Fluorine pollution characteristics and formation mechanism of mine water in Shaanxi and Inner Mongolia contiguous area

The environment and drinking water safety problems induced by high fluoride (F) groundwater with natural origin have attracted widespread attention at home and abroad. Thus， many scholars have carried out some researches on the distribution characteristics and formation mechanism of fluorine in groundwater. However， unlike groundwater， the formation of high fluorine mine water is controlled by some natural causes and human activities. This paper is to study the main pollution characteristics and formation mechanism of fluorine in mine water. Setting the boundary area of Mongolia⁃Shaanxi as the research area， Seventy⁃one groups water samples including surface water， groundwater， mine water and thirty⁃four groups underground aquifer rock samples were collected to study the main pollution characteristics of fluorine in mine water. Piper trigram， Gibbs diagram， rock SEM-EDS and ion relation diagram were used to explore the formation mechanism of high fluorine mine water. Results show that the fluoride concentration of 3-1 coal mine water is different from that of 2-2 coal mine water. The average concentration of fluorine in 2-2 coal mine water and 3-1 coal mine water is 0.29 and 6.36 mg / L， respectively. The average concen⁃ tration of fluorine in 2-2 coal mine roof groundwater and 3-1 coal mine roof groundwater is 0.30 and 4.32 mg / L. Ob⁃ viously， the formation of high fluorine content in 3-1 coal mine water is closely related to fluorine enrichment in 3- 1 coal roof groundwater. And， the main reason of fluoride enrichment in 3-1 coal roof groundwater is the high fluor⁃ ide concentration in rock with mean concentration 609 mg / kg. The dissolution of fluorine⁃bearing minerals including fluorapatite， hornblende and mica is the main source of fluoride ion in groundwater. Besides， the fluoride enrichment is also controlled by fluoride ion desorption from chlorite and illite interbedded with montmorillonite and cation ex⁃ change. Higher concentrations of OH- and HCO-3 compete to adsorb potential on clay surface and promote fluorine de⁃ sorption. In addition， the environment with high Na and low Ca reduces the formation of CaF2 precipitation and pro⁃ motes the enrichment of fluorine. Subsequently， affected by mining， the 3-1 coal roof groundwater flowing into coal roadway or working face through the water⁃conducting fracture zone forms the high fluorine mine water. Then， second⁃ ary full water⁃rock interaction occurs between mine water and goaf rock， which further leads to the increase of fluor⁃ ide concentration in mine water.