Geometric structure and hydrodynamic assessment of mio-pliocene aquifers in the Douala sedimentary sub-basin, Cameroon: First appraisal

Abstract

Effective groundwater management depends on accurately identifying water access points, quantifying available volumes, and understanding subsurface flow dynamics. This is particularly critical in developing countries where groundwater often represents the primary, and sometimes sole source of safe drinking water - especially in densely populated urban centres. In this study, a combination of geophysical methods, lithostratigraphic data, and hydrodynamic parameters was employed to characterize the geometry and properties of the Mio-Pliocene aquifer system the Douala basin in Cameroon. Using the vertical electrical sounding method and the Schlumberger array along a profile of approximately 166 m, combined with data from 20 boreholes including well-log profiles and hydraulic measurements, the underdeveloped aquifers less than 30 m thick, located around 20 m below the surface. More prominent and hydraulically significant sandy layers were detected at depths of 40–50 m, extending down to about 120 m. Three distinct hydrogeological zones were identified: (a) a shallow layer composed mainly of sandy, clayey, and sandy-clay materials; which outcrops in various areas and is located close to river systems; (b) a thinner intermediate layer characterised by gravelly-sandy and sandy-clay sediments; which constitute an underground extension of the unconfined aquifers. (c) A deep aquifer layer with more consistent lateral facies distribution and greater overall thickness. This deep aquifer system also demonstrated better structural continuity and a confined unit that represents the best underground reservoir for useable water. Piezometric levels across the study area ranged from 12.9 to 36.1 m, with well yields between 8.3 and 33.2 m³ /h and drawdowns from 1.19 to 11.3 m. Transmissivity values varied from 0.003 to 0.087 m²/s, generally increasing towards the coastal zone. These values are a potential indicator of the vulnerability of shallow and semi-shallow aquifers to salt water intrusion, from the surface to roughly 40 m. The geology and topography strongly influence groundwater flow behaviour. These findings provide valuable data that can inform future strategies to improve groundwater accessibility and ensure sustainable water resource management. Due to the fact that the Douala basin is completely connected to the Atlantic Ocean and consists mainly of porous and permeable formations, this study provides opportunities not only for a better understanding of its hydrogeological structure, but also for spatial monitoring of its interaction with seawater and intercontinental stresses. • Three separate hydrogeological systems have been identified: a shallow system, a relatively thin intermediate system, and a deeper system, all located within a depth range of 20–120 m. • The Mio-Pliocene sequence comprises five distinct lithofacies: gravel beds, fine to medium-grained sands, clayey sands, sandy clays, and clay layers. • The Mio-Pliocene aquifer in the Douala sub-basin is a multi-layered aquifer with a free, semi-captive to captive character. • The hydrodynamic functioning of Mio-Pliocene aquifers is strongly dependent on topography and subsurface lithology.