Abstract
Fickian and non-Fickian behaviors were often detected for contaminant transport activity owed to the preferential flow and heterogeneity of soil media. Therefore, using diverse methods to measure such composite solute transport in soil media has become an important research topic for solute transport modeling in soil media. In this article, the continuous-time random walk (CTRW) model was applied to illustrate the relative concentration of transport in low-permeability homogeneous and saturated soil media. The solute transport development was also demonstrated with the convection-dispersion equation (CDE) and Two Region Model (TRM) for comparison. CXTFIT 2.1 software was used for CDE and TRM, and CTRW Matlab Toolbox v.3.1 for the CTRW simulation of the breakthrough curve. It was found that higher values of determination coefficient (R2) and lower values of root mean square error (RMSE) concerning the best fits of CDE, TRM, and CTRW. It was found that in the comparison of CDE, TRM, and CTRW, we tend to use CTRW to describe the transport behavior well because there are prevailing Fickian and non-Fickian transport. The CTRW gives better fitting results to the breakthrough curves (BTCs) when β has an increasing pattern towards 2.00. In this study, the variation of parameters in three methods was investigated and results showed that the CTRW modeling approach is more effective to determine non-reactive contaminants concentration in low-permeability soil media at small depths.
Highlights
Transport through a porous medium leads the contaminant to spread into the other fluid medium
The breakthrough curves (BTCs) at each soil column length were individually fitted with continuous-time random walk (CTRW), Two Region Model (TRM), and convection-dispersion equation (CDE)
The tracer BTCs (C/C0) over time in one-dimensional low-permeability homogeneous and saturated media were evaluated with the CDE-based equilibrium model, the non-equilibrium TRM model, and by the CTRW methods
Summary
Transport through a porous medium leads the contaminant to spread into the other fluid medium. This consequence has been well modeled for homogeneous porous media at a large scale, such as 1250 cm (Xioang et al, 2006), only a handful of studies have attempted to understand the solute transport mechanism at a very small-scale. It is necessary to accurately characterize the early time behavior, such as solute introducing time for contaminants to escape from subsurface waste materials for groundwater remediation problems. Low-permeability environments are mostly associated with fine-grained sedimentary deposits such as shales and clays (Neuzil, 1994). There are very few solute transport modeling studies exclusively focusing on transport in low permeability porous media such as heavy silty clay and, to our knowl- The term “low permeability” is applied to media with hydraulic conductivity to water of approximately 10-6 cm/s or smaller (Liu et al, 2013).
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