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Transport of reactive solutes in heterogeneous porous media: Heterogeneous rate-limited mass transfer

机译：反应性溶质在非均质多孔介质中的传输：非均质速率受限的传质

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摘要

The transport of reactive solutes in the subsurface is influenced by a variety of physical and chemical processes. The processes are characteristically heterogeneous and often operate simultaneously at different temporal and spatial scales. In modeling reactive solute transport different models take different approaches, dependent on the scale of the system and the objective of the study. Two major approaches have been used to incorporate heterogeneous rate-limited mass transfer into mathematical models for solute transport. One focuses on processes operative at the microscopic scale and associated grain-scale heterogeneity, while the other stresses the macroscopic variability of the medium and the field-scale behavior of solute transport. In this work, I first examine the conceptual framework and model formulation of these two approaches in an attempt to evaluate potential commonality, then present a two dimensional numerical model that integrates the first approach with traditional stochastic modeling for reactive transport. In this model multiple processes are explicitly accounted for, including spatially variable flow, spatially variable sorption, locally heterogeneous diffusive mass transfer, locally heterogeneous rate-limited sorption, and locally heterogeneous first-order degradation. Finally, the model is used to (1) examine the individual and concurrent effects of multiple heterogeneous processes on reactive transport, and (2) evaluate the impact of microscopic-scale mass transfer heterogeneity on field-scale transport in systems for which hydraulic conductivity is spatially variable. The comparison of the two approaches shows that despite differences in conceptualization and formulation, both microscopic and macroscopic based models produce comparable behavior for smaller-scale systems. However, greater deviations are observed at larger scales. This suggests that caution should be taken when using mathematical modeling for elucidating the specific processes that may be influencing reactive-solute transport for a given system. Results from 2-D simulations of the new model reveal that inclusion of locally heterogeneous mass transfer does not appear to significantly influence the mean transport behavior for systems with field-scale heterogeneity. However, it does appear to influence low-concentration tailing. For simulations of reactive transport over extended distances, models with locally heterogeneous mass transfer may "preserve" the non-equilibrium effects associated with rate-limited mass transfer better than the models incorporating locally uniform mass transfer when both pore-scale and field-scale heterogeneity coexist.
机译：反应性溶质在地下的传输受多种物理和化学过程的影响。这些过程的特征是异构的，通常同时在不同的时间和空间尺度上运行。在对反应性溶质运移进行建模时，不同的模型采用不同的方法，具体取决于系统的规模和研究目的。已经使用两种主要方法将非均质速率限制的质量转移纳入溶质运移的数学模型。一个侧重于在微观尺度上起作用的过程以及相关的晶粒尺度异质性，而另一个则强调介质的宏观变异性和溶质运移的场尺度行为。在这项工作中，我首先检查这两种方法的概念框架和模型表述，以试图评估潜在的共性，然后提出一个二维数值模型，该模型将第一种方法与用于反应性运输的传统随机模型相集成。在该模型中，明确考虑了多个过程，包括空间可变流量，空间可变吸附，局部异质扩散传质，局部异质限速吸附和局部异质一阶降解。最后，该模型用于（1）检查多个非均质过程对反应性输运的单独影响和同时影响，以及（2）评估微观传质非均质性对水力传导率较大的系统中场尺度输运的影响。空间可变。两种方法的比较表明，尽管在概念和配方上存在差异，但基于微观和宏观的模型对于较小规模的系统都具有可比的性能。然而，在更大的尺度上观察到更大的偏差。这表明在使用数学模型阐明可能影响给定系统的反应性溶质运移的特定过程时，应格外小心。新模型的二维模拟结果表明，包含局部异质传质似乎不会显着影响具有现场规模异质性的系统的平均传输行为。但是，它似乎确实会影响低浓度拖尾。对于远距离上的反应输运模拟，与局部不均匀传质的模型相比，当孔隙规模和现场规模不均一时，与结合局部均匀传质的模型相比，可以更好地“保留”与限速传质相关的非平衡效应。共存。

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    Li Zhen;

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  • 年度 2000
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