Spatial variations of groundwater quality in the Machile River Basin, Western Zambia

Abstract

The source of salinity and the geochemical processes that control the groundwater salinization in the Machile River Basin within the Sesheke and Kazungula districts of Western and Southern provinces of Zambia are not well understood and explained. The central part of the basin contains saline groundwater. Deep boreholes and shallow wells delivering saline water are usually abandoned by communities, who resort to open water holes dug out in dry river beds or surface water resources, where available. Therefore a comprehensive geochemical and isotopic study was carried out in the basin to explain the spatial distribution of fresh and saline groundwater quality and processes that influence variability in salinity. Understanding spatial variability and its controls is indispensable for sustainable groundwater management and protection in the basin. Thirty four (34) representative groundwater samples were collected from different boreholes for water chemistry and isotope analyses. The samples were analysed for physicochemical properties, cations and anions. Chemical analyses indicate that the fringes of the basin contain a fresh Ca2+ - Mg2+ - HCO3- type of groundwater and the central part of a saline Na+ - SO42- - Cl- type of groundwater. This classification was confirmed by grouping similar water types together by using Hierarchical Cluster Analysis (HCA) and Principal Components Analysis (PCA) techniques. This spatial variability may be controlled by dissolution of bedded or disseminated evaporites concentrated by evapo-transpiration processes as revealed by stable isotope (2H and 18O) signatures. The isotopic composition for Deuterium (δ2H) and oxygen 18 (δ18O) of the groundwater in the study area ranged from -53.44 to -23.54‰ and from -7.78 to -3.07‰, respectively. These isotope signatures reveal that the groundwater is recharged by precipitation in the fringes of the basin. Furthermore, the fresh groundwater is young in the recharge areas and regularly recharged whereas the saline water is old. The old water was embedded in the Pleistocene (126 ± 11 Ka) thus have saline water in parts of the basin. Exploitation of water for domestic and commercial use in the basin can be through drilling of boreholes of less than 10m to avoid intersecting the saline layers. In areas, where saline water is closer to the surface, rain water harvesting, ultra-filtration vi processing to desalinate the saline groundwater or drilling of boreholes deeper into the basalt that could probably contain freshwater are recommended.