HR: 0800h
AN: H51F-0427    [Abstracts]
TI: Influences of Flow Rate on Axial Diffusion and Dispersion: Experiments in a Cylindrical Macropore Column.
AU: * Haws, N W
EM: nhaws@jhu.edu
AF: Johns Hopkins University, 3400 North Charles Street , Baltimore, MD 21218 United States
AU: Paraskewich, M R
EM: witness@jhu.edu
AF: Johns Hopkins University, 3400 North Charles Street , Baltimore, MD 21218 United States
AU: Ball, W P
EM: bball@jhu.edu
AF: Johns Hopkins University, 3400 North Charles Street , Baltimore, MD 21218 United States
AU: Long, W
EM: longwei@jhu.edu
AF: Johns Hopkins University, 3400 North Charles Street , Baltimore, MD 21218 United States
AU: Hilpert, M
EM: markus hilpert@jhu.edu
AF: Johns Hopkins University, 3400 North Charles Street , Baltimore, MD 21218 United States
AB: Spreading of plumes of solute in porous media is often caused not only by hydrodynamic dispersion but also by mass transfer limitations between the region of relatively high permeability media and another domain of very low permeability. The typical assumptions for modeling the macroscale behavior of such systems is to assume that advective transport and hydrodynamic dispersion of solutes occurs only in the high permeability region (mobile domain) and that solute transfer to and movement within the lower permeable region (immobile domain) is only via diffusion. In this study, the influence of pore-water velocity on the apparent diffusion rate coefficient is evaluated using such mobile-immobile assumptions. A series of tracer (3[H]-water) experiments are conducted in a soil column with a central cylindrical zone of highly permeable sand (macropore region) surrounded by an annulus of relatively impermeable silty-clay loam. The experiments are conducted for pore-water velocities that span two orders of magnitude and for both "long" and "short" solute input pulse lengths. The breakthrough data are interpreted using a dual-porosity Fickian diffusion model, from which an effective diffusion rate coefficient (Dp) is calibrated. The Dp values were relatively insensitive to injection pulse length; however, Dp values exhibited an inverse correlation to the velocity. Our hypothesis is that at lower velocities, the solute concentration within the macropore region becomes more nonuniform, due to slower rates of axial dispersion and this effectively causes an unaccounted slowing of the diffusive flux (i.e. radial dispersion is no longer orders of magnitude greater than diffusion). In addition, there is also reason to believe that the flow field itself may become less uniform as velocity decreases. This suggests a limit to the applicability of the common mobile-immobile macroscale assumption. Pore-scale modeling using a Lattice-Boltzmann technique further explore the observations made in this study.
DE: 1832 Groundwater transport
DE: 1847 Modeling
SC: Hydrology [H]
MN: Fall Meeting 2005