Characteristics of fluoride migration and enrichment in groundwater under the influence of natural background and anthropogenic activities.

Excessive enrichment of fluoride threatens ecological stability and human health. The high-fluoride groundwater in the Chagan Lake area has existed for a long time. With the land consolidation and irrigation area construction, the distribution and migration process of fluoride have changed. It is urgent to explore the evolution of fluoride under the dual effects of nature and human. Based on 107 groundwater samples collected in different land use periods, hydrogeochemistry and isotope methods were combined to explore the evolution characteristics and hydrogeochemical processes of fluoride in typical high-fluoride background area and elucidate the impact of anthropogenic activities on fluoride migration. The results indicate that large areas of paddy fields are developed from saline-alkali land, and its area has increased by nearly 30%. The proportion of high-fluoride groundwater (>2 mg/L) has increased by nearly 10%, mainly distributed in the new irrigation area. Hydrogeochemical processes such as dissolution of fluorine-containing minerals, precipitation of carbonate minerals and exchange of Na, Ca on the water-soil interface control the enrichment of fluoride. The groundwater d-excess has no obvious change with the increase of TDS, and human activities are one of the reasons for the increase of fluoride. The concentration of fluoride is diluted due to years of diversion irrigation in old irrigation area, whereas the enrichment of δH, δO and Cl in new irrigation area indicates that the vertical infiltration of washing alkali and irrigation water brought fluoride and other salts to groundwater. Fertilizer and wastewater discharges also contribute to the accumulation of fluoride, manifesting as co-increasing nitrate and chloride salts. The results of this study provide a new insight into fluoride migration under anthropogenic disturbance in high-fluoride background areas.