Hydro-geochemical processes in an area with saline groundwater in lower Shire River valley, Malawi: An integrated application of hierarchical cluster and principal component analyses

Abstract

Abstract Our ability to adapt to changes in groundwater quality, arising from a changing climate and/or local pressures, is dependent on comprehension of the governing controls of spatial variation in groundwater chemistry. This paper presents results of an assessment of dominant hydro-geochemical processes controlling groundwater chemical composition, using an integrated application of hierarchical cluster analysis (HCA) and principal component analysis (PCA) of a major ion dataset of groundwater from lower Shire River valley, Malawi. The area is in the southernmost part of the western section of the East African Rift System (EARS) and has localised occurrence of saline groundwater. HCA classified samples into three main clusters (C1–C3) according to their dominant chemical composition: C1 (dominant composition: Na–Cl; median TDS: 3436 mg L −1 ), C2 (dominant composition: Na–HCO 3 ; median TDS: 966 mg L −1 ) and C3 (dominant composition: alkali earths–HCO 3 ; median TDS: 528 mg L −1 ). These clusters were in turn described by the principal components PC1, PC3 and PC2, respectively, resulting from the PCA. The results of the PCA and geochemical interpretation suggest that the spatial variation of groundwater quality in the area is influenced by the following processes: C3 samples result mainly from H 2 CO 3 weathering of aluminosilicate minerals by percolating water supersaturated with CO 2 . In addition to aluminosilicate weathering, C2 samples are influenced by the processes of cation exchange of Ca 2+ and Mg 2+ in the water for Na + on clay minerals, and carbonate precipitation. The increase in ionic strength of C2 samples is attributed to mixing with high TDS groundwater in proximity with C2 samples. The saline/brackish C1 groundwater results from the processes of evaporation (for samples with high water table close to the Shire marshes) and dissolution of Cl − and SO 4 -evaporative salts followed by mineralised seep from sedimentary Karoo and Cretaceous Lupata sandstones.