The Longitudinal Dispersion Coefficient of Soils as Related to the Variability of Local Permeability

Abstract

Solute (NaCl) miscible displacement experiments are performed on long disturbed soil columns to determine the hydrodynamic longitudinal dispersion coefficient and correlate it with the variability of the local permeability. The solute concentration, averaged over several cross-sections along the soil column, is monitored by measuring the electrical resistance between rod electrodes. The measured solute concentration breakthrough curves are fitted simultaneously with the one-region and two-region analytical models of the 1-D advection-dispersion equation to estimate the longitudinal dispersion coefficient, D L , as a function of Peclet number, Pe, for common groundwater flow velocities (2<Pe<50). Macroscopic simulations of miscible displacement in 2-D porous media described by a periodic permeability field with low, moderate and high variability are employed to evaluate the predictability of the one-region and two-region models, and the sensitivity of the dispersion coefficients and flow velocities estimated from soil column displacement tests to the variance of local permeability. When the variability of the local permeability becomes high, the one-region model fails, while the two-region model is capable of reproducing satisfactorily the breakthrough curves, and providing reliable values of dispersion coefficients. The two mean pore velocities estimated by the two-region model represent, on average, a fast and a slow mean velocity of the dispersion front, whereas their difference is a measure of the transient evolution of the width of the equiconcentration dispersion front.

Analysis of nickel concentration profiles around the roots of the hyperaccumulator plant Berkheya coddii using MRI and numerical simulations

“…3 A view of the rhizobox placed inside the MRI machine (a) and the root distribution pattern before the Ni 2+ uptake experiment (b). The broken lines indicate the 15 mm thick slices into which MRI signal was recorded which reflects the grain-size of the glass beads we used (0.4-1.0 mm) and agrees with the typical range of dispersivity in soils (Aggelopoulos and Tsakiroglou 2007). The initial Ni 2+ concentration was 10 mg l −1 .…”

The effect of micro-heterogeneity and capillary number on capillary pressure and relative permeability curves of soils

“…The variability of soil grain sizes is reflected in the spatial variation of the local permeability, which affects the solute dispersion transient pattern during miscible displacement tests (Aggelopoulos and Tsakiroglou, 2007). With the variability of grain sizes increasing, the dispersion front (over which solute concentration varies) widens and the one-region model, based on 1-D advective-dispersive solute transport in homogeneous media, is unable to reproduce the solute concentration breakthrough curves (Aggelopoulos and Tsakiroglou, 2007).…”

Quantifying soil heterogeneity from solute dispersion experiments

“…This work was performed under Global Change and Ecosystems contract number SSPI- CT-2003-004017-STRESOIL (2004-2007 supported by the European Commission.…”

“…In this direction, two-region models, subdividing the pore space into a mobile and an immobile zone have widely been used to interpret the experimental results and estimate all pertinent transport parameters (Gwo et al, 1998;Drazer et al, 1999). Alternatively, the two-region model has been used to reproduce the solute breakthrough curves of mildly heterogeneous porous media by estimating not only the dispersion coefficients, but also the pore velocity at each region, the difference of which is proportional to the thickness of the dispersion front (Aggelopoulos and Tsakiroglou, 2007).…”