The applicability of dc resistivity imaging to investigating the feedback mechanism between water quality and transpiration beneath circular islands in the Okavango delta, Botswana: A Case study of Thata Island

2-D Resistivity Survey was carried out in the Thata Island, one of the numerous circular islands in the Okavango Delta to investigate the mechanism governing the interactions between surface water, vegetation and groundwater. Seven profiles were laid across the island and the imaging results indicate that the centre of the island has very low resistivities (less than 10 ohm-m), while outside the island margins resistivity values increase laterally. This lateral resistivity zoning responds to shallow groundwater chemistry below the islands; high solutes load inside and the presence of fresh water outside the islands. Results of borehole to surface resistivity imaging in the island indicate a sinking blob of saline water to depths of 60 m. Groundwater salinities below the island range from 11.7g/l from outside the fringe to 122g/l at the centre of the island. Beyond the depth of 60m, the groundwater salinity drops to about 0.33 g/l at the centre of the island. Depth to water table in all the boreholes is less than 3 m below ground level. Results of the lateral and down-hole imaging as well as water salinity values show a migrating plume of high salinity groundwater from the surface of the island invading a relatively deeper low-density fresh groundwater environment.