Cosolvent-water displacement in one-dimensional soil column

A one-dimensional flow and transport model with dynamic fluid density and v iscosity terms is proposed for modeling cosolvent flushing in a water-satur ated porous medium. Given knowledge of the density and viscosity functions for cosolvent-water mixtures, the model is controlled by two flow parameter s (specific storage and permeability) and one transport parameter (dispersi vity). Sensitivity analysis demonstrates that the model solution is relativ ely insensitive to the flow parameters for the imposed constant head, const ant flow conditions. The dynamic density and viscosity model is tested agai nst the conventional transport model in simulating breakthrough data from s oil column experiments in which water is displaced by pure methanol pulses (or slugs). Methanol slug breakthrough behavior is first predicted using in dependent parameter estimates (dispersivity obtained from tracer tests), th en the dispersivity was adjusted to obtain optimal fits. The dynamic model provided slightly better predictions than the conventional transport model but failed to accurately reproduce methanol breakthrough behavior. Irregula rities in observed slug breakthrough curves suggest that frontal instabilit ies may have been the cause of the discrepancy between the model and observ ations. Cosolvent overriding may have also contributed to the discrepancy i n the horizontal displacement case.