Characterisation of Saline groundwater based on groung-based time-domain electromagnetic soundings in the Machile River Basin, South-Western Zambia

Abstract

Much of southern Africa is semiarid and heavily dependent on groundwater resources. However, access to safe and clean water is an important feature of the natural environment, a human right and a basic requirement for economic development. Like in other parts of the Kalahari Basin, parts of Sesheke and Kazungula districts have significant groundwater salinity problems which affect the use of groundwater as a freshwater resource. This study mapped the spatial distribution of saline groundwater in order to enhance the overall understanding of its existence. The study was undertaken in the Machile River Basin which is a topographic depression located partly in Sesheke District in the Western Province and partly in Kazungula District in the Southern Province of Zambia. The ground-based Time-Domain Electromagnetic (TDEM) method, based on the Maxwell’s equations, was used in the study for data collection and the SiTEM-SEMDI software for data analysis. Other software used include ArcGIS and Geoscene3D for spatial data analysis and geological modelling, respectively. Research findings indicate that rock formation resistivities increase with depth as follows: 3,173m at 10m, 10,616m at 30m, 17,186m at 50m and 19,738Ωm at 80m. The extent of saline groundwater with low resistivities of less than 35Ωm was less at 10m depth and more at 30, 50 and 80m and that salinity was concentrated around the depression bordering the two districts stretching from the Zambezi River going up in the northern direction, suggesting that 10m is generally the appropriate depth to drill for fresh groundwater. It was also observed that the subsurface formation of the Machile River Basin comprises the unconsolidated Kalahari sediments underlain by sandstones of the Barotse Formation of the Kalahari Supergroup which is also underlain by rocks of the Karoo (Batoka Basalts) and Basement Complex such as granites, gneisses and schists. It is therefore concluded that the increase in resistivities of formations with depth indicates the presence of formations ranging from sand, sandstone, basalts, granites and other rocks of the Undifferentiated Karoo and Basement Complex. It is also concluded that saline groundwater mostly exists in unconsolidated Kalahari sediments and sandstones.