The Ohangwena aquifer system in Angola and Namibia – a fresh paleo-groundwater resource in an arid and saline environment

The transboundary Cuvelai-Etosha Basin (CEB), located in Southern Angola and Northern Namibia, is part of the Kalahari Basin, one of the world´s largest intracontinental basins. It contains mostly unconsolidated sandy sediments, often deposited by so-called megafans. The most prominent one in the CEB is the Cubango Megafan (CM). Megafans can contain large aquifers and thus have huge potential for water supply, especially in semi-arid regions. The CM hosts three aquifers: a locally present, perched aquifer, a regional mostly unconfined and a deep confined aquifer.

The regional climate is semi-arid, with very high evapotranspiration (> 2000 mm/yr) that far exceeds the highly seasonal and variable precipitation (400-800 mm/yr). The widespread salinity in the contemporary groundwater of the CM can be attributed to both sub-recent and co-sedimentary evaporative processes.

In the lower parts of the CEB, specifically at the southwestern rim of the megafan towards the Etosha Pan, saline groundwater predominates. Hydrochemical data indicate large-scale and long-term ion exchange processes. Together with environmental tracers, this shows that younger groundwater infiltrating to the north of the basin is slowly displacing the originally brackish-saline pore water. ⁸¹Kr ages range between 40 to 170 ka indicating the fossil nature of this resource. Analysis of stable isotopes and noble gases reveals that climatic conditions during recharge of the confined aquifer were approximately 3 – 5°C colder than today.

Considering the population growth and predicted impact of climate change on water availability, careful management of this vulnerable and only partially replenishable resource is advisable.