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GROUNDWATER FLOW MODELING USING MODFLOW IN THE BASEMENT COMPLEX TERRAIN OF EKITI STATE.
CHAPTER ONE
INTRODUCTION
1.1 Background to the Study
Groundwater is a critical source of fresh water to millions of people around the world, especially in areas where surface water is limited or intermittent. Groundwater is a significant source of drinking water in both rural and urban areas of Nigeria, particularly in crystalline basement rocks terrains where surface water is only available during the wet season. Ekiti State, located in southwestern Nigeria, is mainly underlain by the Precambrian basement complex rocks such as migmatites, granite-gneisses, charnockites, and quartzites that have complex hydrogeology. Groundwater in crystalline basement environments is mostly controlled by secondary porosity and permeability established by the process of weathering and fracturing of rocks (Talabi & Tijani, 2013).
The rising water demand in Ekiti State brought about by its rapid population increase, urbanisation and lack of surface water supply systems has resulted in the huge exploitation of unravelled aquifers. Many boreholes have been identified and constructed throughout the state with varying levels of productivity and success, with many that are low yielding or productive. This calls for a scientific understanding of groundwater flow system dynamics of the basement complex aquifer system in the region (Akinwumiju et al., 2020). Traditional methods of hydrogeological investigation are often used, but may be insufficient in their ability to predict the response of the aquifer to various pumping, recharge and climatic fluctuations.
Groundwater flow modelling has become one of the most valuable techniques in contemporary hydrogeology, offering a quantitative approach to simulate aquifer behaviour and its response to stresses. The MODFLOW family of codes, developed by the United States Geological Survey (USGS), is the international standard for modelling three-dimensional groundwater flow in porous formations (Langevin et al., 2023). MODFLOW uses a finite-difference numerical solution to solve the differential equations describing saturated groundwater flow, making it ideal for large, multi-aquifer systems. The most recent version, MODFLOW 6, includes enhanced functionalities for coupled groundwater/surface-water, solute transport and variable-density flow simulations, significantly broadening its scope to various hydrogeological scenarios (Hughes et al., 2022).
In Ekiti State’s basement complex, use of MODFLOW involves the conceptualisation of a dual-porosity aquifer system which includes the weathered overburden and the fractured bedrock basement. While groundwater occurrence in the basement complex has been reported for some areas in southwestern Nigeria (Ademilua & Eluwole, 2013; Talabi & Tijani, 2013), few studies have provided detailed numerical groundwater flow models incorporating the variability in the spatial distribution of hydrogeological properties in Ekiti State. This study therefore aims to address a crucial knowledge gap through the development and calibration of a groundwater flow model using MODFLOW for typical basement complex terrain in Ekiti State, to better understand the pattern of groundwater recharge, flow trends and estimated sustainable yield of aquifers in the State.
1.2 Statement of the Problem
While the vital role of groundwater to the socioeconomic development of Ekiti State has been established, the management of the resource has been more pragmatic than scientific and proactive. The state is still faced with periodic borehole “droughts”, seasonal falls in the water table and acute pollution of shallow aquifers as a result of unregulated land use practices. An underlying cause of all these issues is the absence of a quantitative description of groundwater flow within the basement complex aquifer system. Such a model is essential to understand aquifer response to increased pumping rates, to estimate reliability of recharge, to characterise water table variations due to climate change, and to design the best pattern and schedules of borehole pumping.
The weathered and fractured basement aquifer system in Ekiti State is highly variable in hydraulic conductivity, storativity and recharge, both horizontally and vertically, making the use of simple mathematical models challenging. And the lack of hydrogeological data such as long-term groundwater level observations, pumping test and recharge estimates further complicate groundwater flow modelling in this region. The ramifications of this lack of knowledge are many, including inappropriate borehole siting, excessive groundwater pumping, and reduction in groundwater quality with adverse effects on human health and wellbeing.
1.3 Objectives of the Study
General Objective
The general objective of this study is to develop and calibrate a groundwater flow model using MODFLOW for the basement complex terrain of Ekiti State, in order to improve the understanding of aquifer dynamics and support sustainable groundwater resource management.
Specific Objectives
The specific objectives of this study are:
(i) Characterise the hydrogeological framework of the study area, including aquifer geometry, boundary conditions, and hydraulic properties.
(ii) Develop a conceptual groundwater flow model based on the hydrogeological characterisation and available data.
(iii) Construct and calibrate a numerical groundwater flow model using MODFLOW against observed groundwater levels.
(iv) Perform sensitivity analyses to assess the influence of key hydraulic parameters on model output.
(v) Simulate groundwater flow under different scenarios of abstraction and recharge to evaluate aquifer sustainability.
1.4 Research Questions
The following research questions guide the study:
- What are the principal hydrogeological characteristics of the basement complex aquifer system in Ekiti State?
- How accurately can MODFLOW simulate observed groundwater level patterns within the basement complex terrain?
- Which hydraulic parameters exert the greatest influence on groundwater flow model output in the study area?
