Thesis
Geochemistry of groundwater of shallow coastal aquifers of Eastern Dahomey Basin, Southwestern Nigeria
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2020
- Thesis identifier
- T16115
- Person Identifier (Local)
- 201657474
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- Coastal basins offer valuable land, water and economic resources and have high economic contribution to national and global development; hence the high population of coastal cities around the world due to their commercial, industrial and agricultural significance. Some of the cities and towns situated in coastal parts of Africa rely on the freshwater of the coastal aquifers to meet the vast shortage in water supply resulting from infrastructural deficiency and decay. Complexity of groundwater occurrence and distribution in basement aquifers lead most of the developing countries to depending on basins to meet their daily water demand as these basins are known for high freshwater potential. The Eastern Dahomey Basin (EDB) is not an exemption. The high rate of population growth, unpredictable rainfall patterns, rise in sea-levels, coupled with over abstraction and seawater intrusion, land use activities, climate conditions, and the geological setting have significant influence on groundwater chemistry and quality. Recently, groundwater resources management has become preferentially higher in the agenda of the sustainable development goal of the United Nations with much attention on developing countries where groundwater remains their source of water demand for various usages. As data and information on groundwater is critical to its sustainable management, especially at a basin scale. This study presents a comprehensive groundwater geochemistry of the Eastern Dahomey basin to contribute to the amount of knowledge available to better increase the effective framework of integrated water resources management in Sub-Saharan Africa. A total of 230 water samples were collected between May 2017 and April 2018, a period which spanned through wet and dry seasons, from the shallow boreholes and hand-dug wells. These 230 groundwater samples (97 in wet season and 133 in dry season) were analysed for essential water quality parameters such as pH, Electrical Conductivity (EC), Total Dissolved solid (TDS), Total Hardness (TH), Ca, Mg, Na, K, HCO3, Cl, SO4, NO3, F and the trace metals As, Cd, Fe, Mn, Pb and Si and stable Environmental Isotopes of δ18O and δ2H. In addition to these, stable isotopes of δ18O and δ2H in precipitation data from three selected GNIP stations, Douala, Cotonou and Kano within the West Africa, were collected for comparative analysis. These data were subjected to evaluation for different quality indices such as GQIswi, SMI, GWQI and ionic ratios, while modelling, plotting and statistical were carried out using analysis using ArcGIS, MATLAB, Geochemist Workbench, SPSS, Origin pro and Microsoft Excel. Electrical resistivity prospecting and borehole logging were carried out in locations with enhanced electrical conductivity around the coastal communities. Three traverses A-B, C-D and E-F were selected along which ERT and IP were carried out in directions perpendicular and parallel to the coastline and correlated with borehole logs. Higher salinities above 1000 μS/cm were observed in wells located around communities in Seme, Lekki, Eleko, Okun-Ajah, Ode-Mahin and Igbokoda. HFE-D revealed that mixed groundwater of Na+Ca-HCO3, Na-Cl and Ca-Cl dominate the area due to gravity-driven flow leading to groundwater freshening inland from the coastline towards the northern part of the basin. The groundwater quality index from SMI and GQIswi show areas within 3 km from the coastline that are more sensitive vulnerable to seawater intrusion. This result therefore guided our selection of areas for electrical resistivity geophysical investigation. Electrical resistivity tomography (ERT) and induced polarisation (IP) indicated a saline water-saturated layer of fine-grained sand and silty clay which is overlaid by the unconsolidated unconfined freshwater aquifer in area around Lekki and Okun Ajjah while low resistivity of clay lenses were found at Igbokoda, Ugbonla and Ode-Mahin. Correlation of selected ERT results with borehole logs further affirmed the suspected lithology from the sections. Results of the hydrochemical model revealed Ca-HCO3 and Na-Cl as dominant water types with other mixing water types such as Ca-SO4, Ca-Cl, Na-SO4 and K-Mg-HCO3 which characterised early stage of groundwater transformation in the shallow coastal basin. In addition, a comparison of the δ18O and δ2H isotopic compositions of groundwater and precipitation in the three selected stations, with their respective D-excess values established low evapotranspiration induced isotope enrichment, which could be due to higher precipitation and humidity in the region resulting in low isotope fractionation, hence, little effect of seasonal variations. Assessing the future use of groundwater for irrigation suggests some parts of the aquifer may have unsuitable quality based on the percentage sodium (%Na), Kelly’s ratio (KR), magnesium ratio (MR) and total hardness. Results of GQWI revealed 44.8, 22.9 and 12.5% of water samples during the wet season fell in the class of excellent, good and moderate quality respectively, while 8.3 %, 1.0 and 10.4 % fell in a class of poor, very poor and nonpotable water quality. Correlating the spatial distribution of the GWQI with the land use pattern map of the area revealed the least potable water is clustered around settlement areas, indicating groundwater quality has been impacted by municipal, industrial and agricultural waste. The linear regression modelling established significant relationships between SWL, SO42−, NO3−, Fe, and Eh for both wet and dry seasons with the p-value falling between 75% and 95%, which can also be seen in the plots of Eh/ORP against Fe2+, Mn2+, SO42−, and NO3−. This study has identified groundwater quality deterioration within the shallow coastal aquifers of the eastern Dahomey basin. Hydrochemical and geophysical approach established saltwater intrusion into freshwater aquifer occurred both as seawater intrusion, dissolution of evaporites within the clay lenses and sea spraying within the close proximity to the sea. Hydrochemical and environmental Isotopes classified as young from a meteoric source, which is still in the early stage of geochemical evolution with short residence time. A combination of groundwater quality indices (GWQI) and biogeochemical (redox) analysis also established the influence of natural and anthropogenic effect of climate change, urbanisation, industrialisation and agriculture on the groundwater quality of the basin. The present study provides information of value to planners and policy-makers for the sustainable management and protection of coastal groundwater resources in the Eastern Dahomey Basin. There is a need for waste management policy review and enforcement to support sustainable groundwater resource management and sustainable development goal number 6 (SDG6). Keywords: Groundwater; Saltwater intrusion; Groundwater quality index; freshwater; Coastal aquifer; Clay lenses; Saline water; Freshwater; Geoelectrical Layers; Hydrogeochemistry; mineral dissolution; Saturation Index; Stable Isotopes; Environmental Isotopes; Precipitation; Groundwater Origin; Residence time; Climate change; Biogeochemical Processes; Redox and Metals mobilisation; land use; seasonal effects; Anthropogenic; water resource management.
- Advisor / supervisor
- Sentenac, Philippe
- Kalin, Robert M.
- Resource Type
- DOI
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