Sources and Transport Mechanisms of Contaminants during Flooding Events

Abstract

Persistent contaminant concentrations in groundwater, especially uranium, present significant challenges, as flooding events can remobilize these contaminants from subsurface hydrogeological compartments. Despite the critical role of these events in contaminant mobilization, the precise sources of residual solid-phase contaminants and their specific transport mechanisms during infiltration remain poorly understood and difficult to quantify. Through a combination of microscale characterization, controlled field-tracer infiltration experiments, and mathematical modeling, this dissertation addresses the fundamental scientific question: What are the mass transport mechanisms that control the fate of contaminants during recharge of contaminant-poor groundwater or surface water to contaminant-rich groundwater? The first chapter identified trace-level solid-phase uranium associations in sediments using fission-track radiography, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. This work revealed that residual solid-phase uranium in source zone aquifer sediments primarily co-occurred with amorphous aluminum-rich and iron-rich coatings, suggesting specific geochemical controls on its persistence. The subsequent chapter elucidated mass and transport mechanisms of contaminants released during flooding through controlled in-situ tracer experiments. This research highlighted the mobilization of contaminant-rich pore water ahead of floodwater and the dissolution of contaminant-rich evaporites with floodwater as key mechanisms, emphasizing the significant role of the vadose zone as a storage and release compartment for contaminants. The final chapter employed a one-dimensional mathematical model to inversely simulate the field data from the infiltration experiments, quantifying the relative contributions of the identified mobilization mechanisms. The modeling results confirmed that advection and dispersion of contaminant-rich pore water, followed by the dissolution of contaminant-rich evaporites (often coupled with desorption), were the dominant transport pathways from the vadose zone. This chapter further solidified the understanding that the vadose zone is the primary source of contaminant release during infiltration events, while transport in the saturated zone is predominantly advection- and dispersion-dominated. Overall, this dissertation significantly deepened our understanding of the complex behavior of contaminants in the vadose zone during flooding events.