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| Titel |
The characterization of a point bar-channel aquifer analogue: from geostatistical simulation to solute transport through hydrofacies connectivity |
| VerfasserIn |
Diana dell'Arciprete, Fulvia Baratelli, Riccardo Bersezio, Fabrizio Felletti, 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 |
250035045
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| Zusammenfassung |
| Ground water flow and solute transport are controlled by the geological structure and the
corresponding heterogeneity and anisotropy of the hydraulic conductivity (K) field. In
alluvial aquifers, a complete interdisciplinary characterization of the reservoir is important
for reliable predictions. The reconstruction of the subsurface heterogeneity cannot be limited
to honor point (e.g., well stratigraphic logs) data, but should also account for the presence of
connected high K hydrofacies, which might form preferential flow paths. To explore these
concepts an aquifer analogue, at the scale of the point-bar/channel depositional
element of a meandering river, was studied. The analogue, exposed in a gravel
pit, belongs to the historical sediments of the terraced meandering valley of the
Lambro River (Po plain, Northern Italy). The study has been conducted in five
steps.
(1) Architectural and sedimentological modelling was based on 31 stratigraphic logs
collected along five quarry faces (four in E-W direction and one in N-S direction)
and a geophysical survey, whereas the hydrostratigraphical characterization was
obtained by permeability analysis of 28 samples. Facies mapping was performed in the
field and supported by the analysis of the photo-composition of the quarry faces to
obtain the geometry, the hierarchy and the internal architecture of sedimentary
bodies. Permeability measurements on undisturbed samples and estimates based on
the grain-size distribution were compared with bibliography values and used to
merge the facies into four hydrofacies: least permeable (very fine sand and silt-clay
respectively from topmost channel-fill, silt/clay plugs, drapes and balls), low permeable
(sand from point-bar and channel fill bedforms), medium permeable (sandy gravel e
gravelly sand from point bars) and most permeable (lower part of lateral accreted
units).
(2) For a test volume of 11.4m à 11.4m à 2.85m 50 equiprobable simulations of the
hydrofacies distribution have been obtained with SISIM (Sequential indicator simulation) and
MPS (Multiple point simulation) on a grid of voxels of 20cm à 20cm à 5cm.
Conditioning data have been extracted from the hydrofacies maps of two crossing quarry
faces.
(3) The connectivity of the four simulated hydrofacies has been quantified with total and
intrinsic indicators: the former measures the degree of connection within the entire
volume, whereas the latter measures the degree of connection of a facies within
itself and is therefore less dependent on the proportion of the facies in the total
volume.
(4) Finite-difference modeling of groundwater flow has been applied to compute the
equivalent hydraulic-conductivity tensor.
(5) Numerical experiments of convective transport of non-reactive solutes have been
performed, in order to map the preferential flow paths and to compute the dispersion
tensor with a Lagrangian approach and the longitudinal dispersion with an Eulerian
approach.
The results show that a multidisciplinary approach permits to reproduce the heterogeneity
of this aquifer analogue, so that the results (strength and weaknesses of different geostatistical
simulation methods, relationship of connectivity indicators with flow and transport
parameters, etc.) obtained for this case study can be generalized to aquifers characterized by
similar geological situations. |
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