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Velocity‐dependent hydrodynamic dispersion during unsteady, unsaturated soil water flow: Experiments
Author(s) -
Bond W. J.
Publication year - 1986
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/wr022i013p01881
Subject(s) - dispersion (optics) , flux (metallurgy) , mechanics , soil water , absorption (acoustics) , flow (mathematics) , flow velocity , attenuation coefficient , constant (computer programming) , magnitude (astronomy) , retardation factor , soil science , materials science , thermodynamics , chemistry , environmental science , physics , optics , chromatography , column chromatography , astronomy , computer science , metallurgy , composite material , programming language
The effect of the velocity dependence of the hydrodynamic dispersion coefficient on solute movement and distribution during unsteady flow of water in unsaturated soils is described mathematically and experimentally. Approximate analytical solutions of the solute transport equation are presented for velocity‐dependent dispersion during constant potential and constant flux horizontal absorption, subject to a step change in solute concentration and a pulse input of solute, respectively. Discussion is confined to the case where there is no chemical interaction between the solute and the soil and no soil water that is inaccessible to the solute. Experiments were designed to demonstrate velocity‐dependent dispersion and were carried out with a fine sand for the conditions outlined above. The effect of the velocity dependence of the dispersion coefficient was to cause the spread in the concentration profile to be greater than would have been expected if there was no velocity dependence. This effect is large at small times and decreases with increasing time for constant potential absorption. During constant flux absorption the magnitude of the effect depends largely on the magnitude of the input flux, increasing as the flux increases, but it also increases slightly with increasing time. The relationship between the dispersion coefficient and pore water velocity was measured for this fine sand by means of steady, saturated breakthrough experiments. This independently measured relationship was then used in the approximate analytical solutions to predict the observed concentration profiles from the unsteady experiments. Very good agreement was found.