Geochemistry of groundwater of shallow coastal aquifers of Eastern Dahomey Basin, Southwestern Nigeria

  • Jamiu Aladejana

Student thesis: Doctoral Thesis

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 aroun
Date of Award7 Jul 2020
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorRobert Kalin (Supervisor) & Phillippe Sentenac (Supervisor)

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