Groundwater resources in northern Namibia

The transboundary Cuvelai-Etosha Basin (Namibia/ Angola) is a semi-arid region. Rising population and water demand increase the stress on already scarce water resources. The local aquifers, the perched KOH-0 and the deeper KOH-1 are of limited use, the former due to low yields and microbiological issues, the latter due to often brackish water. A previously unknown deep freshwater aquifer, KOH-2, with high yield and good water quality was discovered at the end of the 1990s. This new resource is a game changer for the region. Therefore, the geological framework of the CEB and its hydrogeological conditions were investigated by detailed geological (core drilling), geophysical (TEM) and hydrogeological studies. The geological background of the KOH-2 is an inland mega-fan system, reaching from the Angolan highlands in the north down to the Etosha Pan. A 400 m cored borehole revealed three distinct depositional sequences, which cover the last 60 Ma. The lower Olukonda Formation (270-400 m) has poor well yields due to a high degree of heterogeneity. It is overlain by the Andoni Formation, of which the KOH-2 (170-270 m) forms the lower part. The transition between the formations is marked by distinct geochemical changes, indicating different source areas. The KOH-2 is overlain by a sequence of aquitards and aquifers (KOH-1 (80-110 m), KOH-0). The sedimentology of mega-fans strongly affects the distribution and hydraulic properties of aquifers and aquitards. This knowledge will be useful for the further exploration and sustainable development of the KOH-2.