Chemical Characteristics of Shallow High-Fluoride Groundwater in Guanzhong Basin, China

Shallow high-fluoride groundwater with fluoride concentration above 1.0 mg/L occurs widely in Guanzhong Basin, northwestern China. It is mainly distributed in areas on the north of Wei River in the eastern and central parts of the basin. Characterized by the chemical composition of transitional types of groundwater, the high-fluoride groundwater samples commonly belong to dilute water or slightly mineralized water and exist in weak alkaline or alkaline conditions. The correlation between F^- and Na⁺/Ca²⁺ ratio in the high-fluoride groundwater shows that a process conducive to the dissolution of Na⁺ and sink of Ca²⁺ may promote the enrichment of fluoride in groundwater. And the reason why F^- is unrelated to Mg²⁺ is probably the random combination of hydrogeochemical condition advantageous to Mg²⁺ enrichment with that advantageous or disadvantageous to F^- enrichment. In addition, the weak relationship between F^- and Cl^-, together with that between F^- and TDS, suggests that evaporation under the semi-arid climate is just one of the factors controlling the occurrence of high-fluoride groundwater in the study area. Although natural factors are primarily responsible for the high fluoride content in groundwater, the factor of human activities can not be neglected as the relation between F^- and NO₃ ^- shows that most high-fluoride groundwater samples have been influenced by human activities. To protect local people against the fluorosis, the high-fluoride groundwater should be defluoridated or alternative water sources with low fluoride content should be found if necessary.