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DEVELOPMENT OF A TWO-DOMAIN SIMULATION APPROACH USING X-RAY CAT SCANNING TO MODEL SOLUTE TRANSPORT IN A SOIL WITH PREFERENTIAL FLOW PATHWAYS
(1995-1999/ R
esearch Team: J. Perret, S. Prasher)

 

Despite recent research efforts to determine and model the dynamics of macropore flow, it is still not possible to quantitatively predict water and chemical movement in soil containing macropores.  Several researchers have pointed out the potential offered by multi-region modeling.  However, the criteria used to define boundaries between flow regions have been up to now defined arbitrarily.  Therefore, there is a need to develop a reliable technique to isolate and characterize flow domains in soil.

The primary objective of this study was to develop a reliable method for isolating and characterizing flow domains in a large undisturbed soil column using a CAT scanner.  CAT scanning offers tremendous potential for non-destructive quantification of tracer volume concentration inside soil columns during breakthrough experiments. This approach allows for real-time examination of flow mechanisms through soil macropores at various depths along the length of soil columns.  

 

Several computer programs were written in the PV-WAVE language to quantify the three-dimensional geometry of macropore and characterize the spatial distribution of solute at thirteen depths in the soil column.  With knowledge of the macropore structure and the spatial distribution of the solute, breakthrough in the macropore and matrix flow domains was evaluated.  Very soon after the tracer application, the occurrence of macropore flow was detected.  However, a fraction of the flow in macropores was relatively slow.  This was due to the presence of dead-end branches or cavities which, from a geometrical point of view, belong to the macropore domain, but do not contribute to preferential flow. This raises the question of effective flow in the macropore domain and suggests that the macropore domain should be defined both in terms of the geometry of macropores and of their ability to convey tracer preferentially.

Flow in the matrix domain suggested that part of the matrix contains small pores that are connected to macropore networks.  These pores contribute to a rapid tracer build up in the matrix domain.  The breakthrough curves measured in the matrix domain were fitted using the convection dispersion equation (CDE) with CXTFIT 2.0.  The high regression coefficients obtained suggest that the CDE model describes breakthrough in the matrix domain quite well.  Solute transport in the macropore domain was modeled using the analogy between macropore flow and pipe flow.  The macropore domain was divided into two regions, namely the laminar and turbulent regions. 

 

A modified version of Poiseuille’s law was used to model solute breakthrough in the laminar region.  For the turbulent region, a new formula was derived based on Manning’s equation.  The modifications were done so that these simple models would take into account the distribution density functions of macropore size and hydraulic radius.  This approach provides us with a reliable approximation of the overall breakthrough of solutes in the macropore domain.  (Click here for more info)


© 2000 Johan Perret