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<p>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 BTCs in the pool-pipe system from the authors’ previous study in order to analyze the relationships and differences between them. The equations underling the two models are equivalent for simulation of conservative solute transport in the pool-pipe system because the average mobile velocity (<i>v</i><sub><i>m</i></sub>), dispersion coefficient (<i>D</i><sub><i>m</i></sub>), partition coefficient (<i>β</i>) and exchange coefficient (<i>α’</i>/<i>β</i>) in TRM respectively equal the average advective velocity (<i>u</i>), dispersion coefficient (<i>D</i><sub><i>L</i></sub>), main channel proportion (<i>A</i>/(<i>A</i> + <i>A</i><sub><i>S</i></sub>)) and exchange coefficient (<i>α</i>) in TSM. The equivalences of parameters in the TRM and TSM models allow to give appropriate initial estimates for <i>A</i>, <i>A</i><sub><i>S</i></sub> and <i>D</i><sub><i>L</i></sub> in the TSM using initial <i>v</i>, <i>D</i> and <i>β</i> estimates in the TRM, which can be obtained through the parameters (<i>v</i><sub>bar</sub>, <i>D</i><sub><i>chat</i></sub>, and <i>v</i><sub>p</sub>) from Qtracer2. The simplified equations and fewer parameters render the TRM and TSM widely adopted models for simulating solute transport in uniaxial karst conduits. However, the two models are inadequate for simulating more immobile water zones with different exchange intensities and different types of residence time distribution (RTD). TSM outperforms TRM in simulating solute transport in conduit segments (discrete immobile water zones) and in simulating lateral inflow or outflow within karst conduits. Meanwhile, one should opt for TSM over TRM for environmental assessments or remediation strategies because TSM provides more insights into solute transport processes. The results show implications for the selection of the models to simulate solute transport processes in karst conduits.</p>

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

  • 期刊论文

  • 发表日期：

    2025年2月28日

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  • Xiaoer Zhao,
  • Zhaodan Cao,
  • Chengqiu Wu,
  • Xiaofeng Xue

摘要

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 BTCs in the pool-pipe system from the authors’ previous study in order to analyze the relationships and differences between them. 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 TRM and TSM models allow to give appropriate initial estimates for A, AS and DL in the TSM using initial v, D and β estimates in the TRM, which can be obtained through the parameters (vbar, Dchat, and vp) from Qtracer2. The simplified equations and fewer parameters render the TRM and TSM widely adopted models for simulating solute transport in uniaxial karst conduits. However, the two models are inadequate for simulating more immobile water zones with different exchange intensities and different types of residence time distribution (RTD). TSM outperforms TRM in simulating solute transport in conduit segments (discrete immobile water zones) and in simulating lateral inflow or outflow within karst conduits. Meanwhile, one should opt for TSM over TRM for environmental assessments or remediation strategies because TSM provides more insights into solute transport processes. The results show implications for the selection of the models to simulate solute transport processes in karst conduits.