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Effects of Column Conditions on the First-order Rate Modeling of Nonequilibrium Solute Breakthrough: Cylindrical Macropores Versus Spherical Media

D. Young
W. Ball
Journal / Anthology

Water Resources Research
Year: 1997
Volume: 33
Issue: 5
Page range: 1149-1156

When first-order mass transfer models are used to interpret diffusion-based nonequilibrium transport through soil columns, significant misinterpretations may result if experimental impacts on the model fits are not well understood. For the specific case of slow diffusion in spherical sorbent particles, we have previously demonstrated the manner in which estimated rate constants vary with a dimensionless “exposure time” of solute to sorbent. In the current work we extend our quantitative analysis of these concerns to the case of cylindrical macropore flow, where the geometry of the diffusion matrix is that of an angular region surrounding a central flow-permeable cylinder. Our results show that similar to the spherical sorbent case, variations in the first-order coefficient fitted to macropore column data are not necessarily related to variation in underlying diffusion rates and that the variations are similarly dependent upon the column run conditions. However, for the macropore case the proposed nondimensional timescale is even more successful at characterizing the variation in the fitted first-order mass transfer parameter. On the basis of these results we are able to further generalize our conclusions with regard to the effects of experimental conditions such as solute retardation factor (sorption affinity) and solute input conditions (pulse duration). For short exposure times to solute (relative to the characteristic time for diffusion) the ratio of the fitted first-order rate coefficient manner (K ` r -0.4) irrespective of geometry. Only at long exposure time does the fitted K approach a value that we can independently derive on the basis of equivalent second moments.

*Science > Geology and Geophysics