The hydro-geochemistry of groundwater resources in an area with prevailing saline groundwater, lower Shire Valley, Malawi

Abstract

The area considered in this study lies within the western section of the East African Rift System (EARS) and saline groundwater occurs in some parts of the valley plain. Hydrochemical groundwater types were classified into three groups (G1–G3), indicating different stages of groundwater chemical evolution. An overall incongruent weathering of aluminosilicate minerals causes the groundwater solution to generally be in equilibrium with montmorillonite. Ca–(Mg)–HCO3 groundwater (G1), with relatively low TDS (average TDS=548mg/L), are mainly found in areas with Karoo basalt and Precambrian basement complex rocks. This water type is mainly governed by aluminosilicate weathering. Towards the middle of the valley, Na- and mixed cation-HCO3 groundwater (G2; average TDS=1061mg/L) predominates. This water type results from a combination of aluminosilicate mineral weathering, cation exchange and precipitation of clays and carbonates. The increase in ionic strength of G2 samples, in comparison with G1 samples, is attributed to mixing with high TDS groundwater in G3. Brackish and saline groundwaters (G3; average TDS=3457mg/L) are dominated by sodium, chloride and sulphate ions, which is attributable to dissolution of Cl− and SO42- evaporative salts. These are found in clusters and in aquifers with low recharge capacity (low transmissivity) and are attributable to intrusion of mineralised groundwater probably through fault zones from mainly sedimentary Karoo and Cretaceous Lupata formations. Evaporation plays a role in brackish/saline groundwaters found in areas with shallow water table along the Shire River.