CONTAMINANT TRANSPORT FROM CONTAMINATED GROUNDWATER TO THE UPPER VADOSE ZONE

Contaminants in the solid and aqueous phases can persist for extended periods, with contamination potentially accelerating during floods, even after the original contamination source is removed. Characterizing the mechanisms responsible for the transport is particularly challenging in multi-layer hydrogeological systems, which include the aquifer, capillary fringe, and unsaturated zone. This study investigates the initial flow and transport conditions in a multi-compartment system that lead to increased groundwater contamination post-flood. It is hypothesized that contaminants from the groundwater migrate to the vadose zone via upward infiltration before the flood, eventually contaminating the vadose zone. To verify this hypothesis, a 1-D model of a hydrogeological system was constructed, accompanied by visualizations. A two-layer system was used, where the saturated zone and capillary fringe were grouped together, having a moisture content value of 1. Three stress periods were addressed in the model: the first involved no flow conditions, the second involved steady-state flow with no contaminant transport, and the third involved steady-state flow with transport. Parameters such as evaporation flux, contaminant mass, and hydraulic conductivity required for running the model were obtained from an experimental river flooding event. The results from the model indicated that evaporation flux and upward infiltration simultaneously impact migration from the contaminated groundwater to the vadose zone. This process leads to the contamination of the vadose zone, which eventually flushes out during floods and further contaminates groundwater. This study enhances the understanding of pre-flood contaminant mobilization mechanisms and identifies the major compartments responsible for transport.