A comparison of single-continuum and dual-conductivity karst aquifer models during recharge events

Modeling groundwater flow through karst aquifers is a complex problem due to the heterogeneous, dual porosity nature of carbonate formations. This has limited the practice of transport modeling in regions dependent on karst aquifers, and created a need to explore new modeling techniques to address the issue. In this modeling study, we simulate flow in synthetic karst systems that contain stochastically generated conduit networks. For a variety of network geometries, dual-conductivity model simulations are performed using the conduit flow process in MODFLOW. This approach allows for the simulation of turbulent flow, pipe roughness and tortuosity, and matrix-conduit fluid exchange, permitting the development of realistic transient flow systems in response to recharge events. For each conduit network, a corresponding equivalent porous media (single-continuum) model is constructed by assigning representative high hydraulic conductivity values along the conduits. Both model systems were run using identical transient recharge events. The model comparison considers spring discharge at the network outlet, as well as simulated heads throughout the domain. Using the dual-conductivity model as the objective, conduit hydraulic conductivities were adjusted in the single-continuum model. Results indicate that the suitability of the single-continuum approach depends on the magnitude and location of recharge relative to the conduit network.