Abstract
In this study, four homogeneous porous media (HPM1-HPM4), consisting of distinct proportions of sand-sized and clay-sized solid beads, were prepared and used as single fracture infills. Flow and nonreactive solute transport experiments in HPM1-HPM4 under three flow rates were conducted, and the measured breakthrough curves (BTCs) were quantified using conventional advection-dispersion equation (ADE), mobile-immobile model (MIM), and continuous time random walk (CTRW) model with truncated power law transition time distribution. The measured BTCs showed stronger non-Fickian behaviour in HPM2-HPM4 (which had clay) than in HPM1 (which had no clay), implying that clay enhanced the non-Fickian transport. As the fraction of clay increased, the global error of ADE fits also increased, affirming the inefficiency of ADE in capturing the clay-induced non-Fickian behaviour. MIM and CTRW performed better in capturing the non-Fickian behaviour. Nonetheless, CTRW’s performance was robust. 12.5% and 25% of clay in HPM2 and HPM3, respectively, decreased the flowing fluid region and increased the solute exchange rate between the flowing and stagnant fluid regions in MIM. For CTRW, the power law exponent ( β CTRW ) values were 1.96, 1.75, and 1.63 in HPM1-HPM3, respectively, implying enhanced non-Fickian behaviour. However, for HPM4, whose clay fraction was 50%, the β CTRW value was 1.87, implying a deviation in the trend of non-Fickian enhancement with increasing clay fraction. This deviation indicated that non-Fickian behaviour enhancement depended on the fraction of clay present. Moreover, increasing flow rate enhanced the non-Fickian transport based on β CTRW .
Highlights
Filled fractures are ubiquitous in the subsurface
The measured breakthrough curves (BTCs) were analysed to explore the effect of clay on the nature of arrival and elution portions of the measured BTCs
The distinct shaped BTCs, indicating a varying degree of non-Fickian characteristics, were due to microscale heterogeneity induced by the increasing fraction of clay
Summary
Filled fractures are ubiquitous in the subsurface. Despite the narrow nature of such fractures compared to the size of the host rock, the fracture infills, most often sediments, are regarded as porous media through which fluid flow and solute transport occur. The sediment infills are in different forms ranging from purely sand to complex compositions of sand, silt, and clay. Previous studies have shown that sediments that fill the void spaces in single fractures have significant implications for flow and transport [1, 2]. The role of clay, as a proportion of homogeneous mixed sediment infill, on the transport of solutes in single fractures is poorly understood. Are extensive researches about the significance of clay on dissolved contaminant transport in natural and engineered systems
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