Temporal variation of groundwater hydrochemistry and water stable isotopes under long-term mining disturbance in a coal mine, northwest China

Groundwater plays a key role in water supplies in arid/semi-arid regions. Long-term coal mining can change the groundwater flow systems and chemistry, and may consequently lead to re-distribution of water resources and pose the threat to safety of mining. Therefore, identifying the hydrological changes over time due to long-term mining is of great significance for preventing and controlling such negative effects. In this study, the temporal changes of groundwater hydrochemistry influenced by coal mining were systematically evaluated in the Ning-tiaota coal mine, based on the hydrochemical, isotopic and groundwater level data from 2012 to 2019. The results show that the content of the major ions (e.g., Ca2+, Mg2+) of groundwater in overburden aquifers (Zhiluo formation (J2z) and Yanan formation (J2y)) in the southwest area decreased to the lowest in June 2018 during the whole study period. The hydrochemical type evolved from HCO3-Ca & sdot;Mg type in 2012 to HCO3-Na type in June 2018, then it changed to HCO3-Ca & sdot;Mg type again after that. Meanwhile, the water stable isotopes (delta 2H, delta 18O) in J2z and J2y groundwater show obviously consistent depletion in June 2018 (e.g., boreholes J17 and SK16). The changes of hydrochemical and stable isotopic characteristics in J2z and J2y groundwater in June 2018 were probably caused by the increasing recharge from overlying water bodies (includes Quaternary groundwater and surface water). The development of Water Flowing Fractured Zone provides favorable condition for over-lying water infiltration into J2z and J2y aquifers due to long-term mining of underlying coal seam. Furthermore, the heavier rainfall enhanced recharge from overlying water to J2z and J2y groundwater, which could dilute the original hydrochemical composition and lead to isotopic depletion. This study revealed the temporal variation of groundwater hydrochemical and isotopic components and their indication to groundwater recharge under the influence of long-term mining, which could have a signification for the sustainable management of water re-sources and the safety of mining in coal mine areas.