Characteristics and driving factors of fluoride in groundwater in different urban functional area of Lanzhou city

High-fluorine groundwater is a potential threat to ecological environment and human health. The source and distribution of fluoride in groundwater in urbanized areas are complicated due to the double influence of geological background and human activities. It is of great significance to identify the characteristics of fluorine in groundwater to ensure the safety of groundwater.This study analyzed the environmental characteristics and main hydrogeochemical processes of high-fluorine groundwater in different urban functional areas of Lanzhou city, the largest industrial city in the arid region of Northwest China, based on the mathematical statistics, ion ratios and saturation index analysis. Then the influence of human activities on the migration and enrichment of fluoride was revealed.

The results show that the ρ(F⁻) in the groundwater in the study area ranges from 0 to 4.8 mg/L, and 13 high-fluoride water samples exceeded the standard of groundwater quality Class III (1.0 mg/L), with an excess rate of 20.3%. Under the influence of intensity of human activity and sources of human input, the distribution characteristics of fluorine in groundwater in different urban functional areas are prominently different. The fluorine content of groundwater in the Xigu Petrochemical Industrial Zone is the highest, with the 47.4% of high-fluorine groundwater. While in commercial residential areas and new urban areas the fluorine content is relatively low, with the 7.1% and 9.7% of high-fluorine groundwater, respectively. The high-fluorine groundwater in the study area is mainly SO₄•Cl—Na and Cl•SO₄—Na type water, which is low in calcium, rich in sodium and weak in alkalinity. The dissolution of fluorinated minerals, precipitation/dissolution of calcite and dolomite, cation exchange between calcium and sodium on the surface of clay minerals, strong evaporation and salt effect are the main hydrogeochemical processes of fluoride enrichment in groundwater in the study area. Urbanization and industrialization lead to the further deterioration of natural high fluorine water, the industrial wastewater leakage from petrochemical leads to the further dissolution of fluoride in the formation, which is an important driving force for the enrichment of high fluorine groundwater in the Xigu Industrial Zone. The study provides basic information for the fluoride migration and enrichment in the high fluoride background area with human activities.