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HYDROGEOCHEMICAL EVOLUTION AND ISOTOPIC CHARACTERIZATION OF GROUNDWATER IN BORNO STATE
CHAPTER ONE
INTRODUCTION
1.1 Background to the Study
CHAPTER ONE
INTRODUCTION
1.1 Background to the Study
Borno, in northeastern Nigeria, is among the most water-scarce areas in Nigeria. Its semi-arid to arid climatic conditions with scant and highly variable rainfall ranging from 400 mm to 700 mm per year contributes to the seasonal nature of the surface water resources. As such, groundwater extracted from the Chad Formation (CF) and other sandstones deposits is the primary source of drinking water, irrigation and domestic stock watering for the region’s large population. However, the hydrogeochemistry of the groundwater in these resources is poorly documented compared to their significance, and the growing challenges of population, climate, and environmental change have raised concerns about the sustainability and quality of the groundwater resources (Usman et al., 2025).
In this regard, hydrogeochemistry is the study of the chemical characteristics of groundwater and the processes involved in the development of the water chemistry as a result of interaction with aquifer materials, movement through the unsaturated zone, and mixing with other water. The hydrogeochemistry of groundwater offers a wealth of information regarding recharge, flow lines and pathways, and the degree of water-rock interaction, among other things, which are important for aquifer characterization and water quality management (Gibbs, 1970). In semi-arid environments like northeastern Nigeria, the most common hydrogeochemical processes, including evaporation, ion exchange, dissolution of minerals and human activities, contribute significantly to changing salinity and water quality, thereby making it unsuitable for potable and irrigation uses.
Isotopic analyses, including stable isotopes of water (oxygen-18 and deuterium) and radioactive isotopes (tritium and carbon-14), play a valuable role in hydrogeological studies. Natural stable water isotopes can be used as tracers to trace the origin and evolution of water from precipitation to infiltration and groundwater flow, allowing for the determination of groundwater recharge zones, recharge rates and groundwater age and mixing (Fritz et al., 1987; Clark & Fritz, 1997). In Borno State, where groundwater recharge is expected to be intermittent and depend heavily on seasonal flooding and river bank filtration from the Komadugu-Yobe River system, as well as Lake Chad, isotopic data can provide insights into recharge processes that are not revealed by traditional hydrochemical analyses.
Earlier hydrogeochemical studies in northeast Nigeria have documented the complex hydrochemistry of groundwater in the Chad Formation, which can result from both natural processes (such as evaporation, silicate weathering, and cation exchange) as well as human activities such as agricultural use and poorly managed garbage dumps (Usman et al., 2025). But there are no recent extensive isotopic studies that combine stable isotope analyses with hydrochemical investigations in Borno State to reveal the sources and processes of groundwater evolution. This study seeks to fill this gap through the conduct of hydrogeochemical and isotopic characterisation studies.
1.2 Statement of the Problem
The groundwater resources of Borno State are facing an increasing number of threats including drought, overexploitation, chemical contamination from agricultural activity, and water infrastructure decay as a result of decades of military conflict and insecurity within the state. Many communities rely on shallow hand-dug wells and boreholes from the Chad Formation aquifer, but there is little understanding of the water quality. High levels of fluoride, nitrate and total dissolved solids have been documented in boreholes of the area of concern, which presents a serious health risk. An understanding of the hydrogeochemical processes that control groundwater chemistry and the water stable isotope composition of the recharging groundwater is essential for targeted mitigation measures and to anticipate changes in groundwater quality in a warming climate.
In addition, the long-term sustainability of groundwater withdrawal in Borno State is uncertain without understanding the rates of groundwater recharge and groundwater age. Lake Chad is well known to have experienced declines in surface area over the past few decades as a result of reduced flow from its primary tributaries, and increased irrigation abstractions, which affect recharge rates for groundwater in the region. Isotopic evidence can yield important constraints on the rate of recharge and interconnections between surface water and groundwater systems that are required to determine the sustainability of the aquifer.
1.3 Objectives of the Study
General Objective
The general objective of this study is to characterise the hydrogeochemical evolution and isotopic composition of groundwater in Borno State in order to determine the geochemical processes governing water quality and to identify recharge sources, flow paths, and groundwater age.
Specific Objectives
The specific objectives of this study are:
(i) Collect and analyse groundwater samples from boreholes and hand-dug wells for major ion chemistry and physicochemical parameters.
(ii) Identify the dominant hydrogeochemical processes controlling groundwater chemistry using geochemical classification schemes, graphical methods, and geochemical modelling.
(iii) Determine the stable isotopic composition (δ18O and δD) of groundwater and compare with the Global Meteoric Water Line to identify recharge sources.
(iv) Assess groundwater suitability for domestic and irrigation uses based on standard water quality guidelines.
(v) Integrate hydrochemical and isotopic data to develop a conceptual model of groundwater evolution in the study area.
1.4 Research Questions
The following research questions guide the study:
- What are the dominant hydrogeochemical processes controlling the chemical composition of groundwater in the Chad Formation aquifer of Borno State?
