Department of Civil and Environmental Engineering, Colorado School of Mines, Golden, CO 80401, United States of America.
Department of Civil and Environmental Engineering, Colorado School of Mines, Golden, CO 80401, United States of America.
J Contam Hydrol. 2021 Oct;242:103842. doi: 10.1016/j.jconhyd.2021.103842. Epub 2021 May 29.
The processes impacting solute transport through unsaturated porous media have been receiving renewed attention due to their relevance to the transport of emerging contaminants. A set of well-monitored and highly controlled experiments in sand columns were conducted to determine the effect of partial saturation on conservative solute breakthrough in porous media. The results suggest traditional transport parameter estimation methods inadequately account for the pore-scale processes of mass transfer to the immobile zones and the effects of partial saturation on advective transport, even for conservative tracers. Accurate estimation of these basic transport parameters is critical to evaluate the multi-phase partitioning of nonconservative solutes, as any errors in these parameters would bias the estimates of multi-phase partitioning parameters. Herein, we introduced the Mass Transfer Index (MTI), a semi-empirical approach for quantifying the impact of non-Fickian elements of pore-scale unsaturated solute transport (i.e. immobile water, tortuous flow paths, and non-uniform solute distribution), which become increasingly important as the wetting fluid saturation decreases. Importantly, this MTI was determined independently of chemically driven phase partitioning and is supported by experimental data. Based on this conceptualization, the 1-D equilibrium advection dispersion equation was modified to incorporate the MTI as a lumped parameter which quantifies resistance to (MTI > 1) or promotion of (MTI < 1) of advective solute flux. Analytical solutions to the modified advection-dispersion-reaction equation for pulse and step inputs were developed. Conservative tracer experiments were conducted in variably saturated sand columns to validate both the MTI conceptualization and the inversion method used to estimate the MTI. These experiments involved the use of X-ray absorption spectroscopy integrated with sensor-based measurements of soil moisture, temperature, and electrical conductivity for tracer breakthrough. The mathematical model developed herein adapts traditional macroscopic models of solute transport to account for the non-Fickian pore-scale transport behaviors observed in unsaturated porous media with significant advective flux.
由于部分饱和对新兴污染物传输的影响，非饱和多孔介质中溶质传输过程受到了新的关注。在砂柱中进行了一组监测良好且高度受控的实验，以确定部分饱和度对多孔介质中保守溶质突破的影响。结果表明，传统的传输参数估计方法不能充分考虑到质量传递到不可动区的孔隙尺度过程，以及部分饱和度对平流传输的影响，即使对于保守示踪剂也是如此。准确估计这些基本传输参数对于评估非保守溶质的多相分配至关重要，因为这些参数中的任何误差都会偏析多相分配参数的估计。在此，我们引入了质量传递指数（MTI），这是一种半经验方法，用于量化非饱和多孔介质中溶质传输的非菲克元素（即不可动水、曲折的流动路径和非均匀溶质分布）的影响，随着润湿流体饱和度的降低，这些元素变得越来越重要。重要的是，该 MTI 是独立于化学驱动的相分配确定的，并且得到了实验数据的支持。基于这一概念，修改了一维平衡对流弥散方程，将 MTI 作为一个集中参数来包含，该参数量化了对（MTI＞1）或促进（MTI＜1）平流溶质通量的阻力。针对脉冲和阶跃输入，开发了修改后的对流-弥散-反应方程的解析解。在变饱和度砂柱中进行了保守示踪剂实验，以验证 MTI 的概念化和用于估计 MTI 的反演方法。这些实验涉及使用与基于传感器的土壤湿度、温度和电导率测量相结合的 X 射线吸收光谱来进行示踪剂突破。本文所开发的数学模型适应了传统的溶质传输宏观模型，以解释在具有显著平流通量的非饱和多孔介质中观察到的非菲克孔隙尺度传输行为。
