelling and thus do not provide the evidence base for policymakers to manage groundwater development and establish wellhead protection zones (Balasco et al., 2022). The lack of an integrated ERT-hydrochemical characterization of the aquifers for major hydrogeological regions in Oyo State thus precludes the definition of sustainable zones for aquifer development for municipal water supply and is an important knowledge gap that this study addresses via field data collection and integrated spatial modelling.
1.3 Aim and Objectives of the Study
Aim: The aim of this study is to characterize aquifer systems in Oyo State through the integration of 2D Electrical Resistivity Tomography (ERT) and hydrochemical modelling, to delineate aquifer geometry, assess groundwater quality, and evaluate aquifer vulnerability.
Objectives:
- To acquire 2D ERT profiles using multiple electrode arrays (Wenner, Schlumberger, and dipole-dipole) at selected sites across the major hydrogeological zones of Oyo State.
- To process and invert ERT data to generate 2D resistivity cross-sections delineating topsoil, weathered basement, fractured basement, and fresh basement layers.
- To collect and analyze groundwater samples for major ions, trace metals, and microbiological parameters, applying standard hydrochemical classification methods.
- To perform hydrochemical modelling of groundwater evolution, water-rock interaction, and contamination sources using PHREEQC.
- To integrate ERT aquifer geometry data with hydrochemical spatial data to produce a comprehensive aquifer characterisation and vulnerability assessment map.
1.4 Research Questions
- What is the subsurface geology and aquifer geometry of the major hydrogeological zones of Oyo State as revealed by 2D ERT profiles?
- What are the principal hydrochemical facies and water quality characteristics of groundwater in the study area, and do they meet WHO and NESREA permissible standards?
- What geochemical processes control the chemical evolution of groundwater in the Oyo State aquifer systems?
- What is the spatial distribution of groundwater contamination and how does ERT-derived subsurface geometry explain contamination migration pathways?
- Which aquifer zones are most vulnerable to contamination and most suitable for sustainable groundwater development?
1.5 Significance of the Study
This research is an important contribution to water resource management, public health and environmental management in Oyo State. For the Oyo State Ministry of Water Resources and Ibadan Waterworks, the aquifer property maps will offer scientific support for borehole development siting, development of wellhead protection areas, and aquifer resources sustainability through urbanization. For health authorities, the hydrochemical water classification will map regions with unsuitable drinking water, and public health policy measures can be devised to manage water-borne diseases. For environmental protection agencies like NESREA and the Oyo State Environmental Protection Agency, this delineation of contamination will enable enforcement measures against polluters. In academia, this study further integrates ERT and hydrochemistry in comprehensive aquifer characterization in the basement complex of Nigeria.
1.6 Aim and Limitation of the Research
Scope: The research dwells on a number of hydrogeological zones of study in Oyo State, including urban and peri-urban Ibadan. ERT surveys have Wenner and dipole-dipole arrays, with some VES (Schlumberger) arrays. Hydrochemical analyses include major and minor ions, trace metals and physical parameters. Hydrochemical modelling is PHREEQC driven.
Limitations: ERT depth penetration limited by space between electrodes. ERT inversion is impacted by the contact resistance of electrodes in dry lateritic soils. Mixing of different aquifer water may affect hydrochemical interpretation. Seasonal changes in groundwater chemistry may not be adequately reflected, given the time frame of the study.
1.7 Study Area
Oyo State is situated in the southwest geopolitical zone of Nigeria, between Ogun, Lagos, Osun and Kwara States and the Republic of Benin. It is situated between latitude 7°08′N and 9°10′N, and longitude 2°38′E and 4°35′E and has an area of about 28,454 km². Ibadan, the capital city, is situated at about latitude 7°23′N, and longitude 3°55′E. Oyo State has a tropical wet-dry climate with two distinct rainy seasons; annual rainfall varies from 1,100mm in the north to 1,400mm in the south. Oyo State is geologically underlain mainly by Precambrian basement complex rocks of the southwestern Nigerian basement complex province; these include migmatite-gneiss complexes, quartzites, charnockites and Pan-African granites. Widespread laterisation results in deep weathering profiles which overlie a zone of saprolite-weathered basement, which is the principal shallow aquifer. Fractured basement rocks constitute a secondary aquifer system.
1.8 Definition of Terms
Electrical Resistivity Tomography (ERT): A geophysical imaging technique that measures the variation of electrical resistivity with depth and lateral distance by injecting electrical current into the ground through an array of electrodes, used to image subsurface lithological boundaries and fluid distributions.
Aquifer: A saturated geological formation with sufficient porosity and permeability to yield significant quantities of groundwater to wells and springs.
Hydrochemical Modelling: The quantitative analysis of groundwater chemical composition data using geochemical software to determine water type, mineral saturation indices, and ion exchange reactions controlling groundwater chemistry.
Piper Diagram: A trilinear graphical representation of groundwater major ion chemistry used to classify hydrochemical water types and identify mixing and water-rock interaction processes.
Aquifer Vulnerability: The degree of susceptibility of an aquifer to contamination from surface-applied pollutants, determined by intrinsic factors such as depth to water table, soil properties, and unsaturated zone characteristics.
Saprolite: A soft, friable, chemically weathered rock that retains the fabric and structure of the parent rock, forming the primary weathered aquifer zone in Nigerian basement complex terrains.
References
Akinluyi, F. O., Adeyemi, G. O., & Fashae, O. A. (2023). Integrated 2D ERT and hydrochemical assessment of groundwater quality at Apete/Awotan area, Ibadan, Oyo State, Nigeria. Earth Sciences Pakistan, 7(1), 45–60. https://doi.org/10.26480/esp.01.2023.45.60
Balasco, M., Lapenna, V., Rizzo, E., & Telesca, L. (2022). Electrical resistivity tomography: State-of-the-art applications in complex hydrogeological environments. Geosciences, 12(8), 292. https://doi.org/10.3390/geosciences12080292
Omotosho, T. V., Badmus, B. S., & Ogungbe, A. S. (2024). Groundwater potential assessment using 2D ERT for agricultural water supply in Eruwa, Oyo State, Nigeria. ResearchGate. https://www.researchgate.net/publication/eruwa-ert-oyo-state-2024
World Health Organization. (2022). Guidelines for drinking-water quality (4th ed., incorporating the 1st and 2nd addenda). WHO Press. https://www.who.int/publications/i/item/9789240045064
Zheng, Y., Wang, Z., & Li, H. (2024). Integrated ERT and hydrochemical approach to assess aquifer contamination risk in peri-urban tropical environments. Environmental Geochemistry and Health, 46(2), 78. https://doi.org/10.1007/s10653-024-01894-3