Impact of structural heterogeneity on solute transport and mixing in unsaturated porous media: An experimental study

Solute transport in unsaturated porous media plays a crucial role in environmental processes affecting soils, the unsaturated zone, and aquifers lying below. These processes include nutrient and pesticide leaching in soils, contaminant migration to aquifers and degradation in the vadose zone, and nutrient exchange at the soil-river interface, to name a few. Natural porous media are characterized by structural heterogeneity in the pore sizes disorder and their spatial arrangements. The impact of pore size heterogeneity on the spreading and mixing of a solute plume, and the resulting reaction rates, are not well understood for unsaturated flow. In addition, these processes can be affected by incomplete mixing at the pore scale. Thus, direct pore-scale experimental measurements are needed to gain a comprehensive understanding of the mixing state of the system. Our goals are to 1) study the impact of structural heterogeneity on fluid phase distributions and 2) establish how the arrangement of fluid phases impacts solute spreading and mixing. We use micromodel experiments with two-dimensional porous media. The samples are created by placing an array of circular posts in a Hele-Shaw-type flow cell. We vary the heterogeneity by controlling the circular posts’ diameters disorder and correlation length of their spatial distribution. In the first stage of each experiment, we simultaneously inject liquid and air to establish an unsaturated flow pattern with a connected liquid phase cluster. Then, we introduce a conservative fluorescent solute pulse with the moving liquid phase. We track the solute concentration and gradients’ evolution by taking periodic images of the flow cell and analyzing their fluorescence intensity. In addition to unsaturated flow experiments, our system allows us to study the impact of pore size disorder and correlation on solute mixing in saturated porous media and even directly quantifying fast reaction products’ concentrations. Initial results confirm previous findings on the impact of desaturation on enhanced mixing rates for a single porous medium geometry. In addition, our use of a continuous solute pulse highlights regions that maintain a high mixing rate at the interface between mobile and stagnant liquid phase parts. Ongoing experiments explore the impact of increasing pore size disorder and correlation length on fluid phase distributions and mixing rates.