ESTIMATING APPARENT DIFFUSION COEFFICIENT AND TORTUOSITY IN PACKED SAND COLUMNS BY TRACERS EXPERIMENTS

In this work we analyze the spatial and temporal features of the transport parameters characterizing the apparent diffusion coefficient and tortuosity along an experimental setup. To perform this study, we resort to the BEETI device, consisting of a vertical column filled with porous materials and of a dichromatic X-ray spectrometer for detecting solute distribution. X-ray measures are noninvasive and nondestructive, and thus allow determining the solute concentration profiles at several locations along the column, as a function of time. Once the concentration profiles are experimentally measured, the transport parameters v(x) and D(x) are determined at each location by means of the method of temporal moments. Two sets of experimental runs are performed (at high and low flow rates, respectively), in order to single out the effects of mechanical dispersion and molecular diffusion on the overall solute dispersion. Further manipulations allow finally deriving the spatial evolution of the apparent molecular diffusion coefficient and of the tortuosity. The estimated tortuosity values are compared with literature results and show good agreement. This analysis relies on the hypothesis that the traversed porous medium is sufficiently homogeneous and can be used as a benchmark in view of testing our experimental device on more complex soils.