5 research outputs found
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Tracer experiments in a non-uniform porous medium : implications of diffusive mass transfer on the late-time breakthrough behavior
Mass transfer processes were evaluated in an artificial, non-uniform porous medium with a power law distribution of diffusion rate coefficients obtained from physical characterization of solute and porous medium. Breakthrough curves of several pulse experiments conducted at different velocities were compared (1) to analytical expressions for concentrations at mass transfer dominated times, and (2) to simulated breakthrough curves of a 1-D transport model. After correcting diffusion rate coefficients for irregularly shaped particles both analytical and numerical model were able to accurately represent breakthrough curves and latetime concentrations. The main conclusions of this study are the following. First, by connecting normalized breakthrough curves of experiments conducted at different velocities it is possible to obtain the derivative of the residence time distribution of the immobile domain, which completely describes the mass transfer behavior of the column. From the derivative of the residence time distribution, the scaling behavior of mass transfer dominated breakthrough curves can be predicted. Second, in presence of multiple rates of mass transfer the use of a single rate model may lead to velocity-dependent mass transfer rate coefficients. Third, a power law distribution of diffusion rate coefficients is directly related to the late-time slope of breakthrough curves, and, if pore diffusivity is constant for all particles of the porous medium, also to the distribution of grain sizes
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What controls the apparent timescale of solute mass transfer in aquifers and soils? A comparison of experimental results
Estimates of mass transfer timescales from 316 solute transport experiments reported in 35 publications are compared to the pore-water velocities and residence times, as well as the experimental durations. New tracer experiments were also conducted in columns of different lengths so that the velocity and the advective residence time could be varied independently. In both the experiments reported in the literature and the new experiments, the estimated mass transfer timescale (inverse of the mass-transfer rate coefficient) is better correlated to residence time and the experimental duration than to velocity. Of the measures considered, the experimental duration multiplied by 1 + β (where β is the capacity coefficient, defined as the ratio of masses in the immobile and mobile domains at equilibrium) best predicted the estimated mass transfer timescale. This relation is consistent with other work showing that aquifer and soil material commonly produce multiple timescales of mass transfer.Keywords: Groundwater transport, Groundwater quality, Groundwater hydrolog