Advancing the knowledge of solute transport in the presence of bound water in mixed porous media based on low-field nuclear magnetic resonance

Abstract

Bound water (BW) has significant effects on the physical and chemical properties of soils and other geologic formations. However, little is known about its effect on the non-Fickian transport of solute in porous media. This paper investigated the effect of BW on the non-Fickian solute transport characteristics in mixed porous media. Different porous media of quartz sand with variable contents of powdered minerals were prepared. The low-field nuclear magnetic resonance (LF-NMR) was used to quantitatively characterise BW and its spatial distribution. Then solute transport experiments were conducted in different prepared porous media and the measured breakthrough curves (BTCs) were analysed using the advection–dispersion equation (ADE), mobile-immobile (MIM) and continuous time random walk (CTRW) models. The results revealed that all measured BTCs exhibited the characteristics of early arrival and late-time tailing. However, stronger non-Fickian characteristics were observed as BW increased in content, indicating that BW influenced the non-Fickian transport of the solute. The optimised immobile water fraction of the MIM model consistently increased with increasing estimated BW fraction. Moreover, the coefficient of mass transfer rate (α) of MIM decreased as BW increased, meaning that the solute exchange between the mobile and immobile fluids became slower and consequently yielded strongly tailed BTCs. From the CTRW model, the solute transport velocity (vCTRW) increased with increasing BW, leading to a reduction in the breakthrough time of the solute. An increase in BW yielded a decreasing power law exponent (βCTRW) of the CTRW model, implying an enhancement of the non-Fickian transport characteristics. Furthermore, the porous media with BW<16.4% showed moderately non-Fickian solute transport whereas those with BW≥16.4% exhibited highly non-Fickian solute transport.