Mechanism of salinity change and hydrogeochemical evolution of groundwater in the Machile-Zambezi Basin, South-western Zambia

Abstract Machile-Zambezi Basin, South-Western Zambia hosts high salinity groundwater which threatens water security for rural inhabitants. This study investigates the hydrological mechanism that led to high salinity and the geochemical evolution of the groundwater system. The Machile-Zambezi Basin is part of the wider Kalahari Basin which underwent major palaeo-environmental climatic, tectonic and sedimentology dynamics which must have impacted the groundwater salinity. The study examines the groundwater level, hydrochemistry, environmental isotopes ( 18 O/ 16 O, 2 H/ 1 H, 3 H/ 3 He, 14 C/ 13 C). In addition, the sediment cation exchange capacity (CEC) and pore-water chemistry on intact core material were measured. The groundwater chemistry evolved from fresh Ca(Na)HCO 3 to saline Na(Ca, Mg)SO 4 due to dissolution of salts and not evaporation as indicated by stable isotopes. The saline groundwater is old with 14 C ages estimates of more than 1000 years old and stagnant. Geochemical modelling using PHREEQC suggests that ionic exchange due to release of cations from dissolving salts and sulphate reduction were also important processes in this system. High groundwater salinity is therefore associated with Pre-Holocene environmental changes and is restricted to a stagnant saline zone. It will therefore remain unflushed as long as current climatic conditions remain.