Contaminant transport in the vadose zone is influenced by a complex combination of gravity, capillary, and viscous forces. Heterogeneity is known to influence multiphase displacement and solute transport processes, yet gaps exist in our understanding of how heterogeneity impacts capillary-driven transport, such as during spontaneous imbibition. The focus of this seminar is to first introduce positron emission tomography as an experimental imaging technique capable of providing a window into complex transport processes at the sub-core scale. Positron emission tomography and X-ray computed tomography, combined with a newly developed method for conducting spontaneous imbibition experiments, are then used to quantify fluid displacement and solute advection during spontaneous imbibition. Experimental results show increasing solute concentrations in low permeability regions during spontaneous imbibition. Using an experimentally parameterized 2D numerical model, we demonstrate how solutes are pulled into low permeability regions as imbibing water travels through the core to the imbibition front. This process displaces solute from high permeability zones while producing elevated solute concentrations in low permeability zones at the imbibition front. These results and other ongoing projects provide new insights into the role that heterogeneity plays in the transport and long-term retention of contaminants in the vadose zone.
Dr. Christopher Zahasky is an assistant professor at the University of Wisconsin-Madison. Prior to coming to the University of Wisconsin-Madison, he was a postdoctoral scholar at Imperial College London and Stanford University. He completed his PhD and MSc degrees in Energy Resources Engineering at Stanford University were his research focused on using positron emission tomography for fluid transport characterization in geologic materials. He completed his Bachelor of Science degree in Geology at the University of Minnesota. His research interests are focused on understanding the fundamental physics and mechanisms of fluid, gas, and solute transport in geologic systems across length and time scales using experimental observations validated and generalized with analytical and numerical models.