- What is the sustainable yield of the basement complex aquifer system under varying abstraction and recharge scenarios?
- To what extent does seasonal variability in recharge affect water table fluctuations in the modelled domain?
1.5 Research Hypotheses
H₀₁: There is no significant difference between MODFLOW-simulated groundwater levels and observed groundwater levels in the basement complex terrain of Ekiti State.
H₁₁: There is a significant difference between MODFLOW-simulated and observed groundwater levels, indicating that hydraulic parameter heterogeneity substantially influences model accuracy.
H₀₂: Changes in groundwater abstraction rates have no significant effect on water table levels in the basement complex aquifer of Ekiti State.
H₁₂: Changes in groundwater abstraction rates significantly affect water table levels in the basement complex aquifer of Ekiti State.
1.6 Significance of the Study
This study is significant to several stakeholders. The Ekiti State Government and the Rural Water Supply Agency will be benefited by the model outputs that can be used as scientific guide for borehole placement, borehole yield, and water supply infrastructure development. Users affected by boreholes that have failed or those that are currently facing water scarcity will benefit indirectly as a result of better-resourced management decisions informed by the model.
Hydrogeologists and water resources managers will also benefit from the scientific breakthrough of using MODFLOW in modelling hard rock aquifers in Nigeria, where few numerical modelling studies have been conducted. The research findings will also provide information to postgraduate students and researchers on basement complex hydrogeology in West Africa. The results of scenario modelling would be of value for policymakers in climate change adaptation policies in the water sector.
1.7 Scope of the Study
The present study takes the basement complex in Ekiti State, southwestern Nigeria as a case study. The study area covers certain local government areas where hydrogeological data is readily available and problems with water supply are significant. This includes the shallow weathered overburden aquifer and deeper fractured basement aquifer. The study examines steady-state and transient flow in the aquifer, equivalent to variations in the groundwater level during the dry and wet seasons over a designated period.
1.8 Limitations of the Study
The study is subject to several constraints. First, the availability of long-term groundwater level monitoring data in Ekiti State is limited, which may affect the robustness of transient model calibration. Second, the spatial coverage of pumping test data is sparse, introducing uncertainty into the assignment of hydraulic conductivity and storativity values across the model domain.
Financial and logistical constraints may limit the number of field measurement points that can be incorporated into the model, potentially reducing spatial resolution. Additionally, the study is constrained to surface and shallow subsurface processes; deep circulation systems beneath the weathered zone are not within scope. These limitations are acknowledged and addressed through sensitivity analysis and uncertainty quantification within the modelling framework.
1.9 Definition of Terms
Aquifer: A saturated permeable geological unit that can store and transmit sufficient water to supply a well or spring.
MODFLOW: A modular, finite-difference groundwater flow model developed by the United States Geological Survey for simulating three-dimensional groundwater flow in porous media.
Hydraulic conductivity: A measure of the ease with which water can move through pore spaces or fractures in a geological material, expressed in metres per day.
Storativity: The volume of water that a confined aquifer releases from or takes into storage per unit surface area per unit change in hydraulic head.
Recharge: The process by which water enters an aquifer from the surface, typically through infiltration of rainfall or seepage from surface water bodies.
Basement complex: A suite of Precambrian metamorphic and igneous rocks that form the crystalline foundation underlying large parts of West Africa, including Ekiti State.
Calibration: The process of adjusting model input parameters so that model outputs match observed field measurements within acceptable tolerances.
References
Akinwumiju, A. S., Olorunfemi, M. O., & Afolabi, O. (2020). Groundwater potential evaluation in the basement complex terrain of Ekiti East Local Government Area, Southwestern Nigeria. International Journal of Energy and Water Resources, 4(3), 277–295. https://doi.org/10.1007/s42108-019-00053-5
Ademilua, O. L., & Eluwole, A. B. (2013). Hydrogeophysical evaluation of the groundwater potential of Afe Babalola University Ado-Ekiti, Southwestern Nigeria. Journal of Emerging Trends in Engineering and Applied Sciences, 4(1), 77–83.
Hughes, J. D., Russcher, M. J., Langevin, C. D., Morway, E. D., & McDonald, R. R. (2022). The MODFLOW Application Programming Interface for simulation control and software interoperability. Environmental Modelling & Software, 148, 105257. https://doi.org/10.1016/j.envsoft.2021.105257
Langevin, C. D., Hughes, J. D., Provost, A. M., Russcher, M. J., & Panday, S. (2023). MODFLOW as a configurable multi-model hydrologic simulator. Groundwater, 62, 111–123. https://doi.org/10.1111/gwat.13351
Olorunfemi, M. O. (1990). The hydrogeological implications of geology and structures in Ekiti area of Nigeria. Water Resources, 2(1), 15–24.
Talabi, A. O., & Tijani, M. N. (2013). Hydrochemical and stable isotopic characterisation of shallow groundwater system in the crystalline basement terrain of Ekiti area, southwestern Nigeria. Applied Water Science, 3(1), 229–245. https://doi.org/10.1007/s13201-012-0076-3