Impact of Permeability Heterogeneity on Solute Dispersion in Unsaturated Porous Media

Solute dispersion in unsaturated porous media has critical implications for various natural and engineered systems, such as nutrient and contaminant transport in the vadose zone, the unsaturated region between the ground surface and the water table. However, relatively few studies have been conducted on solute dispersion in unsaturated porous media compared to in saturated porous media. This is partially due to the challenges in studying multiphase flows (i.e., co-flow of immiscible fluids) in porous media. The permeability of a porous medium describes the medium’s capability to allow a fluid to flow through it. The permeability of natural geological formations is usually complex, which dictates the solute transport mechanisms in unsaturated flows. Experimental data concerning the role of complex permeability on solute dispersion in unsaturated flows are extremely rare due to the challenges encountered by conventional experimental methods in constructing a well-controlled permeability field in the laboratory. The objective of the research is to use pore-scale numerical simulation, microfluidics, and three-dimensional (3D) printing technologies to overcome the challenges encountered by conventional experimental methods in order to advance the understanding of the impact of complex permeability on solute dispersion in unsaturated porous media. The educational objective is to increase public scientific literacy, engage women and minority students in STEM, enrich undergraduate and graduate curricula, and develop partnerships with the industry and small businesses. A fundamental knowledge gap exists in how the dynamic interaction between fluid co-flow and complex permeability regulates solute dispersion in unsaturated flows. The project aims to close the knowledge gap using pore-scale numerical simulation and state-of-the-art 3D printing. The research tasks are: 1) develop a pore-scale numerical model to directly simulate two-phase flow and solute dispersion in porous media to unravel the impact of water saturation, Peclet number, and capillary number on the dispersivity and develop an empirical model for dispersivity predictions, 2) develop an advanced observation method for in situ solute concentration measurements and conduct two-fluid co-injection and solute dispersion experiments in a microfluidic device to validate the pore-scale simulation results, and 3) use 3D printing to construct a known and well-controlled heterogeneous permeability field at the continuum (meter) scale to study its role on solute dispersion in unsaturated flows. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.