Analysis of ecological water level recovery degree under coal mining subsidence disturbance in ecologically fragile area

The variation degree of ecological water level in ecologically fragile areas is very important for ecological geological environment protection. Ecological water level inevitably undergoes variation under coal seam mining. The current research focuses on the decline of ecological water level under mining induced groundwater leakage, with little consideration given to the ecological water level restoration degree under non leakage of groundwater or disturbance of coal mining subsidence. For this reason, methods such as groundwater dynamics, groundwater level measurement, mathematical statistics, and comprehensive analysis are adopted, and an analytical method was proposed based on the idea of “distinguishing the leakage status of groundwater under coal seam mining → measuring the ecological water level changes in the coal mining face → establishing an ecological water level restoration degree well flow analysis model → predicting different ecological water level restoration times → comparing the analytical and measured values of different ecological water level restoration times” to study the degree of ecological water level restoration under the disturbance of coal mining subsidence in ecologically fragile areas. The results are as follows. ① Based on “key strata layer location + thin plate theory + soil arch effect + descending fissure”, a calculation method for the water-conducting fissure development height under overburden bedrock and soil structure is established, which overcomes the insufficient prediction accuracy caused by the existing empirical formula not considering the soil layer effect. Furthermore, combines the relationship between the thickness of the residual water-retaining layer and the aquifer seepage state, the aquifer seepage state is confirmed. ② Under the disturbance of coal mining subsidence, the measured ecological water level shows a change law of “rapid decline first, slow recovery, and stabilization”, but the ecological water level after coal mining can’t completely recover to the premining state. ③ An analytical model of the recovery degree of ecological water level under the disturbance of coal mining subsidence is established. The analytical values of the recovery time of three different recovery degrees of ecological water level are estimated, and the time errors of both are less than 10% compared with the measured values. ④ The reasons for the incomplete recovery of postmining ecological water level to pre-mining state are discussed from the perspectives of surface topography, atmospheric rainfall recharge, recharge and drainage of phreatic aquifers, and underground water drainage in mining areas.