Effects of different environmental factors on solute transport in the capillary fringe

The capillary fringe (CF), a complex water–gas–solid interaction system, serves as a crucial pathway for solutes before they migrate below the water table. Sandbox experiments and numerical simulations have been employed to assess how tracer injection rates, boundary water levels, and evaporation intensities affect solute transport within the CF. The results showed that when the tracer injection rate was low, the tracer migrated laterally within the CF; however, as the injection rate increased, the tracer’s migration velocity increased, and the contaminant plume expanded, eventually to below the water table. Furthermore, as the hydraulic head difference increased, the ratio of the lateral hydraulic gradient to the vertical hydraulic gradient became larger, resulting in the lateral migration of the tracer within the CF. Conversely, as the hydraulic head difference decreased, the CF became less of a barrier to vertical migration, allowing the tracer to migrate below the water table. Additionally, evaporation was found to significantly influence solute transport by reducing moisture content and unsaturated hydraulic conductivity, which in turn slowed lateral seepage and altered capillary rise dynamics, impacting the downward movement of the tracer. This research highlights the complex factors influencing solute transport in the CF, providing critical insights for accurate water resource evaluation in unsaturated and saturated zones.