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Titel |
Multi-isotopes systematics to constrain surface water-groundwater interactions in an alluvial plain: the Loire River case |
VerfasserIn |
Emmanuelle Pételet-Giraud, Philippe Négrel |
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 |
250049822
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Zusammenfassung |
This work is dedicated to wetlands of the Loire River systems and aims to better understand
the global functioning of the system from the hydrological, geochemical, ecological and
sociological aspects. The present study, using a coupled hydrological and geochemical (based
stable isotopes of the water molecule δ18O & δ2H and Sr isotopes) approach, focuses on the
‘Soulangy’ site with its secondary anastomosing channels just below the confluence of
the Loire and Allier rivers, and also on the ‘Dornant’ site with two unconnected
oxbow lakes 50 km upstream of the confluence. The stable isotopes of water show
that the alluvial (or riverbank) aquifer feeds the Loire River during the summer,
but, contrary to the classical scheme, is not recharged by the river during flood
periods in the winter; the alluvial groundwater thus has a purely local origin from
precipitation. The major elements reveal an anthropogenic input of Cl and more
importantly of nitrate, especially near farms. The 87Sr/86Sr isotopes identify different
groundwater layers in the alluvium, i.e. an upper and a lower alluvial aquifer, and a
perched aquifer at Dornant site, that have relatively complex relationships with the
surface water. The two main rivers (Loire and Allier) present distinct geochemical
characteristics reflecting the different lithologies that they drain upstream. In addition, the
secondary channels, lying parallel to the Loire main stream at the Soulangy site, give
different geochemical signatures, which shows that they are not fed by the same
overflows of the Loire; they are more-or-less well connected to the upper level of
the alluvial plain, and a longitudinal study of one of these channels has revealed a
Loire River influence progressively replaced by a water contribution from the upper
alluvial aquifer. Similarly, the two oxbow lakes at the Dornant site are not supplied
by the same water during the summer months. A conceptual scheme of the Loire
hydrosystem based on δ18O and 87Sr/86Sr suggests that the isotopic variations of
the Loire River can be related to a Massif Central surface-water supply for the
Loire and Allier main streams and to a groundwater supply from the alluvial plains. |
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