Hydrochemical evolution law of mine water under repeated mining disturbance in shallow buried coal seams and its significance

In the shallow coal seam area at the border of Inner Mongolia and Shaanxi, most of the upper coal groups have been mined out after long-term large-scale mining, and the lower coal groups are currently being mined. Following repeated mining disturbances, the mine water inflow in the study area exhibits characteristics of water quality changes and increased water volume. Investigating the causes of the impacts of repeated disturbance conditions on the hydrochemical characteristics and water volume changes of roof mine water in the working faces of the lower coal group is a prerequisite for conducting water prevention and control work and water resource protection. A total of 56 water samples were collected, including surface water, Quaternary loose layer groundwater, bedrock groundwater, goaf water, and mine water. Based on the complete water quality analysis results, mathematical statistics, hydrochemical characteristics analysis, ion proportional coefficient analysis, and hydrogen-oxygen isotope analysis were applied. Combined with the changes in river flow monitoring values, the evolution characteristics of mine water quality were qualitatively analyzed, and the recharge proportions of water-filling sources were quantitatively evaluated. The results show that the hydrochemical type of groundwater in the northern wing changes from HCO₃—Ca·Na type to HCO₃—Na type with increasing burial depth. The hydrochemical types of mine water, aquifer groundwater, and surface water in the southern wing are similar, all being HCO₃—Na type. The formation of their hydrochemical components is mainly controlled by water-rock interaction and also affected by reverse cation exchange adsorption, with the main ionic components derived from the dissolution of silicate minerals. The main recharge sources of mine water in Coal Mine 2 are the bedrock aquifer (66.2%) and the Quaternary aquifer (21.5%). The main recharge sources of mine water in Coal Mine 3 in the southern wing are surface water and shallow groundwater, with corresponding recharge proportions of 23.6% and 29.6% respectively, while the bedrock aquifer recharge proportion decreases to 46.8%. The water-conducting fractured zone formed by repeated disturbances has connected the surface, becoming the main water-filling channel for surface water recharge to mine water. This study presents a recharge path and intensity model for groundwater and mine water applicable to shallow coal seam areas under repeated disturbance conditions, and proposes targeted water disaster prevention and control suggestions integrating “normalized monitoring - flood season early warning - post-mining reconstruction”. The research has important guiding significance for promoting regional water resource protection and ensuring the safety of regional coal mining. In subsequent research, the advancing speed and lateral recharge intensity should be included as research variables to comprehensively consider their influence laws on the quality and quantity changes of mine water inflow.