Groundwater vulnerability mapping of hyper-humid urban aquifers for an improved water management in African megacities, opportunities and limitations in the case of Douala (Cameroon)

In this paper, two methods of mapping and assessment of the intrinsic vulnerability in porous media were applied to an urban case study relevant to illustrate the specificities and difficulties of African megacities concerning the management of the quality of the drinking water supply from groundwater. GOD and DRASTIC models have been used to map the shallow Mio-Plio-Quaternary aquifer vulnerability to pollution in the Tongo-Bassa watershed (Douala, Cameroon). Using GOD model, the results showed only one vulnerability class, the moderate one, extending all over the basin whatever the season. According to DRASTIC model, 25% (15%) of the basin has a moderate class of groundwater vulnerability to contamination, 5% (9%) has a very high vulnerability and 70% (76%) exhibit high vulnerability to contamination in dry (humid) season. There is an increase in high vulnerability zones in the rainy season, while in dry season, moderate vulnerable zones are observed. Vulnerability classes and a selection of chemical elements indicating human pollution are compared. Groundwater with high electrical conductivity corresponds mainly to high vulnerable class, while NO 3 − distribution does not coincide with the intrinsic vulnerability mapping. This raises the concept of pollution legacy as current nitrate load in some megacities is related to the history of urban development and human practices and not to present time conditions. The paper ends with clear recommendations to align land uses, landscape and groundwater protection in order to improve the quality of the drinking water supply for urban populations.