Elaborate simulation and predication of the tunnel drainage effect on karst groundwater field and discharge based on Visual MODFLOW

The groundwater drawdown caused by tunnel excavation can change the hydrogeological flow system in karst areas. However, how to overcome the heterogeneity in karst aquifers and accurately evaluate the impacts of tunnel excavation on karst groundwater flow field and resources remains largely unknown. In this study, through geophysical prospecting, hydrogeological drilling and test, and observations in 4 years, the depth, scale and spatial distribution of underground river conduits and hydrogeological parameters before and after tunnel excavation are obtained, and the groundwater flow field before and after tunnel excavation was established by Visual MODFLOW. Observation and simulation indicate that two ponds, 5 epikarst springs, 1 borehole, 1 railway tunnel and the underground river system were affected during tunnel excavation. The drainage effect of tunnel is more significant in dry season. Three depression cones are formed, and there is a drawdown of 10 similar to 30 m due to the tunnel excavation. The simulation results predict that the drainage effect is more significant when the tunnels are below the groundwater level in the dry season. Driven by the increase of precipitation and tunnel drainage, the seasonal variation of underground river discharge is exacerbated, and the groundwater level near the tunnel in the rainy season is lower than that in the dry season, which is opposite to that in natural state. The groundwater flow budget calculation and prediction suggest that the net loss of groundwater storage was 6.1 x 10(5) m(3) during tunnel excavation, and the net loss of groundwater storage is about 1.7 x 10(5) m(3) one year after the tunnels were completed, and meanwhile the depression cones continue to expand. The increase of precipitation raises the groundwater level and alleviates the drainage effect, but the increased precipitation recharge is less than groundwater discharge and tunnel drainage, so there is still a loss of groundwater storage. It is difficult to restore the groundwater flow field and discharge in a short time. This study provides evidence and a useful method to quantitatively evaluate the effects of tunnel excavation on the groundwater flow field and to inform better measures to protect karst groundwater resources.

Automated summary

This study investigates the impact of tunnel excavation on groundwater flow in karst areas, highlighting significant changes in hydrogeological parameters and flow fields before and after the excavation. The findings suggest a considerable drainage effect during dry seasons, leading to a decrease in groundwater storage and alteration in the seasonal discharge pattern of underground rivers.