A review of the effects of tunnel excavation on the hydrology, ecology, and environment in karst areas: Current status, challenges, and perspectives

•Tunnel excavation-induced karst groundwater drawdown changed hydrological processes.•Tunnel excavation changed the distribution patterns of karst water resources.•Tunnel excavation altered the physical and chemical properties of karst soils.•Tunnel excavation changed physiological processes and growth rates of karst plants.•Tunnel excavation induced karst geological hazards. Due to a lack of awareness of environmental protection at the beginning of tunnel excavation and the unique ecological vulnerability of karst areas, tunnel excavation is beginning to have increasingly serious negative effects on the ecological environments in karst areas, leading to challenges related to regional water resources and ecological security. The groundwater drawdown caused by tunnel drainage has had far-reaching impacts on the hydrology, ecology and environment in karst areas. The most prominent effects of the recent advances in drainage techniques include the destruction of aquifer structures, changes in the distribution patterns of water resources and the groundwater flow field and even the initiation of geological disasters (i.e., collapses) in karst areas. In addition, karst water circulation and hydrogeochemical processes have also been affected. However, due to the complex geological conditions and limited observational data in karst areas, the impacts of tunnel drainage on ecological systems remain poorly understood. With increased understanding, researchers have found that the physical and chemical properties of the soil, the speed of soil erosion, the physiological processes and growth rates of plants, and even the compositions of plant communities are gradually changing in tunnel-affected karst areas, although the understanding of these processes and mechanisms remains far from sufficient. Based on the progress made regarding the understanding of water resource and hydrological process issues resulting from tunnel excavation in karst areas, we expect to experience a worldwide increase in investigations of the eco-hydrogeological effects of tunnel excavation in the future.