The hydrogeochemical characteristics and formation mechanism of high-fluoride mine water.

Introduction

Fluoride is a micronutrient essential for the human body. And moderate fluoride intake is beneficial in the prevention of dental caries or tooth decay. However, excessive intake of F^– for a long time can cause dental or skeletal fluorosis, or even worse, genetic damage, mental retardation or cancer (Ayoob and Gupta, 2006; Yadav et al., 2017; Hossain and Patra, 2020). Abundant in Earth’s crust (625 mg/kg), the element fluorine forms fluoride exists extensively in soil, rocks, and aquifer sediments (Amini et al., 2008). This will lead to fluoride accumulation in fresh groundwater, ultimately.

The main way of human exposure to F is through drinking water especially groundwater (Rashid, et al., 2020). It is estimated that more than 200 million people in 28 countries worldwide (Shah and Danishwar, 2003; Rafique et al., 2009; Souza et al., 2013; Shakir, et al., 2016; Carl et al., 2020; Podgorski and Berg, 2022) depend on groundwater with fluoride concentrations that exceed the national WHO standards of 1.5 mg/L (WHO, 2004). In China, geological high fluoride concentration groundwater is mainly distributed in arid and semi-arid northwestern regions. Approximately 20 million people are negatively impacted by chronic endemic fluorosis in China (Jia et al., 2018; Wang et al., 2020). Hence, the Chinese government has set allowable limit for fluoride in drinking water/groundwater as 1.0 mg/L (Ministry of Health of China, 2010). Meanwhile large coal mines are mainly located in the northwest, and mine water almost comes from groundwater, consequently, high-F-high-fluoride is abbreviated to high-F- mine water is widely distributed in Northwest China (Wang, et al., 2022).

Over the past decades, many scholars have studied on sources, distribution and formation of high-F groundwater (Chae et al., 2007; Guo et al., 2007; Meenakshi et al., 2004; Rashid et al., 2020). Dissolution of fluorine-rich rocks and minerals is an important geogenic source of F^– enrichment in water. Fluorine-bearing rocks are often associated with granites, alkali granites, and hydrothermal deposits (Wang et al., 2021). And Fluoride has been found to be rich in fluorite (CaF₂), topaz (Al₂(SiO₄)F₂), micas [AB₂–3 (X, Si)₄O₁₀(O, F, OH)₂], fluorapatite (Ca₅(PO₄)₃F)- Chae et al., 2007; Jadhav et al., 2015. Usually, F^– is increasingly released and accumulated in the groundwater via water–rock interactions (Shah and Danishwar, 2003). The enrichment of fluoride in groundwater in Sri Lanka is mainly due to the dissolution of hornblende and mica in black gneiss (Young et al., 2011). Controlled by climate conditions, fluorine-rich rock types and anthropogenic activities, the accumulation of fluoride in groundwater exhibits a zonality and significant differences. Horizontally, the concentration of fluoride gradually increases along the groundwater flow path from the recharge area, through the runoff area, to the discharge area (Alarcon-Herrera et al., 2013). Vertically, affected by evaporation, fluoride concentration in shallow groundwater gradually declines with increasing depth (Yadav et al., 2019). Besides, other processes that lead to high-F groundwater include Na–Ca cation exchanges, competitive adsorption and alkaline environment (Schafer, 2020). The exchange is an important hydrogeochemical process in the enrichment of fluorine in groundwater in the Ordos Basin (Su, et al., 2021). Furthermore, under alkalescency and alkaline (high pH) conditions, OH^– and HCO₃^– can mobilize F^– from fluorine-bearing minerals such as clay minerals and Fe/Al oxyhydroxides into groundwater (Alcaine, et al., 2020).

Although the enrichment mechanism of fluorine in groundwater has been studied, no studies have evaluated the formation mechanism of fluorine in mine water controlled by the geologic processes and coal mine activity. In recent years, with the increasing development of coal mining, high-F mine water is more widespread than before in many areas, especially in areas with semi-arid climates, such as Northwest China (Wang, et al., 2022). During coal mining, groundwater gushes into the coal face, goaf, and becomes mine water (Qu, et al., 2020). Thus, high-F mine water is controlled by geologic processes and anthropogenic mining activities. Coal mining changes the original input, output, and groundwater runoff of hydrological systems, thereby causing stronger hydraulic connections between the related aquifers and a stronger water-rock interaction (Zhang, et al., 2022). Besides, goafs are formed after coal mining. In goaf, further water-rock interaction occurs between broken rock and mine water, which promotes the release of fluorine into mine water again. Many physical and chemical reactions are involved in the water–rock interaction process, and it affects both mine water quality and coal or rock masses (Zhang, et al., 2021, 2023). Few researches has focused on release of fluorine in goaf. In conclusion, under coal mining, the fundamental mechanism leading to high-F mine water and controlling factors on release of fluorine in goaf are still not fully understood.

The Inner Mongolia-Shaanxi contiguous area is characterized by an arid and semi-arid climate, with scarce rainfall, and a fragile ecosystem (Qu et al., 2022). The mine water guarantees the water resource supply for production, living and ecology of the local power plants, and chemical industry (Zheng et al., 2021). With the increasing mining depth, and the change of geological conditions, fluoride pollution of mine water is becoming increasingly prominent. This undoubtedly poses potential obstacles to mine water resources utilization. However, there are few research reports the geochemical processes that control F^– enrichment of mine water in the study area. This research aims to (1) characterize the hydrogeochemical properties of high-F mine water using Piper plots. (2) study the lithological features, mineral composition of F-containing rocks by the scanning electron microscope and energy disperse spectroscopy (SEM-EDS). (3) unravel the main water-rock interaction and controlling factors of high-F mine water by laboratory simulation experiment.