Comparison of two-region nonequilibrium model and transient storage model for simulating solute transport in karst conduits

Abstract Two-region nonequilibrium model (TRM) and transient storage model (TSM) are commonly used to interpret the tailing of breakthrough curve (BTC) in karst conduits caused by immobile zones (pool, etc.) for transport studies in conduits. But few studies simultaneously used the two models for comparison. We used the two models to simulate the same BTCs in the pool-pipe system with different pool sizes to analyze the similarities and differences between them. The simulation results show that the BTCs can be well reproduced by both TRM and TSM in CXTFIT and OTIS. The equations underling the two models are equivalent for simulation of conservative solute transport in the pool-pipe system because the average mobile velocity (vm), dispersion coefficient (Dm), partition coefficient (β) and exchange coefficient (α/β) in TRM respectively equal the average advective velocity (u), dispersion coefficient (DL), main channel proportion (A/(A + AS)) and exchange coefficient (α') in TSM. The equivalences of parameters in the two models allow to give appropriate initial estimates for A, AS and DL in TSM using initial v, D and β estimates in TRM. TSM can describe solute transport in conduits with lateral flow or in conduit segments between the sampling points but TRM cannot. In addition, TSM provides more insights into solute transport processes in karst conduits compared with TRM. However, simulating BTC in karst conduits using TRM in CXTFIT is steadier and easier. The results show implications for the selection of the models to simulate solute transport processes in karst conduits.