The severe shortage of water has seriously hindered the development of the energy industry in many coal-producing areas (Chen et al., 2016; Rathi et al., 2017), especially in the Shendong Mining Area (Ma et al., 2013), located in the arid and semi-arid areas of northwest China but with rich coal reserves (Chen, 2016). The protection and rational utilization of water resources have become an important scientific and technological problem to be solved by coal green mining in arid areas of Northwest China (Zhang et al., 2011). In view of the above problems, Gu (Gu, 2015) put forward and successfully developed the coal mine underground reservoir technology using the cavity between the broken rock bodies in the mined space area formed after coal mining (Shi, 2021). The technology solves both the mine water disaster problem and realizes the efficient recycling of the mine water resources (Gu et al., 2016), providing more than 95% of the water use (Chen et al., 2016), which largely solves the production and domestic water problem in the mining area. During the operation of the underground reservoir in the coal mine, the collapsed rocks in the reservoir have a purification effect on the mine water (Gu et al., 2021), and scholars have also studied the purification effect of the rocks in the mined space area on the suspended matter, chemical oxygen demand (COD) (Fang, 2020), dissolved organic matter (DOM) (Han et al., 2020b; Yu et al., 2018) and heavy metals (Jiang et al., 2020; Shao, 2009) and so on. However, the purification mechanism of mine water in the process of water rock interaction between mine water and rock in reservoir storage stage is not clear.
The water rock interaction in the study area is closely related to the actual situation of the site (Jannesar Malakooti et al., 2015), so researchers usually use the field investigation sampling (Ibrahim et al., 2019) and indoor simulation experimental data (Goren et al., 2011; Phan et al., 2018), combined with the ion ratio method (Jia et al., 2020) or hydrogeochemical simulation software to accurately describe the physical and chemical relationship between the water phase composition and the regional environmental background, thus exploring the evolution and migration laws between the water and rock in the stratum (Kumar et al., 2020; Li et al., 2018). At present, some scholars (Han et al., 2020a) have analyzed the hydrochemical characteristics and formation mechanism of underground reservoir by testing the in-situ inflow and outflow water sample data of underground reservoir, combining multivariate statistical method, hydrochemical analysis method and ion ratio method. Some scholars (Fang et al., 2020) have studied the mechanism of water rock interaction and the source of ions in the underground reservoir from the change law of main ions (Zhang et al., 2019) and the characteristics of rock changes (Pearce et al., 2018; Zhang et al., 2020) through static simulation test of water rock interaction. The influence of water rock interaction on the binding characteristics of dissolved organic matter and heavy metals in mine water during the process of mine water flowing in goaf was also studied (Zhang et al., 2021). It can be seen that some achievements have been made in the study of water rock interaction in underground reservoir of coal mine, but there is no study of water rock interaction in reservoir combined with hydrogeochemical simulation.
Reverse geochemical simulation (Dai and Samper, 2006; Embile et al., 2019) is to determine the water rock reaction in the system based on the observed hydrochemical data, that is, to interpret the observed hydrochemical data. The purpose is to find out the complex reaction between groundwater and different minerals and gases, and quantify it under reasonable conditions (Hidalgo and Cruz-Sanjulián, 2001). With the rapid development of computer technology, many hydrogeochemical simulation software appeared (Salcedo Sánchez et al., 2017). Among them, PHREEQC has the most powerful simulation ability among the similar software (Sprocati et al., 2019). It has a huge database of groundwater balance model and ion exchange model (Steding et al., 2020), which can simulate the chemical reaction and migration process of sewage and clean water in the environment, and is widely used in the world (Mahani et al., 2016). In addition, scholars (Chandrasekhar et al., 2018; Korrani et al., 2015) have also conducted a lot of simulation studies on groundwater hydrochemical evolution and water rock interaction in artificial recharge process by using PHREEQC (Sharma and Mohanty, 2018), and the water rock interaction analysis software has been relatively mature (Shabani and Zivar, 2020).
In this study, Daliuta coal mine underground reservoir inlet and outlet water and roof caving rock samples were taken as the research object. Based on the analysis of mineral chemical composition, specific surface area and pore structure of roof caving rock samples and suspended solids in mine water in the study area, as well as the test and analysis of in-situ inflow and outflow water samples of underground reservoir, combined with the reverse geochemical simulation analysis was carried out with PHREEQC software. This paper reveals the process of water rock interaction in coal mine underground reservoir and the purification mechanism of mine water quality, which provides theoretical support and reference for the future application of PHREEQC software to simulate the process of water rock interaction in coal mine underground reservoir and the efficient utilization of reservoir water.