Physical, chemical and isotopic characteristics of groundwater and surface water in the Lake Chilwa Basin, Malawi

Abstract

We measured the physical, chemical and isotopic properties of groundwater and surface water from streams and Lake Chilwa in the Lake Chilwa Basin in southern Malawi. The study objective was to assess water quality and determine the cause of water quality deterioration. Groundwater from 16 boreholes, 5 stream samples and 3 samples from Lake Chilwa were investigated for temperature, pH, electrical conductivity, total dissolved solids (TDS), major ions and stable isotopes of hydrogen and oxygen. Groundwater and water from Lake Chilwa had higher ionic concentrations (e.g., Cl−, HCO3−, Na+, Ca2+, Mg2+) than stream water. Ionic evolution models indicate weathering of silicate minerals as the major control of the ionic concentrations in groundwater. The chemistry of water in Lake Chilwa is markedly affected by evaporation. The concentrations of the TDS, HCO3−, F− and NO3− in groundwater at several locations were above the recommended limits prescribed for drinking water by the World Health Organization. The results of this study show that poor groundwater quality for drinking and domestic use is due to natural water-rock interactions (TDS, HCO3−, F−) and anthropogenic pollution (NO3−). Because stream water is of good quality, and streamflow is supported by baseflow from fresh groundwater, efforts should be made to map fresh groundwater aquifers for domestic, agricultural and industrial use.