Distribution characteristics and cause of fluoride in Dalinuoer Lake in Inner Mongolia

Dalinuoer Lake is located in Inner Mongolia of Plateau in Northern China, which is a typical primary high fluorine area. In order to reveal the distribution characteristics and causes of fluoride in the lake, the overlying water and surface sediments of Dalinuoer Lake were collected, and the spatial distribution and occurrence of fluoride were investigated in detail. The source of fluoride in Dalinuoer Lake and the influence of environmental factors on fluoride in water were discussed. The results indicated that the fluoride content in Dalinuoer Lake ranged from 3.91 to 4.61 mg/L, with an average value of 4.41 mg/L, significantly exceeding Grade Ⅴ threshold (1.5 mg/L) for Environmental Quality Standards for Surface Water (GB 3838-2002). The fluoride content in surface sediments varied from 252.69 to 940.14 mg/kg, with an average value of 643.07 mg/kg. Spatially, the area with high fluoride content was mainly distributed in the southwest and middle areas of Dalinuoer Lake. Regarding forms, the fluoride concentration in various forms generally showed a pattern of residual > organic bound > water soluble > iron-manganese bound > exchangeable, and the water-soluble fluoride was easy to release to the water body under certain environmental conditions, which became the main endogenous source of fluoride in Dalinuoer Lake. The stability risk assessment indicated that the fluoride in the sediment of Dalinuoer Lake was generally at a moderate risk level. The high fluorine water in Dalinuoer Lake was affected by environmental factors such as cold and dry climate, special hydrochemical conditions, the evaporation and concentration characteristics of inflow rivers and internal lakes under high natural background value caused by stratigraphic lithology. The results could provide theoretical support and scientific basis for the ecological environment security evaluation of Dalinuoer Lake and other lakes with high fluoride.