Quantifying Groundwater Exchange Mechanisms and Tunnel Drainage Impacts in a Regional Karst Aquifer: A Coupled GemPy and MODFLOW-CFPv2 Approach

The interaction between the low-permeability matrix and high-permeability conduits is a governing mechanism in karst hydrogeology, yet it remains difficult to quantify, especially under the disturbance of underground engineering. This study investigates the spatiotemporal evolution of groundwater exchange and the impact of tunnel drainage in a typical karst system in Southwest China. We developed a regional dual-permeability model by integrating GemPy for 3D geological structural modeling and MODFLOW-CFPv2 for hydraulic simulation. The model achieved a high reliability with a Pearson correlation coefficient of 0.98 between simulated and observed heads, outperforming traditional MODFLOW-Drain/River methods in capturing non-Darcy conduit flows.

Simulation results under natural conditions reveal significant spatial heterogeneity: while the matrix-to-conduit recharge dominates the spatial extent (37% of the area), the conduit-to-matrix leakage dominates the total exchange volume (58%). Sensitivity analysis identifies matrix hydraulic conductivity and conduit wall permeability as the most critical factors controlling exchange intensity, whereas rainfall intensity primarily influences local flow directions rather than the overall exchange pattern. Furthermore, the study quantifies the disturbance of tunnel drainage. Tunnel construction caused a maximum groundwater drawdown of 5.44 m and a residual drawdown of 2.00 m during the stable phase. Crucially, drainage activities induced flow reversals (transforming matrix recharge zones into leakage zones) and reduced the regional exchange flux by an average of 2,987 m³/d. These findings clarify the non-linear controls on karst flow exchange and provide a robust scientific basis for groundwater management and geo-hazard mitigation in engineering-disturbed karst aquifers.