Détermination du temps de résidence des eaux souterraines : application au transfert d'azote dans les aquifères fracturés hétérogènes

Abstract : Heterogeneous fractured aquifers are extensively developed in crystalline rocks, such as schist
or granite. In these aquifers, porosity ranges from the fractures to the microporosity contained
in pores and fissures of the matrix. A better characterisation of such aquifers and especially
adapted to search for new water reservoir in Brittany, was undertaken through: (1) temporal
chronicles, isotopic analyses and laboratory experiments and (2) by the interpretation of
residence times through CFC analysis.
Groundwater hydrochemistry is related to rock heterogeneity leading to a more important
spatial variability than the temporal or seasonal variability in the catchments. In the aquifer,
nitrogen and sulphur biogeochemical cycles are linked through denitrification coupled to
sulphide oxidation. A denitrification coupled to an iron oxidation in crystalline rock has been
demonstrated in laboratory experiments. Sulphur and oxygen isotopic composition of sulphate
show a strong relationship between the spatial localisation in the watershed and the processes
in any studied site. In the deep part of the aquifer solutes from old circulating fluids are
conserved in the microporosity. These solutes may contribute to the chemical signature of
circulating groundwater.
This type of aquifer can be divided in 3 compartments: recharge areas, weathered part and
deep part. In each compartment, the groundwater residence times are homogeneous at the
scale of Brittany: (1) young water in the recharge areas, (2) old waters (> 50 yr old) in the
deep fractured part and (3) a 18 ± 2 yr old water in the weathered part of aquifers. While the
weathered part supports the water flux to rivers, these results indicate that water management
should to be done at the same timescale.
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Submitted on : Monday, July 31, 2006 - 9:34:15 AM
Last modification on : Friday, April 5, 2019 - 8:03:28 PM
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  • HAL Id : tel-00088100, version 1

Citation

Virginie Ayraud. Détermination du temps de résidence des eaux souterraines : application au transfert d'azote dans les aquifères fracturés hétérogènes. Hydrologie. Université Rennes 1, 2005. Français. ⟨tel-00088100⟩

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