Potential for saline groundwater freshening in a semi-arid environment: Insights from the Machile-Zambezi River Basin

Study area: Machile-Zambezi Catchment. 

Study focus: Saline groundwater is a primary source of water insecurity in arid to semi-arid low recharge environments thus threatens the attainment of the sustainable development goals. In this paper, we investigate the potential of groundwater freshening in the Machile-Zambian Basin, hosting saline groundwater. Various methods were used that include hydro-geochemistry, environmental isotopes (δD/δO¹⁸, ³H/³He and C-13/C-14),and groundwater modelling (with PMWIN). 

New hydrological insights for the region: Fresh groundwater and brackish groundwater had a Ca-Mg-HCO₃ /Na-HCO₃ composition, whereas saline groundwater had a Na-Cl-SO₄ composition. Stable isotopes indicate high evaporation in the saline zone propagating mineralisation under endorheic conditions driven likely by tectonics. The brackish and saline groundwater types had apparent ages of < 10 ka attributed to recharge during pluvial climatic events in the Holocene. Modflow modelling using PMWIN modelling showed that due to the hydraulic conductivity contrast in the host lithologies, fresh water in the basin is forced out through the topographic depressions and consumed by evapotranspiration with little or no interaction with the saline zone except in the transition zone. Aquifer structure, low recharge and high storage limits potential freshening of the saline region. Water security therefore requires investments in deep borehole drilling with an option to explore superficially near paleo-drainage channels.