Geoelectrical imaging of groundwater salinization in the Okavango Delta, Botswana

Soil and groundwater salinization is a major problem in aquatic systems throughout the semi-arid and arid regions. An interesting example of a natural terminal evaporative system is the Okavango Delta, a large inland Delta in Botswana. The system accumulates about 300,000 tons of dissolved solids per year. The majority of the accumulated solutes are deposited on small islands in the wetland. In the centre of the islands, the shallow groundwater is therefore highly saline and displays total dissolved solid (TDS) concentrations around 30 g/l. Fresh groundwater underlying the superficial brines gives rise to a hydrodynamically unstable situation with dense brine perched on less dense fresh water. Instabilities (density fingers) can potentially occur and effectively transport the superficial brines into deeper aquifer units. Electrical resistivity tomography (ERT) was used in this study to directly map the salinity distribution in the subsurface below two different islands in the Okavango Delta. Surface arrays as well as borehole-to-surface layouts were used. Due to differences in the regional hydrologic setting, a density finger could be observed on one island, whereas on the other, the high salinity anomaly was confined to the surface layer. To our knowledge, this is the first time that fingering instabilities were observed under field conditions in natural aquifers.