Hydrochemical evolution of high uranium, fluoride and nitrate groundwaters of Namakwaland, South Africa

Abstract

Water quality globally suffers from overuse and pollution, but desert areas with high evaporation rates and groundwater as the only water source have additional challenges. Namakwaland in the Northern Cape is an arid region with a sparse human population dependent largely upon stock farming, with minor mining and tourism. The complex Proterozoic metamorphic geology is overlain by Cenozoic deposits known for containing secondary uranium mineralisation in places. 86 samples of groundwater were taken over an area of 25 000 km2 and analysed for various parameters in the field and laboratory. The salinity of the water varies from fresh to sea water, averaging 2500 mg/L, with pH in the 7–9 range. Major ion abundances indicate marine aerosol as the main source of Cl, Na, Mg and K, with rock weathering being more responsible for Ca and HCO3. Irrigation water quality tests give mixed results, suggesting careful use of groundwater is possible in some locations. Nitrate is occasionally high, the random distribution indicating farm animals as the source. Fluoride averages 2.4 mg/L and is strongly geologically controlled, but also enriched through evaporation. Uranium, averaging 0.155 mg/L (5 times the guideline), has a complex distribution, poorly correlated to bedrock geology and shows two enrichment trends, one in tandem with other ions and one independent. These two trends are proposed to reflect enrichment through evaporation (other ions also increasing) or precipitation of secondary uranium minerals (limited correlation with other ions). These three parameters are uncorrelated, which emphasizes the variety and complexity of hydrochemical processes taking place. Given the U and other water quality risks, further work into cumulative exposure for plants, animals and humans is warranted.