Knowledge of the source of water in the Lagos coastal basin (LCB) groundwater system was to be found vital to the future development and management of the system. Stable and radioactive isotopic measurements have been employed to unravel the source of recharge and residence time of the shallow groundwater system, based on the sampling conducted in 2016 and 2017 on groundwater, surface water and rainfall. The concentration of tritium in the groundwater samples were very low and ranged from less than 1 to 2.8 TU, while measured 14C contents ranged from 59.1 to 88 pMC. The δ18O values of groundwater samples ranged from 4.81 and 3.98 ‰, while the δ2H values ranged from -24.75 and -19.70 ‰ for the wet and dry seasons, respectively. The obtained results indicated non-existence of paleo recharge; rather all groundwater in the basin were found to be essentially of meteoric origin with intermittent surface water contributions. Moreover, shallow groundwater and surface water have considerable variations in isotopic compositions, reflecting evaporation and preservation of seasonal fluctuation. Though there was an observed generally low tritium content, however, it proved useful in the identification of recent active recharge taking place across the basin. The deduced radiocarbon age reflected the presence of “modern water” and thus supports the presence of present recharge to the groundwater system. Therefore, the source of the shallow groundwater recharge was actively renewable particularly during the wet season and thus water exploitation is potentially sustainable in the basin.
Water security is the central mission of the sustainable development goals. The demand for potable and clean water has skyrocketed due to frequent borehole failures and population expansion, which requires adequate groundwater resource management strategies. Identifying groundwater potential zone, overburden protective index capacity and installations of alternative/artificial storage support mechanisms for water security and sustainability under the growing water challenge and demand is critical. This study identifies; (i) aquifer promising zones (ii) aquifer protective capacity through geophysical investigation and suggests (iii) improving aquifer recovery management strategy as the top three targets for groundwater development. An electric resistivity technique was applied to acquire a total of sixty (60) vertical electrical sounding points with Schlumberger arrays. The study indicates the resistivity layer of the aquifer unit ranges from 21 to 294 Ωm, while the aquifer layer thickness values spread from 8 to 59 m and the overburden thickness overlays the aquifer unit extends from 3 to 20 m. The deeper aquifer zones were encountered between 30 and 59 m, which could be suitable for groundwater development, and the shallow aquifer occurs between 8 and 14 m, which is not encouraging for groundwater development and may be susceptible to surface contaminations. However, for water security and sustainability, multiple boreholes should be sited at the delineated aquifer promising zone where the aquifer is fractured and occurs within a depth of 30 m and above. The weathered/fractured units constitute the regional aquifer units, which are largely responsible for the groundwater potential. The hydraulic conductivity of the regional aquifer was estimated to vary between 0.337 and 10.62 m/day, which invariably enhances the aquifer recovery processing. Groundwater quality and the risk of surface contamination were examined through overburden protective index capacity.
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