## 4 Mechanism of Underground Slope Replenishment through Atmospheric Precipitation  
In its original state, before any mining activity, only certain sections of the Changxing Formation limestone featured dissolution cracks and caves, with minimal crack development. The groundwater permeability coefficient, K, ranged from 0.0000026 m/d to 0.1079 m/d. After atmospheric precipitation infiltration, runoff was sluggish, resulting in a prolonged replenishment cycle for deep aquifers and a limited replenishment volume (Figure 7). Under these groundwater runoff conditions, the Guiyuan coal mine experienced an inflow of \(\sim\)84 m\({}^{3}\)/h, with atmospheric precipitation supplying the mine scope over \(\sim\)10 months. The predominant cation in the runoff retention area was Ca\({}^{2+}\), while Na\({}^{+}\) predominated in the runoff area.  
Figure 6: The curve of the relationship between water inflow and atmospheric precipitation at Guiyuan Coal Mine.  
Figure 7: Water runoff map of the limestone in the Changxing Formation in its original state.  
After the completion of mining operations in the 5\({}^{\#}\) and 9\({}^{\#}\) coal seams, extensive goaf formation occurred in the shallow mines within the mining area, leading to well-developed water-conducting fracture zones. These zones provided ample storage capacity and conduits for atmospheric precipitation to supply the mines. The quality of mine inflow water was correlated with that of surface water, with Na\({}^{+}\)(+K\({}^{+}\)) constituting the primary cation. Atmospheric precipitation smoothly replenished the mining area through the water-conducting fracture zones of the Changxing Formation limestone until the closure of the shallow mine. Despite significant water inflow into the mine, no water-inrush incidents were reported. Furthermore, during this period, atmospheric precipitation and groundwater primarily sustained the shallow mine, with only a fraction contributing to runoff for the deep mine (Figure 8). Under these groundwater runoff conditions, mine water quality remained consistent with surface water, with Na\({}^{+}\)(+K\({}^{+}\)) dominating as the main cation.  
After the closure of the shallow mine, as water discharge ceased, atmospheric precipitation replenished the Changxing Formation limestone, storing water within the original dissolution gaps, caves, and cracks due to mining activities. Mining-induced cracks facilitated the smooth flow of water within the Changxing Formation limestone, serving as a water source and conduit for water inrushes at the deep mining face. Sediment carried by atmospheric precipitation filled the goaf of the fully mined deep mines, coupled with water immersion and coal-bearing strata mud expansion within the goaf. In the event of a water inrush at the working face, goaf water inflow gradually decreases over time. Typically, post-mining completion, goaf water inflow did not exceed 10 m\({}^{3}\)/h. Additionally, accumulated water from the closed shallow mine, goaf, and water-conducting fracture zones in the deep mine flowed into the working face. Thus, during periods in areas with abnormal development of the water-conducting fracture zone before and after the initial pressure on the new working face, in areas where faults develop, in areas where mining is slow, and during periods of mining cessation, these conditions led to another water-inrush incident (Figure 9).  
Figure 8: Atmospheric precipitation groundwater recharge before the closure of shallow mines.