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Lateral Water Diffusion in an Artificial Macroporous System

Modeling and Experimental Evidence

P. Castiglione^*,a, B. P. Mohanty^,a, P. J. Shouse^a, J. Simunek^a, M. Th. van Genuchten^a and A. Santini^b

^a George E. Brown, Jr. Salinity Laboratory, 450 W. Big Springs Road, USDA, ARS, Riverside, CA
^b Department of Agricultural Engineering, University of Naples "Federico II", Italy
^* Corresponding author (paoloc{at}uidaho.edu)

Received 14 June 2002.

In two-domain schematizations of macroporous soils or fractured rock systems, lateral mass exchange between macropores and the soil matrix is generally modeled as an apparent first-order process. With respect to lateral diffusion, the system is thus characterized by a single parameter, the transfer rate coefficient, which is difficult to estimate a priori. We conducted water infiltration experiments in a laboratory column with an artificial macropore. The novel design of the experimental setup allowed us to discriminate between matrix flow and macropore flow, from which we could estimate the water exchange flux between the two domains. Most of the parameters in a dual-permeability model could be determined independently of the experimental data. In particular, a theoretical expression for the transfer rate coefficient was derived by assuming lateral water and solute diffusion to be similar processes. Numerical analysis of the water exchange process revealed that the transfer coefficient depended also on the macropore conductivity. When this dependency was taken into account, the model reproduced the experimental data reasonably well.

Abbreviations: BTC, breakthrough curve • DPM, dual-permeability model • FOA, first-order approximation • MIM, mobile–immobile model • PAM, polyacrylamide

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