Centrifuge modelling of solute transport during partially saturated flow

The concept of “mobile” and “immobile” fluid regions (as incorporated in the Two-Region Model, TRM) is often used in the governing equation to model tailing in solute breakthrough curves (BTC). In this paper, we apply the TRM and its parent equation, the Advection–Dispersion Equation (ADE) to non-equilibrium solute transport in a partially saturated, non-sorbing porous medium during centrifuge modelling. The centrifuge is used to accelerate the processes of non-equilibrium transport and create models, each of which is a scaled replica of the prototype. We discuss possible sources of “immobile” water—such as dead-end pores, boundary layers, and velocity variations—including how these behave during centrifuge modelling and possible effects on the behaviour of the TRM and ADE parameters. Modelling criteria are developed for the TRM parameters using the governing equations. The criteria state that for an N-fold increase in pore velocity V, dispersion D, increases proportionally to N, while the mobile water fraction, β, is constant. The mass transfer rate parameter (describing inter-region transfer between “mobile” and “immobile” zones), α, is proportional to 1/N if it is affected by local advection, but scales to 1 if it is diffusion-controlled. Two types of experiments are reported. One type involved a constant acceleration level, N=100, while in the other the flow rate, Q, was kept constant. For all experiments, the relationship between V and D was linear, irrespective of N. Both α and β are functions of the moisture content and the magnitude of the pore water velocity. The modelling criteria developed and the behaviour of the TRM parameters indicate that centrifuge modelling is a useful tool for investigating solute transport in partially saturated porous media.