Solute transport modelling in karst conduits with slow zones during different hydrologic conditions

Eight tracer tests conducted under different flow conditions in two well-developed karst connections in the Santa Eufemia–Ereñozar karst unit (Basque Country, northern Spain) are considered for transport characterization. Lithium tracer was injected as an impulse release for obtaining well-defined tracer breakthrough curves (TBCs). In order to simulate curve evolution and obtain transport characteristic parameters, two approaches have been applied: a standard analytical solution and a new numerical one that accounts for transient storage processes. The numerical model simulates the effect of localized variations in conduit geometry that give way to dead zones or slow zones in the transport process. This model fits the experimental curves well and explains their tails mainly by diversion processes from the main flow line towards stagnant zones. An increase in dispersion coefficient values is observed in both conduits as flow rates and average flow velocities increase. Stagnation and tracer transient retention processes, which may be quantified by means of the numerical solution, are more important as circulating flow rates decrease and travel times increase. The diversion processes in the two connections follow a similar evolution, with significant changes in the magnitude of the retentions registered depending on the hydrodynamic state of the traced system. Consequently, there is a clear hydrodynamic-controlled tailing effect.