Application of MODFLOW-CFPv2 model in karst tunnel water inrush and its impact on groundwater environment: Example of the Mengkuanggou karst water system in Heqing County, Yunnan Province

Objective In the central Yunnan region, the geological structure is complex and karst formations are prevalent. Sudden water gushing and attenuation of spring flow are among the most difficult problems in tunnel construction. The Mengkuanggou karst water system exhibits a high degree of karst conduitization, with significant differences between karst fissures and karst conduits. Methods This study employs the MODFLOW-CFPv2 dual medium numerical model to simulate the karst water system of Mengkuanggou, providing a detailed description of karst pipeline and diversion tunnel to understand the influence of tunnel construction on groundwater flow and spring flow. Results The results show that (1) The MODFLOW-CFPv2 model can describe the complex geological structure of karst areas, and simulate the dynamic characteristics of the groundwater level and the response characteristics of karst spring flow in the study area. (2) Under the condition of strong discharge, the water inflow per unit length of the tunnel is 164 m3/d·m, with a stable water inflow of 69 m3/d·m, while the Mengkuanggou karst spring flow shows a significant downwards trend. During the simulation period, the average spring flow decreased from 1 578 L/s under natural conditions to 1 098 L/s, an overall decrease of 30.4%, and the peak flow rate decreased from 2 133 L/s to 1 375 L/s, a decrease of 35.5%. The construction of strong discharge conditions will have a significant impact on tunnel construction and the groundwater environment. Under the condition of limited discharge, the maximum water inflow of the tunnel is 39 m3/d·m, and the stable water inflow is 24 m3/d·m. The water inflow of the tunnel is significantly reduced, and the downwards trend of the karst spring flow in Mengkuanggou is also improved to a certain extent. During the simulation period, the average spring flow is reduced to 1 284 L/s, a reduction of 18.6%, and the peak flow is reduced to 1 617 L/s, a decrease of 22.1%. Conclusion The MODFLOW-CFPv2 dual-medium model used in this study accurately describes highly heterogeneous karst water systems with pipelines, karst caves and fissures in karst areas. It can be used to quantitatively evaluate the karst water of Mengkuanggou during Xianglushan Tunnel construction.