Numerical Simulation Study on Solute Transport in Karst Aquifers under the Combined Effects of Dual Conduit Length Ratio and Pool Size

Breakthrough curves (BTCs) in karst aquifers often exhibit anomalous transport characteristics such as long tailing and multiple peaks. However, most existing studies have focused on single factors, making it difficult to explain complex curve patterns. This study simultaneously considers two key factors, the length ratio of dual conduits and pool size, and employs finite element numerical simulations to systematically investigate their combined effects on BTCs. The reliability of the model was first validated against previously published experimental results. Subsequently, four groups of dual-factor simulations were conducted to reveal the mechanisms governing variations in peak time, peak height, tailing intensity, and peak separation under different conditions. The results indicate that: (1) under a fixed length ratio, increasing pool size causes the dual peaks to gradually merge and may lead to a unique pattern in which the first peak is lower than the second; (2) with fixed pool size, the retention effect of the pool weakens the separation between dual peaks; (3) when pool size decreases while the length ratio increases, BTCs evolve from a single peak to dual peaks, with higher peak concentrations and reduced tailing; and (4) simultaneous increases in pool size and length ratio significantly reduce peak concentrations and enhance curve broadening and tailing effects. These findings not only enrich the