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| Titel |
Impact of sedimentologic and structural evolution of the Aquitaine Basin on actual water quality. Eocene sand aquifer, south western France. |
| VerfasserIn |
Pierre Durst, Eline Malcuit, Philippe Négrel, Emmanuelle Pételet-Giraud |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250054707
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| Zusammenfassung |
| In the sedimentary Aquitaine Basin, the Eocene Sand Aquifer system, mostly confined,
represents strategic resources for drinking water, irrigation, gas storage and geothermal
resources. Therefore, its quantity and quality issues are essential for the sustainable
management in this large region that extends over 116,000 km2 (i.e. one-fifth of the French
territory). The Eocene Sand Aquifer system comprises at least five aquifers: Paleocene,
Eocene infra-molassic sands, early Eocene, middle Eocene, and late Eocene. The extension
and thickness of Eocene aquifer layers and negative confined layers vary throughout the
basin, from several tens of metres to a hundred metres. The deposit sequences characterizing
the Eocene Aquifer System are progradational westward from detrital deposits
to carbonates. Eocene sands and Eocene limestones are hydraulically connected
and covered by an aquiclude of up to several hundred metres thick of molassic
sediments.
The groundwater recharge is assumed to occur through the Eocene outcrops located in the
north and north-east, and in the south east in contact with the Montagne Noire as well as by
vertical leakage from the upper and lower aquifers. Another recharge is suspected in the south
near the Petites Pyrenees.
The north and south evolutions of the piezometric surface are different. In the north,
because of years of pumping, a trough in the potentiometric surface has been formed. The
piezometric decline is roughly one meter per year in the depression centre. In the south, the
decline of the water table is roughly half a meter per year.
Two hydrodynamic models exist for the Aquitaine Basin (one in the north and the other in
the south) and allow knowing the flow circulations between the different aquifer
layers in this multi-layer system. Upward or/and downward leakages are observed
notably between the Eocene, Paleocene and Infra-Molassic Sands layers and the
others. However, a major difficulty for the sustainable management is the lack of
knowledge in the central part of this area, due to a lack of geological prospection
data.
The quality of waters induces others management difficulties. The geochemical analyses
reflect the influence of the lithologies (e.g. carbonates, silicates and evaporites). The highest
mineralization is generally encountered in water draining evaporitic formations; but can also
be observed in areas where such lithologies are not yet observed. Thus, many boreholes used
for drinking water supply present waters with sulphate and fluoride concentrations exceeding
the drinking water standards, while secondary resource for dilution is not always
present.
These mineralizations are related to at least three different processes:
In the south, under the effect of tectonic stresses due to the Pyrenean orogenesis,
the evaporitic Triassic deposits have been affected. Phenomena of halokinesis
and diapirism appeared. If some salt diapirs are known today at the surface,
others affect only the first overlying layers, and thus are less characterized.
Borehole near a diapir has a SO4 - Na water type reflecting anhydrite and/or
halite dissolution.
In the Gironde estuary, old brackish or saline water are trapped under the
Flandrian clay, in contact with underlying Eocene aquifer and contaminate some
wells.
In the Entre-deux-Mers and the Medoc, groundwaters of the Eocene aquifer
indicate the presence of evaporates despite there is no diapirism identified in the
vicinity. Some gypsum deposits are known at outcrop in the study area margin
and are assumed to come from non-marine deposits in the framework of endoreic
basins like temporary lakes.
Coupling hydrogeological, geological and geochemical approaches may help
to better understand and constrain management of this complex aquifer system. |
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