Groundwater resources in the lower shire region in Malawi, Southern Africa, show high salinity. The sources of salinity and the mechanism of groundwater salinization in this inland region are not known. The main objective of this study is to clarify sources and extent of groundwater salinization in the lower Shire area.
A total of 33 water samples were collected from lower Shire area. Three of the samples are surface water samples including lower Shire River and the rest are groundwater samples. Major ions, heavy metals, 2H, 18O, and Sr isotopes have been determined for surface water and groundwater in the lower Shire area, Malawi.
Sodium and magnesium are the dominant cations while chloride and/or bicarbonate are the dominant anions in the majority of the groundwater. The waters are mostly classified as sodium-chloride type as shown on the piper diagram whereas some of the groundwater are sodium bicarbonate dominant and characterised as a Na-HCO3-Cl water type. Seven groundwater samples and one surface water sample are Mg(Ca)HCO3 water type. Elevated Na+ concentrations may be due to cation exchange where Ca2+ from groundwater is adsorbed on clayed materials in exchange of Na+. Another possible reason for the higher Na+ concentration could be dissolution of NaHCO3.
In overall the studied area concentration of Ag, B, Ba, Cr, and Li were below the drinking water standards and the concentration of Al in this study except in Jasi (0.4 mg/L) borehole, shows concentration well below the guideline. However Pb content of some samples surpassed the drinking water standard. The heavy metals concentration is less pronounced in lower Shire studied area, Malawi compared to the drinking water standard.
The majority water samples stable isotopic data cluster below the GMWL (∂2H=8 ∂ 18O+10). The deviation of groundwater samples from the GMWL indicates that evaporation enrichment of heavy isotope concentrations has occurred, resulting in a slope of 6.3; somewhat less than the GMWL slope. These samples are characterized by comparatively high chloride contents. Therefore evaporation has an impact on the groundwater salinity of the study area.
The Sr-isotope ratios measured in ground and surface water ranged from 0.708500 (Nyasa) to 0.714417 (Machilka). The average 87Sr/86Sr ratio of 33 water samples is 0.7116. This average is higher than the ratio in sea water of 0.709, probably due to the weathering of minerals.
In conclusion, high spatial variability in groundwater compositions suggest that the groundwater salinity was caused by numerous factors including evaporation, dissolution of evaporates minerals and ion exchanges of Ca for Na. Particularly evaporation has an impact on the ground water of the study area.
Key words: Salinization, groundwater, lower Shire area, Malawi