Distribution characteristics and formation mechanism of fluoride in typical mine water in Shendong Mining Area

Shendong mining area is located in the north of Shaanxi Province and the south of Ordos City, Inner Mongolia, with dry climate, sparse precipitation and water resources scarcity.  A large amount of mine water produced in the process of coal mining is an important source of production and living water. However, the concentrations of fluoride in mine water in different regions of Shendong mining areas present significant differences, while the concentration is relatively high in some areas. Therefore, the distribution characteristics and genesis of high fluoride mine water was analyzed in this study, which was of great significance for the resource reuse of mine water.  Based on the analysis results of 58 groups of mine water quality at different regions in the Shendong mining area, research methods such as mathematical statistics, piper three-line diagram, gibbs diagram, SI simulation, etc. were used. Meanwhile,  XRF analysis was conducted on rock cores and coal seam roof rocks collected from different areas to explore the spatiotemporal distribution characteristics and formation mechanism of fluorine in mine water at different burial depths in the northeastern and central mining areas. The results showed that the concentration of F- in mine water varied from 0.3 to 13.7 mg/L, with an average value of 4.3 mg/L. The pH value of mine water with higher F- concentration was alkaline. In the horizontal direction, the F- concentration in mine water was higher in the north and southwest of Shendong mining area, while it presented lower in the southeast. The fluorine concentration presented an increasing tendency with the enhancement of mining depth in the vertical direction. The F- concentration of mine water replenished by groundwater in different depths of Yan'an Formation aquifer varied greatly. The lower F- concentration was observed in the Yan'an Formation aquifer with a depth of 80-130 meters , whereas the relatively higher value was shown at the 200-500 meters depth. The trend of fluorine concentration in mine water is not obvious with the changes in the season  The XRF analysis of fluorinated minerals in rock samples from the Yan'an Group indicated that the contents of fluorinated minerals in the northern and western central mining areas were relatively higher compared with that in the eastern central mining area. The high F- concentration in mine water had a certain relationship with the hydro-chemical environment. Evaporation and concentration process, cation alternately adsorption, and mineral dissolution caused by water-rock interaction led to the enrichment of F- in groundwater.