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| Titel |
Evolution of isolated caves in porous limestone by mixing corrosion: A model approach. |
| VerfasserIn |
W. Dreybrodt, D. Romanov, G. Kaufmann |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250025483
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| Zusammenfassung |
| When water from the surface, e.g. from a lake flows through porous carbonate rocks down
along some region with high hydraulic conductivity and encounters the water table of a
phreatic aquifer both waters mix by diffusion along their boundary. In a carbonate aquifer,
where both surface and phreatic waters are saturated with respect to calcite, mixing corrosion
causes renewed dissolution capacity Îceq of the carbonate rock in the diffusional mixing
zone extending from the boundary separating the phreatic water from the surface water
encountering it. A numerical model is presented from which the initial change
of porosity in such a diffusional mixing zone is obtained. The initial change of
porosity can be calculated from the local distribution m(x,y)of the mixing ratio m
and the second derivative of Îceq with respect to m. m = V sur-(Vsur + Vphr),
and the V’s assign the corresponding volumes of surface and phreatic water. The
second derivative has been calculated for three geochemical scenarios with differing
CO2-concentrations of surface and phreatic water by use of PHREEQC-2. The
spatial distribution m(x,y) is obtained by using MODFLOW and MT3DMS in a
modeling domain with constant hydraulic conductivity for various flow velocities of the
phreatic aquifer. From the results the time scale of cave evolution is estimated.
Passages of dimensions of about one meter in width and several 10 cm in height,
extending in length along the border line, where surface and phreatic water meet, can
be created in time scales of 10 000 years. These caves are horizontal with blind
ending passages and resemble closely to the isolated caves observed in Central West
Florida. For more realistic modeling we have used a geostatistical local distribution
of hydraulic conductivities in the modeling domain. For a correlation length of
1% of the length of modeling domain the spatial distribution extends deeper into
the flow direction. When the correlation length is increased by a factor of 10 flow
focusing distorts the diffusional mixing zone and enhances the creation of porosity. |
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