Abstrakt
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 delta H-2 and delta O-18 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 parts per thousand. from the LMWL for Windhoek was observed for delta O-18 samples, which was probably caused by the exchange with reservoir rocks. Values of delta S-34(SO4) reflect mixing of two principal sulfate sources, i.e., dissolution of gypsum originating from playas and interaction with sun:tic mineralization in tectonic bedrock zones.
Values of delta C-13(DIC) seem to be affected by a variable vegetation cover and mainly by the input of endogenous CO2. Estimated reservoir temperatures vary from 60 degrees C to 126 degrees 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.