The modeling point
Karst, fractures, and conduits may violate simple porous-media intuition locally, but equivalent porous medium models can still be defensible for regional water-resource questions when the goal is broad hydraulic behavior rather than detailed conduit flow.
Practical caution
Use scale and question to decide defensibility. A regional management model may tolerate equivalent properties, while a local contaminant pathway or spring-response problem may require more explicit structure.