Influence of karst conduit’s geometry and morphology on 3D flow and transport processes: Insights from 3D tracer tests and numerical modelling

•3D mapping of a fully water-filled karst conduit.•3D dye transport experiments in the water-filled karst conduit.•3D numerical simulation of turbulent flow and solute transport.•Influence of karst conduit’s geometry and morphology on flow and transport.•Sensitivity of the breakthrough curve to complex flow circulation. In karst, the characterization of subsurface solute and contaminant transport is a major issue that has been widely investigated due to the high vulnerability of these hydrosystems. In former studies, several researchers addressed these needs with different approaches such as laboratory experiments, field experiments, or groundwater flow and transport simulations. The objective of such approaches is to improve knowledge of transport processes in karst hydrosystems, and propose solutions to limit the downstream hydrogeological risks (contamination of water resources). In this study, we performed the 3D mapping of the karst conduit, which outlet is the Lez spring (southern France) to get the precise geometry and morphology of this fully water-filled karst conduit. Then we injected a dye in the water-filled karst conduit and performed a continuous monitoring of dye tracer recovery at three different cross-sections located downstream the injection spot, at a distance of 40 m, 90 m and 160 m, respectively. In each cross-section, five submersible fluorometer probes were positioned at different locations (top, right, down, left and central parts) along the cross-section of the karst conduit. Experimental data allowed to reconstruct the transient spatial distribution of concentration for each cross-section, which allowed assessing the propagation and evolution of solute plume over time. Monitored data also provided information about dye mixing along the karst conduit and were used to investigate the effect of karst conduit geometry, flow turbulences and velocity profiles on the mixing processes and propagation of solute plume along the conduit with numerical models. Modelling results showed that the consideration of 3D mapped karst conduit geometry and morphology allows a much better reproduction of observed transport processes and breakthrough curves, than the one obtain when considering simplified conduit geometry and morphology.