Hydrogeochemistry of the groundwater in the Namaqualand region, South Africa : implications for surficial uranium mineralization

Abstract

Since the discovery of the Yeelirrie deposit in Australia in 1972, exploration for surficial uranium deposits has been supported through hydrogeochemical studies of groundwater and the calculation of the carnotite solubility index (CSI). This study aims to evaluate groundwater quality and delineate potential areas where surficial uranium mineralization (i.e. carnotite) may take place in the Namaqualand region. Surficial uranium deposits are unconsolidated soils or sediments, usually of Tertiary to Recent age. The most common surficial uranium mineral is carnotite. The study area in Namaqualand is located in the arid to semi-arid Northern Cape Province of South Africa, which comprises a variety of metamorphic rocks of the Namaqua Metamorphic Province, overlain by Cenozoic sediments. A total of 85 water samples were collected from existing boreholes. Physicochemical properties (EC, TDS, pH, DO and Eh) were measured on site and samples were analysed for major ions and trace elements at the Council for Geoscience laboratory in Pretoria. The CSI was calculated using hydrochemical data, and interpolated using inverse distance weighting (IDW) and kriging to produce maps of potential carnotite mineralization. The groundwater of the area is alkaline, with elevated concentrations of EC, TDS, Na+, Cl-, F-and U. The predominant water types are Na-Cl, Na-HCO3 and Mg-HCO3. Most groundwater samples have uranium values that range from 1.2–5 120 ppb, which are above the World Health Organization drinking water quality guideline of 15 ppb. Analysis shows that, in the presence of carbonates, uranium mostly occurs in solution as UO2(CO3)22- and UO2(CO3)34-. The CSI values ranged from -6.71 to -2.99 and those that ranged from 2.99 to -4 were in close proximity to known surficial uranium occurrences. The use of IDW and kriging interpolation methods revealed areas with potential for carnotite mineralization. The existence of suitable uranium source rocks, palaeochannels, climate and geomorphology makes the Namaqualand region prospective for calcrete-hosted uranium deposits. The CSI, used to delineate areas with a potential for surficial uranium mineralization in the Northern Cape, can be extrapolated to other areas with similar geological environments and climatic conditions. More sampling of groundwater is recommended to conduct a medical geology study to ascertain the impacts of uranium and other elements on the people, animals and plants of the area.