Simulation of groundwater flow field evolution and water inrush risks under the deep-well reinjection of high-TDS mine water

Objective The deep-well reinjection of mine water results in a regional groundwater level rise to varying degrees, possibly inducing hydrogeological hazards such as floor water inrushes or water inrushes along faults. In this context, determining the evolutionary pattern of the groundwater flow field under long-term mine water reinjection and conducting reinjection safety assessments play a critical role in ensuring both the safety of mine water reinjection and water conservation-based coal mining.

Methods This study examined the reinjection of mine water in a coal mine within the Juye coalfield, North China. Based on the geological conditions for mine water reinjection in the study area, this study selected the target aquifer for reinjection by referring to indices including mining safety, water quality security, water circulation condition, water exploitation and utilization, and reinjection potential. Using the groundwater modeling system (GMS) software, this study constructed a 3D numerical model of the Ordovician limestone aquifer. Using this model, this study simulated and predicted the regional groundwater level rise, cumulative water storage, and water storage efficiency during long-term mine water reinjection. Accordingly, the primary water inrush patterns that were likely induced by groundwater level rise were determined. In combination with the formulae provided in the Regulations for Mine Water Prevention and Control (also referred to as the Regulations) and the numerical simulation results derived using the FLAC^3D model, this study assessed the impact of the maximum groundwater level rise on mining safety under long-term mine water reinjection. Finally, a safety assessment system for deep-well mine water reinjection was proposed.

Results and Conclusions The mine water in a coal mine within the study area features elevated water inflow and high total dissolved solids (TDS), which leads to high water treatment costs. A comparative analysis reveals that the deep Ordovician limestone aquifer was suitable to use as the optimal reinjection target for minimally treated mine water. Furthermore, the basis for the siting of reinjection boreholes was proposed. The GMS-based simulation results reveal that over a reinjection period of 10 years, single-borehole reinjection at a rate of 200 m³/h led to a maximum groundwater level rise of 10.02 m. In contrast, reinjection using four boreholes with a spacing of 400 m and a reinjection rate of 800 m³/h yielded a maximum groundwater level rise of 25.83 m. The single-borehole reinjection could achieve an annual reinjection volume of up to 1.72 × 10⁶ m³, suggesting considerable potential for groundwater storage and a huge cost advantage of water treatment. Among various water inrushes, those along faults are the most likely triggered by groundwater level rise in the study area. The formulae in the Regulations and the simulation results derived using the FLAC^3Dmodel demonstrate that under conditions where coal pillars are retained as required, the reinjection-induced groundwater level rise will not destroy the structural stability of impermeable rocks, and normal mining activity is less prone to induce secondary water inrushes during mine water reinjection. In this case, safe mining can be maintained. The safety assessment system established for deep-well mine water reinjection underscores that ensuring both water quality safety and mining safety is necessary for the implementation of a deep-well mine water reinjection technology. The results of this study provide critical theoretical and practical guidance for assessing the reinjection safety of mine water in coal mines with typical hydrogeological structures in North China.