Spatial distribution characteristics and prediction of fluorine concentration in groundwater based on driving factors analysis

Excess fluoride (F-) in groundwater can be hazardous to human health. A total of 360 ground water samples was col-lected from northern Anhui, China, to study the levels, distribution, and source of F-. And on this basis, predicting the spatial distribution of F- in a wider scale space. The range of F- was 0.1-5.8 mg/L, with a mean value of 1.2 mg/L, and 26.4 % of the samples exceeded the acceptable level of 1.5 mg/L. Moreover, the water-rock interaction (fluorite dissolution) and cation alternate adsorption were considered to be two main driving factors of high F- in groundwater. To further illustrate the spatial effects, the BME-RF model was established by combining the main environmental factors. The spatial distribution of F- was quantitatively predicted, and the response to environmental variables was analyzed. The R2 of BME-RF model reached 0.93, the prediction results showed that the region with 1.0-1.5 mg/L of F- accounts for 47.2 % of the total area. The predicted F- content of nearly 70 % of groundwater in this area has exceeded 1.0 mg/L, which was dominated by Na+ and HCO3- type. The spatial variability of F-in the study area was mainly affected by hydrogeological conditions, and the vertical distribution characteristics were re-lated to the spatial variation of slope, distance from runoff, and hydrochemical types. The results of the study provide new insights into the F- concentration prediction in underground environment, especially in the borehole gap area.