Abstract

SHEMANG, E.M.; MOLWALEFHE, L. N.  and  MICKUS, K.. Applying DC resistivity imaging to investigating the relationship between water quality and transpiration beneath circular islands in the Okavango Delta, Botswana: a case study of Thata Island. Earth Sci. Res. J. [online]. 2013, vol.17, n.1, pp.25-32. ISSN 1794-6190.

An electrical resistivity survey was carried out on Thata Island, one of the numerous circular islands in the Okavango Delta, to investigate the mechanism governing interactions between surface water, vegetation and groundwater. Seven data profiles were collected across the island and modelling results indicated that the centre of the island had low resistivity values (less than 10 ohm-m), while resistivity values increased laterally outside the island. Such lateral resistivity zoning responded to variation in shallow groundwater chemistry below the islands having a high concentration of solutes inside and the presence of fresh water outside the islands. Borehole to surface resistivity imaging results for the island indicated a plume of saline water sinking to depths of 60 m. Groundwater salinity below the island ranged from 11.7 g/l from the margin of the island to 122 g/l at the centre of the island. Beyond the 60m depth, groundwater salinity dropped to about 0.33 g/l at the centre of the island. Lateral and down-hole imaging results, as well as water salinity values, showed a migrating plume of high salinity groundwater from the surface of the island invading a relatively deeper low-density fresh groundwater environment. Combining geochemical, isotopic and electrical resistivity data indicated that capillary evaporation may have been the most important mechanism in creating high saline water under the centre of the island; however, high transpiration rates along the edges of the island may have been important in producing salts which could migrate toward the island's centre. The results of this study showed how high salinity levels may have formed in near surface aquifers in natural inland delta environments in semi-arid areas.

Keywords : seismic resistivity imaging; salinity; circular island; Okavango delta.

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