Solute transport in karst aquifers is primarily constrained to solutional conduits. An important phenomenon often encountered when interpreting tracer tests in karst aquifers is the occurrence of double-peaked breakthrough curves(BTCs). The double-peaked BTCs are usually attributed to tracer transport through a conduit system consisting of a dual-conduit structure: an auxiliary conduit that deviates from the main conduit at the upstream and converges back at the downstream. In order to understand how the geometric configuration of the dual-conduit structure influences the BTCs, laboratory experiments utilizing plastic tubes were conducted. The looping structures were constructed by varying: 1) the total length of the conduits, while fixing the length ratio; 2) length ratio between the two conduits, while fixing the length of the main conduit; and 3) inlet flow rate.
Results show that the dual-conduit structure causes the double-peaks of BTCs. Keeping the length ratio of the two conduits and increasing their total length leads to a larger separation of the two peaks of the BTCs. Keeping the length of the main conduit while increasing the length of the secondary conduit causes similar effects. Increasing the inlet flow rate reduces the distance between the two peaks and the difference between the two peak values.