Superiority verification of the non-local theory in predicting solute transport behaviour in natural porous media: NaCl tracer experiments in the silica sand and zeolite columns

Here, we investigate transport behaviour through two types of media, silica sand and zeolite, using tracer column experiments and numerical methods. Tracer experiments with sodium chloride (NaCl) were conducted in saturated packed columns with embedded current and potential electrodes to measure co-located bulk electrical conductivity (${\sigma }_b$) and fluid electrical conductivity (${\sigma }_f$) to characterise dual-domain mass transfer (DDMT). Unexpectedly, the silica sand experiments show a hysteretic relationship between co-located ${\sigma }_f$ and ${\sigma }_b$. Field Emission Scanning Electron Microscopy (FESEM) analysis results showed that the observed hysteresis could be due to the presence of aggregated particles. NaCl tracer experiments in the zeolite column confirmed that the intragranular porosity serves as an immobile domain to store solute, resulting in heavy-tailed breakthrough curves (BTCs) and hysteresis between co-located ${\sigma }_b$ and ${\sigma }_f$. The root mean square error (RMSE) between the experimental and simulated ${\sigma }_f$ revealed that the single-rate dual-domain mass transfer (DDMT) model captures the NaCl BTCs much better than the advection-dispersion equation (ADE). Based on a Monte Carlo analysis, the obtained single-rate DDMT parameters were velocity-dependent in such a way that the estimated first-order mass transfer rate coefficient ($\alpha$) increased with an increase in flow rate. The findings of this research confirm the superiority of the non-Fickian theory over the classical model in understanding solute transport in natural porous media. The results also illustrate that in addition to the physical and chemical characteristics of the soil, which leads to the non-Fickian transport, the flow velocity plays an important role in physical solute transport parameters.