- What do stable isotopic signatures (δ18O and δD) reveal about the recharge sources and pathways of groundwater in the study area?
- To what extent does groundwater in Borno State meet national and international water quality standards for domestic and irrigation use?
- How do seasonal variations in rainfall influence the hydrogeochemical character of shallow groundwater in the region?
- What conceptual model best represents the hydrogeochemical evolution and flow dynamics of groundwater in the study area?
1.5 Research Hypotheses
H₀₁: There is no significant variation in the major ion chemistry of groundwater between wet and dry seasons in Borno State.
H₁₁: There is significant seasonal variation in the major ion chemistry of groundwater in Borno State, reflecting the influence of recharge dilution and evaporative concentration.
H₀₂: The stable isotopic composition of groundwater in Borno State shows no significant deviation from the Global Meteoric Water Line, indicating minimal evaporative enrichment.
H₁₂: The stable isotopic composition of groundwater shows significant deviation from the Global Meteoric Water Line, reflecting evaporative enrichment prior to or during recharge.
1.6 Significance of the Study
This study is significant to government agencies responsible for water supply in northeastern Nigeria, particularly the Borno State Rural Water Supply and Sanitation Agency, which will benefit from evidence-based guidance on the safety and suitability of groundwater sources. Communities relying on unmonitored groundwater will benefit indirectly through the identification of contamination hotspots and the development of remediation recommendations.
The study contributes to the scientific literature on the hydrogeochemistry of the Lake Chad Basin, a transboundary aquifer system shared by Nigeria, Niger, Chad, and Cameroon, and will be of relevance to regional water resource management bodies. Public health agencies and non-governmental organisations working on water, sanitation, and hygiene (WASH) in conflict-affected areas of Borno State will also find the water quality assessment results directly actionable.
1.7 Scope of the Study
The study focuses on the major aquifer units of Borno State, including the shallow unconfined aquifer within the Chad Formation and deeper semi-confined aquifers. The spatial scope covers representative parts of the Borno Basin, particularly areas around Maiduguri and its environs. The study encompasses wet season and dry season sampling campaigns to capture seasonal variability in groundwater chemistry and isotopic composition. It does not extend to the Precambrian basement aquifer in the Mandara Mountains.
1.8 Limitations of the Study
The study faces significant logistical and security constraints. Field access in parts of Borno State is restricted due to ongoing insurgency, which may limit the spatial coverage of groundwater sampling to accessible areas around Maiduguri and major towns. This spatial limitation could affect the representativeness of the hydrogeochemical model for remote areas of the state.
Laboratory analysis for stable isotopes and radiocarbon dating involves specialised equipment not available in-country, necessitating sample shipment to certified international laboratories and introducing potential delays. Financial constraints may limit the number of samples that can be submitted for isotopic analysis. Additionally, the absence of an established local meteoric water line for Borno State introduces uncertainty in the isotopic interpretation of recharge sources.
1.9 Definition of Terms
Hydrogeochemistry: The scientific study of the chemical composition of groundwater and the physical, chemical, and biological processes that influence its evolution.
Stable isotopes: Non-radioactive variants of elements (such as oxygen-18 and deuterium) that occur in fixed proportions in natural water and serve as tracers of hydrological processes.
Global Meteoric Water Line (GMWL): An empirical linear relationship between δ18O and δD in precipitation worldwide, used as a reference for interpreting the isotopic composition of groundwater.
Hydrochemical facies: Categories of groundwater composition defined by the dominant ions present, used to characterise water types and infer geochemical evolution.
Cation exchange: A chemical process in which cations in groundwater are exchanged with cations on the surface of aquifer minerals, altering the ionic composition of the water.
Chad Formation: A Quaternary sedimentary unit consisting of alternating sands, silts, and clays deposited in lacustrine and fluvial environments around the Lake Chad basin, forming the principal aquifer of northeastern Nigeria.
References
Clark, I. D., & Fritz, P. (1997). Environmental isotopes in hydrogeology. Lewis Publishers.
Gibbs, R. J. (1970). Mechanisms controlling world water chemistry. Science, 170(3962), 1088–1090. https://doi.org/10.1126/science.170.3962.1088
Moussa, A. B., Zouari, K., Marc, V., Mudry, J., & Sherif, M. (2016). Hydrogeochemical and isotopic investigation of the Lake Chad Basin, West Africa. Environmental Earth Sciences, 75(1), 1–22.
Onwe, I. M., Unigwe, C. O., Amah, J. I., & Chima, C. J. (2023). An integration of hydrochemical data and stable isotopes in groundwater evaluation in Ngboejogu, southern Benue Trough, Nigeria. Modeling Earth Systems and Environment, 10(1), 1021–1039.
Usman, S., Garba, M. L., Abubakar, I. Y., Uzochi, C. A., & Zhou, Q. (2025). Integrated hydrogeochemical and multivariate statistical assessment of groundwater in Gashua, northeastern Nigeria. Modeling Earth Systems and Environment, 11, Article 279. https://doi.org/10.1007/s40808-026-02727-6