Geochemistry and fluoride levels of geothermal springs in Namibia

• Geothermal springs of Namibia have F− concentrations up to 18.9 mg/l.
• Their water is mostly of Na–HCO3–SO4 type.
• Source of water is precipitation originating at the Indian Ocean.
• Calculated reservoir temperatures are up to 126 °C.
• A treatment is necessary to decrease F− concentrations.

A survey of groundwater from six geothermal springs in Namibia showed high concentrations of dissolved fluoride, with values up to 18.9 mg/l. All values are higher than both the WHO limit and the Namibian guideline. High concentrations of fluoride are linked to Na-HCO3 or Na-SO4-HCO3 groundwater types, with increasing sulfate and chloride concentrations towards the south of Namibia. Values of δ2H and δ18O are more negative for the north of the country, and with increasing altitude of springs and distance from precipitation sources towards the southeast from the Indian Ocean. A shift of about 1‰ from the LMWL for Windhoek was observed for δ18O samples, which was probably caused by the exchange with reservoir rocks. Values of δ34S(SO4) reflect mixing of two principal sulfate sources, i.e., dissolution of gypsum originating from playas and interaction with sulfidic mineralization in tectonic bedrock zones. Values of δ13C(DIC) seem to be affected by a variable vegetation cover and mainly by the input of endogenous CO2. Estimated reservoir temperatures vary from 60 °C to 126 °C, with a maximum value at the Ganigobes site. The geothermal springs of Namibia in this study do not meet drinking water standards and thus their water can be used only for other purposes e.g. for thermal spas. Treatment would be necessary to decrease dissolved fluorine concentrations for drinking water purposes.