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The use of Time Domain Electromagnetic method and Continuous Vertical Electrical Sounding to map groundwater salinity in the Barotse sub-basin, Zambia

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Abstract

This paper describes the results from the application of two geophysical exploration techniques, Time Domain Electromagnetic (TDEM) and Continuous Vertical Electrical Sounding (CVES) that have proved effective in mapping groundwater salinity variations within the sedimentary formations of the Barotse sub basin in the Western Province of Zambia. TDEM was used to map groundwater salinity variations on a regional scale, whereas CVES was used at the local scale to investigate freshwater–saltwater distribution in an ephemeral river valley. On a regional scale, salt water occurrence was shown to be present mainly on the south-eastern portions of the basin, which are situated in a rift that forms a tripe junction with the East African Rift Valley. The general geophysical model indicates an aquifer with saline water with a thickness of about 40m with resistivity variations less than 35Ωm (more than 500mg/l of Cl− based on a formation factor of 5), overlain by an unconfined freshwater aquifer of about 10m thickness with resistivities in excess of 70Ωm (i.e. less than 250mg/l of Cl− based on a formation factor of 5). The origin of the saline water is hypothesized to be related to the evapo-concentration of salts in interdune deposits, which were subsequently buried due to dune migration about 32 to 4 thousands of years ago or kilo annums (ka). The occurrence of saline groundwater could also possibly be linked to evaporation of a former Lake Paleo Makgadikgadi, an extensive endorheic lake system that once covered large parts of Southern Africa. Locally, a thin freshwater aquifer was observed in an ephemeral river valley, indicating recent recharge of river water into a pre-existing saline environment.