MASS-TRANSFER IN SOILS WITH LOCAL STRATIFICATION OF HYDRAULIC CONDUCTIVITY

The two-region model was developed originally to describe nonsorbing c hemical transport in soils with dead-end pores based on the concept of mobile and immobile regions in the soil. It has been shown that the m odel can simulate solute transport in soils with local stratification, or inhomogeneity, of hydraulic conductivity. However, the physical ba sis of the model becomes questionable, since the mobile-immobile regio n concept does not apply in stratified soils. In both soil types the n onequilibrium effect is caused by an apparent mass transfer process wi thin the soil, as distinct from advection and diffusion. Where there a re immobile regions, the mass transfer is due to solute interregion di ffusion alone. In stratified soils the nonequilibrium mass transfer pr ocess is affected also by local flow variations. A conceptual model, n umerical simulations, and laboratory experiments are presented to anal yze these effects. For a given soil with fixed local stratification of hydraulic conductivity, it is shown that in the low-velocity range, t he apparent mass transfer rate parameter, alpha, scales as V2/D (V is pore water velocity in the two-region model and D is the longitudinal dispersion coefficient), which implies that the mass transfer process is predominantly affected by local flow variations. When the velocity is relatively high, alpha is-proportional-to D(T)/h2 (D(T) is the inte rregion diffusion coefficient and h is the characteristic thickness of the stratified layers) and the mass transfer process is dominated by interregion diffusion. These scaling relations for alpha reflect the t wo mechanisms controlling the mass transfer process in locally stratif ied soils. They have implications for scaling of time-dependent mass t ransfer from laboratory models to prototype soils. In particular, the relationship alpha is-proportional-to V2/D leads to the conclusion tha t exact physical modeling of nonsorbing chemical transport coupled wit h apparent mass transfer in locally stratified soils may be viable.