In the heart of the South African plateau, the subtropical wetlands of the Okavango Delta (Botswana) represent an oasis of biodiversity within the Kalahari Desert. The Delta lies in an active tectonic trough (the Okavango graben) at the south-western end of the East African Rift. This unique combination of terrestrial and aquatic ecosystems, a UNESCO World Heritage Site since 2014, is closely dependent on the annual flooding of the Okavango River. Every year, the river brings in around 10 km³ of water from the mountains of Angola, 700 km further north. But this exceptional environment, largely untouched by human activity, is based on a fragile balance.

A unique source of biodiversity as well as a key economic resource for Botswana, the Delta is threatened by its own evolution as much as by global change and human pressure. After an initial exploration of the tectonic functioning of the Okavango graben, we have gradually involved geochemists, hydrologists, remote sensing specialists, ecologists, biochemists and more recently doctors and sociologists in a holistic study of the functioning of the Delta and its interactions with the communities that depend on it. The Alkaline Wetlands and CDKu-Link projects, co-sponsored by the CNRS (M. Jolivet, IPGP) and the Okavango Research Institute (N. Mazrui, Univ. Gaborone), are looking in particular at the interactions between a sub-surface saline aquifer (pH of 8 to 10) naturally enriched in metals and metalloids (As, V, Cr, Pb, U, etc) and the surface environment.

We show that since fairly recently, these metals have been absorbed to a very large extent by the flora via surface salt resurgences, and that this natural contamination represents a danger for the fauna and human populations living around the Delta.

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