Douala city, located in the littoral province of Cameroon, receives abundant rainfall quantities due to its geographical position in the gulf of Guinea and bears considerable surface water and groundwater resources. Due to socio-economic development and rapid demographic growth and its consequences of unplanned urbanization and improper sanitation system, these water resources are poorly protected and managed. Streams occurring the Wouri watersheds are mostly used as waste water discharge and hundreds boreholes have been drilled to the aquifer system without any management plan. A detailed geological, hydrodynamic and hydrogeochemistry study in Douala town and vicinity was conducted to get a better insight of the groundwater system functioning. Three field campaigns were carried out with 225 samples collected and analyzed for major ions and silica, stable isotopes (δ18O, δ2H, δ13C), tritium (3H) and 14C.
The system have been reconfigurated through petroleum and hydraulic lithologic boreholes data for a thickness of 200 m and two main geological and hydrogeological formations have been defined such as the superficial system namely Mio-Pliocene/Quaternary aquifer (0 to 70 m of depth) and the intermediate system namely Upper Eocene/Oligocene aquifer (70 to 200 m of depth). The general hydrochemistry is characterized by the chloride water type as the dominant one, mixed water type are also represented and to a lesser extend bicarbonate water type. The Q-mode HCA for both of systems deciphers 4 main clusters C1 to C4 according to the major ions and silica and the hydrogeological conditions. Then, the R-mode FA stand out the mineralization processes. These latters coupled to the isotopical results confirm the general hydrodynamic schema where modern groundwater types indicate silicate minerals weathering as the main geochemical process, allitization and monosiallitization as the principal pedogenetical processes and calcite dissolution (C1 and C2). Whereas sub modern groundwater type mostly show silica deposition, ion exchange and to a lesser extend other carbonate minerals dissolution (C3 and C4) and bisiallitization as the main pedogenetical process occuring. Furthermore, the spatial distribution of stable isotopes exhibit a groundwater enrichment gradient from the recharge areas (NE) to the main catchment (Wouri River). Depleted isotopic contents of superficial aquifers are closed to rainfall signature contents, attesting of direct infiltration. Mixing water characterize vertical flow acting in both systems precisely in the lower part/semi-confined aquifer of the Mio-Pliocene/Quaternary and in the upper part/semi-confined aquifer of the Upper Eocene/Oligocene. Radiocarbon reveal ages from few hundreds to thousands for the Upper Eocene/Oligocene aquifer system. These results refine understanding of the aquifer system functioning which is essential to provide a reasonable basis for effective groundwater control measures and sustainable water management in the study area and the entire sedimentary basin of Douala.
Key words : hydrogeochemistry, groundwater ressources, management, Douala basin, Cameroon