Semi-arid regions are strongly impacted by global change and increasing anthropogenic pressure. In the heart of the Kalahari Desert, Southern Africa, the unique oasis of biodiversity formed by the Okavango Delta wetlands – a UNESCO World Heritage site – is threatened by global change and growing anthropogenic pressure. While global warming decreases the amount of water sustaining wetlands in semi-arid regions, threatening their existence, changes in the quality of that water (increased salinity, potential microbiological contamination, anthropic pollution, etc) also play a major role in the degradation of the ecosystems. On top of climate-driven vegetation shifts, increased evaporation rates or water temperature could also impact the soil and water microbiota which will probably have strong consequences on the water chemistry. Understanding the complex natural processes that govern the fragile equilibrium of such a hot-spot of biodiversity is a major challenge requiring us to increase our scientific capacities to detect, forecast and prevent ecological threats linked to global change.
The Okavango wetlands are sustained by annual floods bringing ~11 km3 of freshwater into the Delta from the highlands of Angola in the North. Because of the very low regional topographic gradient and the distance of about 700 km between the headwaters and the Delta, the inundation occurs during the dry season (May – September). This water input is complemented by about 450 mm/year rainfalls over the Delta during the period November – March. The Delta is separated into three major ecoregions: a permanently flooded area mostly to the north and east, a seasonally flooded region to the centre, extending south to Maun and an occasionally flooded area to the west. It is dotted with about 150,000 islands of all shapes and sizes.The most intriguing characteristic of these islands is the occurrence, at least in some of them, of a highly alkaline (pH>8) aquifer resting some 2 to 3 m below the surface. These waters are extremely enriched in metals and metalloids such as Cr, U, V, or As (the latest reaching concentrations of 35 mg.l-1). However, this contamination is entirely natural
The research team participating in Alkaline Wetlands recently questioned the established model that explains the formation of brine water through evapotranspiration of fresh surface water. We demonstrated that the peculiar Rare Earth Elements (REE) patterns (strongly enriched in Heavy REE) of the brine-type groundwaters could not result from the concentration of the river water through evaporation or evapotranspiration. We suggested that the salted aquifer is completely different and disconnected from the surface aquifer, possibly inherited from a previous lacustrine phase.
Alkaline Wetlands is an original project aiming at revisiting the evaporation/evapotranspiration model widely used to explain the occurrence of alkaline aquifers below the Okavango islands.