Major-ion chemistry and ground-water salinization in ephemeral floodplains in some arid regions of Namibia

Ground-water quality in drinking water sources within ephemeral flood plains in the Namib Desert and north central Namibia displays seasonal and spatial variations. The monthly variation in total dissolved solids (TDS) in individual sources ranges between 5% and 65% in north central Namibia and between 0.5% and 85% in the Namib Desert. A higher monthly TDS variation, close to 500%, is recorded in slightly deeper as well as over-pumped freshwater sources in both study sites. The lowest TDS values are recorded after the rains for north central Namibia sources and only after flood events for sources in the lower rainfall Namib Desert. Ground-water salinization in both sites is generally characterized by a shift from ‘fresh’ to ‘very saline’ ground-water, and a chemical evolution from Na and Ca–HCO3 waters towards Na–Cl ones. The dominant processes that determine these hydrochemical shifts are refreshing by recharge waters, concentration by evaporation, dissolution of saline sediments (mainly evaporites), and mixing with older and more saline ground-water.