Hydro-geochemical appraisal of groundwater quality from weathered basement aquifers in Northern Malawi

Abstract

The obligation to ensure adequate potable water supply to everyone, has necessitated the development of groundwater resources for reliable rural water supply in most developing countries. An understanding of spatial variation and processes affecting water quality is essential in sustaining usable water supplies under changing climate and local environmental pressures. In this study, an assessment of quality and dominant hydro-geochemical processes affecting the quality of groundwater from weathered basement aquifers in Mzimba district, Northern Malawi, has been conducted. Groundwater samples were collected from 172 hand-pumped boreholes, drilled for domestic rural water supply and analysed for major and minor ions, pH and total dissolved solids (TDS). In general, groundwater is of low mineralisation (TDS range: 29–1896mgL−1 for the dry season), with hydro-geochemical facies dominated by Ca–HCO3, which evolves to Ca–Cl water type. Multivariate statistical analysis (HCA and PCA) and geochemical interpretation showed that the Ca–HCO3 groundwater type result from hydrolysis of silicate minerals, which causes the solution to reach equilibrium with kaolinite. The processes of cation exchange of Na+ and K+ in the groundwater for Ca2+ and Mg2+ on clay minerals, carbonate precipitation and evaporation, are shown to modify the chemical composition from Ca–HCO3 types to Ca–Cl types. Groundwater is generally of good quality in both rainy and dry seasons, with little seasonal changes. The United States Salinity Laboratory Staff and Wilcox diagrams showed that most samples were also suitable for irrigation except for 4% (eight samples) of the groundwater samples (with EC>2000μScm−1). These are located in alluviums and colluviums localised near river banks and in inter hill valleys.