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| Titel |
Interpretation of numerical transport tests with single and dual porous medium approaches |
| VerfasserIn |
Fulvia Baratelli, Mauro Giudici, Chiara Vassena |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250035057
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| Zusammenfassung |
| Preferential flow paths (PFP) in alluvial sediments influence the fate of contaminants in
ground water and render the application of the classical Fickian approach doubtful. In order
to improve the comprehension of this remark and its practical effects, numerical
experiments of conservative solute transport were performed on three blocks of
alluvial sediments from the Ticino basin (Northern Italy), described by Zappa et al.
(doi:10.1016/j.jhydrol.2005.10.016). The volume of each block is about 6m3 and the
conductivity field is assigned on cubic voxels with side equal to 2cm. Flow modelling was
conducted with a conservative finite difference scheme and convective transport with particle
tracking (Vassena et al., doi:10.1007/s10040-009-0523-2). The numerical experiments
simulate the evolution of a tracer plume which is instantaneously injected through the
incoming face and travels along a longitudinal average flow bounded by lateral
no flow boundaries. The interpretation of the results is conducted by fitting the
cumulative breakthrough curve (BTC) obtained from the analytical solution of the 1D
convective-dispersive equation to the numerical test data, through the calibration of the
Darcy’s velocity and the dispersion coefficient. Results for the classical single porous
medium (SPM) case show some discrepancies due to the presence of PFP, which could be
taken into account if a dual porous medium (DPM) is considered. In particular the domain is
modelled as the superposition of two porous media, characterised by high and low
conductivity. Most of the DPM models developed earlier neglect the flow rate in the low
K porous medium and include a possibly linear mixing between the two media.
Here the flow in the low K porous medium is taken into account, but mixing is
neglected.
The fit between cumulative BTC obtained from numerical test data and analytical
solutions was conducted with the Levenberg-Marquardt algorithm, supported by the analysis
of the uniqueness of the inverse problem, which is very critical for the DPM approach, since
five parameters have to be calibrated (the Darcy’s velocities and the dispersion
coefficients for both media and the fraction of solute mass travelling in the high K
medium).
The DPM model permits to improve the fit of the cumulative BTC but it requires the
calibration of several parameters. The tests clearly show that the improvement of the results is
particularly important for those blocks where PFP are more evident. Furthermore the
DPM model often reduces the differences between the temporal moments of the
BTC computed from the analytical solution and from the numerical experiment
data. |
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