Geophysical investigation of the groundwater regimes in the lower Okavango Delta, Northwestern Botswana

The Okavango Delta in the northwestern part of Botswana is unique and it is the largest inland delta in the world. Instead of flowing into the sea, the annual flood of fresh water from the highlands flows inland, spreading over 15 000km ² of the arid Kalahari environment and forming several islands and channels that emerge from its waterways. Parts of the Lower Okavango Delta that have been receiving large and frequent flood events have developed extensive fresh water aquifers, while other parts that have been receiving intermittently small floods have limited fresh water aquifers. The present study used aeromagnetic, airborne Electromagnetic (AEM), ground transient electromagnetic (TEM) soundings, drilling and down-hole geophysical logging data to map the distribution and geometry of these fresh water aquifer systems. The results of this study showed that the occurrence of freshwater aquifers in the lower delta are associated with active drainage channels and are bounded below and on their edges by brackish/ saline groundwater. The geometry of these aquifers provides an insight into the long term flooding history of the delta. This can be seen from the Gomoti River, which has had only intermittent flow in recent times, but is one of the largest fresh water aquifers in the lower delta.