Nonuniqueness of hydrodynamic dispersion revealed using fast 4D synchrotron x-ray imaging

Experimental and field studies reported a significant discrepancy between the cleanup and contamination time scales, while its cause is not yet addressed. Using high-resolution fast synchrotron x-ray computed tomography, we characterized the solute transport in a fully saturated sand packing for both contamination and cleanup processes at similar hydrodynamic conditions. The discrepancy in the time scales has been demonstrated by the nonuniqueness of hydrodynamic dispersion coefficient versus injection rate (Peclet number). Observations show that in the mixed advection-diffusion regime, the hydrodynamic dispersion coefficient of cleanup is significantly larger than that of the contamination process. This nonuniqueness has been attributed to the concentration-dependent diffusion coefficient during the cocurrent and countercurrent advection and diffusion, present in contamination and cleanup processes. The new findings enhance our fundamental understanding of transport processes and improve our capability to estimate the transport time scales of chemicals or pollution in geological and engineering systems.

Automated summary

Experimental and field studies have shown a significant difference in time scales between cleanup and contamination processes, with the hydrodynamic dispersion coefficient of cleanup being significantly larger than that of contamination. This nonuniqueness in dispersion coefficients is attributed to concentration-dependent diffusion during advection and diffusion, affecting both contamination and cleanup processes. The findings enhance our understanding of transport processes and improve the estimation of chemical transport time scales in geological and engineering systems.