A Study Of The Occurrence And Origin Of Fluoride In Thermal Groundwater In Some Areas Of China

The exploitation and utilization of geothermal resources have been paid more attention at present. The fluoride concentration is generally high in thermal groundwater in China. In this situation, on the one hand it makes thermal groundwater useful for medical treatment; on the other hand it restricts underground thermal water for drinking. Therefore studies of fluoride concentration in thermal groundwater have practical significance toward the exploitation and utilization of thermal groundwater. Meanwhile, it not only conduces to illuminate the origin of thermal groundwater, but also has practical significance toward seeking geothermal resources.Base on abundant chemical composition data of thermal groundwater, the aquifer lithology and part physical or chemical factors of thermal groundwater are analyzed to discuss the distribution character and source of F^- in thermal groundwater. Conclusions are as follows : when heat reservoir lithology is magmatic rock, the average F^- concentration is about 10.71 mg/L, and the main sources of F– are fluorapatite, biotite and hornblende; when it is carbonate, the average F^- concentration(about 5.29 mg/L) is lower than what is in magmatic rock because of higher Ca²⁺ concentration in hot water; when it is metamorphic rock, F^- concentration lies on country rock's lithology; when it is clastic rock, F^- concentration lies on its mineral component. In addition, F^- concentration shows a positive relation to temperature, pH, (K++ Na+) percentage and H₂SiO₃ concentration, and a negative relation to Ca²⁺ concentration. The correlation between F– and salinity is not significant. Meanwhile,when hydrochemistry style is HCO₃^-Na, F^- concentration is high.The thermal groundwater of 4 places is selected to study deeply according to different heat reservoir lithology. They are Conghua spring in Guangdong, Tangzimiao spring in Hebei, Xiaotangshan spring in Beijing and Wenluo spring in Guangxin. Their corresponding heat reservoirs are composed of magmatic rock, metamorphic rock, carbonate and clastic rock. By comparing rock samples and part of physical or chemical factors, conclusion can be drawn that F^- concentrations of Tangzimiao spring and Conghua spring (respectively are 18.5 mg/L and 9.1125 mg/L) are higher than which of Xiaotangshan spring and Wenluo spring (respectively are 6.0 mg/L and 0.54 mg/L),because fluorine mineral in rock samples of Tangzimiao springand Conghua spring are more than which of Xiaotangshan spring and Wenluo spring, and Ca²⁺ concentration of Tangzimiao spring and Conghua spring are lower than which of Xiaotangshan spring and Wenluo spring. By analyzing rock samples, conclusion can be drawn that the main sources of F^- in Tangzimiao spring and Conghua spring are mica or chlorite. The mechanism is possible that mineral releases F^- into thermal groundwater while biotite mutates to chlorite or that F^- concentration of chlorite is higher because isomorphous replacement is occurred between F and OH in metamorphic rock.