AXISYMMETRICAL TRANSPORT OF WATER AND SOLUTE UNDERNEATH A DISK PERMEAMETER - EXPERIMENTS AND NUMERICAL-MODEL

No analytical solution exists for two-dimensional, axisymmetric flow of both water and solute underneath a disk permeameter. We developed a finite-difference numerical scheme for such flows. Laboratory experiments were also conducted using a box containing repacked sand. A 1/4-sector disk permeameter was located on the surface in one corner. The disk, first containing pure water, was placed on the soil for 200 s. It was then removed, refilled with a KBr solution, and replaced at 225 s. Finally after 12.5 min the disk was removed and soil samples extracted along three radial transects under the disk. In two other experiments, the disk, containing only pure water, was left on the soil surface and soil samples removed at the end i.e., 6 and 14 min. A tensiometer inserted through one face of the box and located just 20 nun under the disk recorded the changing soil water pressure head with time, h(t). Good predictions of both the water content and Br- profiles were achieved with the numerical model, and good renditions of h(t) and the transient flow rate from the disk, q(t). Our results reinforce the need for caution when determining the soil's sorptivity from observations of q vs. t1/2. Care is required in deciding when q has indeed become steady. Numerical models, such as this one might serve as parameter-identification tools when using a tracer-filled disk permeameter to infer the chemical transport properties of soil.