Evidence of Compound-Dependent Hydrodynamic and Mechanical Transverse Dispersion by Multitracer Laboratory Experiments

Abstract

Mass transfer, mixing, and therefore reaction rates during transport of solutes in porous media strongly depend on dispersion and diffusion. In particular, transverse mixing is a significant mechanism controlling natural attenuation of contaminant plumes in groundwater. The aim of the present study is to gain a deeper understanding of vertical transverse dispersive mixing of reaction partners in saturated porous media. Multitracer laboratory experiments in a quasi two-dimensional tank filled with glass beads were conducted and transverse dispersion coefficients were determined from high-resolution vertical concentration profiles. We investigated the behavior of conservative tracers (i.e., fluorescein, dissolved oxygen, and bromide), with different aqueous diffusion coefficients, in a range of grain-related Peclet numbers between 1 and 562. The experimental results do not agree with the classical linear parametric model of hydrodynamic dispersion, in which the transverse component is approximated as the sum of pore diffusion and a compound-independent mechanical dispersion term. The outcome of the multitracer experiments clearly indicates a nonlinear relation between the dispersion coefficient and the average linear velocity. More importantly, we show that transverse mechanical dispersion depends on the diffusion coefficient of the compound, at least at the experimental bench-scale. This result has to be considered in reactive-transport models, because the typical assumption that two reactants with different aqueous diffusive properties are characterized by the same dispersive behavior does not hold anymore.

Helical Flow and Transient Solute Dilution in Porous Media

“…Finally, since transverse dispersion plays a key role to enhance dilution in helical flow fields, a more sophisticated and detailed description of the transverse dispersion coefficient (Chiogna et al 2010;Scheven 2013) may be needed for practical purposes. Indeed, the characteristic timescale of diffusive processes is, in general, large in comparison with the advective timescale, such that mixing is typically incomplete in pore channels Tartakovsky et al 2009;deAnna et al 2014) and Scheidegger's (1961) dispersivity cannot be applied in the dispersion tensor.…”

Stochastic evaluation of mixing-controlled steady-state plume lengths in two-dimensional heterogeneous domains

“…The effect is the largest in the case where D t is assumed to depend linearly on velocity and the transverse dispersivity α t is assumed to scale with ffiffiffi ffi K p (parameterization II). The effect is the smallest when a uniform and K-independent value of α t is assumed (parameterization III, (Scheidegger, 1961)), whereas the nonlinear parameterization of Chiogna et al (2010) leads to an intermediate enhancement of transverse mixing. The different mean mixing-enhancement factors directly lead to different mean plume lengths, whereas the coefficients of variation are hardly affected and have been found to be about unity in all our test cases.…”

“…Finally, the choice of the local-scale dispersion model can affect the plume length prediction. Recent laboratory bench-scale experiments (Chiogna et al, 2010) have shown that a compound-specific, nonlinear parameterization of local transverse dispersion agrees better with experimental results performed in homogeneous porous media than the traditional model of Scheidegger (1961). Scheidegger's (1961) model assumes a linear dependence of the transverse dispersion coefficient on the flow velocity.…”

“…Scheidegger's (1961) model assumes a linear dependence of the transverse dispersion coefficient on the flow velocity. The model proposed by Chiogna et al (2010) accounts for incomplete mixing in the pore channels of the solid matrix, resulting in a less-than-linear dependence on flow velocity and a non-negligible influence of molecular diffusion on transverse dispersion also at high velocities. Chiogna et al (2011a) have demonstrated by numerical experiments that the compound dependence of mechanical transverse dispersion is indeed relevant for reactive transport at field scales.…”

Observations of 3-D transverse dispersion and dilution in natural consolidated rock by X-ray tomography

“…It is important to keep in mind that transverse dispersion is compound dependent [11,12,40]. In principle, the test is general to any compound of interest, as long as the impact on the local atomic density 520 is sufficient to induce attenuation in the X-ray imagery.…”