Influence of dual conduit structure on solute transport in karst tracer tests: An experimental laboratory study

Abstract

We conducted lab-scale experiments to investigate the mechanism of dual-peaked breakthrough curves (BTCs) in karst tracer tests. Three groups of dual conduit structures were constructed by varying: (1) the length ratio between the two conduits for a fixed length of the shorter conduit, (2) the total length of the conduits for a fixed length ratio, (3) the connection angle between the conduits. The BTCs generated by the tracing experiments were fitted by a Dual-Region Advection Dispersion (DRAD) model to derive effective transport parameters.Our results confirm that the dual conduit structure triggers the double-peaked BTCs. Increasing the conduit length for a fixed length ratio or increasing the length ratio increases peak separation. The connection angles between the two conduits θ1 and θ2 also influence the BTCs: increasing θ1 and decreasing θ2 causes the first peak to get smaller and the second peak to get larger. The DRAD model can reproduce the dual-peaked BTCs while its application to the case of single-peaked BTCs may be problematic due to strong interaction between model parameters. A method is proposed for estimating underground karstic conduit lengths from experimental dual-peaked BTCs.