Impact of saturation on continuum-scale conductivity and tracer dispersion in heterogeneous porous media

This study investigates the interplay among water saturation, hydraulic conductivity, and mechanical dispersion in heterogeneous porous media at the continuum scale. Mechanical dispersion of dissolved chemical tracers is influenced directly by water velocity variability, which is governed by the porous structure, the distribution of the water phase within it, and its corresponding conductivity field. Previous studies have either examined the relationships between these factors in fully saturated conditions, or in partial saturation but without considering continuum scale heterogeneity of the media. Through numerical simulations, the analysis here demonstrates how variations in saturation affect the hydraulic conductivity field and, consequently, mechanical dispersion. The study reveals that longitudinal spreading of the tracer plume, when scaled for varying transport times and velocities, shows a non-monotonic relationship with saturation, being least pronounced at an intermediate degree of saturation. These insights contribute to a more nuanced understanding of tracer transport in partially saturated, heterogeneous media, with implications for environmental and engineering applications.