Vista/Abrir - RiuNet

Abstract
Solute dispersion processes in porous media remain as a subject of research due to both
the limitations of available mathematical approaches, and to the need of properly
reproducing the real patterns of the medium heterogeneity. Yet, building models able to
accurately predict transport processes in real formations require dealing with dispersion
processes. Assuming that the classical Advection-Dispersion Equation (ADE) is valid at
some fine scale referred to as Representative Elementary Volume, two main and
important research lines are identified. The first focused on the average ad effective
dispersivity parameter that allows reproducing a solute plume development using the
ADE. As a plume evolves, it flows through different heterogeneities, each of them with
its own effect on the plume. These make the dispersion parameter value to increase until
reaching an asymptotic value. But, different studies show that dispersivity does not
reach an asymptotic value possibly due to the combined effect of different scales of
hydraulic conductivity (K) heterogeneity and to the non-Fickian behavior of transport.
A second line assumes the importance of properly modeling K at different
scales, and that non-Fickian features can be approached by modeling heterogeneity
occurring at very fine scale. Then, some mathematical approaches, such as the multirate
transport formulation, allow incorporating the effects of this fine scale heterogeneity in
a coarse scale, using effective values of dispersivity. Besides, the spatial variability os
this parameter is not addressed in this approaches; only K variability is accounted for.
Both points of view focus on a unique parameter in order to explain the
deviation of real plume behavior from ADE predictions. This parameter is the
heterogeneity of hydraulic conductivity, K. No attention is given to the heterogeneity of
dispersivity distribution. The difficulty to estimate this parameter, and its scale
dependence, has limited the number and scope of reseachs. Though is usual to estimate
an average value for dispersitivy, these methods are not able to predict the details of
local heterogeneties on the plume shape or velocity.
This research is based on a solute transport intermediate scale experiment in
which local and instantaneous values of dispersivity are estimated, using a laboratory
tank. The available means for data monitoring, acquisition and process avoid to gather
exhaustive information of the evolution of a tracer plume evolution experiment. Current
researchs have focused on estimating the dispersivity value using the spatial moments
method or breakthrough curve analysis, mostly to verify different stochastic theories. In
this research, we have built an artificial porous media, within a quasi-2D tank, with
patterns of heterogeneity based on K data from a real non-Gaussian formation (MADE-
2 site, Columbus Air Force base, Mississippi, USA). By controlling the head gradient
between both ends of the tank (1,2 meters long, 0,40 high and 0,05 m deep) steady state
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